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- Front Matter: Volume 10457
- 10457 Laser Components, Systems, and Applications
Front Matter: Volume 10457
Front Matter: Volume 10457
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This PDF file contains the front matter associated with SPIE Proceedings Volume 10457, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.
10457 Laser Components, Systems, and Applications
The effects of pulsed laser parameters on the photoacoustic detection of glucose aqueous solution
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In this study, the photoacoustic detection system was established based on the Q switched Nd: YAG 532nm pumped optical parametric oscillator pulsed laser and the ultrasonic detector. Based on the established photoacoustic detection system, the effects of pulsed laser on the photoacoustic detection of glucose aqueous solutions were experimentally studied. The photoacoustic peak-to-peak values of glucose and pure water were obtained at different output energy of pulsed laser and at the wavelengths from 1300nm to 2200nm. Experimental results show that the photoacoustic peak-to-peak values of glucose linearly increase with the increasing of the output energy of pulsed laser. The photoacoustic peak-to-peak values were compensated because the output energy of pulsed laser exponentially decreased with the increasing of the wavelengths of pulsed laser. The characteristic wavelengths of glucose were determined according to the difference spectral between the compensated photoacoustic peak-to-peak values of glucose aqueous solution and pure water.
Obstacle detection and avoiding of quadcopter
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Recent years, the flight control technology over quadcopter has been boosted vigorously and acquired the comprehensive application in a variety of industries. However, it is prominent for there to be problems existed in the stable and secure flight with the development of its autonomous flight. Through comparing with the characteristics of ultrasonic ranging and laser Time-of-Flight(abbreviated to ToF) distance as well as vision measurement and its related sensors, the obstacle detection and identification sensors need to be installed in order to effectively enhance the safety flying for aircraft, which is essential for avoiding the dangers around the surroundings. That the major sensors applied to objects perception at present are distance measuring instruments which based on the principle and application of non-contact detection technology . Prior to acknowledging the general principles of flight and obstacle avoiding, the aerodynamics modeling of the quadcopter and its object detection means has been initially determined on this paper. Based on such premise, this article emphasized on describing and analyzing the research on obstacle avoiding technology and its application status, and making an expectation for the trend of its development after analyzing the primary existing problems concerning its accuracy object avoidance.
Simulation analysis of debris detection and removal by space-based laser
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With much more attention and utilizing paying to space resource, the detection and removal of space debris, the biggest threatened to the orbiting spacecraft, has become a research hotspot in recent years. In order to protect the important space assets, such as the international space station, it has been realized of simulation and system parameters’ design, which contained debris’ detection and removal by space-based pulse laser. Simulation results show that the determine time of detection laser and removal laser should be considered after judging weather debris is in the “clear window”. As the increasing of detection pulse and removal pulse, the orbit element of space debris has a regular change, with the decreasing of single pulse velocity increment. The system of detection and removal of space debris designed as: detection laser power 50W, removal laser power 150kW, laser wavelength 1064nm, pulse width 10ns, frequency 100Hz. The research has a great significance of detection and removal of debris by space-based laser and engineering application.
A large size vertical cavity surface emitting laser with multiple concentric ring apertures
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A large size vertical-cavity surface-emitting laser (VCSEL) with multiple concentric ring apertures (MCRA) is investigated. Compared with a typical VCSEL with the same outer dimension, the 804nm VCSEL with MCRA has maximal continuous wave(CW) light output power 0.23 W which is about 3 times that of a typical device. The novel laser also exhibits a stable single-lobed far field pattern with low beam divergence angle, which is suitable for free-space optical communication and optical interconnection applications.
Advances in high power linearly polarized fiber laser and its application
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Fiber lasers are now attracting more and more research interest due to their advantages in efficiency, beam quality and flexible operation. Up to now, most of the high power fiber lasers have random distributed polarization state. Linearlypolarized (LP) fiber lasers, which could find wide application potential in coherent detection, coherent/spectral beam combining, nonlinear frequency conversion, have been a research focus in recent years. In this paper, we will present a general review on the achievements of various kinds of high power linear-polarized fiber laser and its application. The recent progress in our group, including power scaling by using power amplifier with different mechanism, high power linearly polarized fiber laser with diversified properties, and various applications of high power linear-polarized fiber laser, are summarized. We have achieved 100 Watt level random distributed feedback fiber laser, kilowatt level continuous-wave (CW) all-fiber polarization-maintained fiber amplifier, 600 watt level average power picosecond polarization-maintained fiber amplifier and 300 watt level average power femtosecond polarization-maintained fiber amplifier. In addition, high power linearly polarized fiber lasers have been successfully applied in 5 kilowatt level coherent beam combining, structured light field and ultrasonic generation.
Optimizing the phase matching in high-order harmonics generation
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This paper presents an experimental method to realize the best high-order harmonics generation (HHG) phase matching in the interaction of strong optical field with gas target. By studying the effects of the relative location between gas target source and the Gaussian-shaped driving femtosecond laser field focus on the harmonics yield, conclusions are obtained that the optimum position of gas target for phase matching is always behind the of the focal point of the driving field, with much lower HHG yield before the focus caused by serious harmonics phase mismatch. Meanwhile, with optimum harmonics phase matching, the high-order harmonics field that resulted has the similar Gaussian-shaped spatial distribution characteristics with the driving field, verifying experimentally the commonly used assumptions for attosecond laser pulse based on HHG. This optimization method is also suitable both for other driving field with different spatial distribution of light intensity and other type of target source. The results here have important guiding significance for high harmonic generation and high harmonic isolated attosecond pulse technology.
Numerical study of temperature in a direct-liquid-cooled Nd:YLF thin disk laser
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The temperature field induced by thermal is investigated numerically with the laminar flow pattern in the direct-liquidcooled Nd:YLF thin disk laser, since it is one of the main reasons of deformation, strain and stress. The convective heat transfer coefficient on the two big surfaces of the disk is analyzed, which affects the temperature distribution directly. The convective heat transfer coefficient is no longer the thermal boundary condition but the analysis result in the analysis process. Moreover, the influences of coolant flow velocity, deposited heat power and channel thickness on temperature field are discussed. The simulation results reveal that the temperature and the cooling capacity of coolant vary on the pump power, flow velocity and channel thickness, which have a significant contribution to the temperature gradient in the disk gain medium.
Twist phase-induced characteristics changes of a radially polarized Gaussian Schell-Model beam in a uniaxial crystal orthogonal to the optical axis
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Based on the extended Huygens-Fresnel integral formula and unified theory of coherence and polarization, we obtained the cross-spectral density matrix elements for a radially polarized partially coherent twist (RPPCT) beam in a uniaxial crystal. Moreover, compared with free space, we explore numerically the evolution properties of a RPPCT beam in a uniaxial crystal. The calculation results show that the evolution properties of a RPPCT beam in crystals are substantially different from its properties in free space. These properties in crystals are mainly determined by the twist factor and the ratio of extraordinary index to ordinary refractive index. In a uniaxial crystal, the distribution of the intensity of a RPPCT beam all exhibits non-circular symmetry, and these distributions change with twist factor and the ratio of extraordinary index to ordinary refractive index. The twist factor affects their rotation orientation angles, and the ratio of extraordinary index to ordinary refractive index impacts their twisted levels. This novel characteristics can be used for free-space optical communications, particle manipulation and nonlinear optics, where partially coherent beam with controlled profile and twist factor are required.
Raman laser amplifier in methane-filled hollow-core fiber
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We report on an ultra-efficient 1.5 μm Raman amplifier in methane-filled negative curvature hollow-core fiber. A 1.5 μm tunable CW DFB seed laser is coupled into the fiber together with a 1064 nm pump laser using a shortpass dichromic mirror, and then stimulated amplified by Raman scattering of methane. The maximum Raman conversion efficiency of 66.4 % was obtained in the 2 bar methane gas filled, 2 m long hollow core fiber with 50 mW coupled pump power and 22.6 mW coupled seed laser power, and the corresponding quantum efficiency is as high as to 96.3 %, which almost approaches the quantum limit. The introduction of the single frequency seed laser not only reduced the Raman threshold from 17.5 mW to 9.5 mW, but also narrowed the Stokes linewidth from 3.4 GHz to 2.1 GHz with a factor of 60%. This kind of gas filled hollow core Raman amplifier can be a potential method to obtain low threshold, narrow linewidth and high efficiency mid infrared laser source in various application.
A calibration method of capacitive displacement sensor based on laser interference
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In order to meet the requirement of frequent in-situ calibration before micro-thrust measurement, a new calibration method based on laser interference is proposed. Based on common optical elements, the interference light path is set up, and the light path adjustment process is simplified by using the visible light of 532nm wavelength as the light source. The calibration principle is that the capacitive displacement sensor and laser interferometer simultaneously measure the movable pyramid prism’s position change resulted by adjusting displacement table, and the measurement result of laser interferometer is viewed as the reference displacement to carry on the sensor's calibration. By comparing with the sensor output, the method of calculating the number of fringes corresponding to optical path difference is analyzed. The practicability and accuracy of the calibration device is verified by experiment, and the calibration results and the relative error are analyzed at last.
Complex soliton bunching patterns induced by nonsaturable absorption
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A fiber laser with a semiconductor saturable absorption mirror (SESAM) and a graphene-polymer composite (GPC) film is constructed for achieving different soliton bunching patterns. The SESAM is used as a mode locker for self-started pulse generation, while the GPC provides a nonsaturable absorption effect for achieving a bunching in the laser cavity. There are three extra temporal patterns observed in the experiments through adjusting polarization controllers. They are chaotic bunching, weak coherent bunching and modulated harmonic bunching. The experimental investigation shows that dynamic nonsaturable absorption effect of an absorber is beneficial for generating different bunching patterns. Our work can have a deeper understanding of the formation of the complex soliton bunching patterns in pulsed lasers.
Spectroscopic properties of nanosecond laser-ablated ZnO: Mg thin film plasma
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Mg doped ZnO (MZO) thin films were prepared by magnetron sputtering and laser induced breakdown spectroscopy (LIBS) were characterized by Q-switched nanosecond 1064nm Nd:YAG laser pulse. Element characteristic spectral lines from MZO thin films with Mg concentration of 0.1 at%, 0.26 at% and 0.49 at% are illustrated by LIBS system. The results show that Mg (I) emission lines are observed corresponding the relatively high excitation with increasing Mg doped concentration. It can be mainly interpreted as more crystallization planes produced by high Mg doping concentration radiate different atomic spectral lines. The results are in relative agreement with XDR patterns. We calculated the electron density of 8.08×1022 cm-3, 7.70×1022 cm-3 and 7.99×1022 cm-3 inferred by measuring the Starkbroadened line profile. The electron temperature of 21875.85 K, 42941.49K and 28985.51K was determined using the Boltzmann plot method through the acquired data.
Correction of phase delay caused by mixer and lowpass filter in heterodyne interferometer
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Phase response is an essential parameter of vibration sensors in primary vibration calibration, heterodyne interferometer was usually used to obtain this parameter. To reduce sampling rate of heterodyne interferometer signal and sampling number, analog mixer and lowpass filter are used to down-convert the interferometer signal. However, the using of analog devices will introduce additional phase delay for the measurement of the phase response, it leads to the measurement by the calibration is unreliable. A novel correction of phase delay method was proposed to precisely measure the phase response of the vibration sensors. Experiment results show the performance of the proposed correction method.
High-energy master oscillator power amplifier with near-diffraction-limited output based on ytterbium-doped PCF fiber
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With the development of fiber technologies, fiber lasers are able to deliver very high power beams and high energy pulses which can be used not only in scientific researches but industrial fields (laser marking, welding,…). The key of high power fiber laser is fiber amplifier. In this paper, we present a two-level master-oscillator power amplifier system at 1053 nm based on Yb-doped photonic crystal fibers. The system is used in the front-end of high power laser facility for the amplification of nano-second pulses to meet the high-level requirements. Thanks to the high gain of the system which is over 50 dB, the pulse of more than 0.89 mJ energy with the nearly diffraction-limited beam quality has been obtained.
Efficient self-seeding in a long cavity-length discharge-excited excimer laser system
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A self-seeded discharge-excited ArF excimer laser oscillator using a dual-cavity configuration with long cavity lengths was developed and characterized. Proper designs for effective mode-locking in long cavity-lengths are proposed and demonstrated experimentally, which will significantly improve the energy and monochromaticity. By using the techniques of efficient mode-locking, self-seeded laser outputs with nearly the same linewidths with the seeders and larger energies are obtained. And the mode-locking can be achieved at different laser linewidths and wavelength centers.
Experimental investigation of thermal characteristics of erbium doped distributed feedback fiber laser output power
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The temperature dependence of erbium doped distributed feedback fiber laser (DFB-FL) output characteristics is experimentally investigated. The output power decreased linearly as temperature increased from 30° to 120° for 980nm and 1480nm pumped DFB-FL while 1480nm pumped DFB-FL showed higher thermal sensitivity. Comparison of the output variation under heating between four DFB-FLs of different phase shift location revealed that asymmetric grating design is more suitable to resist high temperature induced reflectivity change of the laser resonant cavity.
