Proceedings Volume 9255

XX International Symposium on High-Power Laser Systems and Applications 2014

Chun Tang, Shu Chen, Xiaolin Tang
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Proceedings Volume 9255

XX International Symposium on High-Power Laser Systems and Applications 2014

Chun Tang, Shu Chen, Xiaolin Tang
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Volume Details

Date Published: 10 February 2015
Contents: 7 Sessions, 170 Papers, 0 Presentations
Conference: XX International Symposium on High Power Laser Systems and Applications 2014
Volume Number: 9255

Table of Contents

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Table of Contents

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  • Front Matter: Volume 9255
  • Solid State Laser Sources and System Engineering (Including Disk, Fiber, Hybrid Lasers)
  • High Power Diode Laser
  • Ultrashort Pulsed Laser Systems, Theory, and Simulation (UV, VUV, EUV Lasers)
  • Gas and Chemical Lasers (Including DPALs)
  • Resonators and Laser Beam Control
  • Applications, Laser Materials (Crystal, Ceramic), Novel Approaches
Front Matter: Volume 9255
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Front Matter: Volume 9255
This PDF file contains the front matter associated with SPIE Proceedings Volume 9255, including the Title Page, Copyright information, Table of Contents, Authors, Introduction, and Conference Committee listing.
Solid State Laser Sources and System Engineering (Including Disk, Fiber, Hybrid Lasers)
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Theoretical and experimental study on reabsorption effect and temperature characteristic of a quasi-three-level 946nm Nd:YAG laser
The influence of temperature and incident pump power on reabsorption loss is theoretically discussed. Temperature characteristic and reabsorption loss rate of a diode-pumped quasi-three-level 946 nm Nd:YAG laser are investigated. Reabsorption effect has a significant impact on laser performance. The results indicate that reabsorption loss increases as the working temperature rises and decreases with the increased incident pump power.
1540-nm single frequency single-mode pulsed all fiber laser for coherent Doppler lidar
Xin Zhang, Weifeng Diao, Yuan Liu, et al.
A single-mode single frequency eye-safe pulsed all fiber laser based on master oscillator power amplification structure is presented. This laser is composed of a narrow linewidth distributed laser diode seed laser and two-stage cascade amplifiers. 0.8 m longitudinally gradient strained erbium/ytterbium co-doped polarization-maintaining fiber with a core diameter of 10 μm is used as the gain fiber and two acoustic-optics modulators are adopted to enhance pulse extinction ratio. A peak power of 160 W and a pulse width of 200 ns at 10 kHz repetition rate are achieved with transform-limited linewidth and diffraction-limited beam quality. This laser will be employed in a compact short range coherent Doppler wind lidar.
A linearly-polarized directly diode-pumped L-shaped Er:YAG laser at 1617 nm with average output power scaling
Zhenzhen Yu, Mingjian Wang, Xia Hou, et al.
We report a linearly-polarized 1617 nm Er:YAG laser pumped by 1532 nm fiber-coupled laser diodes. An L-shaped resonator was employed incorporating two Er:YAG crystals which were pumped independently. In continuous-wave operation, a maximum output power of 7.73 W was obtained with an optical conversion efficiency of ~15.2% (versus incident pump power). To the best of our knowledge, this was the highest conversion efficiency ever demonstrated for a directly diode-pumped Er:YAG laser. In Q-switched operation, pulses with energy of 7.8 mJ and pulse duration of 80 ns were yielded at a repetition rate of 500 Hz, and the corresponding peak power was ~ 97 kW at 1617 nm.
Progress on the suppression of FM-to-AM modulations in SG-III laser facility
D. P. Xu, R. Zhang, X. C. Tian, et al.
FM-to-AM modulations are harmful to output characteristics of large-scale laser facility. In SG-III laser facility, some key techniques have been employed to suppress FM-to-AM modulations. Firstly, phase modulator was arranged at the end of fiber laser injection system to avoid GVD and PMD in the SM fiber system. Secondly, a fiber-based polarization rotated filter was proposed to suppress FM-to-AM modulations independently on each beam. Finally, less wave plates were employed and liquid crystal modulators were coated for decreasing weak etalon effect in preamplifier system. The results indicated that it can reduce the modulation depth less than 10% at a modulation frequency of 2.488GHz in SG-III laser facility.
High purity efficient first Stokes Raman laser
Xiaomeng Liu, Qinyong Liu, Daijun Li, et al.
The subject of the solid-state Raman frequency conversion to the yellow frequency spectra has been an active topic since the mid 1990’s, because of its application in bio-medical and astronomy fields. However, the yellow laser performance is often limited because of the cascade conversion to second or higher Stokes. This cascade conversion not only limits the conversion efficiency and the output power of the first Stokes, but also degrades the pulse and the beam profile of the first Stokes. We present a type of polarization coupled Raman resonator, in which the higher order ( the second Stokes and higher ) laser output can be dramatically suppressed. Our Raman resonator is pumped by a Q-switched and frequency doubled slab laser, and we can get an almost pure (P559/(P559 +P532)>99%) 559 nm yellow light output with an efficiency over 39% from 532 nm to 559 nm. The resonator includes a high reflection rear mirror, a KGW crystal, a polarization coupled input/output element, and a high reflection output coupler of 559 nm (R559 nm = 0.6). Furthermore, we have proposed an improvement of this polarization coupled Raman resonator. The theoretical calculations of the temporal and spatial dependent Raman conversion equations show that the conversion efficiency of the first order Stokes is greatly enhanced with an additionalλ/2 waveplate for 589 nm and the BBO crystal.
High power mid-infrared continuous-wave optical parametric oscillator pumped by fiber lasers
3~5μm mid-infrared laser has many important applications, such as gas detection, spectral analysis, remote sensing, medical treatment, and also in the military laser radar, infrared countermine, and so on. Optical parametric oscillator (OPO) is an efficient way to generate laser in this wavelength range, which has attracted the eyes of many people. In this paper, the recent development of mid-infrared OPO is overviewed. Meanwhile, detailed introduction on our recent work is given. Maximum idler output power of 34.2W at center wavelength of 3.35μm was obtained, to our knowledge, which is the new power record of the international public reporting for the continue-wave (CW) mid-infrared OPO. It is worth mentioning that the pump source, the quasi single-frequency (SF) narrow line width fiber laser, was also developed by our groups. According to the current status of research, some solutions is proposed in order to achieve higher power, narrower line width, and compact volume mid-infrared OPO in a wide tunable range.
1047nm 270mJ all solid state diode pumped MOPA at 50 Hz
Jian Ma, Qi Yang, Tingting Lu, et al.
A diode-pumped nanosecond Master Oscillator Power Amplifier (MOPA) system based on Nd:YLF crystal slabs has been demonstrated. The seed pulses with pulse duration of 11 ns were generated in an EO Q-switched Nd:YLF laser, with single pulse energy of 10 mJ. The 1047 nm signal pulses were amplified in a double-pass amplification system. Maximum output pulse energy of 270 mJ at a repetition rate of 50 Hz has been achieved with effective optical-to-optical efficiency of 14.5%.
Passively Q-switched high-energy all-solid-state holographic Nd:YAG laser with a multiloop cavity
V. F. Lebedev, A. P. Pogoda, A. S. Boreysho, et al.
A high-energy, high-beam-quality laser based on a single Nd:YAG rod with laser diode side-pumping and multiloop self-adaptive reciprocal cavity is presented. The optimized laser cavity geometry allowed to change the laser oscillation from single 200-ns self-Q-switched pulse followed by low-intensity free-running lasing to repetitive high-intensive pulses by periodically writing and erasing holographic gain gratings in the active Nd:YAG medium. The intensified self-Q-switching pulse train oscillation was realized by the interaction with a diffusely reflecting target placed in the focal plane of a lens at the laser output. Effects of shortening of the repetitive laser pulses down to 70-ns duration, stabilization of the pulse repetition period (~ 5 μs), and mode-locking temporal modulation of the pulses were observed. The use of a passive F2–:LiF Q-switch resulted in stable giant pulse train oscillation with total output energy of up to 0.43 J, individual pulse energy of 50 mJ, and its peak power of 4 MW at the individual pulse duration of 12 ns.
1KHz high average power single-frequency Nd:YAG laser
Xiaolei Zhu, Xiuhua Ma, Shiguang Li, et al.
A laser-diode-pumped master oscillator and power amplifier was developed with high efficiency, high average power and high beam quality. The oscillator is an injection-seeding, fiber coupled diode-end-pumped E-O Q-switched Nd:YAG laser, producing single frequency pulse laser output with pulse energy of 8mJ and pulse width of 11ns at a pulse repetition rate of 1KHz,The 1KHz was divided into four chains with frequency of 250Hz, through E-O modulation technology, The power amplifier utilizes conductively-cooling Nd:YAG zigzag slab with two sides’ pump architecture at bounce point. Pulse energy of more than 800mJ with pulse widths of 12.6ns was obtained at repetition rate of 250Hz in every amplifier chain, the frequency-doubled pulse energy of 360mJ when KTP crystal was used was obtained at a repetition of 250Hz.
1.4kW all-fiber narrow-linewidth polarization-maintained fiber amplifier
We report on the high power amplification of narrow-linewidth polarization-maintained (PM) laser in all-fiber PM MOPA configuration, which can operate with linewidth around 2GHz at above 1kW power level. Pump-limited maximum narrow-linewidth output power is 1.4kW without SBS effect, and the linearly-polarization single-mode output power was limited to ~550W by mode instabilities (MI). The beam quality of the fiber amplifier (M2 ) was measured under different power, which degrades gradually from ~1.4 to ~2.2 after the onset of MI. The polarization extinction rate is measured to be about 90% before the onset of MI and reduce dramatically after MI sets in. The experimental results are analyzed based on a novel semi-analytical model, which has taken the effects of gain saturation into account. The theoretical results agree well with the experimental results. Mitigating MI by coiling the gain fiber has been analyzed and demonstrated numerically. It shows that, by tight coiling of the gain fiber to a radius of 4cm, the MI threshold can increase to 2.5 times higher than that without coiling or loose coiling, which means that the amplifier have the potential to achieve MI-free 1.4kW output power.
Thermal management of liquid direct cooled split disk laser
The thermal effects of a liquid direct cooled split disk laser are modeled and analytically solved. The analytical solutions with the consideration of longitudinal cooling liquid temperature rise have been given to describe the temperature distribution in the split disk and cooling liquid based on the hydrodynamics and heat transfer. The influence of cooling liquid, liquid flowing velocity, thickness of cooling channel and of disk gain medium can also be got from the analytical solutions.
160mJ and 9ns electro-optics Q-switched conductively cooled 1047nm Nd:YLF laser
Qi Yang, Jian Ma, Tingting Lu, et al.
A compact diode side-pumped conductively cooled 1047 nm Nd:YLF slab laser with high energy and short pulse width is developed. Through ray tracing method, we design a home-made pump module to homogenize the pump intensity. Based on the Possion equation, a thermal conduct model of side-pump laser is established. The temperature distribution in laser crystal is obtained, and the thermal lens is caculated. With the absorbed pump energy of 818 mJ, the maximum output energy of 228 mJ is achieved in free-running mode. At a repetition rate of 50 Hz, 160 mJ, 9 ns 1047 nm infrared light is obtained under the maximum absorbed pump energy, and the slope efficiency is 27.8%.
Precise tuning technique of frequency conversion crystals of SG facility
Fan Zhang, Wei Zhong, Huai Ting Jia, et al.
To assure that the frequency conversion crystals of SG facility are always being in the best shot position, and have the highest energy conversion efficiency with various beam conducting directions, we researched the precise tuning technique of frequency conversion crystals with the SG facility. The on-line quick tuning method of target point detecting with preshot was used in precise tuning of crystal match angle, and helped to correct the off-line match angle satisfactorily. With crystal alignment technique and crystal accompany technique, the precise tuning of crystal match angle can assure the crystals to maintain a stable high efficiency in a long experiment period.
Investigation on graded reflectivity mirror unstable laser resonators with larger intra-cavity phase aberrations
Yu-Xin Wei, Hua Su, Xiao-Jun Wang
In this paper, the operation properties of unstable resonators with graded reflectivity mirror (GRM-UR) are studied numerically in a solid-state thin-disk laser in terms of beam quality and power threshold. By comparing to traditional unstable resonator with same output coupling fraction, results show that the GRM-UR is advantageous to suppress ASE effect, but hard to achieve better beam quality when the phase aberrations cannot be well compensated within the cavity.
Development of large-aperture electro-optical switch for high power laser at CAEP
Xiongjun Zhang, Dengsheng Wu, Jun Zhang, et al.
Large-aperture electro-optical switch based on plasma Pockels cell (PPC) is one of important components for inertial confinement fusion (ICF) laser facility. We have demonstrated a single-pulse driven 4×1 PPC with 400mm×400mm aperture for SGIII laser facility. And four 2×1 PPCs modules with 350mm×350mm aperture have been operated in SGII update laser facility. It is different to the PPC of NIF and LMJ for its simple operation to perform Pockels effect. With optimized operation parameters, the PPCs meet the SGII-U laser requirement of four-pass amplification control. Only driven by one high voltage pulser, the simplified PPC system would be provided with less associated diagnostics, and higher reliability. To farther reduce the insert loss of the PPC, research on the large-aperture PPC based on DKDP crystal driven by one pulse is developed. And several single-pulse driven PPCs with 80mm×80mm DKDP crystal have been manufactured and operated in laser facilities.
Design and analysis on thermal adaptive clamping device for PPMgLN crystal used in solid state laser
Conglin Yan, Yongliang Chen, Wei Zhang
The quality of clamping device for PPMgLN crystal has a vital influence on the optical property of solid-state laser. It has highly requirements of work stability and environmental adaptation ability, especially the thermal adaptation under high temperature differences. To achieve thermal adaptation, structural stiffness will be unavoidably weakened. How to keep both enough stiffness and thermal adaptation as far as possible is the key design point and also difficult point. In this paper, a kind of flexible thermal release unit which can work permanent under 130±10°C is studied. Thermal compensation principle and flexible thermal release theory are applied. Analysis results indicate that this device can effectively decreased the thermal stress of the crystal from 85MPa to 0.66MPa. The results of the vibration resistance test on the optical axis direction of the crystal indicate that the device can provide at least 5.62N to resistant 57.2g impact vibration and 18.5g impact vibration in the side direction, well satisfied the requirements of ability to resistant 6g impact vibration.
The study of the thermally expanded core technique in end-pumped (N+1)×1 type combiner
Tapering will raise the signal loss in an end-pumped (N+1)×1 type combiner. In this paper, the Thermally Expanded Core (TEC) technique is used in the signal loss optimization experiment with the tapering ratio of the pump combiner is 0.6. The experimental results indicate that the coupling efficiency of the 1.55μm signal light increases from 81.1% to 86.6%, after being heated 10 minutes at the homo-waist region of the tapered signal fiber with an 8mm wide hydroxygen flame. Detail analysis shows that the TEC technique can both reduce the loss of the LP01 mode and the LP11 mode in the signal fiber.
Novel front end design for synchronized output pulses with zero timing jitter in XG-III laser facility
Song Zhou, Lei Zhao, Kainan Zhou, et al.
XG-III laser facility is a petawatt laser which has a unique feature of three synchronized pulses output for various pump-probe experiments. To realize the synchronization with zero timing jitter, we have designed and implemented a novel front-end system based on super-continuum injected femtosecond optical parametric amplification (fs:OPA). Critical parameters of fs:OPA were optimized for the best conversion efficiency. Experimental results verified that major design specifications such as pulse energy, central wavelength and spectral width were fully accomplished and a high pulse contrast ratio was also achieved by the fs:OPA process.
Research of high brightness 1018nm ytterbium doped fiber laser
Yanshan Wang, Weiwei Ke, Yinghong Sun, et al.
The effect of the gain fiber parameters for the design of a short wavelength ytterbium doped fiber laser (YDFL) is analyzed theoretically, and based on these analyses, we find viable approaches to improve the laser performances. High power 1018 nm YDFL system was constructed, output characteristics of 1018 nm laser with the core/cladding, doping concentration and gain fiber length are experimentally investigated. It is found that increasing the core/cladding area ratio, reducing the doping concentration and shortening the fiber length is effective way to suppress the amplified spontaneous emission (ASE) gain and ensure the laser operation at 1018 nm. A record output power of 154 W at 1018 nm is obtained, with a brightness of 5.7GW/cm2 -sr, optical-optical conversion efficiency is 71% when the laser operated at maximal power. Meanwhile, ASE and spurious oscillation are suppressed efficiently.
Experimental design of double-cladding planar waveguide laser amplifier
Juntao Wang, Xiaojun Wang, Tangjian Zhou, et al.
An end-pumped laser amplifier with high efficiency and compactness was designed. The thermal stress of symmetrical single-cladding and double-cladding planar waveguide with Nd:YAG as its core were analyzed theoretically, and the maximum thermal load for each were also obtained according to the stress fracture limit. For different inner cladding thickness, the maximum pump power that could be absorbed was calculated. The laser medium was chosen to be a symmetrical double-cladding planar waveguide with Nd:YAG as its core, the dimensions of the gain area were 50mm×12mm×100μm, and the waveguide were 60mm×12mm×2mm. The single thickness of inner cladding YAG and the outer cladding sapphire were 250μm and 700μm respectively. The size of LDA pump light was 18.9mm and 10mm respectively in fast axis and slow axis. The seeder was coupled into the waveguide from one end, and the outer cladding were welded with two heat sink for heat transfer. By theoretical calculation, if the seeder into the waveguide is 0.1W, the outputting power of 1651W could be obtained when the pump power from the LD array was 3600W, with the optical-optical efficiency of 46%.
An improved method for stripping cladding light in high power fiber lasers
Tenglong Li, Juan Wu, Yinghong Sun, et al.
