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- Front Matter: Volume 11046
- Fifth International Symposium on Laser Interaction with Matter
Front Matter: Volume 11046
Front Matter: Volume 11046
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This PDF file contains the front matter associated with SPIE Proceedings Volume 11046 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.
Fifth International Symposium on Laser Interaction with Matter
Multispecies combustion diagnostics using tunable diode laser absorption spectroscopy
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Multi-species measurements of N2O, CO, NO within an ammonium dinitramide (ADN) based thruster are presented using quantum cascade lasers, while gas temperature are obtained using near-infrared H2O two-line thermometry. ADN monopropellant represents a new generation of green propellant for spacecraft propulsion. Measurements using the developed mid-infrared diagnostic system provide an access to characterize the combustion process inside the ADN based thruster, while theoretical study of ADN monopropellant combustion is still under study. Both steady-state firing and pulse-mode firing for ADN based thruster operation are measured in the present experiments. Results of multispecies concentration agree well with the mechanism of the combustion process and divide the whole process into decomposition stage and combustion stage. Reaction (R1, NH4N( NO2)2→ NH3+N2O+HNO3 ) is the primary reaction branch as N2O generation is much higher and faster than NO generation in decomposition stage at the measured position. To the author’s knowledge, this work represents the first multispecies simultaneous measurements in an actual ADN based thruster using mid-infrared laser absorption spectroscopy. The time-resolved multispecies results aid in an improved understanding of the ADN monopropellant combustion and demonstrate mid infrared diagnostic technique’s practicality for small-sized ADN based thruster.
Quantitative research on component failure by using Monte-Carlo method
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The thermal effect is one of the main effects of directed energy interaction with materials. Currently, research on thermal effects between directed energy and materials have been extensively developed. However the system level thermal effect and thermal failure evaluation is relatively less, especially the system level thermal failure evaluation lacks of effective analysis method. In this paper, the method of reliability analysis based on fuzzy inference and Monte-Carlo method is proposed to quantitatively evaluate the thermal failure process of the system. Based on the above method, simulation verification is performed. The component model is established and the boundary conditions of the directed energy are applied. Through the simulation, the thermal failure quantitative evaluation results of the system are obtained. The thermal failure rules of the system are studied and the results can be used for system level radiation effect research.
Laser-induced plasma in a single droplet
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Water clouds are considered as the common dispersion systems suspending in the air. In general, droplets can be treated as a transparent dielectric for the laser field. However, once the intensity of laser exceeds the breakdown threshold, laserinduced plasma will be generated in the droplet. This plasma is able to significant influence the propagation of laser field. Since the water clouds are constituted by the countless droplets, it is reasonable to study the interaction between intense laser field and a single droplet as the starting point. The laser-induced plasma is usually generated instantaneously in a micro spatial volume. It is difficult to reveal the detail structure of plasma. We will provide a transient coupling model to study the time-evolution of the laser-induced plasma propagating in a single droplet. Using the above model, we will bring insight into the breakdown threshold firstly. On the other hand, there are abundant features in the both plasma and its coupling laser field. The plasma is initially generated at the location where laser field propagates away from the droplet, if the intensity of the laser is not very high. Meanwhile, after the plasma generates, the droplet becomes opaque. Large amount of laser energy then is deposited in the droplet. The saturation of the energy deposit can be revealed in our model. We would expect this transient coupling model is helpful to estimate the propagation length of the intense laser pulse while it is passing through the water cloud.
SBS suppression in high brightness, all-fiberized and linearly polarized amplifiers by rectangular spectrum
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Stimulated Brillouin scattering (SBS) suppression in high power, narrow linewidth and all polarization-maintained amplifiers with near diffraction limited (NDL) beam quality by rectangular optical spectrum is demonstrated. Rectangular spectrum is generated by using cascaded phase modulations. In the preliminary experiment, output power of 509 W with spectral linewidth of ~ 3 GHz is obtained. At maximal output power, the polarization extinction ratio (PER) is measured to be ~ 14 dB and the beam quality (M2 factor) is M2 ~1.2. The technique presented give useful reference to control the spectral linewidth in high brightness fiber amplifiers. By combining with other linewidth controlling techniques, several kilowatt-level output power with spectral linewidth of < 10 GHz could be expected.
Diode-pumped passively Q-switched mode-locking Tm:LLF laser with graphene oxide saturable absorber
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By employing Graphene Oxide (GO) grown by vertical growth method as a saturable absorber, we first demonstrated a stable passively Q-switched mode-locked (QML) all-solid-state Tm:LLF laser. when the LD pump power is higher than 8.58W, the laser operation gets into a stable Q-switched mode-locked state, corresponding mode-locked pulse repetition frequency is 104.2MHz. The modulation depth of mode-locked pulses in the Q envelope is close to 100%. The results show that graphene oxide is a promising SA for QML solid-state laser in the 2μm wavelength
Mid-infrared upconversion imaging pumped by sub-nanosecond micro-cavity laser
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Based on the polarization period of 22.67um magnesium-oxide-doped lithium niobate (MgO: PPLN) Crystal, a 4f midinfrared imaging system was constructed. The electronic enhanced charge-couple device (EMCCD) was used to receive the mid-infrared images to improve the sensitivity of the systerm. The resolution tablets of USAF 1951 were used to confirm the performance of the optical system. In this experiment, mid-infrared upconversion from 3412 nm to 3482 nm were achieved by changing the crystal temperature and the wavelength of the signal light. The upconversion imaging at 3482nm was attained by a silicon based CCD camera with the conversion efficiency of 3.8×10-6 and the resolution of 2.52/2 lp/mm.