The method for scanning reshaping the spectrum of chirped laser pulse based on the quadratic electro-optic effects
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T A new method for scanning reshaping the spectrum of chirped laser pulse based on quadratic electro-optic effects is proposed. The scanning reshaping scheme with a two-beam interference system is designed and the spectrum reshaping properties are analyzed theoretically. For the Gaussian chirped laser pulse with central wavelength λ0=800nm, nearly flat-topped spectral profiles with wider bandwidth is obtained with the proposed scanning reshaping method, which is beneficial to compensate for the gain narrowing effect in CPA and OPCPA. Further numerical simulations show that the reshaped spectrum is sensitive to the time-delay and deviation of the voltage applied to the crystal. In order to avoid narrowing or distorting the reshaped spectrum pointing to target, it is necessary to reduce the unfavorable deviations.
With the rapid and wide applications of ultra-short laser pulse supported by some latter research results including photo-associative formation of ultra-cold molecules from ultra-cold atoms[1-3], laser-induced communications[4], capsule implosions on the National Ignition Facility(NIF)[5-6], the control of the temporal and spectral profiles of laser pulse is very important and urgently need to be addressed. Generally, the control of the pulse profiles depends on practical applications, ranging from femtosecond and picosecond to nanosecond. For instance, the basic shaping setup is a Fourier transform system for ultra-short laser pulse. The most important element is a spatially patterned mask which modulates the phase or amplitude, or sometimes the polarization after the pulse is decomposed into its constituent spectral components by usually a grating and a lens[7].
One of the generation techniques of ultra-short laser pulse is the chirped pulse amplifications(CPA), which brings a new era of development for high energy and high peak intensity ultra-short laser pulse, proposed by D. Strcik and G. Mourou from the chirping radar technology in microwave region since 1985[8]. The other generation technique of ultra-short pulse is the optical parametric chirped pulse amplification(OPCPA) invented by Dubietis et al. in 1992, which combined the respective superiorities of CPA and optical parametric amplification(OPA). However, there are disadvantages for the both technologies such as gain narrowing, gain saturation effects, and even spectrum shift. The first one among the three is the most significant which narrows the spectrum after amplification so that it limits the minimum durations of ultra-short laser pulse.
This paper proposed a approach for scanning reshaping the spectrum of chirped laser pulse to compensate for the gain narrowing effect, according to the characteristics of the chirped laser pulse, i.e. the frequency varies with time linearly. The spectral characteristics of the scanning reshaping was analyzed quantitatively. Furthermore, the influence of the time-delay and deviation of the controlling voltage employed on the electro-optic crystal on the reshaped spectrum was also been discussed in detail.
Research on an algorithm for visibility based on lidar
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Lidar is used to measure atmospheric aerosol, and horizontal visibility is retrieved based on lidar measurement data. In the process of visibility retrieval, it’s manually ascertaining linear region that existed in the curve of data, which is difficult for computer. An algorithm is presented to ascertain the linear region and gives out the horizontal visibility automatically. A group of lidar measurement data is processed by the algorithm, and the results show its practicability.
Test and analysis of background sunlight's influence on laser receiver
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Background sunlight has more or less influence on optical receiver operating in the wild. Laser receiver usually has poor performance in sunlight than designed. The paper’s intention is to test and evaluate the influence of background sunlight on laser receiver by rule and line. The measurement method was studied and measuring system was put up. The outdoor experiments were carried out at plateau. The sensitivity of laser receiver was tested separately at noon and night. The test proves that the sensitivity of laser receiver at noon is down 37.77 per cent, compared with the sensitivity at night. The analytical model of maximum operating distance was founded. With the model, the maximum operating distances at noon and night were calculated. The calculation proves that the maximum operating distances at noon falls more than 12 per cent compared with the distance at night. The test and calculation show that background sunlight has a great influence on performance of laser receiver. It’s very necessary to consider the impact of sunlight when testing, evaluating, and using laser receiver.
Uncertainty analysis of distortion measurement of laser differential confocal by different evaluation methods
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The focal length is one of the important parameters in the optical element, and the high precision measurement of the focal length has become a key problem in the processing and use of the optical element. The laser differential confocal length measurement system is introduced, and the uncertainty of the two sets of focal length measurement is evaluated. The relative error (K = 2) is better than 4.77×10-5, and the relative error is 0.00025%.
Dual-wavelength external cavity laser device for fluorescence suppression in Raman spectroscopy
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Raman spectroscopy has been widely used in the detection of drugs, pesticides, explosives, food additives and environmental pollutants, for its characteristics of fast measurement, easy sample preparation, and molecular structure analyzing capability. However, fluorescence disturbance brings a big trouble to these applications, with strong fluorescence background covering up the weak Raman signals. Recently shifted excitation Raman difference spectroscopy (SERDS) not only can completely remove the fluorescence background, but also can be easily integrated into portable Raman spectrometers. Usually, SERDS uses two lasers with small wavelength gap to excite the sample, then acquires two spectra, and subtracts one to the other to get the difference spectrum, where the fluorescence background will be rejected. So, one key aspects of successfully applying SERDS method is to obtain a dual-wavelength laser source. In this paper, a dual-wavelength laser device design based on the principles of external cavity diode laser (ECDL) is proposed, which is low-cost and compact. In addition, it has good mechanical stability because of no moving parts. These features make it an ideal laser source for SERDS technique. The experiment results showed that the device can emit narrow-spectral-width lasers of two wavelengths, with the gap smaller than 2 nanometers. The laser power corresponding to each wavelength can be up to 100mW.
Detection on vehicle vibration induced by the engine shaking based on the laser triangulation
Wenxue Chen,
Biwu Yang,
Zhibin Ni,
et al.
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The magnitude of engine shaking is chosen to evaluate the vehicle performance. The engine shaking is evaluated by the vehicle vibration. Based on the laser triangulation, the vehicle vibration is measured by detecting the distance variation between the bodywork and road surface. The results represent the magnitude of engine shaking. The principle and configuration of the laser triangulation is also introduced in this paper.
Ultrafast broadband reverse saturable absorption based on two-photon induced singlet state
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The broadband reverse absorption in a multi-branched conjugated compound TPPh is investigated. Transient absorptive spectra of TPPh solution is recorded and a broadband excited-state absorption (475~780 nm) is discovered. The lifetime of this broadband excited-state absorption was measured to be about 20 ns. Transient fluorescence experiment was conducted to confirm that the long-lived broadband excited-state absorption is established on the first singlet state. Optical limiting with extremely high linear transmittance is achieved under the excitation of 532 nm, 21 ps pulses. Theoretically analysis showed that both two-photon absorption and excited-state absorption played a part in it, optical limiting capability is thus enhanced via cooperating effects of two-photon absorption and excited-state absorption. All of the findings suggested that TPPh is a good broadband nonlinear absorptive material and could be further optimized for optical limiting applications.
Research on microstructure properties of the TiC/Ni-Fe-Al coating prepared by laser cladding technology
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In this paper, the laser cladding method was used to preparation the TiC reinforced Ni-Fe-Al coating on the Ni base superalloy. The Ti/Ni-Fe-Al powder was preset on the Ni base superalloy and the powder layer thickness is 0.5mm. A fiber laser was used the melting Ti/Ni-Fe-Al powder in an inert gas environment. The shape of the cladding layer was tested using laser scanning confocal microscope (LSCM) under different cladding parameters such as the laser power, the melting velocity and the defocused amount. The microstructure, the micro-hardness was tested by LSCM, SEM, Vickers hardness tester. The test result showed that the TiC particles was distributed uniformly in the cladding layer and hardness of the cladding layer was improved from 180HV to 320HV compared with the Ni-Fe-Al cladding layer without TiC powder reinforced, and a metallurgical bonding was produced between the cladding layer and the base metal. The TiC powder could make the Ni-Fe-Al cladding layer grain refining, and the more TiC powder added in the Ni-Fe-Al powder, the smaller grain size was in the cladding layer.
Generation of picosecond optical pulse based on chirp compensation
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Picosecond optical pulses are widely used in optical communication systems, such as the optical time division multiplexing (OTDM) and photonic analog-to-digital converter (ADC). We have proposed and demonstrated a simple method to generate picosecond optical pulse using the mach-zehnder modulator (MZM), phase modulator (PM) and single model fiber (SMF). The phase modulator is used to generate a frequency chirp which varies periodically with time. The MZM is used to suppress the pedestal of the pulse and improve the performance of the pulse. The SMF is used to compensate the frequency chirp. We have carried out theoretical analysis and numerical simulation for the generation process of the picosecond optical pulse. The influence of phase shift between the modulation signals loaded on the MZM and PM is analyzed by numerical simulation and the conditions for the generation of picosecond optical pulse are given. The formula for calculating the optimum length of SMF which is used to compensate the linear chirp is given. The optical pulses with a repetition frequency of 10 GHz and a pulse width of 8.5 ps were obtained. The time-bandwidth product was as small as 1.09 and the timing jitter is as low as 83 fs.
Experiment of optical axis angle of electro-optic crystal by conoscopic interference and x-ray diffraction method
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Owing to the advantages of low loss, high spatial uniformity and high damage threshold, plasma electrode pockels cell (PEPC) is the key element of multi-pass amplifying technology in large laser facilities. Properties of PEPC is directly affected by the optical axis angle of the electro-optic crystal. Therefore, high precision measurement of the optical axis angle is indispensable. X-ray diffraction analysis method is a traditional way to determine the direction of optical axis of crystal, which is presented. By using conoscopic interference technique, a measurement system for optical axis angle of electro-optic crystal is introduced. The principle of conoscopic interference method is described in detail, and a series of techniques are implied in this measurement system to improve the accuracy. The optical axis angle two different electro-optic crystal is measured by X-ray diffraction analysis method and our conoscopic interference measurement system, respectively. The absolute error is less than 0.01mrad, while the relative error is nearly 2%.
A fiber-laser-pumped four-wavelength continuous-wave mid-infrared optical parametric oscillator
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In this paper, a four-wavelength continuous-wave mid-infrared optical parametric oscillator was demonstrated for the first time. The pump source was a home-built linearly polarized Yb-doped fiber laser and the maximum output power was 72.5 W. The pump source had three central wavelengths locating at 1060 nm, 1065 nm and 1080 nm. Four idler emissions with different wavelengths were generated which were 3132 nm, 3171 nm, 3310 nm and 3349 nm under the maximum pump power. The maximum idler output reached 8.7 W, indicating a 15% pump-to-idler slope efficiency. The signal wave generated in the experiment had two wavelengths which were 1595 nm and 1603 nm under the maximum pump power. It was analyzed that four nonlinear progresses occurred in the experiment, two of them being optical parametric oscillation and the rest two being intracavity difference frequency generation.
Observation of the dispersion of wedge waves propagating along cylinder wedge with different truncations by laser ultrasound technique
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The research focuses on study the influence of truncations on the dispersion of wedge waves propagating along cylinder wedge with different truncations by using the laser ultrasound technique. The wedge waveguide models with different truncations were built by using finite element method (FEM). The dispersion curves were obtained by using 2D Fourier transformation method. Multiple mode wedge waves were observed, which was well agreed with the results estimated from Lagasse’s empirical formula. We established cylinder wedge with radius of 3mm, 20° and 60°angle, with 0μm, 5μm, 10μm, 20μm, 30μm, 40μm, and 50μm truncations, respectively. It was found that non-ideal wedge tip caused abnormal dispersion of the mode of cylinder wedge, the modes of 20° cylinder wedge presents the characteristics of guide waves which propagating along hollow cylinder as the truncation increasing. Meanwhile, the modes of 60° cylinder wedge with truncations appears the characteristics of guide waves propagating along hollow cylinder, and its mode are observed clearly. The study can be used to evaluate and detect wedge structure.
Time-domain properties of femtosecond laser diffracted by transmitting blazed gratings
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Ray tracing method is used to calculate additional phase and group delay dispersion (GDD) of femtosecond laser diffracted by transmitting blazed gratings. The dependence of GDD with grating parameters is discussed. For the pulse durations of 100fs and 30fs, the pulse broadening and the propagation process through the transmitting blazed grating are analyzed with the Fourier optics. Stimulation results show that the pulse broadening is related to the grating period and the propagation distance diffracted from the transmitting blazed grating.
Investigations on the carbon contaminations on the alkali cells of DPAL with hydrocarbon buffer gas
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Diode pumped alkali laser (DPAL) with hydrocarbon buffer gases has the features of low threshold and high efficiency. The chemical reaction between alkali and hydrocarbon gases affects the life time of DPAL. In this paper, a method based on Fourier transform infrared spectroscopy and Lambert-Beer law is adopted to find a safe temperature at which DPAL runs for a long term. A theoretical model is established to figure out ways to reduce the peak temperature in the cell window. The results indicates that 170 °C is a safe temperature. Although the absorbance of the cell window to the pump light and alkali laser is lower, there is temperature increase. Small light-transmitting area and air blowing on the windows can reduce the peak temperature effectively. Cooling the cell window is essential and critical in a long-term running DPAL.