In order to ensure the high power all-fiber laser reliability and excellent beam quality, it is necessary to strip the unwanted cladding light. The common method for stripping cladding light is to recoat the double cladding fiber with a high index gel, but localized heating and low thermal conductivity of the recoating gel are the prime factors limiting the power-handling capability of the cladding power stripper(CPS). An improved fabrication technique to manufacture the CPS is presented. Light stripping section of the fiber is fused with a transparent quartz tube, by applying different amount of etchant along the quartz tube, frosted surface is created and uniformly removal of the cladding light is achieved. The quartz tube is joined to water-cooled thermal enclosure tightly without the gel to avoid heat aggregation. The power-handling capability of the device is tested under 200W of cladding light, and attenuation of 20 dB is achieved.
Study on the thermal distribution and thermal management of high average power fiber lasers
Yongliang Zhang, Lei Zhao, Xiaobao Liang, et al.
The thermal problems of CPS and YDF were studied. And the thermal management technologies are developed separately to the problems. Experimental results showed that the thermal management technologies worked well.
Home-made high efficiency side-pumped fiber amplifiers
Zhihe Huang, Jianqiu Cao, Shaofeng Guo, et al.
The side-coupled cladding-pumped fiber has many advantages in terms of system design, ease of pump injection, signal extraction and power scalability over traditional double cladding fiber. We demonstrate a 290-W all home-made side-coupled cladding-pumped fiber laser with the slope efficiency of 67.8%. To the best of our knowledge, the slope efficiency is the highest for the published high power fiber laser based on the DSCCP fiber. The slope efficiency of the home-made side-coupled cladding-pumped fiber amplifier is mainly limited by the background loss, pump light coupling and pump light absorption. The results show that the power ratio of signal light to residual pump light is near 20 dB. The pump light is not absorbed enough that the slope efficiency can be further improved by optimizing the amplifier structure.
Numerical analysis of the convergence speed of the SPGD algorithm with two different perturbation methods in coherent beam combination using active segmented mirror
Yi Tan, Xinyang Li, Wen Luo, et al.
An introduction was given to the active segmented mirror(ASM) used in coherent beam combination firstly. Based on the actuator distribution of ASM, the computational formula between the perturbation voltage of each actuator and the perturbation of piston or tip/tilt was acquired. Then the numerical analysis of correction process with two different perturbation ways in SPGD algorithm was carried on. By comparing the two correction processes, it was found that the convergence speed of the second method is 25 times as fast as the first one when there only existed piston error between the two beamlets, while the convergence speed was almost the same when there only existed tip/tilt error, and the convergence speed is 5 times when there existed both piston and tip/tilt error.
Numerical analysis to four-wave mixing induced spectral broadening in high power fiber lasers
Yujun Feng, Xiaojun Wang, Weiwei Ke, et al.
For powers exceeding a threshold the spectral broadening in fiber amplifiers becomes a significant challenge for the development of high power narrow bandwidth fiber lasers. In this letter, we show that the spectral broadening can be partly caused by four-wave mixing(FWM) process in which the power of the central wavelength would transfer to the side ones. A practical FWM induced spectral broadening theory has been derived from the early works. A numerical model of fiber amplifier has been established and FWM process has been added to the model. During the simulation process, we find that when a 10 GHz, several watts narrow bandwidth laser is seeded into a few modes fiber laser amplifier, the FWM induced spectral broadening effect might continually increase the FWHM of the spectra of the continuum laser to 100 GHz within the amplification process to several hundred watts which has been convinced by our experiments. Some other results have also been analyzed in this paper to complete the four-wave mixing induced spectral broadening theory in fiber amplifiers.
Nd:YAG thin-disk laser with large dynamic range unstable resonance
Jianli Shang, Yi Yu, Xiangchao An, et al.
In this paper, based on the self-reproduction condition of laser wavefront curvature, the influences of disk defocus on laser parameters can be calculated. The laser-pumping overlapping efficiency will decrease by 9%; the magnification will rise to 2.3, and the intra-cavity loss will be high to 30% due to a laser beam size mismatch when each disk has focal length of -100 m in a positive-branch confocal unstable resonator containing four disks with magnification of 1.8. Therefore, the optical conversion efficiency and stability will be reduced significantly. Several methods defocus compensation of are compared, it can be found that inserting variable-focus lens in resonant is useful in large dynamic range. In experiment, the defocus values are measured in different pumping power. A lens group, used for compensate components according to the single pass probe, is carefully designed. Under this compensation, the pulse energy can be maintained in 10 J from 1 Hz to 100 Hz. The output power can be improved 2.33 times compared to non-compensation condition.
Experimental characterization of beam quality of a Yb:YAG thin disk laser
In this paper we investigate the effects of cooling water temperature and pumping diode laser beam profile on the disk laser beam quality. The results show that both issues are important and can influence the beam quality but at the conditions of our experiment these issues do not affect the beam quality significantly.
Numerical approach to temperature and thermal stress in direct-liquid-cooled Nd:YAG thin disk laser medium
Zhibin Ye, Zhen Cai, Bo Tu, et al.
A Nd:YAG thin disk is end-pumped by two high power laser diodes and the fluid flows in a narrow channel to cool it directly. The forced convection occurs between the fluid and disk. A system is designed to measure the convective heat transfer coefficient with different flow rate. With the measured coefficient, the temperature and thermal stress in the disk are numerically analyzed. The maximum permissible thermal load is calculated, which increases with the increasing flow rate. Furthermore, the optical path different distribution is numerically calculated by considering of the thermo-optical effect, and thermal expansion at the maximum permissible thermal load. These results are useful for design of a direct-liquid-cooled Nd:YAG thin disk laser.
All-fiber wavelength-tunable Tm/Ho-codoped laser between 1727 nm and 2030 nm
Lasers in the eye-safe 2 μm spectral region are attracting significant interest due to a variety of applications such as atmospheric lidar sensing and medical treatment, which require laser sources matching the absorption lines of various molecules in the 2 μm wavelength region. We demonstrate an all-fiber Tm/Ho-codoped laser operating in the 2 μm wavelength region with a wide wavelength tuning range of more than 300 nm. The Tm/Ho-codoped fiber laser (THFL) was built in a ring cavity configuration with a fiberized grating-based tunable filter to select the operating wavelength. The tunable wavelength range of the THFL was from 1727 nm to 2030 nm. To the best of our knowledge, this is the widest tuning range that has been reported for an all-fiber rare-earth-doped laser to date. Efficient short wavelength operation was also achieved. The output power of the THFL was further scaled up from 1810 nm to 2010 nm by using a stage of Tm/Ho-codoped fiber amplifier (THFA), which exhibited the maximum slope efficiency of 42.6% with output power of 408 mW at 1910 nm.
High Power Diode Laser
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HiLASE: development of fully diode pumped disk lasers with high average power
M. Divoky, M. Smrz, M. Chyla, et al.
An overview of Czech national R&D project HiLASE (High average power pulsed LASEr) is presented. The HiLASE project aims at development of pulsed DPSSL for hi-tech industrial applications. HiLASE will be a user oriented facility with several laser systems with output parameters ranging from a few picosecond pulses with energy of 5 mJ to 0.5 J and repetition rate of 1-100 kHz (based on thin disk technology) to systems with 100 J output energy in nanosecond pulses with repetition rate of 10 Hz (based on multi-slab technology).
A 3000W 808nm QCW G-stack semiconductor laser array
With the improvement of output power, efficiency and reliability, high power semiconductor lasers have been applied in more and more fields. In this paper, a conduction-cooled, high peak output power semiconductor laser array was studied and developed. The structure and operation parameters of G-Stack semiconductor laser array were designed and optimized using finite element method (FEM). A Quasi-continuous-wave (QCW) conduction-cooled G-Stack semiconductor laser array with a narrow spectrum width was fabricated successfully.
Packaging of complete indium-free high reliable and high power diode laser array
High power diode lasers have been widely used in many fields. For many applications, a diode laser needs to be robust under on-off power-cycling as well as environmental thermal cycling conditions. To meet the requirements, the conduction cooled single bar CS-packaged diode laser arrays must have high durability to withstand thermal fatigue and long lifetime. In this paper, a complete indium-free bonding technology is presented for packaging high power diode laser arrays. Numerical simulations on the thermal behavior of CS-packaged diode laser array with different packaging structure were conducted and analyzed. Based on the simulation results, the device structure and packaging process of complete indium-free CS-packaged diode laser array were optimized. A series of high power hard solder CS (HCS) diode laser arrays were fabricated and characterized. Under the harsh working condition of 90s on and 30s off, good lifetime was demonstrated on 825nm 60W single bar CS-packaged diode laser with a lifetime test of more than 6100hours achieved so far with less 5% power degradation and less 1.5nm wavelength shift. Additionally, the measurement results indicated that the lower smile of complete indium-free CS-packaged diode laser arrays were achieved by advanced packaging process.
Research on 9xx nm diode laser for direct and pumping applications
J. Sebastian, R. Hülsewede, A. Pietrzak, et al.
High-power laser bars and single emitters have proven as attractive light sources for many industrial applications such as direct material processing or as pump sources for solid state and fiber-lasers. There is also a great interest in quasi-CW laser bars for high-energy projects. These applications require a continuous improvement of laser diodes for reliable optical output powers, high electrical-to-optical efficiencies, brightness and costs. In this paper JENOPTIK presents an overview of recent research for highly efficient CW and quasi-CW laser devices emitting in a wide wavelength range between 880 nm and 1020 nm. The last research results concern the 9xx single emitters and laser arrays. The 9xx nm 12 W single emitters and 976 nm 55 W laser arrays have efficiencies above 65%. New life time tests for single emitter devices currently exceed 1300 hours of reliable operation at room temperature and over 1500 hours at 45°C. Because of the small far field distribution of the optical power, the high output power and the small near field the 55 W arrays show a brightness of 75 MW x cm-2sr-1 with 95% power content. The technology for new generation 940 nm high fill-factor bars has been currently extended to emission wavelengths of 976 nm and 1020 nm with excellent results: 200 W output power with 63% efficiency using passive cooling. The innovative design of the laser structure enables, moreover, the realization of 500 W 880 nm quasi-CW laser bars with wall-plug efficiencies of 55% and a narrow fast-axis divergence angle of 40° (95% power content).
Generation of 6.05J nanosecond pulses at a 1Hz repetition rate from a cryogenic cooled diode-pumped Yb:YAG MOPA system
Xiaojin Cheng, Jianlei Wang, Zhongguo Yang, et al.
Diode-pumped solid state laser system based on cryogenic Yb:YAG active-mirror scheme are presented with recent energy output. With improved optical design, 6.05J/1Hz pulse energy is achieved and a conceptual design with 30J output energy is theoretical simulated. The doubling efficiency of YCa4O (BO3)(YCOB) crystal is also discussed in this paper.
Ultrashort Pulsed Laser Systems, Theory, and Simulation (UV, VUV, EUV Lasers)
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Progress on the XG-III high-intensity laser facility with three synchronized beams
The paper presents the technical design and progress on a special high-power laser facility, i.e. XG-III, which is being used for high-field physics research and fast ignition research. The laser facility outputs synchronized nanosecond, picosecond and femtosecond beams with three wavelengths, i.e. 527 nm, 1053 nm and 800 nm respectively, and multiple combinations of the beams can be used for physics experiments. The commissioning of the laser facility was completed by the end of 2013. The measurement results show that the main parameters of the three beams are equal to or greater than the designed ones.
Production of petawatt laser pulses by backward Raman amplification in plasma
Zhaohui Wu, Yanlei Zuo, Jingqin Su, et al.
Backward Raman amplification (BRA) in plasma has been demonstrated an effective way to produce high power laser pulses. However, most experiments of BRA are carried out around the pump wavelength of 800 nm. In recent years, the 1053 nm pump pulse becomes more and more essential as the development of the chirped pulse amplification (CPA) around this wavelength. Here we design an experiment of BRA with a 1053 nm, 20 ps pump pulse and a 1200 nm, 50 fs seed pulse based on the facility of XG III. The simulation results obtained by a 1-d particle-in-cell (PIC) code show that the amplified peak seed intensity of ∼ 5 × 1016 W/cm2 is obtained, with an energy transfer up to 16.8%. An output pulse of petawatt power is theoretically demonstrated feasible.
Generation of high average power supercontinuum involve visible spectrum
Lei Zhao, Xiaobao Liang, Chao Li, et al.
A high average-power all-fiber supercontinuum laser source is constructed. By integrating series techniques together, the output average power achieves 65W with the spectrum range covering two octaves from 540nm to 2200nm. To our knowledge, there has been never reported similar supercontinuum source with such high average power, broadband spectrum and picosecond pulse width.
Spectral shaping of the amplified signal in optical parametric chirped pulse amplification
Jing Wen, Xiaoming Zeng, Song Zhou, et al.
A spectral shaping method during the optical parametric chirped pulse amplification is presented. The relationship between the temporal shape of the signal pulse and the pump pulse is analyzed theoretically, which shows that the temporal shape of the signal pulse can be modulated through modulation of the pump pulse. This is proven by our verification experiment. And we have successfully used this method in the pre-amplifier of the XG-III laser facility to modulate the signal spectrum to match the requirements of the main amplifier.
Application problems of object-image-grating self-tiling in ultra-short pulse lasers
Meter-sized gratings are urgently required by ultra-short ultra-high lasers, especially petawatt and multi-petawatt lasers, in the world wide. Object-image-grating self-tiling method gives a way to achieve an ideal grating tiling condition by completely eliminating three tiling errors within a traditional tiled grating. However, some application problems challenges it actual application in laser systems. In this paper, we will discuss the influence of two important problems: the stability problem and the spectrum amplitude modulation problem. And related simulations and analyses are developed to promote the application of this method in large-scaled lasers systems.
Method to realize real-time monitoring and control of a tiled-grating compressor for the XGIII laser facility
Zhilin Li, Xiao Wang, Jie Mu, et al.
In a high-energy chirped-pulse-amplified laser system, grating tiling technology provides an effective means to increase the aperture of the gratings and to scale the energy and irradiance of short-pulse lasers. The difficulties lie in controlling tiling errors accurately between the sub-gratings and keeping long time stability. In this paper, a two-pass full-tiled grating-compressor (TGC) with real- time control unit is developed for the first time. The far-field distributions of the 0th order and -1 st order diffracted beams of the two pairs of tiled gratings are monitored by the same CCD system, with the main laser chain being not disturbed. In this way, we realize online real-time control of tiling errors. Through a method of locking the far-field image to compensating the temporal drift, we can realize the automation of the assembly. The TGC has successfully applied in the multi-function XGIII laser facility, and focusing focal spot and output pulse width are obtained.
Improvement of temporal contrast by 2 order of magnitude based on the deformable mirrors
Bo-peng Wang, De-en Wang, Kai-nan Zhou, et al.
Chirp pulse amplification (CPA) has been promoted as an effective way to explore the intensity frontier. High order dispersion induced by the stretcher and materials in the CPA system, which deteriorates both the pulse duration and temporal contrast, however, can not be absolutely compensated by the compressor. Placed at the Fourier plane of a 4f zero-dispersion stretcher consisting of a grating, the deformable mirror (DM) has been demonstrated as the modulator to compensate high order dispersion. Using the method of ray tracing, the relation between spectrum and position on DM has been obtained. It shows that the resolution of the deformable mirror can be controlled by adjusting the focal length and incident angle. We have simulated a typical Ti:sappire CPA system to revise the spectral phase by the DM. The result illustrates that if the spectral phase can be compensated, the temporal contrast will be improved by 2 order of magnitude.
Array element auto-tiling based on capacitive displacement sensor
Jie Mu, Feng Jing, Xiao Wang, et al.
Array element tiling is one of the key technologies for the coherent beam combination in a high-power laser facility. In this paper, we proposed a method of the array element auto-tiling based on capacitive displacement sensor. The method was verified on a double-pass tiled-grating compressor in XG-III laser facility. The research showed that the method is an effective way to control the misalignment errors automatically, with high precision and long-term stability.
Amplitude instability of the dissipative soliton mode-locked fiber laser
Chao Li, Yongliang Zhang, Xiaobao Liang, et al.
The singular point of the dissipative soliton mode-locked fiber laser is demonstrated experimentally. Mode-locked pulses are severely disturbed under certain pump power. The peak-valley (P-V) of the output power reaches up to 26.5% under the pump power of 918mW. However, mode-locked fiber laser can operate stably under higher or lower pump power. A numerical model based on nonlinear Schrödinger equation (NLSE) is established. And the singular point of the mode-locked state is theoretically proved.
Effect of acoustic waveguide properties on the Brillouin gain spectrum in multi-mode fibers
Wei-Wei Ke, Xiao-Jun Wang, Xuan Tang
With a recent developed mode-coupling model, the Brillouin gain spectra (BGS) of multi-mode fibers (MMF) are investigated and compared with the corresponding experiment. It is found that the calculation results are coincident well with the experiment data. Furthermore, the BGS are found to be very sensitive to the index fluctuation. Such phenomenon is demonstrated by introducing a small index hump or dip in the center of the fiber core. And it can be explained by that the index fluctuation may influence the acoustic mode greatly.
Eigenmodes of large-mode-field laser resonators with intra-cavity phase aberrations
Hua Su, Xiao-Jun Wang, Yu-Xin Wei, et al.
A eigenmode expansion method (EME) is proposed to solve the laser eigenmode of optical resonator with intra-cavity phase aberration (ICPA) semi-analytically. In this model, the eigen-equation of OR, so called self-reappearance condition is translated to be a linear eigen-value problem, and it is proved that all eigen-modes can be obtained for any resonators. The linear eigen-value problem is solved numerically, and it gives out the transverse distribution and corresponding eigen-value of each eigenmode, which describe the light field and diffraction loss, respectively. Compared with traditional methods, EME is a semi-analytical method which is unlimited by the order of phase aberration, and it can be solved without numerical iteration. The existing of local modes (LM) in OR with ICPA is proved with EME, which may be the source of local damage on solid medium. And the use of output coupler with transmission, such as graded reflectivity mirror (GRM), can prevent the appearance of LM and improve beam quality. Specially, for the ICPA coupled with laser extraction, the linear eigen-value equations become a nonlinear problem, which are numerically solved by the finite-difference Jacobian method. The result shows that the optical resonator exhibits transverse modal instability (TMI) with certain cavity parameters.
Theoretical study of mode evolution properties in a 3×1 adiabatic tapered single-mode fiber combiner
Xuanfeng Zhou, Zilun Chen, Hang Zhou, et al.
We study the mode evolution properties in a 3 x 1 adiabatic tapered single-mode fiber combiner (ATSMFC) in theory. The fabrication of the combiner for single mode fibers based on adiabatic tapered fused bundle (TFB) technique with the assistant of low index glass capillary is introduced. The whole taper region can be seen as three phase: single-mode fibers, multi-core fiber and multi-mode fiber. Supermodes of three-core fiber with scalar mode results are derived based on coupling mode theory. The analysis is verified with numerical examples by fully vectorial finite element mode solver (Cosmol Multiphysics). Simulation results show that the three input core modes in single-mode fibers gradually evolve into three supermodes in three-core fiber and then evolve into three low-order modes in the multi-core fiber. Effective indices for different modes are calculated which can depict the evolution process vividly. The results may be useful for practical high power fiber laser systems.
Analysis of thermal stresses in HfO2/SiO2 high reflective optical coatings for high power laser applications
Chunxue Gao, Zhiwei Zhao, Zhuoya Zhu, et al.
HfO2/SiO2 high reflective optical coatings are widely used in high power laser applications because of their high laser damage resistance and appropriate spectral performance. The residual stresses strongly influence the performance and longevity of the optical coatings. Thermal stresses are the primary components of the residual stresses. In the present work, the distribution of thermal stresses in HfO2/SiO2 high reflective optical coatings was investigated using two different computational methods: finite element method (FEM) and an analytical method based on force and moment balances and classical beam bending theory. The results by these two methods were compared and found to be in agreement with each other, demonstrating that these two methods are effective and accurate ways to predict the thermal stresses in HfO2/SiO2 optical coatings. In addition, these two methods were also used to obtain the thermal stresses in HfO2/SiO2 optical coatings with different layer number to investigate the effect of the layer number on the thermal stresses of the HfO2/SiO2 optical coatings. The results show that with the increase of the layer number, the stresses in the substrate increase, while the stresses in the respective SiO2 and HfO2 layers decrease. Besides, it was also found that the radius of curvature of the coating system decreases as the layer number increases, leading to larger bending curvature in the system.
Investigation of defect rate of lap laser welding of stainless steel railway vehicles car body
In order to resolve the disadvantages such as poor appearance quality, poor tightness, low efficiency of resistance spot welding of stainless steel rail vehicles, partial penetration lap laser welding process was investigated widely. But due to the limitation of processing technology, there will be local incomplete fusion in the lap laser welding seam. Defect rate is the ratio of the local incomplete fusion length to the weld seam length. The tensile shear strength under different defect rate and its effect on the car body static strength are not clear. It is necessary to find the biggest defect rate by numerical analysis of effects of different defect rates on the laser welding stainless steel rail vehicle body structure strength ,and tests of laser welding shear tensile strength.
Research on power spectral density after nonlinear propagation in high-power solid state laser
Wan-Qing Huang, Ying Zhang, Wen-Yi Wang, et al.
The Power Spectral Density (PSD) is the specification of mid-spatial-frequency phase error in high power solid-state laser facilities. Its evolution during propagation and connection with laser performance have not been fully discussed due to the complexity of nonlinear propagation. We present in this paper the analytic forms of PSD of output phase and intensity modulation, which could be obtained from the input phase PSD introduced by imperfect optics. Moreover, we connect the PSD with the important laser performance parameters: the relative intensity of wings on the focal spot and the contrast ratio of the near-field.
Laser superposition in multi-pass amplification process
Physical model was established to describe the pulse superposition in multi-pass amplification process when the pulse reflected from the cavity mirror and the front and the end of the pulse encountered. Theoretical analysis indicates that pulse superposition will consume more inversion population than that consumed without superposition. The standing wave field will be formed when the front and the end of the pulse is coherent overlapped. The inversion population density is spatial hole-burning by the standing wave field. The pulse gain and pulse are affected by superposition. Based on this physical model, three conditions, without superposition, coherent superposition and incoherent superposition were compared. This study will give instructions for high power solid laser design.
Simulation code of high power discharge for iPhones
Takuro Hatsugai, Takuya Sugimoto, Manabu Taniwaki, et al.
The new simulation code of high power discharge has successfully developed. The code is not executed by mainframe computer, PC, nor Macintosh, but by iPhones. The details, such as development environment and process are described in this paper below.
High precision pointing system based on Risley prism: analysis and simulation
Jin-ying Li, Qi Peng, Ke Chen, et al.
This paper presents the key issues for high precision pointing system using Risley prism. An iterative optimization algorithm is proposed to solve the inverse problem of Risley Prism and anti-achromatic Risley Prism. Different error sources, particularly Risley prism's rotation errors and rotation axis jitters are considered. Error propagation formula between pointing accuracy and rotation position is deduced. Finally, a beam steering simulation system including target position creating module, Risley Prism inverse solution module, Risley Prism rotation control module and beam pointing module is established. The simulation results show that it can achieve better than 1 arcsec pointing accuracy.
Numerical analysis of the beam quality and spectrum of wavelength-beam-combined laser diode arrays
Xuan Tang, Xiao-Jun Wang, Wei-Wei Ke
In this paper, a numerical model is presented to simulation the performance of the wavelength-beam-combined laser diode arrays (LDA) system. The eigen mode expansion method is used to describe the two-dimensional optical amplification and the strength of field feedback of external cavity. To describe the mode competition in laser diodes, the gain saturation effect is considered. The two-dimension distributions of the carrier concentration, recombination rates, and optical gain are calculated for solving the laser dynamic equation. The Fresnel integration, grating equation and mode overlap integration are used to obtain the feedback coefficient of extent cavity diffraction. Quantum noise is considered to evaluate the spectral linewidth of semiconductor laser. Based on the numerical model, the impact of the mutual optical feedback on the beam quality and spectrum of the LDA is present and analysis.
Theoretical study of iterative pump number of diode pumped solid state laser
W. Liu, J. Guo, F. Sang
A novel conception of iterative pump number was introduced in this work for the first time. Based on the conservation of energy, the equivalent model for the lasering of solid state laser was built up, the iterative pump number was calculated, and a formula for the output power of laser was given. This formula presented the relationships among the output power of laser, pumping power of diode and the thickness of laser medium. The output power predicted by this formula is consistent with experimental results, so this formula could be an important tool for the designing of parameter for diode pumped solid state laser.
Compensation analysis of low frequency distorted wave front of laser beam
Youjun Feng, Rongzhu Zhang
Because of low frequency distorted wave front impacting on beam quality in ICF system, a thin mirror with diameter 300mm is optimized in order to compensate the distorted wave front. Meanwhile, the influences of low frequency distorted wave front made to laser beam quality are discussed in detail. Far field beam quality before and after compensating distorted wave front is discussed in detail, too. The analysis results show whether the space scale parameter of the low frequency wave front is altered or the amplitude of the wave front is changed, the amplitude of the wave front decrease obviously, phase gradient reduce during the whole spatial period, the peak of far field intensity increase, power focusability of focal spot is improved obviously.
Research of beam control system component simulation and separation method of the kinematics coupling
EasyLaser is component-based laser system simulation software. Beam control system simulation is a main part of EasyLaser, which can be used for systems with multi-optical paths, multi-wavelength beams, and multi-controllers. A new numerical method about general kinematics separation is proposed for beam control system simulation. It provides axis rotation conversion relationships due to orientation data of apparatus of system, such as gimal, sensor and optical mirror. It gives their coupling and uncoupling matrixes in kinematics and controller model. The matrixes could change every iterative time automatically during the dynamic tracking process. The main advantage of the method is more suitable to solve the problems that the gimbal movement and geometry optical transmission are considered simultaneously. By using the method, sensor images and undershoot data are updated automatically. And further the kinematic driver or controller signals are separated automatically. Therefore the tracking and beam control can be designed without consideration of the system kinematical composition. Then the beam control system simulation has the virtues of generality, flexibility, and usability. No matter what kinds of gimbal and optical path, designer needs only to consider tracking and beam control aspects. In addition, a union beam control example for atmosphere transmission correction is given. It includes tracking tilt mirror and adaptive optics system. Simulation results show that the low-frequency fluctuation is restrained effectively and the high-frequency fluctuation is corrected obviously.
Research on 3D simulation of coupling model among flow field, optical field and heating effects of mirrors
Y. Li, Y. Y. Du, S. X. Li
When chemical oxygen-iodine laser (COIL) runs longtime, the influence on heating effects of mirrors has attracted attention abroad. For obtaining accurate estimate on it, relying on past experience, we carry on coupling simulation among flow field, optical field and heating effects of mirrors. At present computational condition, three-dimensional model about nozzle flow is constructed; by analyzing simulation data, coupling simulation result is obtained between flow field and optical field. Finally, the influence on heating effects of mirrors is investigated. The focus is solving the convergence of iteration between flow field and optical field. By particular analysis on the physical mechanism, coupling style is adjusted; finally, stable result is obtained, coupling iteration times is reduced greatly. The simulation result indicate if considering actual flow field disturbance, facular fragmentation is distinct; the disturbance exist from the beginning, so it influences on beam quality in the whole course.
Theoretical analysis and comparison of Tm-doped fiber lasers with different pump bands
Mengmeng Tao, Fei Wang, Zhenbao Wang, et al.
Modeling of Tm-doped fiber lasers pumped with 793 nm, 1.6 μm and 1.9 μm is presented and compared. Output performance of three different pump schemes with active fiber length, pump power and output reflectivity is investigated. Numerical simulation shows that, with 793 nm pump, the cross relaxation process is of vital importance for high efficient operation of Tm-doped fiber laser. And, 1.9 μm pump scheme is more likely to offer even higher output compared with 793 nm pump and 1.6 μm pump.
Influence of the neodymium glass parameters on the amplified spontaneous emission in slab amplifier
Bingyan Wang, Junyong Zhang, Yanli Zhang, et al.
Amplified spontaneous emission (ASE) causes the decrease of the inverted population density and the nonuniformity of gain in slab amplifier for high power laser systems. In this paper, a three dimension model, based on the data in SG-II, in which the residual reflection in the cladding and the ASE process are taken into consideration, is built to analyze the space distribution and time evolution of small signal gain coefficient using Monte Carlo algorithm and ray tracing. This model has been verified by comparing with the experimental data. The traverse size of slab is 68.2cm×36.3cm, which is usually decided by the clear aperture and the manufacture. By means of the model, the impact of thickness, residual reflectivity and the stimulated cross section of neodymium glass to the ASE are analyzed in detail.
Mathematical simulation of heating effects in a static diode-pumped vapor rubidium cell
Wei Zhang, You Wang, Juhong Han, et al.
Diode-pumped alkali lasers (DPALs) have attracted a lot of interests in the recent years because of their high Stokes efficiency, good beam quality, compact size, and near-infrared emission wavelengths. Until now, the thermal features have been only analyzed in an open alkali cell. In this report, we established a mathematical model to examine the heating effect of a static sealed rubidium cell by means of a Finite Difference (FD) procedure. After assuming the absorption coefficient of the alkali vapor, the temperature distributions of a real sealed rubidium vapor cell have been acquired for different powers and beam waists of the pump. The analytic conclusions would be helpful in designing a feasible DPAL.
Efficient fourth harmonic generation of Nd: glass lasers in ADP and DKDP crystals
Fang Wang, Fu-quan Li, Xiang-xu Chai, et al.
Experimental research on non-critical phase-matching fourth harmonic generation with ADP and DKDP crystals are reported. The characteristics of 2ω-to-4ω efficiency as function of incident angle, crystal temperature and input 2ω intensity have been investigated in detail, and the 2ω-to-4ω conversion efficiency has been demonstrated up to 84.1% with ADP crystal and 85.1% with DKDP crystal, respectively. Nevertheless, the spatial non-uniformity of rapid grown DKDP crystal has to be improved while the temperature control uniformity should be upgraded for ADP crystal to realize large aperture high efficiency fourth harmonic generation.
Gas and Chemical Lasers (Including DPALs)
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DPAL activities in Japan
Activities on diode pumped alkali laser (DPAL) in Japan is reviewed. We have started alkali laser works in 2011, and currently, we are the only players in Japan. Our interests are application oriented, and it is not only defense but also industrial. DPAL is a good candidate as a source of remote laser machining, thanks to its scalability and extremely good beam quality. We are studying on scientific and engineering problems of Cs DPAL with a small-scale apparatus. A commercial diode laser with volume Bragg grating outcoupler is used to pump the gain cell longitudinally. A 6.5 W continuous-wave output with optical to optical efficiency of 56% (based on the absorbed power) has been achieved. Numerical simulation codes are developed to understand the physics of DPAL and to help future developments.
Molecular singlet delta oxygen quenching kinetics in the EOIL system
A. P. Torbin, P. A. Mikheyev, A. A. Pershin, et al.
The development of a discharge oxygen iodine laser (DOIL) requires efficient production of singlet delta oxygen O21 Δ) in electric discharge. It is important to understand the mechanisms of of O2α1 Δ) quenching in these devices. To gain understanding of this mechanisms quenching of O2]1 Δ)in O/O2/O3/CO2/He mixtures has been investigated. Oxygen atoms and singlet oxygen molecules were produced by the 248 nm laser photolysis of ozone. The kinetics of O21 Δ) quenching were followed by observing the 1268 nm fluorescence of O2α1 Δ → X3 Σ transition. It is shown that vibrationally excited ozone O3(υ;) formed in the three-body recombination O + O2 + M →O3(υ) + M is an important O/O2/O3 quenching agent in O/O2/O3 systems. The process O3(υ ≥2) + O2(a1 Δ)→ 2O2 + O is the main O21 Δ) deactivation channel in the post-discharge zone. If no measures are taken to decrease oxygen atom concentration, the contribution of this process into overall O21Δ) removal is significant even in the discharge zone. It was found in experiment that addition of species that are good quenchers of O3(υ;) decrease O2(a1 Δ) deactivation rate in the O/O2/O3 mixtures.
Precision beam pointing control with jitter attenuation by optical deflector exhibiting dynamic hysteresis in COIL
Yan-Hua Ma, Zeng-Bao Zhang, Zhi-Guo Zhang, et al.
Due to the existence of various disturbances during the lasing process of the chemical oxygen iodine laser (COIL), the optical beam pointing performance is severely degraded. In this paper, an adaptive control methodology is proposed for the precise pointing control of the optical beam with active beam jitter rejection using a giant magnetostrictive optical deflector (GMOD) which exhibits severe dynamic hysteresis nonlinearity. In particular, a least square support vector machine (LS-SVM) based fast compensator is employed to eliminate the dynamic hysteresis without the inverse model construction. Then an improved feedforward adaptive filter is developed to deal with jitter attenuation when the full-coherent reference signal is unavailable. To improve the stability and overall robustness of the controller, especially when a large initial bias exists, a PI controller is placed in parallel with the adaptive filter. Experimental results validate the precise pointing ability of the proposed control method.
O2 (1Δg) detection using broadband CARS
1 Δg oxygen was the active medium of chemical oxygen iodine laser (COIL), the concentration and distribution of 1 Δg oxygen was important for the output power and beam quality. However, the current test technique, such as fluorescence detection method, absorption spectrum method could not get accurate 1 Δg oxygen information, due to the interference from the iodine fluorescence or the rigorous request of the laser source and optics and detection elements. The anti-stokes Raman spectrum of 1 Δg oxygen was regarded as a potential technique to obtain desirable signal, and the coherent anti-stokes Raman scatter (CARS) was the most feasible technique to get better signal to noise ratio (SNR). In this paper, we reported a broadband nanosecond coherent anti-stokes Raman scatter (CARS) detecting system built up for the detection of the concentration and distribution of O2( 1 Δg) in COIL:The second harmonic of a Nd: YAG pulse laser was separated into two parts, one part was used to pump a broadband nanosecond dye laser to generate light of 578-580 nm, which covered both stokes lines of O2 ( 1 Δg)and O2 (3 ∑); The other part was combined with dye laser output by a dichroic mirror, and then introduced into the detection region of COIL through a focus lens. CARS signals for O21 Δg)and O23 ∑)have different wavelengths, and their intensity was proportional to the square of the concentration of O21 Δg) and O2( 3 ∑). By changing the focus spot of pump and stokes laser, the concentration distribution of O21 Δg) and O23 ∑)at different position could be obtained.
Study on key operating parameters of diode-pumped Cs vapor laser
Fei Chen, Fei Gao, Yan Xu, et al.
Kilowatts class diode-pumped Cs vapor laser (DPCL) has been realized and this kind of lasers have military applications potentially for its high output power with high efficiency. Pumped by a fiber coupled laser diode, the key operating parameters of a DPCL are studied, including the spot size of focused pumping light, pressure ratio of buffer gases, vapor cell length, temperature of Cs vapor and reflectivity of output coupler. The spot size is properly chosen in the consideration of both the intensity scalability and mode matching. Pressure ratio is optimized under a modest pressure of mixed gases of helium and ethane. Under the optimized pressure ratio, the Cs vapor can absorb the pumping energy and convert it into laser energy efficiently. Besides, the temperature and reflectivity are also optimized to operate the DPCL in optimum state. The results have significant instructions for the experimental design of DPCL.
DPAL research in Changsha
Hongyan Wang, Xiaojun Xu, Zining Yang, et al.