Efficient yellow-light generation based on a Q-switched frequency-doubled self-Raman laser
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Efficient 588-nm yellow laser with high pulse energy and peak power is generated based on an intracavity frequency doubled Q-switched Nd:YVO4 self-Raman laser. The cavity is elaborately designed to ensure high conversion efficiencies in both Raman and second harmonic generation processes, and meanwhile mitigate the thermal load deposited in gain medium so as to help improve power scalability. At the pulse repetition frequency of 60 kHz, 6.33 W of 588-nm yellow output is obtained under 30.3-W incident pump. The pulse energy of 167 μJ and peak power of 33.4 kW are also obtained at 30 kHz.
Design the laser wrecker system based on unmanned aerial vehicle
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A laser obstacle avoidance system based on UAV platform is designed to achieve long-distance removal of foreign matter in overhead transmission lines. The system has a total mass of 3.25kg, of which 80W eye safe fiber laser is the critical load of the whole system. The laser output from the fiber pigtail irradiated to 5.5 meters where foreign matter gravity support, winding or adhesion sites through the optical system, the parts temperature of the foreign matter absorbed laser led to a sharp rise to 515 °C, which result in a drop to achieve the purpose of remote removal of foreign objects.
Error analysis and experimental verification of a fiber based displacement interferometer
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The error structure of fiber based displacement interferometers was studied, and an error compensation system developed. Experimental verification was achieved, contrasting gravitational acceleration between theoretical and measured results. The effective measurement velocity range was large with excellent precision.
Multiwavelength mode-locked cylindrical vector beam fiber laser based on mode selective coupler
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We propose and demonstrate a multiwavelength mode-locked fiber laser with cylindrical vector beam generation for the first time, to the best of our knowledge. The mode-locking mechanism is nonlinear polarization rotation, and the multiwavelength operation is contributed to the in-line birefringence fiber filter with periodic multiple passbands formed by incorporating a section of polarization maintaining fiber into the laser cavity with a polarizer. Furthermore, using the mode selective coupler, which acts as mode converter from fundamental mode to higher-order mode, multiwavelength mode-locked cylindrical vector beams have been obtained, which may have potential applications in mode-division multiplexing optical fiber communication and material processing.
Measurements of loss and gain of optically pumped InGaAs semiconductor lasers based on the photoluminescence spectra from dual facets
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In this paper, the loss and gain characteristics of optically-pumped InGaAs/GaAs quantum well lasers are measured based on the photoluminescence spectra from dual facets of a single laser device. The device is pumped by 808nm fiber coupled semiconductor lasers controlled with pulsing signal and beam shaping system to reduce the thermal effect. The result of loss spectra is consistent with gain spectra well. In addition, the special double-peak configuration in the loss and gain spectra is observed and analyzed, in term of the strain mechanism and band structure of InGaAs quantum well. The results will be very helpful to the study and design of the InGaAs semiconductor lasers
Control algorithms and applications of the wavefront sensorless adaptive optics
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Compared with the conventional adaptive optics (AO) system, the wavefront sensorless (WFSless) AO system need not to measure the wavefront and reconstruct it. It is simpler than the conventional AO in system architecture and can be applied to the complex conditions. Based on the analysis of principle and system model of the WFSless AO system, wavefront correction methods of the WFSless AO system were divided into two categories: model-free-based and model-based control algorithms. The WFSless AO system based on model-free-based control algorithms commonly considers the performance metric as a function of the control parameters and then uses certain control algorithm to improve the performance metric. The model-based control algorithms include modal control algorithms, nonlinear control algorithms and control algorithms based on geometrical optics. Based on the brief description of above typical control algorithms, hybrid methods combining the model-free-based control algorithm with the model-based control algorithm were generalized. Additionally, characteristics of various control algorithms were compared and analyzed. We also discussed the extensive applications of WFSless AO system in free space optical communication (FSO), retinal imaging in the human eye, confocal microscope, coherent beam combination (CBC) techniques and extended objects.
Broadband spatial optical filtering with a volume Bragg grating and a blazed grating pair
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A broadband spatial optical filtering system is presented in this paper, which is composed of a Volume Bragg Grating (VBG) and a blazed grating pair. The diffraction efficiency and filtering properties are calculated and simulated by using Fourier diffraction analysis and Coupled Wave Theory. A blazed grating pair and VBG structures are designed and optimized in our simulation. The diffraction efficiency of filtering system shows more than 77.2% during the wavelength period from 953nm to 1153nm, especially 84.1% at the center wavelength. The beam quality is described with near-field modulation (M) and contrast ratio (C). The M of filtering beam are 1.44, 1.49 and 1.55, respectively and the C of filtering beam are 10.1%, 10.2% and 10.5% , respectively and the beam intensity distribution is great improved. The cut-off frequencies of three filtering systems are 1.57mm-1 , 2.06 mm-1 and 2.38 mm-1 , respectively from power spectral density (PSD) curve. It’s clear that the cut-off frequency of filtering system is closely related to the angular selectivity of VBG, and the value of cut-off frequency is decided by VBG’s Half Width at First Zero (HWFZ) and center wavelength.
Effects of gap width on droplet transfer behavior in ultra-narrow gap laser welding of high strength aluminum alloys
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Ultra-narrow gap laser welding is a novel method for thick high strength aluminum alloy plate for its lower heat input, less deformation and higher efficiency. To obtain a perfect welding quality, it is vital to control the more complex droplet transfer behavior under the influence of ultra-narrow gap groove. This paper reports the effects of gap width of groove on droplet transfer behavior in ultra-narrow gap laser welding of 7A52 aluminum alloy plates by a high speed camera, using an ER 5356 filler wire. The results showed that the gap width had directly effects on droplet transfer mode and droplet shape. The droplet transfer modes were, in order, both-sidewall transfer, single-sidewall transfer, globular droplet transfer and bridging transfer, with different droplet shape and transition period, as the gap width increased from 2 mm to 3.5mm. The effect of gap width on lack of fusion was also studied to analyze the cause for lack of fusion at the bottom and on the sidewall of groove. Finally, with a 2.5 mm U-type parallel groove, a single-pass joint with no lack of fusion and other macro welding defects was successfully obtained in a single-sidewall transfer mode.
Spectrum properties and Judd-Ofelt analysis of Er3+ doped P2O5 -based glasses
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40P2O5-20Al2O3-(30-x)Na2O-10BaO-xEr2O3 (PANB) glasses with different Er3+ concentrations were fabricated by the traditional melt quenching method and their spectroscopic properties were investigated. Glass thermal stability is investigated by differential scanning calorimetry (DSC) (Tx-Tg=138°C) which indicates a better recrystallization performance than that of fluorophosphates glass and fluorotellurite glass. The derived Judd-Ofelt intensity parameters of Er3+ doped PANB glass (Ω2=18.8, Ω4=15.9, Ω6=5.38(×10-20cm2 )) indicate higher asymmetry and stronger covalent environment when compared with Er3+ doped tellurite, fluoride and fluorotellurite glasses. Compared with Er3+ transitions in other glass hosts, large stimulated emission cross-section and broad full wave at half maximum (FWHM) were found to be 4.58×10-20cm2 and 60 nm centered at ∼1550 nm, respectively. It implies a potential material for infrared lasers and amplifiers.
Design and fabrication of lithium niobate based single mode Y-branch APE waveguide
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In this paper, we report on a minized length, compact, low loss Y-branch waveguide based on lithium niobate optical crystal. The Y-branch waveguide device was fabricated using annealed proton exchange technique to achieve single mode guiding at 1550 nm wavelength. The simplified Y-branch structure has a total length of 18.7 mm and calculated bending loss of 0.5 dB has been realized with a splitting ratio found to be 1.25 at 1550 nm wavelength. Such easy to fabricate waveguide could find applications in future photonic communication networks, sensing, and quantum information technology.
Development of a fluorescence lidar for biological aerosol detection in the air
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In order to investigate biological aerosols in the air, a fluorescence lidar has being developed at Laser Radar Center of Remote Sensing of Atmosphere, Xi’an University of Technology. The fluorescence lidar is constructed with a pulsed Nd:YAG laser, employing at based harmonic (1064 nm), second harmonic (532 nm) and fourth harmonic (266 nm) simultaneously, with a repetition rate of 10 Hz. A 250 mm diameter custom telescope is used to collect optical spectra ranging from 260-1100 nm. In the Infrared detection, an avalanche diode (APD) is used, and two photomultiplier tubes (PMTs) for two linear orthogonal polarization detection at a wavelength of 532 nm. Range-resolved fluorescence signals are collected in 32 channels of compound PMT sensor coupled with Czerny–Turner spectrograph. Based on the current configurations, we performed a series of numerical simulations to estimate the maximal detectable ranges and the minimal detectable concentrations of biological aerosols with various conditions. With a relative error of less than 10%, simulated results show that the system is able to monitor biological aerosols within detected distances of 1.3 km and of 2.0 km at daytime and nighttime, respectively. The developing fluorescence lidar is also capable to identify a minimum concentration of bio-aerosols at about 150 particlesL-1 with daytime operation and 100 particlesL-1 with nighttime at a distance of about 0.1 km. We truly believe that the fluorescence lidar could be spread in the field of remote sensing of biological aerosols in the near future.
HAZ characteristics of laser cladding remanufacturing ductile iron component
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Ductile cast iron is a ferrous alloy characterized by spheroidal graphite, and it is difficult to be remanufactured own to the complicated phase evolution . In this work, laser cladding using Ni-Cu based alloy power as the cladding material has been completed. The grain morphology of cladding and related phase evolution in partially melted zone (PMZ) and heat affected zone (HAZ) by single pass and multi layer cladding have been investigated. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive microanalysis (EDS) were used to identity the microstructure and phase composition of cladded layers and interfaces. Microhardness of the samples was evaluated after laser cladding. The result revealed that entirely different phases were formed by single pass and multi layer cladding progress. Nickel, carbon, and copper elements migrated apparently across the interface. It was also found the microhardness of the substrate was lower than that of the cladded layers, PMZ and HAZ. In samples processed with single pass and multi layer cladding, microhardness in PMZ reached up to 900 and 800 HV respectively.
Fabricating micro-nano structures on stainless steel surface by picosecond laser
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This paper is mainly about the micro-nano structure on stainless steel surface was fabricated by picosecond laser under the static experimental condition, and the surface morphology after ablation is observed and measured by laser scanning confocal microscope (LSCM) and scanning electron microscope(SEM). The technological disciplinarian of stainless steel ablation with different laser parameters, such as power percentage and processing times, was studied. The results show that the change of average power has a main influence on the energy density of laser pulse. With the increase of average power, the energy density of single pulse raises linearly, and the ablation rate increases exponentially. At the same time, the width and depth of micro-nano structures rises with the increase of average power, and the multiscale structure improves gradually. And change the number of processing mainly affects the laser pulse number, the pulse numbers increase linearly with the improve of the processing times. When processing times increase gradually, the width of the micro-nano structures remains the same and then increases and eventually keeps it steady; the depth raises firstly, then decreases, and continues growing finally, while the ablation rate decreases with the power function.
Research on improving the precision of 1064nm fiber Doppler lidar in detecting rotating targets
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Fiber Doppler lidar systems the advantage of high velocity precision over conventional differential velocity measurement. However, the velocity precision of fiber Doppler lidar systems may be degraded by the complex motion of the target, the roughness of the target and the discrete digital data processing. In this paper, a setup of 1064 nm fiber Doppler lidar system is proposed to measure the velocity of radial moving targets. Detailed theoretical analysis was conducted with the influence factors on the velocity precision of the system, and simulations were conducted with an optimized design. A prototype was constructed in the laboratory and experiments were carried out with the prototype. The experimental velocity was 2.65 cm/s, and the relative velocity precision was superior to 0.3%, which proved the validity of the research.
Simulation of NLOS (non-line-of-sight) 3D imaging system
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This passage studies on theory and scene simulation of NLOS imaging. Based on math and physical properties, a simulation platform is built for a NLOS imaging system, including a femtosecond laser, a scanning galvo system, a lambertian surface, several hidden scenes, an ultrafast photodetector to transfer the intensity of laser echo signal to voltage value and a TCSPC module to produce intensity-time histograms. By the simulation platform on MATLAB, precise imaging of the scenes is accomplished. Results show that multi-path analysis using echo signal intensity versus time provides enough proof to reconstruct 3D geometry of a hidden scene.
Random fiber lasers based on artificially controlled backscattering fibers
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The random fiber laser (RFL) which is a milestone in laser physics and nonlinear optics, has attracted considerable attention recently. Most previous RFLs are based on distributed feedback of Rayleigh scattering amplified through stimulated Raman/Brillouin scattering effect in single mode fibers, which required long-distance (tens of kilometers) single mode fibers and high threshold up to watt-level due to the extremely small Rayleigh scattering coefficient of the fiber. We proposed and demonstrated a half-open cavity RFL based on a segment of a artificially controlled backscattering SMF(ACB-SMF) with a length of 210m, 310m or 390m. A fiber Bragg grating with the central wavelength of 1530nm and a segment of ACB-SMF forms the half-open cavity. The proposed RFL achieves the threshold of 25mW, 30mW and 30mW, respectively. Random lasing at the wavelength of 1530nm and the extinction ratio of 50dB is achieved when a segment of 5m EDF is pumped by a 980nm LD in the RFL. Another half-open cavity RFL based on a segment of a artificially controlled backscattering EDF(ACBS-EDF) is also demonstrated without an ACB-SMF. The 3m ACB-EDF is fabricated by using the femtosecond laser with pulse energy of 0.34mJ which introduces about 50 reflectors in the EDF. Random lasing at the wavelength of 1530nm is achieved with the output power of 7.5mW and the efficiency of 1.88%. Two novel RFLs with much short cavities have been achieved with low threshold and high efficiency.
Single higher-order mode ring-shaped VCSEL with surface relief
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A novel vertical-cavity surface-emitting laser (VCSEL) with single mode, high-power, low divergence, and temperature stability is presented. The most prominent structural feature of the device is that the high optical loss region is formed by an anti-phase surface relief above the top Distributed Bragg Reflectors (DBRs) and the light-emitting aperture is ringshaped with larger region. The simulation results show the device with 15μm oxide aperture and 5μm width ring light emitting region achieves stable single-higher-order transverse mode emission with a side mode suppression ratio (SMSR) of more than 80dB. The maximum continuous-wave (CW) single mode power is up to 15.2 mW and far-field divergence angle (FWHM) is lower than 4.5°. Moreover, the VCSEL maintains CW single mode emission up to a record high temperature of 450K.
Analysis of polarization-sensitive resonator based on vectorial eigenvector method
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Polarization selective devices are commonly utilized as rear mirrors to generate high power cylindrical vector (CV) beams in the resonators. The modes of these polarization sensitive resonators are very significant for the generation and application of CV beams. Upon the scalar eigenvector method, a vectorial eigenvector method (VEM) was used to compute the CV modes of polarization-selective resonators. Then, different polarization dependent resonators were simulated with the VEM. We can find that both mode TE01* and mode TEM00 are the eigen modes of polarization sensitive symmetric confocal sphere resonator and when Rte=0.99 and Rtm=0.93, TE01* most possibly appears in the cavity with polarization sensitivity due to its lowest loss. In addition, the VEM was used to guide our design of a 45-degree three-fold cavity structure for high power radially polarized laser. The axicon mirrors with azimuthal polarization selection and four λ/4 phase shifters are used to obtain the output of radially polarized light.
Preliminary study of the influence of polarization orientation on bulk damage resistances of doubler KDP crystals
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The investigation of the influence polarization orientation on damage performance of type I doubler KDP crystals grown by the conventional growth method under under 532nm pulse exposure is carried out in this work. The obtained results point out the pinpoint density (ppd) of polarization parallels the extraordinary axis is around 1.5× less than that of polarization parallels the ordinary axis under the same fluence, although polarization has no influence on size distribution of pinpoints. Meanwhile, crystal inhomogeneity is observed during experiment.
Study on double end-pumped high power slab laser with laser diode arrays
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In diode-pumped solid-state laser system, the performance of pumped coupling system directly determines the output power and the beam quality of laser. we report a pump coupling system for sandwich structured slab gain medium. Image relaying along the slow axis is used to obtain the uniform distribution of pumping light in the slab width direction and the methods of spatial multiplexing and polarization multiplexing are used to increase the density of pump power. When the pump power is 11.2kW, the continuous-wave output power of 4.55kW is obtained through the plane-parallel resonator. Meanwhile, the influence of the methods of spatial/polarized composite splicing on the pump efficiency of the pump coupling system is analyzed, and the experiment results provide the basis for optimization design to high power slab laser.
Optical amplification in Er:Yb co-doped fiber and their applications in fiber laser
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With the development and popularization of optical communication in recent years, optical fiber, which has low transmission loss in C band, has been broadly employed in all kinds of optical systems. It facilitates the applications in long-haul optical information transmission, whereas the signal attenuation as well as optical noise arises greatly in the fiber system. In this paper, we investigated the optical amplification effect of the Er:Yb co-doped fiber in a DBR fiber laser system. To increase the optical signal power, the analysis focusing on the relationship between laser power and system attenuation was also built up.
Cylinder surface test with Chebyshev polynomial fitting method
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Zernike polynomials fitting method is often applied in the test of optical components and systems, used to represent the wavefront and surface error in circular domain. Zernike polynomials are not orthogonal in rectangular region which results in its unsuitable for the test of optical element with rectangular aperture such as cylinder surface. Applying the Chebyshev polynomials which are orthogonal among the rectangular area as an substitution to the fitting method, can solve the problem. Corresponding to a cylinder surface with diameter of 50 mm and F number of 1/7, a measuring system has been designed in Zemax based on Fizeau Interferometry. The expressions of the two-dimensional Chebyshev polynomials has been given and its relationship with the aberration has been presented. Furthermore, Chebyshev polynomials are used as base items to analyze the rectangular aperture test data. The coefficient of different items are obtained from the test data through the method of least squares. Comparing the Chebyshev spectrum in different misalignment, it show that each misalignment is independence and has a certain relationship with the certain Chebyshev terms. The simulation results show that, through the Legendre polynomials fitting method, it will be a great improvement in the efficient of the detection and adjustment of the cylinder surface test.
Fiber up-tapering and down-tapering for low-loss coupling between anti-resonant hollow-core fiber and solid-core fiber
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In this paper, we demonstrate a novel method for the low-loss coupling between solid-core multi-mode fibers (MMFs) and anti-resonant hollow-core fibers (AR-HCFs). The core/cladding diameter of the MMF is 50/125μm and the mode field diameter of the AR-HCFs are 33.3μm and 71.2μm of the ice-cream type AR-HCFs and the non-node type ARHCFs, respectively. In order to match the mode field diameters of these two specific AR-HCFs, the mode field diameter of the MMFs is increased or decreased by up-tapering or down-tapering the MMFs. Then, according to the principle of coupled fiber mode matching, the optimal diameter of tapered fiber for low-loss coupling is calculated. Based on beam propagation method, the calculated coupling losses without tapering process are 0.31dB and 0.89dB, respectively for a MMF-HCF-MMF structure of the ice-cream type AR-HCFs and the non-node type AR-HCFs. These values can be reduced to 0.096dB and 0.047dB when the outer diameters of the MMF are down-tapered to 116μm and up-tapered to 269μm, respectively. What’s more, these results can also be verified by existing experiments.
A study on the transmission characteristics of laser in the rain by utilizing Modtran software
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Laser transmission is influenced by environment particularly large, especially in fog, rain, snow and other complex weather conditions.This article mainly from two aspects of theory research and Modtran software simulation discussed the laser transmission characteristics in the rain.Different laser is theoretically discussed mainly in a variety of transmission attenuation degree of rainfall conditions.Using Modtran simulation software to get the theoretical calculation results of simulation and error analysis.
Research on Rb-DPAL for the pumping of SERF rubidium magnetometers
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A continuous wave diode pump rubidium vapor laser (Rb-DPAL) is developed in this paper. It has long operating time at output power of 1 W. The Rb-DPAL, with optical efficiency of 18.9% and slope efficiency of 24.0%, has the characteristics of high stability and good beam quality. It is expected to be a kind of good pump source of SERF rubidium magnetometer.
Simulation of dissipative-soliton-resonance generation in a passively mode-locked Yb-doped fiber laser
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We present a numerical investigation of dissipative-soliton-resonance (DSR) generation in an all-normal-dispersion Ybdoped fiber laser mode-locked by a real saturable absorber (SA). In the simulation model, the SA includes both the saturable absorption and excited-state absorption (ESA) effects. The intra-cavity pulse evolution is numerically simulated with different transmission functions of SA. When omitting the ESA effect, the transmissivity of SA increases monotonically with the input pulse power. The noise-like pulse (NLP) operation in the cavity is obtained at high pump power, which is attributed to the spectral filtering effect. When the ESA effect is activated, higher instantaneous power part of pulse encounters larger loss induced by SA, causing that the pulse peak power is clamped at a certain fixed value. With increasing pump, the pulse starts to extend in the time domain while the pulse spectrum is considerably narrowed. In this case, the NLP operation state induced by the spectral filtering effect is avoided and the DSR is generated. Our simulation results indicate that the ESA effect in the SA plays a dominant role in generating the DSR pulses, which will be conducive to comprehending the mechanism of DSR generation in passively mode-locked fiber lasers.
Ultrafast pulse erbium-doped fiber laser with a graphene/WS2 heterostructure saturable absorber
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We report on ultrafast-pulse generation in an erbium-doped fiber ring laser mode-locked by a graphene/WS2 van der Waals heterostructure saturable absorber. Atomic-layered WS2 is first synthesized on SiO2/Si substrate by the chemical vapor deposition (CVD) method, and then graphene/WS2 heterostructure is fabricated by transferring graphene onto the CVD-grown layered WS2. Taking advantage of excellent saturable absorption properties of the fabricated graphene/WS2 heterostructure, stable soliton pulses are successfully generated in the laser with a 3-dB spectral width of 2.3 nm and a pulse duration of 1.12 ps. Numerical simulations reproduce the mode-locked pulse emission in the experiment. Our research provides a new insight for tailoring versatile two-dimensional heterostructures so as to develop ultrafast photonic applications.
Simulation of laser scattering by typical marine aerosol
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Maritime environment own numerous models of the aerosols which are typically comprised of dust, water-soluble aerosol and sea salt aerosol. A Maritime environment aerosol model is developed in the simulation for laser beam from visible to near infrared. Mie theory is used to calculate optical parameters, such as scattering coefficient, Aledo and average asymmetry factor, for different models of aerosols of variable size with gamma distribution. The simulation results show that dust aerosol and water-soluble aerosol have the largest absorption coefficient. Aged Sea-salt aerosol and near-surface Sea-salt aerosol have the largest scattering coefficient. The model solving the optical properties of marine aerosol problem is capable of handling light detection and ranging.
Improved power and efficiency for tapered lasers with optimized photonic crystal structures
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High power and high beam quality laser sources are required in numerous applications such as nonlinear frequency conversion, optical pumping of solid-state and fiber lasers, material processing and others. Tapered lasers can provide a high output power while keeping a high beam quality. However, the conventional tapered lasers suffer from a large vertical beam divergence. We have demonstrated 2-mm long tapered lasers with photonic crystal structures. A high beam quality and a narrow vertical divergence are achieved.
In this paper, we optimized the photonic crystal structure and fabricated a 4-mm long tapered laser to further increase the output power and the wall-plug efficiency. Compared with our precious wafer, the optimized structure has a lower doping level to reduce the internal loss. The period of the photonic crystal structure and the thickness of the upper cladding are also reduced. The device has a 1-mm long ridge-waveguide section and a 3-mm long tapered section. The taper angle is 4°. An output power of 7.3 W is achieved with a peak wall-plug efficiency of 46% in continuous-wave mode. The threshold current is around 500 mA and the slope efficiency is 0.93 W/A. In pulsed mode, the output power is 15.6 W and the maximum wall-plug efficiency is 48.1%. The far-field divergence with full width at half maximum is 6.3° for the lateral direction at 3 A. The vertical far-field beam divergence is around 11° at different injection levels. High beam qualities are demonstrated by beam quality factor M2 of 1.52 for the lateral direction and 1.54 for the vertical direction.
Applications of OALCLV in the high power laser systems
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This paper introduces the recent development of our integrated optical addressed spatial light modulator and its applications in the high power laser systems. It can be used to convert the incident beam into uniform beam for high energy effiency, or it can realize special distribution to meet the requirements of physical experiment. The optical addressing method can avoid the problem of the black matrix effect of the electric addressing device. Its transmittance for 1053nm light is about 85% and the aperture of our device has reached 22mm× 22mm. As a transmissive device, it can be inserted into the system without affecting the original optical path. The applications of the device in the three laser systems are introduced in detail in this paper.
In the SGII-Up laser facility, this device demonstrates its ability to shape the output laser beam of the fundamental frequency when the output energy reaches about 2000J. Meanwhile, there’s no change in the time waveform and far field distribution. This means that it can effectively improve the capacity of the maximum output energy.
In the 1J1Hz Nd-glass laser system, this device has been used to improve the uniformity of the output beam. As a result, the PV value reduces from 1.4 to 1.2, which means the beam quality has been improved effectively.
In the 9th beam of SGII laser facility, the device has been used to meet the requirements of sampling the probe light. As the transmittance distribution of the laser beam can be adjusted, the sampling spot can be realized in real time. As a result, it’s easy to make the sampled spot meet the requirements of physics experiment.
Hundred-watt level highly stable passively Q-switched fiber laser based on graphene saturable absorber
Hanshuo Wu,
Jiaxin Song,
Jian Wu,
et al.
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We demonstrate a monolayer graphene based passively Q-switched fiber laser with cascaded amplifiers that can deliver 84.1 W average power at 1064 nm, with pulse energy of 1.67 mJ. To the best of our knowledge, this is the first time for a high power passively Q-switched fiber laser in the 1 μm range reported so far. More importantly, the Q-switched fiber laser operates stably during a week few-hours-per-a-day tests, which proves the stability and practical application value of graphene in high power pulsed fiber lasers.