Alkali laser has been one of the most promising paths to high energy laser during past dozen years. As the first group realized DPAL and XPAL lasing in China, we had done lots of theoretical and experimental works to further clarify the mechanism of alkali lasers, such as exploring scaling parameters design balance and MOPA configuration amplified spontaneous emission suppression in DPAL based on our self-developed fast converging algorithm, XPAL’s continuous wave operation threshold, performance degradation of VBG narrowed diode laser array and stacks due to conductive thermal flow, heat deposition induced gas dynamic parameters variation estimation, local atomic number density change measurement with single frequency tunable diode laser, ionization and other higher level nonlinear effects with opto-galvanometer method. Based on above research works, preliminary c onsiderations and conclusions for alkali laser scaling are given.
Continuously wavelength tunable high pressure CO2 lasers
Hubertus von Bergmann, Francois Morkel
Results obtained from a small discharge cross section (10×10 mm), high pressure (10 bar) TE CO2 laser are presented demonstrating continuous wavelength tunability of the laser. Two arbitrary wavelength regions in the 9P and 10P branches are chosen to demonstrate the continuous tunability of the laser wavelength. Furthermore stability of the laser output is demonstrated over extended periods of operation. Other output parameters of the high pressure laser such as temporal pulse profile and peak pulse power were also measured. Preliminary results will be presented of a discharge system scaled to larger discharge cross sections intended for high pressure amplifiers in ultra short pulse terawatt laser systems. Electrode separations of up to 50 mm have been investigated with measured discharge widths of 80 mm. The system has been operated at gas pressures of up to 3.5 bar with various CO2 laser gas mixtures. Discharge stability studies and gain measurements are reported.
Features of power extraction in EOIL
The single-frequency two-level (SFTL) generation model was applied to predict the power extraction efficiency in electrically driven oxygen-iodine laser (EOIL). The analysis of the calculated and published experimental data showed that power extraction from the cw EOIL is limited by the rate of energy transfer from singlet oxygen molecules O2(1Δ) to iodine atoms I(2P3/2) and about half of the produced singlet oxygen molecules were carried out from the resonator. To increase power extraction efficiency it will be necessary to substantially extend concentrations of singlet oxygen molecules and iodine atoms in the active medium of EOIL.
Semi-analytical and CFD model calculations of subsonic flowing-gas DPALs and their comparison to experimental results
Application of two- and-three dimensional computational fluid dynamics (2D and 3D CFD) models to subsonic flowing-gas DPALs is reported. The 2D model is applied to a DPAL with optical resonator-flow field coaxial configuration and the 3D model is applied to an optical axis transverse to the flow configuration. The models take into account effects of temperature rise and losses of alkali atoms due to ionization. The 2D CFD model is applied to 1 kW flowing-gas Cs DPAL [A.V. Bogachev et al., Quantum Electron. 42, 95 (2012)] and the calculated results are in good agreement with the measurements. Comparison of the 2D CFD to semi-analytical model [B. D. Barmashenko and S. Rosenwaks, J. Opt. Soc. Am. B 30, 1118 (2013)] shows that for low pump power both models predict very close values of the laser power; however, at higher pump power, corresponding to saturation of the absorption of the pump transition, the laser power calculated using the 2D CFD model is much higher than that obtained using the semi-analytical model. At high pump power, the heat convection out of the laser resonator is more efficient for the transverse case than the coaxial case, the beam temperature is lower and consequently the calculated laser power is higher. Optimization of the Cs DPAL parameters, using 3D CFD modeling, shows that applying high flow velocity and narrowband pumping, maximum lasing power as high as 40 kW can be obtained at pump power of 80 kW for transverse flow configuration in a pumped volume of ~ 0.7 cm3.
Hydrogen fluoride chemical laser cascade dynamics under cavity loss modulation
Lei Li, Hongyan Wang
Under cavity loss modulation, enormous dynamic behaviors will come up in B type cascade laser. In this paper, we built a two-wavelength cascade transition balanced equation model for CW HF chemical laser, 2P(5) and 1P(6) line were taken as cascade pair. We carried out cavity loss modulation experiment on a small scale supersonic discharge driven hydrogen fluoride chemical laser. From the view of phase space, we explained the characteristics by nonlinear time series analysis tools.
The research on the technology of surface discharge optical pumping source
This paper describes systematically the properties of the surface discharge pumping source. The pre-ionization technology was investigated. The characteristics of the surface discharge and the photodissociation wave of exciting media have been studied. The temporal and Spatial Stability of discharge were measured and analyzed. The discharge spectral and ablation rate of three different materials were obtained.
Progress on high power excimer laser in NINT
Xueqing Zhao, Jingru Liu, Aiping Yi, et al.
Excimer laser has been shown to be efficient tools in plasma physics and material science. Recent progress on techniques of beam control in excimer laser system required for energy scaling are overviewed, Configuration and initial results of a 100J/10ns, 18 beam excimer laser system are given.
Theoretical investigation of diffraction features of a planar wave using a Reflecting Volume-Bragg-Grating (RVBG)
Liangping Xue, You Wang, Wei Zhang, et al.
A theoretical procedure has been developed for evaluation of the diffraction efficiency of a Reflecting Volume-Bragg-Grating (RVBG) in accordance with Kogelnik’s theory. The diffraction features have been investigated for the different grating thicknesses, refractive index modulation, and grating spatial frequencies, respectively. The linewidths of a diffracted light and the effective diffraction efficiency have been studied while a certain linewidth in spectrum of an incident LD beam is taken into account in the calculation. The results are thought to be useful in the applications of single-longitudinal-mode (SLM) lasers and linewidth narrowing by means of RVBGs.
A quasi-CW linearly polarized rubidium vapor laser pumped by a 5-bar laser diode stack
We report a quasi-CW linearly-polarized rubidium vapor laser. The pumping source consists of 5 laser diode bars and its linewidth is reduced from the raw 1.8 nm to 0.2 nm by a bulk Volume Bragg Grating (VBG). Instead of adopting the “quasi-waveguide structure” gain cell, the pumping light of the rubidium vapor laser propagates freely in the vapor cell. The pumping light with polarization perpendicular to one of the rubidium laser is coupled into the resonator cavity by the PBS. Output power of 10 W with good beam quality is realized. This laser configuration is suitable for a convection-cooling DPAL.
Research on COIL employing no-flake-nozzle and CO2 as buffer gas
Mingxiu Xu, Fengting Sang, Yuelong Zhang, et al.
The supersonic nozzles lower temperature to 170-180 K better for the small signal gain coefficient. But at this temperature, the CO2 buffer gas may become liquid state. A chemical oxygen-iodine laser (COIL) employing CO2 as buffer gas and no-flake-nozzle was studied. Some mathematical simulation in three-dimensional computation fluid dynamics was adopted first to validate its usability. New nozzles gave the temperature higher than 400 K and considerable small signal gain coefficient. In the same conditions as simulation, experiments gave a 23% of chemical efficiency and 2.5 kW of output power. And it have got rid of “black area”, which was familiar in the supersonic COIL both in simulation and experimental results.
Chemical oxygen-iodine laser with a cryosorption vacuum pump with different buffer gases
Mingxiu Xu, Benjie Fang, Fengting Sang, et al.
A traditional pressure recovery system is the major obstacle to mobile chemical oxygen-iodine laser (COIL) for its huge volume. A cryosorption vacuum pump was used as the pressure recovery system for different buffer gases. It made COIL become a flexible, quiet and pressure-tight. Experiments were carried out on a verti- COIL, which was designed for N2 and energized by a square-pipe jet singlet oxygen generator (JSOG). The output power with CO2 was 27.3% lower than that with N2, but the zeolite bed showed an adsorption capacity threefold higher for CO2 than for N2 in the continuous operation. The great volume efficiency interested researchers.
Modeling of lasing possibility in XeF(C-A) amplifier of the THL-100 laser system
Results of experimental measurements and numerical simulations of the N2 and XeF2 gas pressure effects on the gain characteristics are presented in the paper. It is experimentally and theoretically is shown that maximum total gain (5-6)×104 achieved at a pump energy E = 240 J, 0.2 Torr XeF2 and 0.5 atm N2 pressure. Increasing and decreasing of N2 pressure leads to decrease of the gain. The possibility of the XeF(C-A) amplifier operation in a generator mode is discussed and the theoretical study results of generation are presented. It is shown that an annular laser radiation with energy up to 8.5 J can be obtained. The study results of the influence on the laser energy of N2 and XeF2 pressure and reflectance coefficient of output mirror are presented.
Small signal gain measurement of liquid oxygen under different wavelength laser pump
Zhe Shi, Hui Li, Canhua Zhou, et al.
Oxygen molecules existed in pairs under liquid condition, the radiation from vibrational ground state of 1 Δ state to the first vibrational excited state of 3 ∑ state was electronic dipole moment transition allowed, and a photon with wavelength of 1580 nm was emitted. In our experiment, dye laser with wavelength of 581 nm, 634 nm, 764 nm was used to excite liquid oxygen to different excited states, while a tunable OPO was used as the seeder laser, and the small signal gain was measured to be 0.23 cm-1, 0.3 cm-1 and 0.076 cm-1 respectively. The small signal gain (pump by photon of 634 nm) was significantly higher than that of common solid state lasers and chemical lasers. When the fundamental output of a Q-switched Nd:YAG laser was used as the pump source, the corresponding small signal gain was 0.12 cm-1. The profiles of small signal gain form 1579.2 nm to 1580.8 nm were also presented. These results were consistent with theoretical calculation. The high positive gain indicated that the liquid oxygen was a potential medium for high energy laser. A comprehensive parameter optimization was still necessary in order to improve the mall signal gain.
Single line oscillation of continuous wave HF chemical laser with a grating cavity
Yuanhu Wang, Liping Duo, Yuqi Jin, et al.
An effective single line continuous wave HF chemical laser operation has been demonstrated using a Littrow-mounted diffractive grating cavity experimentally. Selection of spectral lines of the laser was investigated when the grating used as a reflector and an output mirror respectively. The feedback factor of the cavity is demonstrated important parameter of line selection. Output power of single line from grating cavity is obviously stronger than the same line within the full spectrum of the laser that operated without line selection.
Data acquisition and control system with a programmable logic controller (PLC) for a pulsed chemical oxygen-iodine laser
A user-friendly data acquisition and control system (DACS) for a pulsed chemical oxygen -iodine laser (PCOIL) has been developed. It is implemented by an industrial control computer,a PLC, and a distributed input/output (I/O) module, as well as the valve and transmitter. The system is capable of handling 200 analogue/digital channels for performing various operations such as on-line acquisition, display, safety measures and control of various valves. These operations are controlled either by control switches configured on a PC while not running or by a pre-determined sequence or timings during the run. The system is capable of real-time acquisition and on-line estimation of important diagnostic parameters for optimization of a PCOIL. The DACS system has been programmed using software programmable logic controller (PLC). Using this DACS, more than 200 runs were given performed successfully.
Optical cavity temperature measurement based on the first overtones spontaneous emission spectra for HF chemical laser
An optical cavity temperature test method has been established for the HF chemical laser. This method assumes that in HF optical cavity the rotational distribution of vibrationally excited HF molecules meets the statistical thermodynamic distribution, the first overtones (v = 3-1 and 2-0) spontaneous emission spectral intensity distribution is obtained by using OMA V, the optical cavity temperature is calculated by linear fitting the rotational thermal equilibrium distribution formula for each HF vibrationally excited state. This method is simple, reliable, and repeatable. This method can be used to test the optical cavity temperature not only without lasing, but also with lasing.
Optical resonator with nonuniform magnification for improving beam uniformity of chemical oxygen iodine lasers
Kenan Wu, Yang Sun, Ying Huai, et al.
Unstable resonator with nonuniform magnification for improving the beam uniformity of chemical oxygen iodine lasers is explored for the first time. The magnification of the resonator is a function of the radial coordinate of the polar coordinate system on the front mirror surface. A resonator was designed to have a lower magnification at the center of the resonator than at the edge. The resonator consists of two aspherical mirrors. Method for designing the resonator is given. The energy conservation law and the aplanatic condition were used to derive the designing principle of the two aspherical mirrors. The design result was fitted to polynomial form which is suitable for manufacturing. Numerical experiment was carried out to evaluate the performance of the resonator. The computation was based on coupled simulation of wave optics model and computational fluid mechanics model. Results proved the effectiveness of the design method. The design tends to enhance the intensity near the center of the output beam and cripple that near the edge. Further analysis revealed that this effect is induced because rays of light are reflected more densely at the center of the pupil than at the edge. Therefore, this design affords for a potential approach for improving the near field uniformity of chemical oxygen iodine lasers.
Spectral analysis of cavity chemiluminescence of a combustion-driven HF laser fueled by NF3
Liucheng Li, Liping Duo, Yuanhu Wang, et al.
The visible and near infrared spectra of cavity chemiluminescence of a combustion driven HF laser fueled by NF3 were collected and analyzed. The spectral line at 529 nm for the green chemiluminescence was attributed to electronic excited NF molecules in b1∑ state, i.e. NF(b). The diffuse bands from 570 nm to 700 nm were attributed to the N2(B-A) emission. The spectral lines from 850 nm to 1000 nm were attributed to the HF Δυ = 3 emission bands. At the end of every experiment, the spectral line at 874 nm would be observed, which was attributed to the electronic excited NF molecules in a1 Δ state, i.e. NF(a). The NF(a-X) emission was found experimentally to be always avoiding the HFΔυ = 3 emission bands. It was also found experimentally that the NF(b-X) emission always accompanied the HF Δυ = 3 emission bands and their emission intensities had the same trends as a function of experimental time. Whereas the NF(a) molecules was produced in the optical cavity directly by the reaction of H atoms with NF2 molecules in the incomplete combustion effluents, the NF(b) molecules were suggested to be produced mainly by the near resonant energy transfer from vibrational excited HF(v<=2) molecules to NF(a) molecules. In other words, the vibrational excited state HF(v<=2) molecules can be efficiently deactivated by the NF(a) molecules by near resonant V-E energy transfer process. Therefore we concluded that incomplete dissociation of NF3 might be harmful to the HF(v<=2) population.
Oxygen assisted iodine atoms production in an RF discharge for a cw oxygen-iodine laser
Pavel A. Mikheyev, Andrey V. Demyanov, Nikolay I. Ufimtsev, et al.
Results of experiments and modeling of CH3I dissociation in a 40 MHz RF discharge in a discharge chamber of original design to produce iodine atoms for cw oxygen-iodine laser are presented. In experiments a substantial increase in CH3I dissociation efficiency due to addition of oxygen into Ar:CH3I mixture was observed. Complete CH3I dissociation in Ar:CH3I:O2 mixture occurred at 200 W discharge power. Fraction of discharge power spent on iodine atoms production was equal to 16% at 0.17 mmol/s CH3I flow rate. The rate of carbon atoms production as a function of molecular oxygen and water contents in CH3I:Ar mixtures was studied with the help of numerical modeling. It was found that addition of water vapor resulted in increase while addition of molecular oxygen and HI in decrease of the rate of carbon atoms production. Due to diffusion most of carbon atoms had enough time to deposit on the walls of the discharge chamber. However, contrary to the situation in a DC discharge, in the RF discharge accumulation of carbon on the walls of the discharge chamber did not hamper discharge stability and iodine production, as it was observed in our experiments.
The amplification of stimulated Raman scattering in H2 pumped by a Q-switched Nd:YAG laser
The experimental study of the amplification of stimulated Raman scattering (SRS) in high purity H2 gas was demonstrated employing a Q-switched Nd:YAG laser at 1064 nm as the pump source. A part of the 1064 nm pump light (20% in energy) was focused into the first H2 gas cell to generate the backward first Raman Stokes light (BS1), which is taken as the Raman seed light. The BS1 seed light combined to the residual pump light were focused into the second H2 gas cell to get the amplification of the S1 1900 nm infrared Raman light. In this study, the maximum quantum conversion efficiency of the S1 light was estimated to be 76%. Under the condition of the same pump energy, especially for the low pump energy (lower than 40 mJ), the quantum conversion efficiency of the S1 light with the Raman seed light was significantly increased comparing to the single focus geometry (without the Raman seed light).
Blue satellites of absorption spectrum study of sodium based excimer-pumped alkali vapor laser
Shu Hu, Baodong Gai, Jingwei Guo, et al.
Sodium based excimer-pump alkali laser (Na-XPAL) is expected to be an efficient method to generate sodium beacon light, but the information about the spectroscopic characters of Na-XPAL remains sparse so far. In this work, we utilized the relative fluorescence intensity to study the absorption spectrum of blue satellites of complexes of sodium with different collision partners. The yellow fluorescence of Na D1 and D2 line was clearly visible. After processing the fluorescence intensity and the input pumping laser relative intensity, we obtained the Na-CH4 system’s blue satellites was from 553nm to 556nm. Meanwhile, we experimentally demonstrated the Na-Ar and Na-Xe system’s wavelength range of blue satellites. Also, it was observed that the Na-Xe system’s absorption was stronger than the other two systems.
H2 stimulated Raman scattering in a multi-pass cell
Stimulated Raman Scattering (SRS) is an effective means of laser wavelength conversion. Hydrogen is an excellent Raman medium for its high stimulated Raman gain coefficient and good flowability which can rapidly dissipate the heat generated by SRS process. In this paper we reported the H2 SRS in multiple-pass cell pumped by the fundamental frequency output of a Q-switched Nd: YAG laser. Two concave reflection mirrors (with 1000 mm curvature radius and 50 mm diameter) were used in our experiment, both mirrors with a hole near the edge and were positioned to form co-center cavity, therefore the laser could repeatedly pass and refocus in the Raman cell to achieve a high SRS conversion efficiency and reduce SRS threshold for pump laser. By changing the pass number (1~17) of optical path in the Raman cell and the pump power(0~2.5MW), the Stokes conversion efficiency is optimized. Experimental results indicated that the Raman threshold was 0.178MW and the highest photon conversion efficiency was 50 %.
Iodine flow rate measurement for COIL with the chemical iodine generator based on absorption spectroscopy
Weili Zhao, Yuelong Zhang, Peng Zhang, et al.