Estimation of mode instability threshold based on local thermal load
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The behavior of the mode instability (MI) threshold in the double cladding Yb-doped fiber amplifier when the amplifier with different local heat load is studied theoretically and experimentally. A theoretical model is constructed, and the effects of different laser parameters on the local thermal load of the gain fiber are analyzed theoretically, such as pump direction, pump linewidth, thermal conductivity of cooling medium. An experimental structure is described. The effects of different local heat load on the MI threshold when the fiber amplifier have the same total or average heat load is studied. The theoretical and experimental results reveal that the MI threshold can be estimated by the local heat load of the gain fiber.
Research on temperature characteristics of laser energy meter absorber irradiated by ms magnitude long pulse laser
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The research on temperature characteristics for large-energy laser energy meter absorber is about continuous wave (CW) laser before. For the measuring requirements of millisecond magnitude long pulse laser energy, the temperature characteristics for absorber are numerically calculated and analyzed. In calculation, the temperature field distributions are described by heat conduction equations, and the metal cylinder cavity is used for absorber model. The results show that, the temperature of absorber inwall appears periodic oscillation with pulse structure, the oscillation period and amplitude respectively relate to the pulse repetition frequency and single pulse energy. With the wall deep increasing, the oscillation amplitude decreases rapidly. The temperature of absorber outerwall is without periodism, and rises gradually with time. The factors to affect the temperature rise of absorber are single pulse energy, pulse width and repetition frequency. When the laser irradiation stops, the temperature between absorber inwall and outerwall will reach agreement rapidly. After special technology processing to enhance the capacity of resisting laser damage for absorber inwall, the ms magnitude long pulse laser energy can be obtained with the method of measuring the temperature of absorber outerwall. Meanwhile, by optimization design of absorber structure, when the repetition frequency of ms magnitude pulse laser is less than 10Hz, the energy of every pulse for low repetition frequency pulse sequence can be measured. The work offers valuable references for the design of ms magnitude large-energy pulse laser energy meter.
Generation of high-output yellow light by intracavity doubling Nd:YAG-Nd:YVO4 hybrid gain Raman laser
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A promising method for generation of relatively high-peak-power and high-energy yellow light based on doubling actively Q-switched Nd:YAG-Nd:YVO4 hybrid gain intracavity Raman laser at 1176 nm has been proposed and experimentally demonstrated for the first time. Under the incident pump power of 42.0 W, pulse energy of 0.37 mJ and peak power of 75 kW at 1176 nm were generated in our experiment.
Study on modulation amplitude stabilization method for PEM based on FPGA in atomic magnetometer
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Atomic magnetometer which uses atoms as sensitive elements have ultra-high precision and has wide applications in scientific researches. The photoelastic modulation method based on photoelastic modulator (PEM) is used in the atomic magnetometer to detect the small optical rotation angle of a linearly polarized light. However, the modulation amplitude of the PEM will drift due to the environmental factors, which reduces the precision and long-term stability of the atomic magnetometer. Consequently, stabilizing the PEM’s modulation amplitude is essential to precision measurement. In this paper, a modulation amplitude stabilization method for PEM based on Field Programmable Gate Array (FPGA) is proposed. The designed control system contains an optical setup and an electrical part. The optical setup is used to measure the PEM’s modulation amplitude. The FPGA chip, with the PID control algorithm implemented in it, is used as the electrical part’s micro controller. The closed loop control method based on the photoelastic modulation detection system can directly measure the PEM’s modulation amplitude in real time, without increasing the additional optical devices. In addition, the operating speed of the modulation amplitude stabilization control system can be greatly improved because of the FPGA’s parallel computing feature, and the PID control algorithm ensures flexibility to meet different needs of the PEM’s modulation amplitude set values. The Modelsim simulation results show the correctness of the PID control algorithm, and the long-term stability of the PEM’s modulation amplitude reaches 0.35% in a 3-hour continuous measurement.
Comparison of SP-LIBS and DP-LIBS on metal and non-metal testing based on LIBS
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Laser-induced breakdown spectroscopy (LIBS) technology for metal and nonmetallic detection accuracy is the key technology to be solved in LIBS measurement, Due to metal elements and non-metallic elements in the lively, atomic structure and the degree of excitation of the laser are totally different, so the laser induced plasma evolution and spectral intensity are absolutely different. Among the many factors that affect measurement accuracy, the single and double pulse of the laser has a great influence on the measurement accuracy of metal and non-metal, they both have their own advantages, but also have their own shortcomings. In order to compare the effect of SP-LIBS and DP-LIBS on the measurement results of different elements, in this experiment, we put the metal element aluminum and non-metallic element carbon as the sample, the laser energy as a variable, using the high-speed camera shooting SP- LIBS and DP- LIBS plasma images. Using the spectral analyzer to record the spectral intensity of the elements, by calculating the relative RSD of the signal intensity and comparing the spectral intensity and the signal stability for different elements, develop an optimized experimental program. The experimental results show that under the same energy condition, the metal aluminum ion image under the DP- LIBS and the non-metallic carbon ion image under the SP- LIBS are the most suitable images. By considering the stability of the line intensity and the signal stability, we find that the sensitivity and stability of the signal strength of the metal elements under the double pulse are better than that of the single pulse, and for the non-metallic element, the single pulse laser is better than the double pulse.
Measurement of the wavefront distribution characteristics of a 400-1700nm supercontinuum light source
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In order to find out the wavefront distribution characteristics of supercontinuum light source, adding wavefront distortion description into the evaluation of supercontinuum light source beam quality, a comprehensive and accurate measurement about the wavefront distribution characteristics of a 400-1700nm supercontinuum light source developed by National University of Defense Technology (NUDT) is carried out in this paper. According to the experimental results, wavefront distribution characteristics in different wavelength bands of this 400nm-1700nm supercontinuum source are basically the same, mainly composing of defocus and astigmatism, however, values of distortions are different in specific wavelength bands.
Heat-affected zone microstructure and mechanical properties evolution for laser remanufacturing LZ50 axle steel
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In this article, the laser cladding on LZ50 axle steel was conducted to investigate the evolution of microstructure and mechanical property of HAZ. Based on the quality of cladding formation and thermal damage evaluation in the HAZ, the optimal process parameters was obtained as 7.8g/min powder feeding rate,420mm/min scanning rate and other constant settings. The microstructure of HAZ was characterized by means of OM and SEM. Meanwhile hardness distribution in HAZ and tensile property of cladding-HAZ-substrate samples were measured. The results indicate that, Two types of HAZs were observed under different cladding strategies: gradually changed microstructure during thin-layer cladding process and relatively uniform microstructure during multi-layer cladding process..Due to different maximum temperature of thermal cycle, HAZ and substrate have partial surface hardening during the laser cladding process. The mechanism of hardening was discussed. The final microstructure after complex thermal cycles result hardening behavior in both HAZ and substrate.
Efficient phase locking of two dual-wavelength fiber amplifiers by an all-optical self-feedback loop
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Efficient phase locking of two dual-wavelength fiber amplifiers has been demonstrated by using a self-feedback coupling and intracavity filtering configuration, and the effect of bandwidth and wavelength spacing on their phase locking performances have been investigated in experiment. Two independent fiber lasers with different operating wavelength were combined incoherently by a 3 dB fiber coupler to form a dual-wavelength seed source laser, which was injected into the fiber amplifiers’ coupling array through the self-feedback loop. The effect of bandwidth and wavelength spacing was researched by altering the seed laser’s pump power and operating wavelengths respectively. As long as the feedback loop and the single-mode fiber filtering configuration were well constructed in the unidirectional ring laser cavity, stable phase locking states and high fringe visibility interference patterns could always be obtained in our experiment. When the spacing of two operating wavelength was varied from 1.6 nm to 19.6 nm, the fringe visibility decreased slightly with the increase of wavelength spacing, and the corresponding fringe visibility was always larger than 0.6. In conclusion, we believe that efficient phase locking of several multi-wavelength laser sources is also feasible by passive self-adjusting methods, and keeping the component laser beams’ phase relationship stable and fixed is more important than controlling their operating wavelengths.
An expander system of high stable laser beam for cesium atomic fountain clocks
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The quality of the cold atoms sample plays an important role to improve the performance of cesium atomic fountain clocks. The preparation of cold atoms sample has a strict requirement for cooling lasers characteristics such as power stability, polarization, collimation, uniformity and verticality. This work implements a design of an expander system of high stable laser beam for the cold atom preparation. The cooling laser characteristics can be adjusted by the design, and the use of non-magnetic materials to avoid the magnetic field on the impact of cold atom. The cold atom sample with the number of 2×108 and temperature of 5μk was obtained by expander system. The result meets the requirements of the preparation of cold atoms for cesium atomic fountain clocks.
Interaction of ultrashort pulse with two-level medium beyond the slowly varying envelope approximation
Da-qing Wen Jr.,
Long-zhao Lu Jr.,
Xiang-yang Yu
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By solving the Maxwell-Bloch equations beyond the slowly varying envelope approximation (SVEA), we investigate the interaction of ultrashort pulse with two-level medium. When the pulse width decreases to the order of the optical period, the evolutions of the 2π, 4π and 6π ultrashort pulses deviate from the results obtained using the SVEA and present some new features, such as the frequency shift occurring on the trailing edge of the pulse. Moreover, a factor is defined to quantify the envelope deviation between the cases of the SVEA and the non-SVEA in order to quantitatively evaluate the error caused by the SVEA, then we discuss the applicable condition of the approximation.
Theoretical study on the absorption features of a diode side-pumped alkali laser
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In this paper, we introduce a new model to analyze the absorption efficiency of the laser medium for a diode side-pumped alkali laser (DSPAL). In the model, a ray trace method is employed to analyze the pump laser propagating route inside a diffusing chamber. In addition, the method, which is used to determine the total absorbed power of an alkali vapor cell, is named as the infinite convergence approach (ICA) while the random reflection is assumed to take place at the inner surface of a ceramic reflector. By considering the increase of a slit size will give rise to both increase of the input power and decrease of the reflection of the ceramic wall, we deduce that there must be an optimum slit width corresponding to the maximum absorption efficiency.
The ultra-stable microwave based on ultra-stable laser
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An ultra-stable microwave based on ultra-stable laser and fiber optical frequency comb (FOFC) is built at the National Institute of Metrology China (NIM). The repetition rate (fr) of FOFC is stabilized by stabilizing the sum of the carrier envelope of set frequency (fceo) of FOFC and the beating frequency between the ultra-stable laser and FOFC. A current feedback with 30 kHz bandwidth and a piezoelectric transducer (PZT) with 1 kHz bandwidth are applied for the feedback locking. The frequency jitter of the stabilized sum frequency is about 4 Hz. The short time stability of fr is calculated better than 1E-14. This ultra-stable microwave will be applied as local oscillator for NIM5 Cs fountain to improve its short term stability.
Laser beam shaping design based on micromirror array
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In the practical application of the laser, it is necessary to use the laser beam shaping technology to shape the output beam of laser device to the uniform light intensity distribution. The shaping divergent optical system of compound eye integrator way is composed of beam expanding mirror group and lens array. Its working principle is to expand the output laser to a certain size of caliber, and then divide the beam with lens array into multiple sub beam, where the lens unit of lens array can control the divergence angle of sub beam through the design of focal length, with mutual superposition of the sub beam in far field, to make up for the nonuniformity of beam, so that the radiant exitance on the radiated surface may become uniform. In this paper, we use a reflective microlens array to realize the laser beam shaping. By through of the practical optical path model established, the ray tracing is carried out and the simulation results for single-mode Gaussian beam with noise circumstance is provided. The analysis results show that the laser beam shaping under different inputs can be effectively realized by use of microlens array. All the energy is within the signal window, with a high energy efficiency of more than 90%; The measured surface has a better uniformity, and the uniformity is better than 99.5% at 150m.
Subsurface damage distribution and processing method of ground fused silica
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Subsurface damage (SSD) has been identified as a main initiator of laser-induced damage in fused silica, and the most of SSD is produced during grinding process. The distribution and morphology of SSD in fused silica samples ground with loose abrasive are investigated by magneto-rheological finishing (MRF) dimpling and buffered oxide etch (BOE) etching method. The results demonstrate that the SSD depth is most responsive to the loose abrasive size and the BOE etching is good for removing the SSD. Based on these results, an efficient grinding technique combined with BOE etching is proposed to reduce the SSD of fused silica, and the damage threshold is obviously improved by this routine as a result.
Diffraction characteristics of hump volume Bragg grating in photo-thermo-refractive glass
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The diffraction characteristics of hump volume Bragg grating in photo-thermo-refractive glass are analyzed with coupled-wave theory. Results show that the diffraction efficiency of hump volume Bragg gratings (HVBGs) is increasing with the product of refractive index modulation and grating thickness. Keeping the refractive index modulation and grating thickness, the interval between the hump peaks enlarges with the grating period increasing. Keeping the grating period and the product of refractive index modulation and grating thickness, the interval between the hump peaks enlarges with the grating thickness decreasing. Moreover, the angular selectivity of HVBGs is getting better with the larger interval.
Output characteristics of the mode-locked thulium-doped fiber laser near 2 um based on MoS2
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The output characteristics of the mode-locked thulium-doped fiber laser with MoS2 as saturable absorber are simulated based on rate equation and nonlinear Schrodinger equation. With small signal gain efficient as variable, the effect of pump power, length of gain fiber, doping concentration and other parameters on output characteristics are analyzed. The theoretical results show an optimum doping concentration or fiber length exists to obtain maximum gain and average output power for given doped fiber. In experiment a thulium-doped fiber laser is setup and the experimental output characteristics of the laser are compared with theoretical counterpart, as a result, theoretical results are well in consistence with experimental results.
Studies on low-loss coupling of non-node anti-resonant hollow-core fiber and tapered fiber
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Up to now, near almost optical fiber gas lasers employ/adopt the scheme of free-space coupling, which increases the difficulty to adjust the optical path, and has poor stability. All-fiber structure fiber-gas lasers are important development directions in the future. We established the numerical model of SMF-28 type tapered single-mode fiber and non-node hollow-core fiber. When the SMF-28 type single-mode fiber has a waist diameter of 40μm when the light source is LP01 fundamental mode with 1550nm wavelength, the mode field diameter is the largest. Meanwhile, we simulated that the equivalent mode field diameter of non-node anti-resonant hollow-core fiber is about 75μm at the same 1550nm wavelength light source. Then, we use different waist diameters of SMF-28 type tapered fibers injected to the non-node anti-resonant hollow-core fiber in simulation and experiments. In the scheme of the single-ended low-loss coupling, the simulation results indicate that the best waist diameter of tapered fiber is 40μm, and the calculated maximum coupling efficiency is 83.55%. Meanwhile, the experimental result of maximum coupling efficiency is 80.74% when the best waist diameter of tapered fiber is also 40μm. As for the double-ended low-loss coupling, the calculated maximum coupling efficiency is near 83.38%.
Intensity and frequency stabilization of a laser diode by simultaneously controlling its temperature and current
Show abstract
Nuclear magnetic resonance gyroscope (NMRG) detects the angular velocity of the vehicle utilizing the interaction between the laser beam and the alkali metal atoms along with the noble gas atoms in the alkali vapor cell. In order to reach high precision inertial measurement target, semiconductor laser in NMRG should have good intensity and frequency stability. Generally, laser intensity and frequency are stabilized separately. In this paper, a new method to stabilize laser intensity and frequency simultaneously with double-loop feedback control is presented. Laser intensity is stabilized to the setpoint value by feedback control of laser diode’s temperature. Laser frequency is stabilized to the Doppler absorption peak by feedback control of laser diode’s current. The feedback control of current is a quick loop, hence the laser frequency stabilize quickly. The feedback control of temperature is a slow loop, hence the laser intensity stabilize slowly. With the feedback control of current and temperature, the laser intensity and frequency are stabilized finally. Additionally, the dependence of laser intensity and frequency on laser diode’s current and temperature are analyzed, which contributes to choose suitable operating range for the laser diode. The advantage of our method is that the alkali vapor cell used for stabilizing laser frequency is the same one as the cell used for NMRG to operate, which helps to miniaturize the size of NMRG prototype. In an 8-hour continuous measurement, the long-term stability of laser intensity and frequency increased by two orders of magnitude and one order of magnitude respectively.
Dependence of output features of a micro-cavity laser on the cavity structure
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In this study, we analyze the characteristics of a micro-cavity laser with the size one-order larger than the lasing wavelength by employing the finite-difference time-domain (FDTD) methodology. The simulation results have been obtained under the conditions with different materials and structures of the oscillator. It is seen that the power leakage from the side wall depends on the material and structure of a micro-cavity laser system. The wall material of the micro-cavity is assumed to be BK7 glass, silver, and copper, respectively. The results indicate that the side power leakage with the wall material of BK7 glass is much more serious than those with the wall materials of silver and copper. In addition, it is demonstrated that the cavity structure is also a key factor that influences the output features of such a laser.
Influence of axial temperature distribution to optical parametric gain in CW-OPO
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We use the fiber-pumped MgO:PPLN crystal to realize the MIR CW-OPO operation, and observe the nonuniform temperature distribution on the central axis of the crystal. Then we use the heat transfer model in COMSOL software to simulate the temperature distribution in the crystal and find the near linear temperature gradient on the central axis of the crystal. Input the axial temperature distribution to our SRO model based on MATLAB and the simulation results show that the linear axial temperature gradient distribution will not only cause the center wavelength shift of the signal light, but also reduce the parametric gain of the signal light, the uniform temperature along the crystal axis will get the maximum gain. This feature limits the prospect of the single OPO in high-power narrow linewidth laser.
Effect of ridge structure on electro-optical characteristics of ridge-waveguide lasers with low vertical divergence based on photonic crystal structure
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Ridge-waveguide (RW) lasers based on photonic crystal structure were fabricated and measured. We investigated the effect of residual layer thickness (corresponding to etching depth) and ridge width on electro-optical characteristics of RW lasers. For deep-etching RW lasers, although lateral beam quality factor M2 is better than that of shallow-etching RW lasers, the other characteristics such as output power are much less than that of shallow-etching RW lasers. The calculating results indicate that RW lasers with ridge width w ≥ 8 μm will operate in mixing mode. The experimentally results of various ridge width RW lasers show that RW laser with 7 μm ridge operated in single mode over the whole measurement range and RW laser with 8 μm ridge change from single-mode operation to mixing-mode operation with the increasing of driving current. The device with 7-μm-wide ridge and 3-mm-long cavity obtain 2 W single-transverse-mode optical power and 59% maximum power conversion efficiency. The lateral beam quality factors M2 values are less than 1.7 over the whole measuring range.
Study on on-machine defects measuring system on high power laser optical elements
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The influence of surface defects on high power laser optical elements will cause some harm to the performances of imaging system, including the energy consumption and the damage of film layer. To further increase surface defects on high power laser optical element, on-machine defects measuring system was investigated. Firstly, the selection and design are completed by the working condition analysis of the on-machine defects detection system. By designing on processing algorithms to realize the classification recognition and evaluation of surface defects. The calibration experiment of the scratch was done by using the self-made standard alignment plate. Finally, the detection and evaluation of surface defects of large diameter semi-cylindrical silicon mirror are realized. The calibration results show that the size deviation is less than 4% that meet the precision requirement of the detection of the defects. Through the detection of images the on-machine defects detection system can realize the accurate identification of surface defects.
An accurate method for investigation of laser-induced damage of optical component at 351nm
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A multipurpose laser damage test facility delivering pulses from 1ns to 20ns and designed to output energy 40 Joule at 351nm is presented. The laser induced damage threshold (LIDT) measurement and test procedure are performed. The original system consist of the online detection system based on the microscopy and an energy detection device based on the scientific grade Charge Coupled Device (CCD) which provides the method to measure the LIDT with high accuracy. This method is an efficient way that allows measuring a small area fluence which the defect exposed. After complete test procedure and data treatment the damage position of the defect has been found. Then we can obtain the local fluence of small area when the damage occurred. This procedure provides a straightforward means of laser-damage threshold obtained from the test method. Damage correlation of measures is discussed in connection with present theoretical understanding of laser damage phenomenon. The damage process in transparent dielectric materials being the results of complex processes involving multi-photon ionization, avalanche ionization, electron-phonon coupling, and thermal effects. Those complex processes lead to the damage on the optical surface. We performed a method to measure the local fluence which defects irradiated with high accurate.
2μm all fiber multi-wavelength Tm/Ho co-doped fiber laser
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A 2 μm all fiber multi-wavelength Tm/Ho co-doped fiber laser based on a simple ring cavity is experimentally demonstrated. Compared with other 2 μm multi-wavelength Tm/Ho co-doped fiber lasers, the multi-wavelength fiber laser is obtained by the gain saturation effect and inhomogeneous broadening effect without any frequency selector component, filter component or polarization-dependent component. When the pump power is about 304 mW, the fiber laser enters into single-wavelength working state around 1967.76 nm. Further increasing the pump power to 455 mW, a stable dual-wavelength laser is obtained at room temperature. The bimodal power difference between λ1 and λ2 is 5.528 dB. The fluctuations of wavelength and power are less than 0.03 nm and 0.264 dB in an hour, which demonstrates that the multi-wavelength fiber laser works at a stable state. Furthermore, a research about the relationship between the pump power and the output spectra has been made.
Multi-wavelength Tm/Ho co-doped mode-locked fiber laser with a simple cavity
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A multi-wavelength Tm/Ho co-doped mode-locked fiber laser achieved without any frequency selector, filter component, or polarization-dependent component is proposed in this paper. The fiber laser outputs 56 ps pulses at 7.09 MHz. As increasing the pump power from 26.12 to 29.50 dBm, the tuning range of the three wavelengths are 1918.641-1947.527 nm, 2179.852-2205.147 nm, and 2200.163-2226.420 nm, respectively. The fluctuations of wavelength and power are less than 0.400 nm and 0.512 dB within 45 minutes, which demonstrates a stable operation. Such a 3-wavlength fiber laser has the advantages of long wavelength, simple configuration, high stability, and low cost.
The coherent combination of multi-wavelength fiber laser
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The feasibility and influencing factors of the coherent combination of multi-wavelength and multi-channel fiber laser is studied by simulation and experiment. The experiment has obtained the effective coherent combination of multi-wavelength laser. The results have shown: The number of channels between multi -wavelength light beams has a great influence on the intensity distribution of coherent combination. With the increase of the number of channels, the coherent fringes become narrow and the energy concentration increases. Theoretical simulation and experimental results proved that the multi-wavelength laser can also effectively be coherent combined.
Investigation of numerical simulation on all-optical flip-flop stability maps of 1550nm vertical-cavity surface-emitting laser
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Based on the extended spin-flip model, the all-optical flip-flop stability maps of the 1550nm vertical-cavity surface-emitting laser have been studied. Theoretical results show that excellent agreement is found between theoretical and the reported experimental results in polarization switching point current which is equal to 1.95 times threshold. Furthermore, the polarization bistable region is wide which is from 1.05 to 1.95 times threshold. A new method is presented that uses power difference between two linear polarization modes as the judging criterion of trigger degree and stability maps of all-optical flip-flop operation under different injection parameters are obtained. By alternately injecting set and reset pulse with appropriate parameters, the mutual conversion switching between two polarization modes is realized, the feasibility of all-optical flip-flop operation is checked theoretically. The results show certain guiding significance on the experimental study on all optical buffer technology.
Mode-locked 2μm fiber laser with a Tm/Ho-doped fiber saturable absorber
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A mode-locked 2 μm fiber laser with a simple ring cavity is experimentally demonstrated. By inserting the Tm/Ho-doped fiber saturable absorber into a laser cavity pumped by a 793 nm diode laser, a stable mode-locked pulse at a central wavelength of 2010.11 nm was obtained. The repetition rate, maximum average output power, 3 dB spectral bandwidth, pulse width, and signal-to-noise ratio are 3.76 MHz, 9.63 mW, 0.16 nm, 28.2 ns, and 37 dB, respectively. To the best of our knowledge, this is the first reported mode-locked 2 μm fiber laser using a Tm/Ho-doped fiber saturable absorber.
High-efficiency fiber-coupled module based on multi-single emitter photonic-band-crystal laser diodes
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High-efficiency, high-power and high-brightness, fiber-coupled modules based on semiconductor laser diodes have been important sources in many fields, such as fiber laser pumping, material processing and defense applications. The coupling efficiency of fiber-coupled module has been limited due to the large vertical divergent angle of conventional semiconductor laser diodes. We present a high coupling efficiency module by using photonic-band-crystal (PBC) laser diodes with narrow vertical divergent angle. Fourteen PBC single-emitter laser diodes are combined into a fiber with core diameter of 200 μm and numerical aperture (NA) of 0.22. A high and stability coupling efficiency of 88% and peak otuput power of 47W with the injection current of 5 A are obtained. A comparison with the coupling efficiency of conventional laser diodes module is also presented. And there is a 5% increase of fiber-coupled efficiency based on PBC laser diodes module compared to conventional semiconductor laser diodes module.
Multimode fiber modal decomposition based on hybrid genetic global optimization algorithm
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Numerical modal decomposition (MD) is an effective approach to reveal modal characteristics in high power fiber lasers. The main challenge is to find a suitable multi-dimensional optimization algorithm to reveal exact superposition of eigenmodes, especially for multimode fiber. A novel hybrid genetic global optimization algorithm, named GA-SPGD, which combines the advantages of genetic algorithm (GA) and stochastic parallel gradient descent (SPGD) algorithm, is firstly proposed to reduce local minima possibilities from sensitivity initial values. Firstly, GA is applied to search the rough global optimization position based on near-far-field intensity distribution with high accuracy. Upon those initial values, SPGD algorithm is afterwards used to find the exact optimization values based on near-field intensity distribution with fast convergence speed. Numerical simulations validate the feasibility and reliability.