A dual-components absorption method based on absorption spectroscopy was described in the paper. It can easily eliminate the influence of the serious contamination and aerosol scattering on IFR measurement by utilizing the absorptions of iodine vapor and chlorine on two different wavelengths respectively. According to the character that there is no other gaseous product in the reaction besides iodine vapor, IFR in real time can be obtained by the connections of the pressure and the flow rate among chlorine remainder, iodine vapor, and the buffer gas. We used this method to measure IFR for the first time at the exit of a chemical iodine generator. The average of IFR is coincident with that calculated by chemical weighting mass.
Investigation on the separation performance of strongly swirling flow singlet oxygen generator
Xiaobo Xu, Wenwu Chen, Dapeng Hu, et al.
Separation performance of a new spray-type singlet oxygen generator (SOG), named strongly swirling flow singlet oxygen generator (SSF-SOG), is studied by computational fluid dynamics (CFD) analysis and experiments. The flow field and the separation performance of the SSF-SOG are analyzed using Fluent® soft firstly, and the results show that the separation performance of SSF-SOG is effective for liquid droplets whose diameters are greater than 5μm. Then, using Phase Doppler Particle Analyzer (PDPA), the diameters of liquid droplets in reaction zone and gas outlet of SSF-SOG are measured. The results show that in the reaction zone the diameters of the liquid droplets mainly concentrated on 10μm, and only a little of droplets of which the diameter are under 5μm are detected in the gas outlet of SSF-SOG, which are well consistent with the results of the CFD analysis on SSF-SOG.
Investigation on regeneration of basic hydrogen peroxide by electrochemical methods
Changchun Ke, Wenwu Chen, Xiaobo Xu, et al.
Two electrochemical methods for regeneration of Basic Hydrogen Peroxide (BHP) were investigated in this paper, which could be called one-step method and two-step method, respectively, distinguished by the number of steps during the regeneration process. The one-step method converts potassium chloride solution and oxygen directly to chlorine and BHP by a modified chlor-alkali cell with an oxygen cathode. For the one-step method, two reactors of different structure and corresponding regenerating process were designed. The experimental results showed that, for the continuous-type reactor, the highest peroxide concentration was 0.042 mol/L, while for batch-type reactor the highest peroxide concentration was 0.563 mol/L. The two-step method accomplishes the regeneration of BHP by a conventional chlor-alkali cell combined with a fuel cell reactor which could convert hydrogen and oxygen to peroxide in alkaline potassium hydroxide solution. A peroxide concentration of 2.450 mol/L was obtained for the two-step method.
Research on reaction zone structure of high-press gravity-independent singlet oxygen generator
Yushi Liu, Wenwu Chen, Jinglong Wang, et al.
High press, gravity-independent, singlet oxygen generator (HGSOG) [1] with small reaction zone and high chemical efficiency was designed and fabricated. The mixing, reaction and separation processes happened simultaneously in the reaction zone of HGSOG. The size and the configuration of reaction zone are very important for HGSOG. In this paper, p-τ value was calculated to determine the upper limit of the volume of reaction zone. The condition of gas-liquid separation was calculated to determine the lower limit of the volume of reaction zone. The utilization rate of Cl2 achieved 90% and the yield of O2(1Δ) reached 70%.
Experimental performances of pre-swirling jet singlet oxygen generator
Zhendong Liu, Wenwu Chen, Xiaobo Xu, et al.
Singlet oxygen generator (SOG) is the key part of chemical oxygen iodine laser (COIL) which supplies chemical energy for chemical oxygen iodine laser. A novel pre-swirling jet singlet oxygen generator (PJSOG) with rapid separation technology is put forward. In this paper, experimental performances of PJSOG have been studied by means of theoretical and experimental analysis. With excellent phase dispersion and rapid separation performances, PJSOG has several or several decade times specific area than traditional SOGs and high separation efficiency. It can reach more than 95% chlorine utilization and about 60% O2 ( 1Δ) yield. The operating stability has also been studied in this paper. The stable running time of PJSOG increased with liquid flow structure improvement. With high separation efficiency, high O2 ( 1Δ) output coefficient, high utilization coefficient of Cl2 and excellent operating stability, the novel PJSOG must be a promising technology in further research.
Multi-photon processes in alkali metal vapors
Baodong Gai, Shu Hu, Hui Li, et al.
Achieving population inversion through multi-photon cascade pumping is almost always difficult, and most laser medium work under 1-photon excitation mechanism. But for alkali atoms such as cesium, relatively large absorption cross sections of several low, cascading energy levels enable them properties such as up conversion. Here we carried out research on two-photon excitation alkali fluorescence. Two photons of near infrared region are used to excite alkali atoms to n 2 D5/2, n 2 D3/2 or higher energy levels, then the blue fluorescence of (n+1) 2 P3/2,(n+1) 2 P1/2→n 2 S1/2 are observed. Different pumping paths are tried and by the recorded spectra, transition routes of cesium are deducted and concluded. Finally the possibility of two-photon style DPALs (diode pumped alkali laser) are discussed, such alkali lasers can give output wavelengths in the shorter end of visual spectroscopy (400-460 nm) and are expected to get application in underwater communication and material laser processing.
Feasibility study of a novel pressure recovery system for CO2-COIL based on chemical absorption
Qingwei Li, Yuqi Jin, Zicai Geng, et al.
A chemical oxygen-iodine laser (COIL) is an electronic transition, low pressure, high throughput system. The use of this laser demands a suitable pressure recovery system. This paper proposed a novel pressure recovery system based on chemical absorption and the feasibility for COIL with CO2 as buffer gas (CO2-COIL) was investigated. The novel pressure recovery system works by chemisorbing the CO2-COIL effluents into two fixed-beds maintained at initial temperature of around 293-323K. Compared with the cryosorption system for N2-COIL based on physical absorption, the novel chemisorptions based pressure recovery system has a simpler logistics and a shorter run-to-run preparation time. Two kinds of solid chemo-sorbents were designed and synthesized. One was used for chemisorbing the oxidizing gases such as O2 ,Cl2 and I2, another was used for chemisorbing the acidic gas such as CO2. The capacities of the two sorbents were measured to be 3.12 mmol(O2)/g and 3.84 mmol (CO2) /g, respectively. It indicated that the synthesized sorbents could effectively chemosorb the CO2-COIL effluents. Secondly, analog test equipment was set up and used to study the feasibility of the novel pressure recovery system used for CO2-COIL. The test results showed that the novel pressure recovery system could maintain the pressure under 6 Torr for tens seconds under the continuous gas flow. It showed that the novel pressure recovery system for CO2-COIL based on chemical absorption is feasible.
Measuring laser beam quality by use of phase retrieval and Fraunhofer diffraction
Wenbo Shi, Zengbao Zhang, Xin He, et al.
We demonstrate the use of phase retrieval and Fraunhofer diffraction as a method for the measurement of laser beam quality. This technique involves using two CCD cameras to record a pair of conjugated light intensity images in defocus plane and one near field measurement instrument to record the light intensity image in near field. The wavefront is then retrieved using an optimization jointly constrained by them. Thereafter, combining with the known light intensity image in near field, light intensity image in focus plane can be figured out. After that, laser beam quality will be obtained by comparing with ideal light intensity distribution in focus plane. As light intensity images in defocus plane can be measured with higher resolution and lower CCD dynamic range than that in focus plane, this method is expected to give a precise laser beam quality.
420nm alkali blue laser based on two-photon absorption
Yan-nan Tan, Yi-min Li, Tong Liu, et al.
Based on two-photon absorption, a 420nm blue laser of alkali Rb vapor was demonstrated, and a dye laser was used as the pumping laser. Utilizing the energy level structure of Rb atom, lasering mechanism and two-photon absorption process are analyzed. Absorbing two 778.1nm photons, Rb atoms were excited from 52 S1/2 to 52 D5/2, then relaxed to 62 P3/2 with mid infrared photon radiation. 420nm blue laser was achieved by the transition 62 P3/2→52 S1/2. To improve efficiency of the blue laser, two-photon resonant excitation pumped alkali vapor blue lasers are proposed, which will be good beam quality, high efficiency and scalable blue lasers. The development of diode pumped alkali vapor blue laser is expected.
Tuning laser output characteristics of a pyrotechnically pumped free-running Nd:YAG laser in terms of pumping kinetics
Xiaoli Kang, Fan Yang, Jiangshan Luo, et al.
Using light radiation directly produced by combustion of some pyrotechnics as pumping sources of solid state lasers is a potentially effective way to obtain compact and high energy lasers. Kinetics of this kind of pumping is studied in terms of pulse energy and pulse time characteristics as well as laser output energy. Pumping kinetics is turned through changing fabrication methods of the pumping modules. It was found that the useful light energy and pulse time for the pyrotechnic pumping light showed opposite changing trend. Compressing pulse duration from 45 ms to about 10 ms would simultaneously cause 20%~ 50% decreases in useful light radiation energy. However, the laser output energy produced by these pumping sources only had a variation 9%, ranging from 427 mJ to 468 mJ. Reasons were related to the decrease in fluorescence loss in pumping energy below the threshold for the pyrotechnic modules having shorter pulse duration but higher radiation power.
The mode-matching model of diode-end-pumped alkali vapor lasers
Diode-pumped alkali vapor lasers are famous in the field of laser for their significant advantages such as very high quantum efficiency (Cs 99.5%, Rb 98.1%, K 95.2%), good thermal management performance and excellent beam output quality etc. A rate equation model fully considering the spatial distributions of pumping light and oscillating light is established under the hypothesis of quasi-two-level energy system of DPALs in this paper. Meanwhile, expressions of threshold pumping power, mode-matching efficiency and output power and slop efficiency in low pumping and strong pumping, respectively, are obtained. Then, the influences of mode-matching efficiency on working performance of DPALs are discussed and analyzed. Results show that mode-matching efficiency mainly impacts on threshold pumping power, output power and slop efficiency in low pumping but that nearly has no effects in strong pumping. Therefore, this model benefits the further research of DPALs.
High repetition rate solid state switched CO2 TEA laser employed in industrial ultrasonic testing of aircraft parts
Hubertus von Bergmann, Francois Morkel, Timo Stehmann
Laser Ultrasonic Testing (UT) is an important technique for the non-destructive inspection of composite parts in the aerospace industry. In laser UT a high power, short pulse probe laser is scanned across the material surface, generating ultrasound waves which can be detected by a second low power laser system and are used to draw a defect map of the part. We report on the design and testing of a transversely excited atmospheric pressure (TEA) CO2 laser system specifically optimised for laser UT. The laser is excited by a novel solid-state switched pulsing system and utilises either spark or corona preionisation. It provides short output pulses of less than 100 ns at repetition rates of up to 1 kHz, optimised for efficient ultrasonic wave generation. The system has been designed for highly reliable operation under industrial conditions and a long term test with total pulse counts in excess of 5 billion laser pulses is reported.
Efficient gas lasers pumped by run-away electron preionized diffuse discharge
Alexei N. Panchenko, Mikhail I. Lomaev, Nikolai A. Panchenko, et al.
It was shown that run-away electron preionized volume (diffuse) discharge (REP DD) can be used as an excitation source of active gas mixtures at elevated pressures and can produce laser emission. We report experimental and calculated results of application of the REP DD for excitation of different active gas mixtures. It was shown that the REP DD allows to obtain efficient lasing stimulated radiation in the IR, visible and UV spectral ranges. Kinetic model of the REP DD in mixtures of nitrogen with SF6 is developed allowing to predict the radiation parameters of nitrogen laser at 337.1 nm. Promising prospects of REP DD employment for exciting a series of gas lasers was demonstrated. Lasing was obtained on molecules N2, HF, and DF with the efficiency close to the limiting value. It was established that the REP DD is most efficient for pumping lasers with the mixtures comprising electro-negative gases.
Tunable diode-laser spectroscopy (TDLS) of 811.5nm Ar line for Ar(4s[3/2]2) number density measurements in a 40MHz RF discharge
Pavel A. Mikheyev, Alexander K. Chernyshov, Nikolay I. Ufimtsev, et al.
A hardware and a computative technique for tunable laser spectroscopy was developed for simultaneous measurement of Gaussian and Lorentian components of line broadening by fitting Voigt line profile. The technique was tested in measurements of pressure broadening coefficient for 811.5 nm Ar absorption line in a 40 MHz discharge in the pressure range 15-75 Torr with the help of tunable diode laser with a short external resonator. The obtained values for this coefficients reduced to 300 K are: ξAr-Ar = (2.85±0.1)×10-10 s-1cm3 for broadening in the parent gas and ξAr-He = (3.3±0.1)×10-10 s-1cm3 for broadening in helium. A good agreement with published results is observed. Measured Ar(1s5) number density amounted to 10-11cm-3 for the discharge power density ~10 W cm-3.
Discharge initiated high power repetitively pulsed HF/DF laser
Ke Huang, Aiping Yi, Ying Tang, et al.
The design and performance of a closed cycle, repetitively pulsed HF/DF laser is described. For obtained higher stable laser pulse energy and running frequency, discharge stability with different electrode profile and different gas circulation structure are researched. The functional relations of laser pulse energy with electric field strength (E) and gas mixture pressure (P) for various gas flow velocity are studied. It is shown that with preliminary optimized of the gas injection segment structure before pump region and optimal E/P conditions, maximal running frequency of 100Hz is obtained and operating stability keeps well. Under these conditions, the laser average power is 40W and peak power is 4MW.
Resonators and Laser Beam Control
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Beam quality active control of a slab MOPA solid state laser
Rujian Xiang, Kai Zhang, Jing Wu, et al.
The wavefront of the output high power laser beam generated by a MOPA laser system with slab gain medium was measured and analyzed by an H-S wavefront sensor, the wavefront measured consisted the static aberration of the whole optic chain and the dynamic component caused by the laser driver. Analyzing results showed that the temporal frequency of the beam aberration was mostly less than 10Hz, but the spatial distribution of the aberration was more challengeable to the phase compensation. Due to the narrow rectangle shape of the laser gain medium and the none uniformity of the pumping and cooling, it had some local big slope cross the beam section, that caused a lot of difficulties to the aberration measuring and wavefront correcting, there were many high order components in the wide direction cross the beam, but the phase aberration was more smooth in the vertical direction. A beam expander which has different ratio in the x/y directions had been used for matching the beam aperture and the valid controlling range of the deformable mirror. An adaptive optical system consisted of an H-S wavefront sensor and a deformable mirror (DM) with 67 valid actuators which arranged in two dimensions as hexagon designed to compensate the phase aberration. The driving vector of the deformable mirror was calculated from the spot array which sampled by the H-S wavefront sensor from the target beam, the surface response of all actuators were pre-calibrated and saved as a matrix. The matrix invert method was used to calculate the driving vectors in the close loop steps. The beam quality factor β was adapted to evaluate the output beam from the adaptive optical system. Simulation and experiment results of the close loop correction about the adaptive optical system showed that the aberration was compensated to a very low level and the far field beam quality of the high power laser could reach 1.67xDL.
Thermal blooming effects of gas on laser propagation in a closed tube
Huahua Yu, Peng Hu, Jianzhu An, et al.
Thermal blooming effect of inner optical path remarkably affects far-field beam quality and energy distributions which should be taken into account in high energy laser (HEL) system. A physical model of thermal blooming is established. Based on the model, numerical simulations are carried out to study both the influences of absorptions of laser energy and tube structures on laser propagation in a closed tube. The natural convection of gas is numerically simulated by computational fluid dynamics (CFD) method. Gas temperature distributions, additional phase differences (APDs), variations of beam quality and drifts of mass center in far-field under different absorptions of laser energy and tube structures (Z-shaped and U-shaped) are compared, respectively. By analysis of numerical simulation results, the switch time of heat conduction and heat convection in gas is distinguished, which significantly affects the variations of beam quality and drifts of mass center in far-field. In addition, it also indicates that less absorption of laser energy improves beam quality and delays the switch time of beam quality between two heat transfer mechanisms. Therefore, it is significant to control the absorptions of laser energy for HEL system in practice. Different tube structures owning different beam paths change the distributions of APDs and thus influence beam quality. APDs of the two horizontal sections are the same (superposition effect) for Z-shaped tube while inverse (compensation effect) for U-shaped tube. It is shown that drifts of mass center in far-field are greatly suppressed for U-shaped tube than that of Z-shaped tube and beam quality is also improved.
A new method of frequency-doubling with converging cavity
Dong Liu, Jingwei Guo, Dongjian Zhou, et al.
A converging cavity is introduced in the frequency-doubling experiment to increase the efficiency. In this experiment, an annular collimated laser beam produced by Nd:YAG laser with confocal resonator is introduced into the converging cavity as fundamental light. A BBO is positioned in the converging cavity. As propagating in the converging cavity, the fundamental beam profile becomes smaller and smaller. In the theory, the conversion efficiency could be approach 100%. Our experimental result shows that the conversion efficiency is improved significantly compared with the single pass configuration.
Thermal lens effect induced by high power diode laser beam in liquid ethanol and its influence on a probe laser beam quality
In this paper, we present results of experimental investigation of the thermal lens effect in ethanol by a dual beam arrangement. The influence of thermal lens strength on a probe beam passing through this thermal lens medium was analyzed. Thermal lens was induced by focusing the beam of a high power diode laser in ethanol and the profile of a He-Ne laser as a probe beam was inspected. The comparison of the He-Ne laser beam profile in the condition of various diode laser beam powers shows that the beam quality of the probe can be significantly affected and the its deformed profile depends on the strength of thermal lens effect.