Passively q-switched fiber lasers based on concave gold bipyramids saturable absorbers
Jiaxin Song,
Hanshuo Wu,
Jian Wu,
et al.
Show abstract
In our experiment, a kind of novel gold nanoparticles – concave gold bipyramids (CAuBPs) withlarge electromagnetic-field enhancement and broader plasmonic tuning rangehave been used as a saturable absorber (SA). Passively Q-switched thulium-doped and ytterbium-doped fiber lasers have been demonstrated based on CAuBPs SAs, the longitudinal surface plasmon resonance (LSPR) bands of which are located at 1 μm and 2 μm. In the 1 μm region, the maximum average output power of 9.61 mW is obtained with the shortest pulse width of 1.83 μs at the pulse repetition rate of 97.47 kHz.In the 2μm waveband, 9.72 mW average output power is obtained with the minimum pulse width of 4.56μs at the repetition rate of 19.9 kHz. To the best of our knowledge, it’s the first time that CAuBP has been used as SA, which verifies the saturable absorption properties of CAuBPs in the infrared range experimentally.
Integrated double-clad photonic crystal fiber amplifier
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This paper studies and fabricates an integrated double-clad photonic crystal fiber amplifier, which overcomes the shortcomings of space application and makes full use of excellent property of double-clad photonic crystal fiber. In the experiment, the (6 + 1) × 1 end-pump coupler with DC-PCF is fabricated. The six pump fibers are fabricated with 105 / 125μm (NA = 0.22) multi-mode fiber. The signal fiber is made of ordinary single-mode fiber SMF-28. Then we spliced the tapered fiber bundle to photonic crystal fiber. At last, we produce double-clad photonic crystal fiber with an end-cap that are able to withstand high average power and protect the system. We have fabricated an integrated Yb-double-clad photonic crystal fiber amplifier.
250W continuous-tunable all-fiberized single-frequency polarization-maintained amplifiers with wavelength spanning from 1065 nm to 1090 nm
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In this manuscript, we demonstrate an all-fiberized, single-frequency and polarization-maintained (PM) amplifiers with wavelength tuned from 1065 nm to 1090 nm. The ASE is suppressed by a signal to noise ratio of higher than 27 dB, and each wavelengths can be amplified to be 250 W output power. The stimulated Brillouin scattering (SBS) effect in such high power amplifiers is suppressed by employing a high dopant fiber (10 dB/m). The polarization extinction ratio (PER) of the amplifier is over 20 dB at the maximum output power. It should be noted that although the experiments are conducted at the wavelength from 1065 nm to 1090 nm with a step of 5 nm, the wavelength can also be continuously tuned.
Study on VCSEL laser heating chip in nuclear magnetic resonance gyroscope
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In recent years, atomic gyroscope has become an important direction of inertial navigation. Nuclear magnetic resonance gyroscope has a stronger advantage in the miniaturization of the size. In atomic gyroscope, the lasers are indispensable devices which has an important effect on the improvement of the gyroscope performance. The frequency stability of the VCSEL lasers requires high precision control of temperature. However, the heating current of the laser will definitely bring in the magnetic field, and the sensitive device, alkali vapor cell, is very sensitive to the magnetic field, so that the metal pattern of the heating chip should be designed ingeniously to eliminate the magnetic field introduced by the heating current. In this paper, a heating chip was fabricated by MEMS process, i.e. depositing platinum on semiconductor substrates. Platinum has long been considered as a good resistance material used for measuring temperature The VCSEL laser chip is fixed in the center of the heating chip. The thermometer resistor measures the temperature of the heating chip, which can be considered as the same temperature of the VCSEL laser chip, by turning the temperature signal into voltage signal. The FPGA chip is used as a micro controller, and combined with PID control algorithm constitute a closed loop control circuit. The voltage applied to the heating resistor wire is modified to achieve the temperature control of the VCSEL laser. In this way, the laser frequency can be controlled stably and easily. Ultimately, the temperature stability can be achieved better than 100mK.
Analysis of detection performance of multi band laser beam analyzer
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Compared with microwave radar, Laser radar has high resolution, strong anti-interference ability and good hiding ability, so it becomes the focus of laser technology engineering application. A large scale Laser radar cross section (LRCS) measurement system is designed and experimentally tested. First, the boundary conditions are measured and the long range laser echo power is estimated according to the actual requirements. The estimation results show that the echo power is greater than the detector's response power. Secondly, a large scale LRCS measurement system is designed according to the demonstration and estimation. The system mainly consists of laser shaping, beam emitting device, laser echo receiving device and integrated control device. Finally, according to the designed lidar cross section measurement system, the scattering cross section of target is simulated and tested. The simulation results are basically the same as the test results, and the correctness of the system is proved.
Suppression of high order modes employing active self-imaging mode filter in large mode area strongly pumped fiber amplifier
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To suppress high order modes and improve the beam quality, an active self-imaging mode filter based on multimode interference and self-imaging effect is proposed in large mode area (LMA) fiber amplifier. With this filter structure, transverse mode competition and individual transverse mode power distributions in strongly pumped fiber amplifiers are theoretically demonstrated. Employing this mode selection technique in 30/400 LMA strongly pumped fiber amplifier, the percentage of the fundamental mode rises from 27.8% (without filter) to 96.3%. By the modal power decomposition, the M2 parameter of beam quality decrease dramatically from 2.24 to 1.11 (0 relative phase) and from 3.01 to 1.24 (π/2 relative phase). This study provides a new method to achieve single mode in LMA fiber amplifier and this filter would be extended to larger mode area fiber amplifier to improve the beam quality.
Ultra-long-period grating as a novel tool for multi-wavelength ultrafast photonics
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Here, we demonstrate the six-wavelength mode-locking and hybrid mode-locking operation in an erbium-doped fiber laser (EDFL) with an ultra-long-period grating (ULPG) by properly adjusting the pump power and the cavity parameters. The ULPG is fabricated by using the fused biconical method with a GPX-3000 glass processing system. Study found that, the ULPG exhibits dual-function, that is, mode-locker and multiwavelength filter. Our finding implies that apart from its fantastic sensing application, the ULPG may also possess attractive nonlinear optical property for ultrafast photonics.
A homogeneous cooling scheme investigation for high power slab laser
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The forced convective heat transfer with the advantages of reliability and durability is widely used in cooling the laser gain medium. However, a flow direction induced temperature gradient always appears. In this paper, a novel cooling configuration based on longitudinal forced convective heat transfer is presented. In comparison with two different types of configurations, it shows a more efficient heat transfer and more homogeneous temperature distribution. The investigation of the flow rate reveals that the higher flow rate the better cooling performance. Furthermore, the simulation results with 20 L/min flow rate shows an adequate temperature level and temperature homogeneity which keeps a lower hydrostatic pressure in the flow path.
A single-longitudinal-mode Tm, Ho:YAG laser
Li Li D.V.M.,
Youlun Ju II,
Fang Chen
Show abstract
A single-longitudinal-mode Tm, Ho:YAG laser was demonstrated by using a diode-pumped L-shaped twisted-mode-cavity. The single-longitudinal-mode laser generated a maximum output power of 202 mW at 2090.9 nm under the pump power of 5.9 W, corresponding to a slope efficiency of 6.95% with respect to launched pump power. The beam quality M2 factor was measured about 1.17.
Laser frequency-offset locking based on the frequency modulation spectroscopy with higher harmonic detection
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We design a fiber electro-optic modulator (FEOM)-based laser frequency-offset locking system using frequency modulation spectroscopy (FMS) with the 3F modulation. The modulation signal and the frequency-offset control signal are simultaneously loaded on the FEOM by a mixer in order to suppress the frequency and power jitter caused by internal modulation on the current or piezoelectric ceramic transducer (PZT). It is expected to accomplish a fast locking, a widely tunable frequency-offset, a sensitive and rapid detection of narrow spectral features with the 3F modulation. The laser frequency fluctuation is limited to ±1MHz and its overlapping Allan deviation is around 10-12 in twenty minutes, which successfully meets the requirements of the cold atom interferometer.
Compensation for the phase-type spatial periodic modulation of the near-field beam at 1053 nm
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A phase-only spatial light modulator is used to provide and compensate for the spatial periodic modulation (SPM) of the near-field beam at the near infrared at 1053nm wavelength with an improved iterative weight-based method. The transmission characteristics of the incident beam has been changed by a spatial light modulator (SLM) to shape the spatial intensity of the output beam. The propagation and reverse propagation of the light in free space are two important processes in the iterative process. The based theory is the beam angular spectrum transmit formula (ASTF) and the principle of the iterative weight-based method. We have made two improvements to the originally proposed iterative weight-based method. We select the appropriate parameter by choosing the minimum value of the output beam contrast degree and use the MATLAB built-in angle function to acquire the corresponding phase of the light wave function. The required phase that compensates for the intensity distribution of the incident SPM beam is iterated by this algorithm, which can decrease the magnitude of the SPM of the intensity on the observation plane. The experimental results show that the phase-type SPM of the near-field beam is subject to a certain restriction. We have also analyzed some factors that make the results imperfect. The experiment results verifies the possible applicability of this iterative weight-based method to compensate for the SPM of the near-field beam.
Control of metal ablation by temporally shaped femtosecond laser pulse
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Temporally shaped femtosecond laser pulse control of two basic heat transportation processes in metals, i.e., electron-phonon coupling and electronic heat diffusion is investigated experimentally and theoretically. It is revealed that the transformation of single-pulse irradiation into double-pulse irradiation, or an adjustment of pulse-separation for double-pulse irradiation, in all case keeping the energy constant, can exert very different influence on electron-phonon coupling and electronic heat diffusion for different metals. This can lead to an unusual ablation/damage effect for nickel, i.e., an ablation/damage enhancement after double-pulse irradiation with respect to single-pulse irradiation, in comparison with the ablation/damage suppression for various other metals as widely reported. We conclude that control of heat transportation processes with tailored femtosecond pulses is suitable for robust manipulation of metal excitation and thus control of the initial steps of laser processing of metallic materials.
Characteristic analysis of atmospheric boundary layer and particulate matter in Beijing
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Raman lidar has been designed for the measurement of vertical and temporal distribution of aerosol optical properties, atmospheric temperature and water vapor. In order to investigate characteristics of aerosol boundary layer (ABL) height in Beijing, the lidar system had been installed in the University of Chinese academy of sciences from November 2014 to January 2015. The data obtained by Raman lidar have been used to derive the ABL height (ABLH) based on the gradient method and the ABL height is compared with particulate matter (PM) data provided by the Ministry of Environmental Protection of the People’s Republic of China. A total of 15 days of haze, 27 days of pollution and 24 days of clean occurred through the entire period of observation. On haze, pollution and clean days, the average ABLH were 0.6~0.9 km, 0.9~1.3 km and 1~1.9 km, respectively. In contrast to clean days, haze days have lower ABLH, and gradient changes are faster. The measurement results show the height of ABL has a negative correlation with the concentration of surface PM. The rate of PM concentration variations increase gradually with the height of ABL in clean, pollution and haze days. The rate of PM2.5 average concentration in haze days (-242.4 μg·m-3 /km) is more than 2 times than the rate in pollution days (-114.8 μg·m-3 /km), 3 times than the rate in clean days (-77.4 μg·m-3 /km). The rate of PM10 average concentration in haze days (- 224.2 μg·m-3 /km) is more than 2 times than the rate in pollution (-117.6 μg·m-3 /km) and clean days (-90.4 μg·m-3 /km).
PT-symmetry of coupled fiber lasers
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In this work, we propose a concept of a coupled fiber laser exhibiting PT-symmetry properties. We consider a system operated via Raman gain. The scheme comprises two identical fiber loops (ring cavities) connected by means of two fiber couplers with variable phase shift between them. We show that by changing the phase shift one can switch between generation regimes, realizing either PT-symmetric or PT-broken solution. Furthermore, the paper investigates some peculiarities of the system such as power oscillations and the role of nonlinear phase shift in fiber rings.
Nanosecond pulse pumped, fiber-integrated narrow linewidth linearly polarized Raman amplifier at 1120 nm
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In this manuscript, we demonstrated a watt-level average power narrow linewidth linearly polarized fiber-integrated Raman amplifier core pumped by a pulsed laser with center wavelength of 1064 nm. The gain medium in the amplifier stage is a 100-meters-long polarization-maintaining (PM) single-mode fiber. Seeded by a single-frequency (S-F) distributed feedback (DFB) diode laser, ~3.2 ns duration tunable 1120 nm laser is obtained with peak power of ~360 W. The average power of the Raman laser are 0.62W, 1.22 W and 1.83W with repetition rates of 500 KHz, 1 MHz and 1.5 MHz, respectively. More than 50% slope efficiency and 42 dB amplification is achieved. The polarization extinction rate (PER) of the Raman laser is measured to be ~21 dB.