Influence of distribution of optical component surface defects on near field beam quality
International standard ISO 10110-7 sets a strict limit on the size and quantity for surface defects of an optical element. For high-power laser, sub-beams caused by defects with different distributions interfere with each other in the transmission process, causing beam quality complex changes. So it is necessary to make a clear limitation on relative position of defects, thereby giving the standard a more comprehensive supplement. Based on the diffraction theory, the changes of beam modulation are studied. The influence of scratch depth on the distribution of near field beam modulation is also taken into account. Results demonstrate that when two parallel scratches are on the same or different surfaces of an element, they produce more severe modulation than single scratch, and the maximum modulation can be increased to 1.5 times. Meanwhile more strict requirements for scratch depth are put forward. The results could provide reference for the determination of defects specifications for large-diameter optical elements in high-power laser systems.
Simulation research on beam steering technology based on optical phased array
Junlin Tian, Xudong Pan
The principle of beam steering technology based on optical phased array (OPA), which is composed of individual phase-modulating units, is introduced. By use of Fraunhofer diffraction and Fourier transformation, the OPA models are established. The influence of main parameters of OPA on beam steering efficiency, including duty ratio (ratio of effective unit size to total unit size), total unit size, unit number, and steering angle, is simulated and analyzed. It shows that beam steering efficiency of OPA is improved with larger duty ratio, smaller total unit size, and smaller steering angle, while the number of units has a very small impact on beam steering efficiency.
The application of ptychography in the field of high power laser
Haiyan Wang, Cheng Liu, Xingchen Pan, et al.
The commonly used interferometer and Hartmann-Shack sensor are not ideally qualified for the phase or wavefront measurement in the field of high power laser because the optical elements always have large aperture, the steep phase gradient and very irregular surface profile. The ptychography, which is a newly developed coherent diffraction method for the imaging with short wavelength, can be a perfect alternative to traditional technologies due to its outstanding advantages. The complex transmittance of the optical element can be obtained by measuring its transmitted and incident fields with ptychography and calculating their phase difference. Since ptychography can realize measurement with a resolution comparable to that of interferometry, it can find lots of applications in the field of high power laser such as the measurement of the complex transmittance of large optical element, the thermal distortion of the gas-cooled Nd:glass amplifier, and the focal length of the lens array etc.
Dependence of the influence function on laser-induced thermoelastic deformation of deformable mirrors
Binbin Wei, Wenguang Liu, Kun Xie, et al.
Influence function is one of the most important parameters for the correction of aberrations with deformable mirrors. When deformable mirrors are loaded with laser irradiation, the influence function will be affected by the heat-induced thermoelastic deformation, which would lower the aberration correction ability and effectiveness. On occasions when they are applied in laser systems, the influence of laser-induced thermoelastic deformation is significant, it would cause a considerable change in the influence function. We have analyzed the changes in the surface shape of a 37-element deformable mirror caused by laser induced thermoelastic deformation, and calculated the changes of influence function. The dependence of influence function on the arrangement of actuators, material properties of the actuators and the base and the heat flux is analyzed, with the actuator layout being square and hexagonal, different material properties and the heat flux varied from 0.005W/mm2 to 0.01W/mm2, using the finite element method. The results indicate that all of the previous factors will affect the influence function. Thus, in order to reduce the change of influence function caused by laser induced surface shape, the material and parameters of the deformable mirror should be chosen carefully.
Analysis of atmospheric turbulence anisoplanatism
The image quality of space object or the Laser propagation characteristics in atmosphere will be degraded by the effect of turbulence. Adaptive optics can be used to correct atmospheric turbulent aberration, but the anisoplanatic effect of atmospheric turbulence is unavoidable. Adopting the special spectral filtering method, different type of anisoplanatism in different scene are systemically modelled, and the formulae that describing different type of anisoplanatic variance and their Zernike model components are obtained. According to HV turbulent outline, the characteristics of Zernike model components of different type of anisoplanatic variance are analyzed.
Phase-distortion correction based on stochastic parallel proportional-integral-derivative algorithm for high-resolution adaptive optics
Yang Sun, Ke-nan Wu, Hong Gao, et al.
A novel optimization method, stochastic parallel proportional-integral-derivative (SPPID) algorithm, is proposed for high-resolution phase-distortion correction in wave-front sensorless adaptive optics (WSAO). To enhance the global search and self-adaptation of stochastic parallel gradient descent (SPGD) algorithm, residual error and its temporal integration of performance metric are added in to incremental control signal’s calculation. On the basis of the maximum fitting rate between real wave-front and corrector, a goal value of metric is set as the reference. The residual error of the metric relative to reference is transformed into proportional and integration terms to produce adaptive step size updating law of SPGD algorithm. The adaptation of step size leads blind optimization to desired goal and helps escape from local extrema. Different from conventional proportional-integral -derivative (PID) algorithm, SPPID algorithm designs incremental control signal as PI-by-D for adaptive adjustment of control law in SPGD algorithm. Experiments of high-resolution phase-distortion correction in “frozen” turbulences based on influence function coefficients optimization were carried out respectively using 128-by-128 typed spatial light modulators, photo detector and control computer. Results revealed the presented algorithm offered better performance in both cases. The step size update based on residual error and its temporal integration was justified to resolve severe local lock-in problem of SPGD algorithm used in high -resolution adaptive optics.
Double deformable mirrors' control based on voltage decoupling for adaptive optics system
Rui Zhou, Feng Shen, Hongwei Ye, et al.
The paper presents a control algorithm based on double deformable mirrors' (DM) voltage decoupling for an adaptive optics system. In the method there includes two DMs, and one DM has large stroke, and another has high spatial frequency. This paper presents the theoretic model about DM's voltage decoupling, designs control system that uses inertial element, and analyzes the performance on the control algorithm. Simulation and experimental results indicate the method can effectively correct aberrations that include large-scale low order aberrations and high spatial frequency aberrations in adaptive optics system at the same time, and improve the AO control system's performance.
A fast high voltage driver for the piezoelectric fast steering mirror
Muwen Fan, Linhai Huang, Mei Li, et al.
This paper describes High Voltage Driver (HVD) development for Piezoelectric Fast Steering Mirror (PFSM). Piezoelectric actuator's highly capacitive nature and its influence on high voltage driver are discussed. Since the PFSM tends toward higher speed and larger angle, a multi-paralleling power booster output stage linear high voltage amplifier is described that can output significantly higher current than single output stage linear power amplifier. Test results are presented for a fast high voltage driver that can drive a 300mm diameter PFSM (2.5μF piezoelectric actuator) at 100Hz 1000Vpp, also at 5000Hz 20Vpp.
Investigation of anisoplanatic effect in adaptive optics for atmospheric turbulence correction
Xi Luo, Xinyang Li, Li Shao, et al.
Laser Guide Star (LGS) is an artificial atmospheric turbulence probing source of adaptive optics (AO) for compensating for the wave-front error of interested object in real time, and for providing approximate diffraction-limited resolution recovery. Actually the unavoidable anisoplanatic error resulting from different light experience between the LGS and the object of interest through turbulent atmosphere will lead to an incomplete wave-front distortion compensation of the object. In this paper the statistics of anisoplanatic errors and their associated Zernike-modal variances have been systematically investigated for different LGS sources by means of numerical simulation, including Rayleigh LGS and Sodium LGS. The numerical results show that the probed wave-front expanded Zernike-modal decorrelation versus angular deviation between the LGS and the object of interest becomes much more sensitive for the higher altitude LGS. For minor angular deviations with LGS focal spots being still within the ray path from the object to the telescope, the reduction of the error from turbulence above the LGS altitude is still a leading cause to decrease the residual error variance after AO correction. However, for the greater angular deviations with LGS focal spots moving on the outside of the ray path from the object to the telescope, higher-altitude LGS could lead to an increasing residual error variance after AO complete correction with its wave-front as reference. At this point the adopted LGS operation mode and the AO system modal correction optimization should be taken into account for achieving a desired residual wave-front error.
Wavefront reconstruction with total-least square method in an adaptive optic system with HS wavefront sensor
Rujian Xiang, Kai Zhang, Jing Wu
Adaptive optic systems with HS Wave Front Sensor (WFS) and Deformable Mirror (DM) to compensate the aberration are used to improve the imaging quality and the far field quality laser beam widely, to get the correction driver vector that the deformable mirror needed to produce the conjugated reflecting surface, the matrix equation A⋅ x =ϕ should be solved. The least square method is the most choice used to solve the equation, but the result of the least square consider only about the error of vector ϕ , the solution vector x is no longer suitable while the response matrix A contain any error. A new method base on the total-least square to reconstruct the wavefront is presented, different from the traditional wavefront reconstructing with the least square method, the TLS method takes into account not only the error of vector ϕ , but also the error about the response function matrix A. For testing the characteristics of the solution vector x with the TLS method, an Adaptive Optical (AO) system with 67 actuators DM and 40x40 sub-apertures Hartmann-Shack (HS) WFS is used to simulate the wavefront reconstructing process, and result of simulation and experiments show that the driver vector calculated with TLS method is more stably and the wavefront aberration residual is more reasonable.
A transformation approach for aberration-mode coefficients of Walsh functions and Zernike polynomials
Shuai Wang, Ping Yang, Lizhi Dong, et al.
Walsh functions have been modified and utilized as binary-aberration-mode basis which are especially suitable for representing discrete wavefronts. However, when wavefront sensing techniques based on binary-aberration-mode detection trying to reconstruct common wavefronts with continuous forms, the Modified Walsh functions are incompetent. The limited space resolution of Modified Walsh functions will leave substantial residual wavefronts. In order to sidestep the space-resolution problem of binary-aberration modes, it’s necessary to transform the Modified-Walsh-function expansion coefficients of wavefront to Zernike-polynomial coefficients and use Zernike polynomials to represent the wavefront to be reconstructed. For this reason, a transformation method for wavefront expansion coefficients of the two aberration modes is proposed. The principle of the transformation is the linear of wavefront expansion and the method of least squares. The numerical simulation demonstrates that the coefficient transformation with the transformation matrix is reliable and accurate.
Experimental study on return signals detection of pulsed sodium laser beacon
Wenchao Zhou, Tianjiang Chen, Yanhua Lu, et al.
Using pulsed Sodium laser beacon,the exploration experiment of the return light measurement is carried out. The wavefront of Sodium laser beacon of one pulse and the wavefront of natural star of the same direction are measured using two Shack-Hartmann wavefront sensors simultaneously. The results of experimental data analysis show that the return photons numbers received of Shack-Hartmann wavefront sensor’s one subaperture are more than 200, and the intensity of Sodium laser beacon’s return light is even and steady. The signal-to-noise ratio of light spot distribution is about 3~5. The statistical variance of each order Zernike coefficient of Sodium laser beacon’s wavefront and natural star’s wavefront is consistent in the gross, and the pertinency of each order Zernike coefficient is well.
The entire beam wavefront control of high power laser facility
Wanjun Dai, Deen Wang, Dongxia Hu, et al.
Experiment of entire beam wavefront compensation was carried out in a beamline of a high power laser facility, and two adaptive optics systems with different intentions were applied in the chosen beamline. After correction, the far-filed irradiance distribution is concentrated evidently and the entrance rate of 3ω focal spot to a 500-μm hole is improved to be about 95% under number kilojoules energy.
Unload control of double fast steering mirrors of the control system
T. Tang, Y. Tan, G. Ren
Double fast steering mirror (FSM) are made of a fast steering mirror driven by piezoelectric actuator and a fast steering mirror by voice coin motor, which provides not only high control bandwidth and high resolution but also a wide range of angle for the tracking control system. A classical control system of double fast steering mirror is that each CCD sensor provides line of sight error to control relevant FSM respectively, resulting in complexity and weakening optical energy. The proposed system works by depending on single CCD sensor, which implies that it is crucial to decouple the control double fast steering mirror .An unload technique is designed to solve this problem .The FSM driven by piezoelectric actuator is driven using line of sight from CCD, while the other FSM is followed it. Error attenuation transfer function is modeled and used to analyze stabilization and performance of the control system, which is optimal only if the control bandwidth of FSM driven by piezoelectric actuator is ten times more than that of the FSM driven by the voice coin motor. The experimental results are in great accordance with theoretical analysis.
Simulations of far-field optical beam quality influenced by the thermal distortion of the secondary mirror for high-power laser system
Ruhai Guo, Ning Chen, Xinyu Zhuang, et al.
In order to research the influence on the beam quality due to thermal deformation of the secondary mirror in the high power laser system, the theoretical simulation study is performed. Firstly, three typical laser power 10kW, 50kW and 100kW with the wavelength 1.064μm are selected to analyze thermal deformation of mirror through the finite element analyze of thermodynamics instantaneous method. Then the wavefront aberration can be calculated by ray-tracing theory. Finally, focus spot radius,beam quality (BQ) of far-filed beam can be calculated and comparably analyzed by Fresnel diffraction integration. The simulation results show that with the increasing laser power, the optical aberration of beam director gets worse, the far-field optical beam quality decrease, which makes the laser focus spot broadening and the peak optical intensity of center decreasing dramatically. Comparing the clamping ring and the three-point clamping, the former is better than the latter because the former only induces the rotation symmetric deformation and the latter introduces additional astigmatism. The far-field optical beam quality can be improved partly by simply adjusting the distance between the main mirror and the secondary mirror. But the far-field power density is still the one tenth as that without the heat distortion of secondary mirror. These results can also provide the reference to the thermal aberration analyze for high power laser system and can be applied to the field of laser communication system and laser weapon etc.
Discriminatively trained part based model armed with biased saliency
Huapeng Yu, Yongxin Chang, Pei Lu, et al.
Discriminatively trained Part based Model (DPM) is one of the state-of-the-art object detectors. However, DPM complies little with real vision procedure. In this paper, we try arming DPM with biologically inspired approaches. On the one hand, we use Gabor instead of Histogram of Oriented Gradient (HOG) as low level features to simulate the receptive fields of simple cells. We show Gabor outperforms or is on par with HOG. On the other hand, we learn biased saliency of the object with the same Gabor features to simulate the search procedure of real vision. We combine DPM and biased saliency in a single Bayesian framework, which at least partially reflects the interactions between top-down and bottom-up vision procedures. We show these biologically inspired procedures can effectively improve the performance and efficiency of DPM. We present experimental results on both challenging PASCAL VOC2007 dataset and publicly available sequences.
Automatic alignment technology in high power laser system
Xin Zhang, Wan-jun Dai, Yuan-cheng Wang, et al.
The high power solid laser system is becoming larger and higher energy that requires the beam automatic alignment faster and higher precision to ensure safety running of laser system and increase the shooting success rate. This paper take SGIII laser facility for instance, introduce the basic principle of automatic alignment of large laser system. The automatic alignment based on digital image processing technology which use the imaging of seven-classes spatial filter pinholes for feedback to working. Practical application indicates that automatic alignment system of cavity mirror in SGIII facility can finish the work in 210 seconds of four bundles and will not exceed 270 seconds of all six bundles. The alignment precision promoted to 2.5% aperture from 8% aperture. The automatic alignment makes it possible for fast and safety running of lager laser system.
High-accuracy arithmetic for cavity mirror automatic alignment in multi-pass beam system
Yuancheng Wang, Wei Zhou, Xingquan Xie, et al.
Beam alignment of multi-pass amplification is based on cavity mirror alignment. To optimize multi-segmental parallel cavity mirror alignment arithmetic of high power solid-state lasers, propose a new type of arithmetic of multi-pass beam path cavity mirror based on diffraction symmetry, and the accuracy of multi-pass amplification beam path alignment is improved by 10μrad up to 3.96μrad. The arithmetic avoids low accuracy of CM alignment caused by poor image quality, It makes SG-Ⅲ facility operate long term and properly.
A novel multi-view object recognition in complex background
Yongxin Chang, Huapeng Yu, Zhiyong Xu, et al.
Recognizing objects from arbitrary aspects is always a highly challenging problem in computer vision, and most existing algorithms mainly focus on a specific viewpoint research. Hence, in this paper we present a novel recognizing framework based on hierarchical representation, part-based method and learning in order to recognize objects from different viewpoints. The learning evaluates the model’s mistakes and feeds it back the detector to avid the same mistakes in the future. The principal idea is to extract intrinsic viewpoint invariant features from the unseen poses of object, and then to take advantage of these shared appearance features to support recognition combining with the improved multiple view model. Compared with other recognition models, the proposed approach can efficiently tackle multi-view problem and promote the recognition versatility of our system. For an quantitative valuation The novel algorithm has been tested on several benchmark datasets such as Caltech 101 and PASCAL VOC 2010. The experimental results validate that our approach can recognize objects more precisely and the performance outperforms others single view recognition methods.
Simulation of phase noise for coherent beam combination
Qi-qi Hu, Zhi-meng Huang, Xuan Tang, et al.
Active coherent beam combination has been a hot area of research for several years. Particular algorithm module is used to stabilize the phase difference between beamlets, and make them coherent. The phase noise increases with the raising power of laser output. Under low power condition, we simulate the phase noise of high power laser amplifier by the Arbitrary Function Generators (AFGs), and send them to the phase modulators to destabilize the phase, to test the performance of the phase lock algorithm. The experimental results show the feasibility.
Simulation study on fiber-coupling efficiency for quantum communication through atmospheric turbulence
Xuejun Long, Bo Qi, Ge Ren, et al.
Quantum communication is a high secure and high efficiency communication method. The received laser beam in the quantum communication system must be coupled into the fibers, by which the communication signal can be detected, amplified and processed for the latter devices. However, atmospheric turbulence will degrade the spatial coherence of the laser beam and limits the fiber- coupling efficiency. In the paper, the propagation model of the laser beam through atmospheric turbulence is established and the fiber-coupling efficiency for the laser beam distorted by atmospheric turbulence is evaluated, and the optimal coupling geometry parameter a=1.12 is given, with which the theoretical maximal coupling efficiency is reached. Then the relationship between the atmospheric coherence constant r0 and the fiber-coupling efficiency is analyzed. The simulation also indicates that the fast steering mirror (FSM) can reduce the influence of the atmospheric turbulence and improve the coupling efficiency distinctly.
Investigation on the influence of spectral linewidth broadening on beam quality in spectral beam combination
Fei Tian, Hong Yan, Li Chen, et al.
Spectral beam combination (SBC) is a promising method to combine multiple fiber outputs for further power scaling with the capability of maintaining high beam quality, but the beam quality will be degraded with spectral linewidth broadening, because it could result in additional angular spread in the output beam. In this paper, we described theoretical calculation as well as experimental investigation on the influence of spectral linewidth broadening on beam quality. The results show that in single SBC system the spectral linewidth should be limited to less than a few GHz in order to avoid beam quality degradation, but the linewidth requirement could be decreased to more than hundreds of GHz using a pair of parallel gratings, which reveals a feasible way to increase the stimulated Brillouin scattering (SBS)-free power output of single fiber laser for overall output scaling and high beam quality.
Characteristic analysis of a polarization output coupling Porro prism resonator
An Electro-optical Q-switched Nd:YAG slab laser with a crossed misalignment Porro prism resonator for space applications has been theoretically and experimentally investigated. The phase shift induced by the combination of different wave plates and Porro prism azimuth angles have been studied for creating high loss condition prior to Q-switching. The relationship of the effective output coupling reflectivity and the employed Q-switch driving voltage is explored by using Jones matrix optics. In the experiment, the maximum output pulse energy of 93 mJ with 14-ns pulse duration is obtained at the repetition rate of 20 Hz and the optical-to-optical conversion efficiency is 16.8%. The beam quality factors are M 2 x = 2.5 and M 2y = 2.2, respectively.
Vibration characteristic of high power CO2 laser
High power CO2 laser is widely used in various scientific, industrial and military applications. Vibration is a common phenomenon during laser working process, it will affect the working performance of high power CO2 laser, vibration must be strictly controlled in the condition where the laser pointing is required. This paper proposed a method to investigate the vibration characteristic of high power CO2 laser. An experiment device with vibration acceleration sensor was established to measure vibration signal of CO2 laser, the measured vibration signal was mathematically treated using space-frequency conversion, and then the vibration characteristic of high power CO2 laser can be obtained.
Applications, Laser Materials (Crystal, Ceramic), Novel Approaches
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Laser space debris removal: now, not later
Small (1-10cm) debris in low Earth orbit (LEO) are extremely dangerous, because they spread the breakup cascade depicted in the movie “Gravity.” Laser-Debris-Removal (LDR) is the only solution that can address both large and small debris. In this paper, we briefly review ground-based LDR, and discuss how a polar location can dramatically increase its effectiveness for the important class of sun-synchronous orbit (SSO) objects. No other solutions address the whole problem of large (~1000cm, 1 ton) as well as small debris. Physical removal of small debris (by nets, tethers and so on) is impractical because of the energy cost of matching orbits. We also discuss a new proposal which uses a space-based station in low Earth orbit (LEO), and rapid, head-on interaction in 10- 40s rather than 4 minutes, with high-power bursts of 100ps, 355nm pulses from a 1.5m diameter aperture. The orbiting station employs “heat-capacity” laser mode with low duty cycle to create an adaptable, robust, dualmode system which can lower or raise large derelict objects into less dangerous orbits, as well as clear out the small debris in a 400-km thick LEO band. Time-average laser optical power is less than 15kW. The combination of short pulses and UV wavelength gives lower required energy density (fluence) on target as well as higher momentum coupling coefficient. This combination leads to much smaller mirrors and lower average power than the ground-based systems we have considered previously. Our system also permits strong defense of specific assets. Analysis gives an estimated cost of about $1k each to re-enter most small debris in a few months, and about 280k$ each to raise or lower 1-ton objects by 40km. We believe it can do this for 2,000 such large objects in about four years. Laser ablation is one of the few interactions in nature that propel a distant object without any significant reaction on the source.
Mean flow aero-optic effect of airborne optical platform
The simulation of the mean flow over airborne laser fairing as well as its aero-optic analysis is presented. The simulation reveals that the aero-optic effect with respect to the mean flow strongly depends on the beam direction, and the aberration is mainly low-order effect. The thickness of mean flow around the platform is defined and estimated based on aero-optical effect. A rough estimation formula about the thickness of flow field around the platform, the platform speed, and the characteristic time of the aero-optic effect, is brought about. The calculation results verify the reliability of the rough estimation formula.
Peculiarities of radiation formation in laser systems for remote sensing
Yury N Panchenko, M. V. Andreev, V. F. Losev, et al.
The study results of a high-quality radiation formation in the UV and IR spectral ranges for using in lidar systems to detect the vapors of nitric oxide and complex molecules, including data radicals, are discussed. The influence of the existing cavity losses on the radiation formation in the dispersion cavity is studied.The development of KrF laser with a broad spectral band adjustment of 247.5-249.5 nm and high energy are presented about. The possibility of pulse formation with duration of 30 ns in CO2 laser is shown.
Laser assisted die bending: a new application of high power diode lasers
D. Schuöcker, T. Schumi, O. Spitzer, et al.
Nowadays high power lasers are mainly used for cutting of sheet metals, for welding, hardening and rapid prototyping. In the forming of sheet metals as bending or deep drawing lasers are not used. Nevertheless a few years ago a new application of high power lasers has been invented, where bending of materials that break at room temperature becomes possible by heating them along the bending edge with high power lasers thus allowing their treatment without cracks and rupture. For this purpose a large number of diode lasers are arranged in the bottom tool of a bending machine (a V-shaped die) which heat up the initially flat sheet metal during the bending process what is performed by pressing it into the die with a knife shaped upper tool where due to the laser heating the material is softened and thus cracks are avoided. For the technical realization of the new process of laser assisted die bending, modules equipped with numerous laser diodes and a total beam power of 2,5 kW are used. The light emitted by these modules enters a tool with a length of 15cm and is deflected towards the workpiece. By using ten of these modules with adjacent dies and by integrating those in a bending press a bending edge of sheet metals with a length of 1500mm can be realized. Such a bending press with laser assistance also needs energization with a power of practically 50kW, a respective water flow, a heat exchanger system and also a control for all functions of this system. Special measures have also been developed to avoid radiating of those tools that are not covered by a workpiece in the case of bending edges shorter than the full length of the bending tools whereas individual short circuiting of diode modules can be performed. Specific measures to ensure a safe operation without any harm to the operational person have been realized. Exploitation of the bending process has been carried out for titanium, where material thicknesses up to 3mm have been bent successfully.
Interaction of extreme ultraviolet laser radiation with solid surface: ablation, desorption, nanostructuring
Karel Kolacek, Jiri Schmidt, Jaroslav Straus, et al.
The area, where interaction of focused XUV laser radiation with solid surface takes place, can be divided according to local fluency into desorption region (if fluency is larger than zero and smaller than ablation threshold) and ablation region (if fluency is equal or larger than this threshold). It turned out that a direct nanostructuring (e.g. imprinting diffraction pattern created on edges of windows of proximity standing grid) is possible in the desorption region only. While for femtosecond pulses the particle (atom/molecule) removal-efficiency η in the desorption region is very small (η < 10%), and hence, it can be easily distinguished from the ablation region with η ~ 100%, for nanosecond pulses in desorption region this η rises at easily ablated materials from 0% at the periphery up to ~90% at the ablation contour and, therefore, the boundary between these two regions can be found with the help of nanostructuring only. This rise of removal efficiency could be explained by gradually increased penetration depth (due to gradually removed material) during laser pulse. This is a warning against blind using crater shape for fluency mapping in the case of long laser pulses. On the other hand it is a motivation to study an ablation plum (or ablation jet) and to create a knowledge bank to be used at future numerical modeling of this process.
Photoluminescence defects on subsurface layer of fused silica and its effects on laser damage performance
Hongjie Liu, Jin Huang, Fengrui Wang, et al.
Subsurface defects of polished fused silica optics are responsible for igniting laser damage in high power laser system. A non destructive measurement technique is developed to detect subsurface photoluminescence defects of fused silica. The fused silica samples polished by different vendors are applied to characterization of subsurface defects and measurement of damage performance. Subsurface photoluminescence defects of fused silica are evaluated by confocal fluorescence microscopy system. Laser induced damage threshold and damage density are measured by 355 nm nanosecond pulse laser. The results show a great differential subsurface quality of fused silica samples. Laser induced damage performance has a good correlation with subsurface defects. This paper shows a new non destructive measurement technique to detect photoluminescence defects on the subsurface layer of polished fused silica. It is very valuable to increasing laser damage performance and improving production-manufacturing engineering of optics.
Crystalline micro/nanostructures fabrication on silicon using femtosecond laser
The laser induced micro/ nano-meter size surface structures are fabricated by multi linear polarized femtosecond laser pulses (pulse duration τ=35 fs, wavelength λ=800 nm) irradiation at room temperature(25 ℃ ) and 400 ℃. The structures fabricated at these two temperatures show distinct temperature dependence. The grooves, which are parallel to the polarization of the laser light, can be clearly observed at almost all the structured area formed at 400 ℃ while laser induced period structures(ripples) are the most pronounced surface structure in the crater formed at room temperature. The crystallinity of these surface structures are investigated by using Raman spectroscopy. The Raman spectrum shows that all the structured area formed at 400 ℃ is crystalline(or poly-crystalline) while amorphous silicon can be observed within the structures formed at room temperature (25 ℃). These results indicate that temperature is an important parameter to be tuned to tailor the micro/nano-structure fabrication.
Regular sub-wavelength surface structures induced by femtosecond laser pulses on nickel
In this research, the formation of laser-induced periodic surface structures (LIPSS) on the nickel surface by femtosecond laser pulses was investigated. In the experiment, we used a commercially available amplified Ti:sapphire laser system that generated 164 fs laser pulses with a maximum pulse energy (Ep) of 1 mJ at a 1 kHz repetition rate and with a central wavelength λ= 780 nm. To obtain a fine periodic ordering of surface nanostructures, the laser beam, through a 0.2 mm pinhole aperture positioned near the 10× objective lens, was focused onto the sample. The samples were mounted on an XYZ-translation stage and irradiated in static and line-scanning experiment. The morphology of the induced periodic structure was examined by scanning electron microscopy. The surface profile was measured by atomic force microscopy. LIPSS with a period of around 700 nm entirely covered the irradiated area. Large area of LIPSS in the nickel surface was produced in line-scanning experiment. The mechanism of the formation of LIPSS in the entire irradiated area in static irradiation was discussed. The function of a 0.2 mm pinhole aperture was studied. The regular LIPSS on the nickel surface changed the optical property of the surface. The regular LIPSS on nickel surface could be also applied on the micro-mould fabrication.
Study of high power laser mirror shape maintenance technology
Gang Li, Feng Wang, Shunfu Liu, et al.
With the laser power and operation time increasing, the surface distortion of the laser cavity mirrors has been a critical problem to be resolved. In order to maintain the mirror shape under intense laser irradiation, in this paper, a new type of micro channel silicon water cooled mirror is given. The water cooled mirror includes three layers, the first layer is mirror seat made of Invar materials with water inlet and outlet, the second layer is water manifold unit made of silicon with millimeter channels; the third layer is reflection plate with micro-channels. These three layers are bonded together by vacuum soldering. Then the thickness of the reflection plate is reduced to 0.5mm by grinding, and polished by traditional pitch polishing method. In order to reduce the coating stress, conventional all dielectric coating is replaced by a metal dielectric film stack. At last, a 100mm×100mm water cooled mirror is fabricated with surface figure 0.22λ@632.8 nm. The mirror surface distortion is 0.12λ、0.24λ、0.33 λ respectively corresponding to 7.3w/cm2 、11.9 w/cm2 、17.6 w/cm2 heat load.
Microstructure and properties of welds between 5754 Al alloys and AZ31 Mg alloys using a Yb:YAG laser
Sana Bannour, Michel Autric, Jean-Eric Masse, et al.
Dissimilar laser beam welding between A5754 Al alloys and AZ31 Mg alloys with the plate thickness of 2 mm was investigated. Complex flow pattern characterized by a large volume of intermetallic compounds Al12Mg17 and Al3Mg2 is formed in the fusion zone. Microhardness measurement of the dissimilar welds presents an uneven distribution due to the complicated microstructure of the weld, and the maximum value of microhardness in the fusion zone is much higher than of the base materials.
Femtosecond laser induced surface structuring on silicon by diffraction-assisted micropatterning
Femtosecond laser micropatterning of silicon with nanometric surface modulation is demonstrated by irradiating through a diffracting pinhole. The irradiation results obtained at fluences above the melting threshold are characterized by optical and scanning electron microscopy and reveal a good agreement with Fresnel diffraction theory. LIPSS have been generated in the micropatterning surface. We found Ripples spacing were found of 550-680 nm. Based on the Sipe and Drude model, the theoretical period of LIPSS is closer to experimental measurements. Due to the diffraction, the LIPPS having a different period appear in a diffraction micropatterning.
A novel description based on skeleton and contour for shape matching
Jinlong Hu, Xianrong Peng, Chengyu Fu
In computer vision field, feature extraction plays a critical role in shape matching, image alignment, object recognition and tracking etc. Generally speaking, feature extraction consists of three steps: feature detection, feature description and feature matching. In the second step, the detected features (e.g. gray value, SIFT, Harris corners) are converted to vectors or the form that can be described mathematically such that feature can be matched correctly. How to construct an efficient descriptor to realize accurate shape matching under a variety of transformations is still a challenge. To this end, a novel shape descriptor based on skeleton for shape matching is proposed in this paper. Firstly, the image is smoothed with Gaussian filter to remove the influence of the noise. Secondly, the smoothed image is segmented with Fuzzy C-means Cluster (FCM) to obtain a binary image. Thirdly, the binary image’s skeleton is extracted with Medial Axis Transform (MAT), thus the skeleton’s endpoints and joint-points locations are acquired. Furthermore, the object’s contour is extracted with contour coding. In the construction of skeletal descriptor, the relative location vectors of the skeletal endpoints to each contour point are computed. Being similar to shape context, statistical histogram is constructed in log-polar coordinate. Consequently, shape matching is performed via two histograms’ similarity measurement. Experiments on standard MPEG7 dataset show that the proposed shape description method allows translation, scale and rotation invariance.
Mechanical analysis of photo-electricity measure equipment shafting in mobile-platform
Weiwei Jiang, Hongyi Hu, Yi Tan, et al.
Along with the development of technology, photo-electricity measure equipment has been from ground-fixed to mobile-platform borne, So it is need to mechanical analysis of shafting for adapt movement environment of mobile-platform. First listed three kinds of shafting form, then analysis mechanical station of the foremost component bearing in vertical load, radial load, swing load station, last work out the formulas, which was the theory for the shafting design in mobile-platforms photo-electricity equipment.
Realization of LOS (Line of Sight) stabilization based on reflector using carrier attitude compensation method
Yao Mao, Jing Tian, Jia-guang Ma
The techonology of LOS stabilization is widely applicated in moving carrier photoelectric systems such as shipborne, airborne and so on. In application situations with compact structure, such as LOS stabilization system of unmanned aerial vehicle, LOS stabilization based on reflector is adopted, and the detector is installed on the carrier to reduce the volume of stabilized platform and loading weight. However, the LOS deflection angle through reflector and the rotation angle of the reflector has a ratio relation of 2:1, simple reflector of stable inertial space can not make the optical axis stable. To eliminate the limitation of mirror stabilizing method, this article puts forward the carrier attitude compensation method, which uses the inertial sensor installed on the carrier to measure the attitude change of the carrier, and the stabilized platform rotating half of the carrier turbulence angle to realize the LOS stabilization.
PCIE interface design for high-speed image storage system based on SSD
This paper proposes and implements a standard interface of miniaturized high-speed image storage system, which combines PowerPC with FPGA and utilizes PCIE bus as the high speed switching channel. Attached to the PowerPC, mSATA interface SSD(Solid State Drive) realizes RAID3 array storage. At the same time, a high-speed real-time image compression patent IP core also can be embedded in FPGA, which is in the leading domestic level with compression rate and image quality, making that the system can record higher image data rate or achieve longer recording time. The notebook memory card buckle type design is used in the mSATA interface SSD, which make it possible to complete the replacement in 5 seconds just using single hand, thus the total length of repeated recordings is increased. MSI (Message Signaled Interrupts) interruption guarantees the stability and reliability of continuous DMA transmission. Furthermore, only through the gigabit network, the remote display, control and upload to backup function can be realized. According to an optional 25 frame/s or 30 frame/s, upload speeds can be up to more than 84 MB/s. Compared with the existing FLASH array high-speed memory systems, it has higher degree of modularity, better stability and higher efficiency on development, maintenance and upgrading. Its data access rate is up to 300MB/s, realizing the high speed image storage system miniaturization, standardization and modularization, thus it is fit for image acquisition, storage and real-time transmission to server on mobile equipment.
Investigation into the initiation of hexanitrostilbene by laser-driven composite flyer plates
Meng Wang, Qiu-bo Fu, Yao Wang, et al.
A laser-driven mini-flyer priming system has been a primary focus for research into advanced initiators in recent years. Due to the low transmission rates and efficieny in the utilization of lasers, it is difficult to apply such a system in practice. Improving the flyer structure’s technology is a key to increasing energy efficiency. We calculate and design the thickness of each layers in the composite flyer plates, then the composite flyers with different parameters are prepared using magnetron sputtering. Taking advantage of a photonic doppler velocimetry (PDV) high-speed testing system, the accelerating processes of flyers with different parameters are analyzed. An experimental comparison is made on HNS-IV donation between laser-driven single-layer aluminium flyer and composite flyer
The structure design and performance analysis for damping system of the airborne equipment
Wei Zhang, Chun-xia Wu, Cong-lin Yan, et al.
Vibration is an important factor that could affect the performance of airbone optical system, the damping device based on the wire-rope vibration isolators was designed in this paper, in which the optical system mounted on the helicopter was taken as an example. The transmissibility of the damping device was about 40% which obtained by finite element method, the transmissibility of the damping device was about 36% which obtained by vibration platform test, the result obtained by finite element method was proved by vibration platform test. The vibration of the optical system could been reduced significantly as a result of the device with good damping effect, thereby the stability of the optical system could be enhanced.