Design of laser diode driver with constant current and temperature control system
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A laser Diode (LD) driver with constant current and temperature control system is designed according to the LD working characteristics. We deeply researched the protection circuit and temperature control circuit based on thermos-electric cooler(TEC) cooling circuit and PID algorithm. The driver could realize constant current output and achieve stable temperature control of LD. Real-time feedback control method was adopted in the temperature control system to make LD work on its best temperature point. The output power variety and output wavelength shift of LD caused by current and temperature instability were decreased. Furthermore, the driving current and working temperature is adjustable according to specific requirements. The experiment result showed that the developed LD driver meets the characteristics of LD.
Research on method of obtaining multi wavelength correction coefficient of laser power meter
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Due to the detector's response to different wavelength laser is different, the correction coefficients are also not the same, and however, it is not realistic to calibrate all the wavelengths to obtain the corresponding correction coefficient. It is studied on a new method for obtaining these data under different light sources, based on one wave length correction coefficient and the spectral response curve of the detector. It is solved the calibration problem of multi wavelength laser power meter. The experiments are made and the results show that the correction coefficient obtained by this method agrees well with the one obtained by the traditional method.
The entangled photons generation in third order nonlinearity of spontaneous parametric down conversion by whispering gallery mode resonator
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We manipulated the simulation and apparatus to generate the entangled quantum photons by the enhanced higher quality factor in waveguide of whispering gallery mode resonator in silica microsphere. As the several nonlinear optics effects have been validated in micro-disk (lithium niobate materials based), others micro-cavity (microfiber and micro ring on the chip) and second harmonic generation (SHG) on the surface of silica microsphere because of the characterization of enhanced higher quality factor Q and smaller volume mode in these resonator. However until now for the second third nonlinearity of spontaneous parametric down conversion (SPDC), third order nonlinearity of spontaneous parametric down conversion (TOSPDC) and spontaneous four wave mixing (SFWM) in whispering gallery mode (WGM) resonator of silica microsphere rarely have not been fully investigated and verified to generate the triple and pair entangled photons where are widely applied on the applications of biosensor, quantum communications and spectroscopy, respectively. Specially, the features of silica microsphere have attracted many applications due to the simple fabrication, simplified materials melted by silica fiber. The work we demonstrated in this paper based on the breaking of the dispersion rules to make perfect phase matching in normal dispersion in silica microsphere depending on the blue laser spectrum in visible spectrum, then manipulated the modified size of microsphere to detune the pump laser of free spectral range (FSR) and both shift the geometrical dispersion are characterized in the variation of FSR given by (see PDF for equation), where n is refractive index, R is the microspheres radius and m is mode numbers in resonator, to compensate the materials dispersion given by (see PDF for equation), where c is the speed of light and λ is pump laser wavelength to fulfill the perfect phase matching in parametric down conversion regimes and the modeling fabrication coupling results also will be presented.
Localization in disordered potential in photonic lattice realized in time domain
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We describe theoretically and realize experimentally Anderson localization for optical pulses in time domain, using a photonic mesh lattice with random phase modulation implemented with coupled optical fiber loops. We demonstrate that strongest degree of localization is limited and increases in lattices with wider band-gaps.
Photonic modes in synthetic photonic lattices localized due to nontrivial gauge field circulation
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One of concepts giving opportunities for studying of topological insulators in non-magnetic materials, or creating scattering-immune in optical waveguides is creation of synthetic gauge fields in photonic systems. It was shown that gauge fields shift the band-gaps of optical waves, which can be applied to implement one-way nonreciprocal waveguides, even though both the waveguide core and cladding are in a topologically trivial state [1]. In our work we propose a method to create a gauge field in a synthetic photonic mesh lattice – an optical device proved its high versatility for optical experiments [2]. We demonstrate presence of localized modes due to nontrivial gauge field circulation.
Verification and calibration of laser Doppler flowmetry (LDF) prototype for measurement of microcirculation
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Laser Doppler Flowmetry (LDF), a non-invasive microcirculation measurement equipment, is designed to be used in measuring microcirculation and perfusion in the skin. LDF is very applicable to healthcare. However, the cost of commercial LDF prevents its prevalence and popularity. In this paper, continuing previous researches, a LDF prototype was built from the combination of the off-the-shelf electronic components. The raw signals acquired from the proposed LDF prototype is validated to be relevant to the microcirculation flux.
Furthermore, we would like to verify the consistency between the signals measured from both model, and find an implicit transformation rule to transform the LDF prototype signals. For the purpose of verification and calibration of the LDF prototype signal feature, we first collected a parallel database consisting of flux signals measured by commercial and prototype LDF at the same time. Second, we extract signals with specific frequency of normalized signals as features and use these features to establish a model to allow us to map signals measured by LDF prototype to the commercial model. The result of the experiment showed that after we used the linear regression models to calibrate physiological feature, the correlation coefficient reached nearly 0.9999, which is close to a perfect positive correlation. The overall evaluation results showed that the proposed method can verify and ensure the validity of the LDF prototype. Through the proposed transformation, the flux signals measured by the proposed LDF prototype can successfully be transformed to its parallel form as if it is measured by commercial LDF.
Monolithic wavelength-tunable thulium-doped fiber laser mode-locked by semiconductor saturable absorber mirror with 120nm tuning range
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A monolithic widely wavelength-tunable TDFL mode-locked by SESAM is experimentally demonstrated in our contribution. The laser features a broad wavelength tuning range of 121 nm (1862 - 1983 nm). The central wavelength is tuned by a grating-based tunable filter in the ring laser cavity. To the best of our knowledge, this is so far the most widely wavelength-tunable TDFL mode-locked by SESAM.
Ultra-long duration and ultra-high duty cycle dissipative soliton resonance in a mode-locked thulium-doped fiber laser
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Dissipative soliton resonance (DSR) exists in a dissipative system with narrow cavity parameter space in mode-locked fiber lasers. In this work, DSR generation with >800ns pulse width and >84% duty cycle is demonstrated in a mode-locked TDFL. To the best of our knowledge, this result presents the longest DSR pulse duration for 2μm fiber lasers, and the highest pulse duty cycle that has been achieved for any fiber laser operating in the DSR regime.
Effects of Y3+ codoping on the spectral properties of Nd:CaF2 crystals
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Nd:Ca1-xYxF2+x(x=0%,2%,5%,10%) crystals were grown by Temperature gradient technique. The emission spectrum and absorption spectrum were tested. The emission cross section, emission bandwidth and absorption cross section were calculated. The influence of codoping ions on the spectral performance was compared and analyzed. These crystals have better spectral parameters than Nd:CaF2 crystals. By increasing the concentration of Y3+, spectral properties could be optimized. Among these crystals, the crystal with x=10% has the longest fluorescence lifetime (283.5 μs). The crystal with x=5% has the largest emission cross section(2.90× 10-20 cm2) and the largest absorption cross section(2.18× 10-20 cm2).
Effects of Gd3+ on the photoluminescence properties of Nd3+-doped SrF2 crystal
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The effect of doping concentration of Nd3+ and co-doping Y3+ on the spectroscopic properties are investigated systematically. Due to the particular clustering effect, the quench effect was demonstrated in lightly doped NdxGd0.03Sr0.97-xF2.03+x (x=0.0005,0.0015,0.0065,0.01) crystals. For a 3% Gd:SrF2 crystal, the fluorescence lifetime at 1054 nm decrease from 380.9 to 159.8 μs by doping Nd3+ from 0.15 at.% to 1 %at.%, while the emission cross section decreases to 4.12 × 10−20cm2 at 1054 nm. However, the absorb cross section were increased when the concentration of Nd3+ increase from 0.5 % to 0.65 %. Thus, there is an optimum doping concentration of Nd3+. According to the research, the optimum doping concentration of Nd3+ is 0.15 % in 3% Gd:SrF2 crystals.
Modulated visible spectra properties of Pr:Ca1-xRxF2+x(R=Y, La, Gd) crystals
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The spectroscopic properties of the 1.0 at.%Pr:Ca0.97R0.02F2.03(R=Y, La, Gd) crystals are investigated. X-diffraction and room temperature absorption spectra have been registered and analyzed. The emission spectra and decay curves of the crystals were obtained at room temperature. The photoluminescence intensity in the visible region is significantly enhanced by co-doping R3+ ions in Pr:CaF2 crystal. The different effects among the R3+ (Y3+, La3+ and Gd3+) regulating ions on the crystals were observed and compared. Pr:Ca0.97La0.02F2.03 and Pr:Ca0.97Y0.02F2.03 crystals have substantially strong emission at blue and orange region, while the Pr:Ca0.97Gd0.02F2.03 crystal is more suitable for the red emission emitting.
Modeling the mode competition of Yb-doped fiber amplifiers in consideration of photodarkening
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In this paper, the numerical steady-state model of mode competition in high power ytterbium-doped fiber amplifiers (YDFA) which considering the distributed photodarkening (PD) loss has been studied and developed for the first time. According to the relationship between PD loss and the upper-state population fraction, the formula of saturated PD loss which can distinguish different modes is established. Based on the rate-equation model considering transverse spatial-hole burning, we calculate the longitudinal distribution and quantitative results of each mode’s PD loss for the first time to the best of our knowledge. The dependence of parameters such as pumping direction, fiber length, and fiber doping composition on YDFA is also investigated and discussed. According to our simulation results, the output power of YDFA is strongly affected by PD loss. Meanwhile, PD increases the loss of higher-order mode (HOM), as well as enhances the ratio between fundamental mode (FM) and HOM. Thus, HOM suppression and improvement of beam quality can be expected within PD effect. In our work, the mode competition of high power YDFA under varying degrees of PD loss has been simulated. Obviously the output power will decrease with increasing the PD loss. However the ratio between FM and HOM has no evident changes with varying the PD loss, which providing guideline on appropriately using PD loss to regulate the mode competition between FM and HOM aiming at effective single-mode output of YDFA.
Compact and efficient blue laser sheet for measurement
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Compact and efficient blue laser sheet has important applications in the field of measurement, with laser diode end pumped Nd:YAG directly and LBO intracavity frequency doubling, a compact and efficient CW 473nm blue laser sheet composed of dual path liner blue laser is realized. At an incident pump power of 12.4W, up to 1.4W output power of the compound blue laser is achieved, the optical-to-optical conversion efficiency is as high as 11.3%.
Analysis of thermal effects in high power Yb doped fiber amplifier by distributed pumping
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We have introduced a theoretical modeling based on arbitrary pump mode, derived the analytic functions of distributed amplifier along the fiber, and constructed a steady state heat equations of Yb-doped double-clad fiber amplifiers with both quantum defect and propagation losses being considered. Thermal effects and output power characteristics of kilowatt all fiber master-oscillator power amplifier (MOPA) are investigated by distributed pumping. Proper designs for reducing the temperature at the end of the fiber are proposed, the effect of the pumping mode and fiber length on the output performance and the temperature of fiber amplifiers is discussed and proposed a method to alleviate the thermal effect and improve amplifier efficiency. At last we also analyzed the output power and temperature characteristics by changing the power of seed laser, the results show that with the increasing the power of seed laser, the output power and temperature increase.
Theoretical study of Raman fiber laser and random fiber laser
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We present results on Raman fiber laser and random fiber laser. We theoretically analyzed the optical conversion efficiency in core and cladding pumped Raman fiber lasers. For cladding pumped Raman fiber laser, we change the cladding area and core area ration, fiber length, reflectivity of output FBG, Raman gain for improving the laser efficiency. At last we present a study of power output characteristics of random fiber lasers based on the half-open cavity by one or two fiber Bragg gratings, through our theoretical study we can know how to change the laser structure or fiber parameters to obtain the laser power and laser wavelength we want.
Simultaneous three wavelength mid-IR laser by utilizing MgO:PPLN crystal
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A multi-wavelength mid-infrared laser based on a multi-period doped MgO periodically poled lithium niobate (MgO:PPLN) was reported in this letter. The pump source was 1.064μm Q-switched Nd:YVO4 laser with a pulse repetition rate of 15kHz and pulse duration around 30ns. Three domain periods of 28.5μm, 29μm and 29.5μm in series were fabricated in a 2mm-thick z-cut MgO:PPLN, and the length of each domain period was 20mm. The extra-cavity singly resonant optical parametric oscillator had been demonstrated with a compact two-mirror cavity. Three idler wavelengths of 3825nm, 4004nm and 4165nm in the mid infrared were obtained at the same time, and the total output power was 139mW at 1.064μm pump power of 3.09W corresponding to optical-to-optical conversion efficiency of 4.5％ and the pump threshold was 1.04W for the 60mm-long MgO:PPLN.
Coaxial monitoring of temperature field in selective pulsed laser melting
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Selective Laser Melting is a rapid manufacturing technology which produces complex parts layer by layer. The presence of thermal stress and thermal strain in the forming process often leads to defects in the formed parts. In order to detect fabricate errors and avoid failure which caused by thermal gradient in time. An infrared thermal imager and a high speed CCD camera were applied to build a coaxial optical system for real-time monitoring the temperature distribution and changing trend of laser affected zone in SLM forming process. Molten tracks were fabricated by SLM under different laser parameters such as frequency, pulse width. And the relationship between the laser parameters and the temperature distribution were all obtained and analyzed.