Study about the 6-DOF parallel tracking platform
Sen Tan, Ge Ren, Yi Tan
We designed one 6 degrees of freedom (DOF) parallel platform to track and observe in a certain range according to requirements of a telescope. First of all, considering about the application requirements of telescope, we determined the specific structure. After that we carried on theoretical analysis about kinematics and dynamics of the platform. Then, we build parameterized 3D model and optimized it with Adams software to ensure the platform to achieve best dynamic performance and minimum space size. Finally we did co-simulation making use of Adams and Simulink software (Matlab) to observe the platform’s tracing ability. And we analyzed the factors affecting the tracking error.
Laser erosion diagnostics of plasma facing materials with displacement sensors and their application to safeguard monitors to protect nuclear fusion chambers
Koichi Kasuya, Shinji Motokoshi, Seiji Taniguchi, et al.
Tungsten and SiC are candidates for the structural materials of the nuclear fusion reactor walls, while CVD poly-crystal diamond is candidate for the window material under the hazardous fusion stresses. We measured the surface endurance strength of such materials with commercial displacement sensors and our recent evaluation method. The pulsed high thermal input was put into the material surfaces by UV lasers, and the surface erosions were diagnosed. With the increase of the total number of the laser shots per position, the crater depth increased gradually. The 3D and 2D pictures of the craters were gathered and compared under various experimental conditions. For example, the maximum crater depths were plotted as a function of shot accumulated numbers, from which we evaluated the threshold thermal input for the surface erosions to be induced. The simple comparison-result showed that tungsten was stronger roughly two times than SiC. Then we proposed how to monitor the surface conditions of combined samples with such diamonds coated with thin tungsten layers, when we use such samples as parts of divertor inner walls, fusion chamber first walls, and various diagnostic windows. We investigated how we might be able to measure the inner surface erosions with the same kinds of displacement sensors. We found out the measurable maximum thickness of such diamond which is useful to monitor the erosion. Additionally we showed a new scheme of fusion reactor systems with injectors for anisotropic pellets and heating lasers under the probable use of W and/or SiC.
Interaction of high-power ultrashort laser pulses with air
New concept of self-focusing on the basis of great number of laboratory experiments and numerical simulations is developed. The results of the field experiments on the control of parameters of a terawatt ultrashort laser radiation in order to obtain the self-focusing effect at a specified part of atmospheric path is presented. The numerical simulations the nonlinear propagation of TW laser pulse at 10,6 mkm carrier wavelength in air are carried out.
Research of beam smoothing technologies using CPP, SSD, and PS
Rui Zhang, Jingqin Su, Dongxia Hu, et al.
Precise physical experiments place strict requirements on target illumination uniformity in Inertial Confinement Fusion. To obtain a smoother focal spot and suppress transverse SBS in large aperture optics, Multi-FM smoothing by spectral dispersion (SSD) was studied combined with continuous phase plate (CPP) and polarization smoothing (PS). New ways of PS are being developed to improve the laser irradiation uniformity and solve LPI problems in indirect-drive laser fusion. The near field and far field properties of beams using polarization smoothing were studied and compared, including birefringent wedge and polarization control array. As more parameters can be manipulated in a combined beam smoothing scheme, quad beam smoothing was also studies. Simulation results indicate through adjusting dispersion directions of one-dimensional (1-D) SSD beams in a quad, two-dimensional SSD can be obtained. Experiments have been done on SG-III laser facility using CPP and Multi-FM SSD. The research provides some theoretical and experimental basis for the application of CPP, SSD and PS on high-power laser facilities.
Research on auto-focusing of high signal to noise ratio image with weak light
Hao-yu Yuan, Wen-feng Wang, Jun Tang, et al.
For souped-up of Optical Damage Online Inspection system and obtain damage information of the optic exactly, this paper describes an auto focusing method for weak spot which on high signal to noise ratio image in darkroom. We used different calculation strategy to target pixel and adjoining pixel, compute the gray different that on x and y, compared the result of two focusing evaluation function which the gray difference absolute sum function and the gray difference quadratic sum function, select the best calculation strategy and focusing evaluation function. Test results indicate that, the gray difference quadratic sum function can be more exactly in darkroom and under weak light
A multi-beam alignment method at target area for ICF laser facility based on invariant sub-area configuration of the alignment sensor
Ning-Bo Cheng, Fu-Quan Li, Bin Feng, et al.
A multi-beam alignment method is proposed to reduce the total time for aligning at the target area all the laser beams of an ICF laser facility. A number of sub-areas with invariant size and position are extracted from the image acquired by the alignment sensor. An alignment route is comprised of a certain part of those sub-areas, and several alignment routes can cover all the sub-areas. The invariant layout of the sub-areas and the alignment routes is called an invariant sub-area configuration of the alignment sensor. The focused spots of the alignment beams are adjusted in a specific sequence along the alignment routes, and finally reach the desired position on the alignment sensor. The adjustment of all the spots inside each sub-areas is carried out concurrently, and the adjustment along one route for a spot moving from one sub-area into the next sub-area is carried out consecutively. The estimated total time for aligning all the laser beams at target area shows that the proposed multi-beam alignment method has a much higher efficiency.
A target positioning method for ICF laser facility without translational kinematic coupling
Ning-Bo Cheng, Bin Feng, Fu-Quan Li, et al.
The target position system (TPS) is one of the important subsystems of an ICF laser facility. However, TPS shows to have kinematic coupling problem in practice. This necessitates iterative adjustment of the Stewart 6-DOF manipulator to make the pose of the target as expected. In every iteration, the pose of the target must be measured, making TPS incompetent in some scenarios which call for only one step to position a target. To handle this problem, this paper proposes a target positioning method focusing on translational kinematic coupling. This method have a significant advantage that it has no relation with both the geometric parameters and the mounting of the target. This makes the proposed positioning method featured by a good practicality. Experiment results show that the proposed method can greatly reduce the position error when positioning a target by only one step.
Experimental research on high-contrast measurement for ns high-power laser shaped pulse
Jun Dong, Zhi-hong Sun, Zhi-tao Peng, et al.
Based on the multiple pulses joining, the cascaded photodetection is experimentally researched on high-contrast measurement of ns high-power laser pulse. The ultrafast photodetectors with the saturation characteristics are used. A joining method for multi-pulse waveforms in the nonlinear region is put forward. The experimental results for ns step-pulse at SG-III laser system show the contrast of ~ 400:1 is achieved, in accord with designed contrast value.
Lifetime of high-power GaAs photoconductive semiconductor switch triggered by laser of different power density
Yi Liu, Wei Wang, Yi Shen, et al.
Conduction modes of GaAs photoconductive semiconductor switch (PCSS) and their conditions are expounded. Laser diode and high-power picosecond Nd:YAG lasers are used as triggers for nonlinear mode and quasi-linear mode respectively in high-power conduction experiment. GaAs PCSS`s failure mechanisms and factors influencing lifetime in both modes are analyzed. It is found that the power density of laser at trigger time determines in which mode GaAs PCSS operates. Low-power laser triggers a nonlinear mode conduction in which GaAs PCSS`s lifetime is only 103, while high-power laser triggers a quasi-linear mode conduction in which GaAs PCSS`s lifetime is up to 105. According to the findings, the compact high-power pulsed power system based on mass of GaAs PCSSs demands for miniature high-power laser generators.
Optical transmission and reflection of a plasma produced in nanosecond laser induced air breakdown
In this paper, we investigate dynamics of optical breakdown in air induced by a pump- probe laser beam technique. The optical breakdown was generated by an Nd:YAG laser beam (operating at 10 ns pulse duration and 1064 nm wavelength). A small part of the beam was separated from the main beam by an optical splitter and was aligned to probe the breakdown plasma transversely. Monitoring the time resolved reflectivity from the breakdown plasma may give information on critical density and together with its transmittivity through the plasma provides information on plasma absorptivity. The results of this experiment can be also used to describe dynamics of breakdown plasma evolution.
Three-dimensional footprint of optical breakdown in transparent dielectrics induced by femtosecond pulsed lasers and the effect of laser energy absorption
M. H. Mahdieh, Z. Mortezaei
In this paper optical breakdown process in dielectrics induced by femtosecond pulsed lasers was calculated numerically and 3-dimensional footprint of the breakdown region was simulated. Rate equation was used to calculate the temporal and spatial evolution of free electron density. In these simulations the role of tunneling ionization in free electron generation has been considered. Also absorbed energy from the pulse via multiphoton and inverse bremsstrahlung absorption was calculated numerically. The results show that the absorption of the propagating laser through the initially transparent dielectric is important and its ignorance may results in significant error in estimating the breakdown geometry, and volume. According to the simulation results in compare with our previous calculations, smaller and asymmetrical breakdown footprint is obtained if the absorption is considered in the calculations.
Research on high-power laser diode used for triggering photoconductive semiconductor switch
Wei Wang, Yi Liu, Yi Chen, et al.
A kind of laser diode with high power and short duration is described in this paper. The laser diode is used for triggering gallium arsenide photoconductive semiconductor switch (GaAs PCSS) in the experiment. The driver of the laser diode is based on RF MOSFET and it provides an ultra-fast pulse current, which has the rise-time, FWHM and peak current are 4ns, 17ns and 130A, respectively. The characteristics of the laser diode have been researched, including laser pulse waveform, optical field distribution, and limiting drive current. Using a combination of two laser diodes, the PCSS has a better performance than being triggered by single laser diode in the experiment.
Energy transmission by laser
Laser spark obtained by using a conical optics is much more appropriate to form conducting channels in atmosphere. Only two types of lasers are actively considered to be used in forming high-conductivity channels in atmosphere, controlled by laser spark: pulsed sub-microsecond gas and chemical lasers (CO2, DF) and short pulse solid-state and UV lasers. Main advantage of short pulse lasers is their ability in forming of superlong ionised channels with a characteristic diameter of ~ 100 mkm in atmosphere along the beam propagation direction. At estimated electron densities below 1016 cm-3 in these filaments and laser wavelengths in the range of 0.5 - 1.0 mm, the plasma barely absorbs laser radiation. In this case, the length of the track composed of many filaments is determined by the laser intensity and may reach many kilometers at a femtosecond pulse energy of ~ 100 mJ. However, these lasers could not be used to form high-conductivity long channels in atmosphere. The ohmic resistance of this type a conducting channels turned out to be very high, and the gas in the channels could not be strongly heated (< 1 J). An electric breakdown controlled by radiation of femtosecond solid-state laser was implemented in only at a length of 3 m with a voltage of 2 MV across the discharge gap (670 kV/m). Not so long ago scientific group from P.N. Lebedev has improved that result, the discharge gap -1m had been broken under KrF laser irradiation when switching high-voltage (up to 390 kV/m) electric discharge by 100-ns UV pulses. Our previous result - 16 m long conducting channel controlled by a laser spark at the voltage - 3 MV - was obtained more than 20years ago in Russia and Japan by using pulsed CO2 laser with energy - 0.5 kJ. An average electric field strength was < 190 kV/m. It is still too much for efficient applications.
Temperature dependence of Raman scattering in KDP crystal
Xiangxu Chai, Xinguang Xu, Fuquan Li, et al.
We measured the Raman spectra of the ν1 mode in KDP crystal over the temperature range from 285.3 K to 345.2 K. And the temperature dependence of the Raman characteristics (Raman shift, FWHM and intensity) were well analyzed. The result reveals that with temperature increasing the ν1 mode displays a red-shift and the linewidth broadens, but the scattering intensity shows no obvious tendency. The stimulated Raman scattering (SRS) gain coefficient of the ν1 mode decrease about 12% at 345.2 K.
Measurement of crystal defects using phase retrieval technique
Yudong Yao, Junyong Zhang, Yanli Zhang, et al.
In high power laser systems, crystal defects introduced by manufacturing have significant impact on quality of light beams; finally affect the output status of high power laser system. The phase retrieval algorithm can precisely measure the crystal defects, such as the residual periodic perturbations in a relatively large area and the relatively small point defects, with the resolution of micrometer magnitude. At the same time, the multiple near-focus intensity measurements algorithm used here can retrieve the morphology of focal spot, which is modulated by the defects and cannot be directly measured due to its high power. In addition, the algorithm has been improved in order to use less measurement planes and less iteration times to complete retrieval.
Design and simulations of CAEP THz FEL resonator
Yuhuan Dou, Xiaojian Shu, Derong Deng, et al.
A high power China Academy of Engineering Physics(CAEP) THz free electron laser (FEL) is designed and optimized in a radiation frequency range of 1~3 THz and average output power of about 10 W. The main work focuses on the optimization of different schemes through physical analysis. The wiggler peak field strength and electron beam energy have been selected with eleven frequencies ranging from 1 THz to 3 THz. It is found that the values of the gain and output power of the cavity are largest at 2.6 THz. So we can test the facility at this frequency. While the value of the output power is less than the design goal at the lower frequency region of about 1.0 THz due to the serious slippage between the electron bunch and radiation pulse. To increase the output power at the lower frequency region, the scheme of elliptical hole-coupling optical resonator is proposed to solve this problem. The simulation results show that the elliptical hole-coupling output is effective and applicable for the THz FEL and the output power can be increased by more than 30%.
High-peak-power terahertz sources pumped by high power laser and single-shot measurement of terahertz temporal waveform
Sen-Cheng Zhong, Zhao-Hui Zhai, Li-Guo Zhu, et al.
Optical rectification of laser pulses in LiNbO3 crystal pumped by high power laser is one of the most powerful way to generate this high-peak-power terahertz pulses. It enhances the laser-terahertz transform efficiency by tilted-pulse-front pumping(TPFP) to fulfill phase match in the LiNbO3 crystal. However, comprehensive theoretical analysis is still lack. In this work, a detailed theoretical model to investigate the THz generation efficiency by using nonlinear susceptibility tensor of LiNbO3 crystal was presented. Based on femtosecond laser system, a setup to generate high-peak-power terahertz pulses and a time domain spectroscopy system are established. The property of generated terahertz pulses was analyzed by using terahertz camera and THz time domain system. We also realized the single-shot measurement of terahertz temporal waveform by using this terahertz source.
Aspects for efficient wide spectral band THz generation via CO2 laser down conversion
Yu. N. Panchenko, Yu. M. Andreev, G. V. Lanskii, et al.
Detailed model study of THz generation by CO2 laser down-conversion in pure and solid solution crystals GaSe1-xSx is carried out for the first time. Both forward and backward collinear interactions of common (eo-e, oe-e, oe-o, oo-e, ee-o) and original (ee-e, oo-o) types are considered. Possibility of realization, phase matching angles and figure of merits are estimated for line mixing within 9 μm and 10 μm emission bands, as well between them. Dispersion properties of o- and e-wave refractive indices and absorption coefficients for GaSe, GaS and GaSe1-xSx crystals were preliminary measured by THz-TDS, approximated in the equation form and then used in the study. Estimated results are presented in the form of 3-D figures that are suitable for rapid analyses of DFG parameters. The most efficient type of interaction is eo-o type. Optimally doped (x = 0.09-0.13) GaSe1-xSx crystals are from 4 to 5 times more efficient at limit pump intensity than not doped GaSe crystals.
Study on conversion efficiency of optical-to-terahertz in optical rectification
Optical rectification of ultra-short laser pulses is an attractive technique for efficient generation of terahertz pulses. An analytical expression of the optical-to-terahertz conversion efficiency in optical rectification by the use of the method of tilted-pulse-front pumping has been derived and used to simulate the maximum optical-to-terahertz conversion efficiency. The variations of the conversion efficiency of optical-to-terahertz with the length of crystal, the intensity and the pulse duration of the pumping ultra-short laser have been investigated, both in numerical and analytical solutions. In addition, the absorption of terahertz wave in LiNbO3 has also been taken into account. The results show that, the optical-to-terahertz conversion efficiency increases with the intensity of pump pulse and the length of crystal, whereas decreases with the increasing of the pulse duration for the case of the relatively large pulses duration.
3D coordinate transform model of optical images fusing vector distance information
Huanhuan Ran, Yihua Huo, Zili Huang
For reducing the error of affine transform while matching the three-dimensional targets in optical images, the model of optical images matching was extended to three dimension using distance information of high characteristics in optical images (vector distance information), the three-dimensional (3D) coordinate transform was proposed. Theoretical analysis shows that when the optical imaging model was simplified to pinhole imaging model, the error of 3D coordinate transform didn’t exist, while avoiding the nonlinear problem. The amount of calculation of 3D coordinate transform was analyzed using least squares estimation and RANSAC estimation as examples; the amount of calculation of 3D coordinate transform is only four times of affine transform, twice when using RANSAC estimates. The simulation analysis of matching tracking algorithm based on SIFT feature points using 3D coordinate transform was taken by the visual simulation software VegaPrime and MATLAB, and the advantages of 3D coordinate transform has been verified.
Radiative characteristics of high-power halogen tungsten lamp used in calibration of high-energy laser energy meter
Ji Feng Wei, Yan Chang, Li Qun Sun, et al.
The calibration method using high-power halogen tungsten lamp as calibration source is very fit for calibration of high-energy laser energy meters. However, high-power halogen tungsten lamps after power-off still reserves much residual energy and continually radiates energy. The radiation efficiency of the halogen tungsten lamp after power-off can be quantitatively measured by a fast response radiation detector. The results show that the halogen tungsten lamp's radiation efficiency was improved with power-on time, but did not change under constant power-on time/energy. All the tested halogen tungsten lamps reached 89.3% of radiation efficiency at 50 s after power-on. After power-off, the residual energy in the halogen tungsten lamp gradually dropped to less than 10% of the initial radiation power, and the radiation efficiency changed with time. The final total radiation energy was decided by the halogen tungsten lamp's radiation efficiency, the radiation efficiency of residual energy, and the total energy consumption. The measuring uncertainty of total radiation energy was about 2.4% (k=2).