Modeling and optimization of actively Q-switched 2.8 μm Er3+-doped ZBLAN fiber laser
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A theoretical model based on rate equations for actively Q-switched Er3+-doped ZBLAN fiber laser is built. The operation behaviors and output characteristics of the actively Q-switched fiber laser at 2.8 μm are analyzed. Effects of some important laser parameters, such as pump power levels, reflectivity of the laser output coupler, fiber lengths, Er concentrations, etc., on laser output were investigated. The model and simulating results are useful for design and optimization of actively Q-switched fiber laser at 2.8-μm region.
Experimental investigation of heat dissipation of Fe:ZnSe crystal
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Crystal thermal characteristic is a key factor to affect output laser property. In some applications, the facets of crystal will be contaminated by dust in the air, which will enhance the heat absorption of laser and cause local thermal unbalance. Therefore a novel crystal heat dissipation method is proposed in this paper. Crystal is mounted in a specially designed heat sink, heat conducts between the contacting surfaces of crystal and heat sink. Pump incident laser irradiates from the end facet of crystal. The end facet of crystal is cooling by convection heat transfer with flowing protect gas. The experiment device is established, the pump laser is Hydrogen Fluoride laser with the wavelength of 2.8μm, pulse energy of 600mJ, and repetition rate of 50Hz. The crystal is Fe: ZnSe with the dimension of 20mm× 20mm× 6mm. The beam quality is measured in the condition with and without heat sink for comparison, the results indicate that the heat dissipation method proposed in this paper is benefit for improving the beam quality.
CO concentration measurements using tunable diode laser absorption spectroscopy behind the shock waves in Martian atmosphere
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Shock tube experiments are carried out to study the physical and chemical processes during a vehicle entry into the Mars atmosphere using tunable diode laser absorption spectroscopy (TDLAS) and optical emission spectroscopy (OES). CO concentration distributions are diagnosed behind a shock wave in a CO2-N2 mixture with three different conditions of initial pressure and velocity. The strong shock wave is established in a shock tube driven by combustion of hydrogen and oxygen. Time-resolved spectra of the Δv = 0 sequence of the B2Σ+ →X2Σ+ electronic transition of CN have been observed through OES. A precise analysis of the CN violet spectra is performed and used to determine rotational and vibrational temperatures. Two absorption lines in the first overtone band of CO near 2.33 μm, are selected from a HITRAN simulation to calibrate laser wavelength and detect the CO concentration. Combined with these temperature results using OES, CO concentrations in the thermal equilibrium region are derived, which are 2.91 × 1017 cm-3, 7.46 × 1017 cm-3 and 1.01 × 1018 cm-3, corresponding to equilibrium temperatures equal to 7000 ± 400 K, 7400 ± 300 K, 6000 ± 300 K in the low, medium and high pressure conditions, respectively.
The absorption characteristic of titanium alloys irradiated by quasi-continuous-wave laser
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The absorption characteristic is an important factor for laser machining. A laser absorption rate experimental platform of titanium alloy is established using the integrating sphere-photodiode system. A Quasi-Continuous-Wave (QCW) pulsed Nd: YAG laser is used as the laser source, operating at a repetition rate of 400 Hz and pulse duration of 200μs. The purpose of our study was to show the characteristic of QCW. Experimental results show that higher absorption efficiency is caused by higher peak power of the pulse, which would benefit for the performance of laser machining.
The enhancement of the InGaAs solar cells by the thermoelectric generation technology under the continuous laser exposure
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The temperature rise of the InGaAs solar cells which under the continuous laser exposure is theoretically calculated, and experimentation,correspondingly designed to bismuth telluride thermoelectric power generation and cooling system,thereby enhancing the overall photovoltaic system integrated photoelectric conversion efficiency.
Conical ring beam achieved via degenerated optical parametric generation in tightly focused quasi-phase matching conditions
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Ring or conical light beams show great potential in many applications. We report on achievement of multi-ring-shaped conical beams through degenerated optical parametric generation (OPG) with tightly focused Gaussian pump beam in periodic polarized lithium niobate (PPLN) crystal. We observed continuous beam spatial pattern variation from multi to single ring and normal non-hollow beams by crystal temperature tuning. We attributed the experimental results to the effective polarized period changing under tightly focusing. This unique feature would exist in any quasi-phase matching processes and may find applications in optical trapping, manipulation, and even quantum field.
Experimental study on the influence of backward ASE on SBS process in single-frequency amplifier
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We have constructed an all-fiber single-frequency amplifier system and an additional all-fiber amplified spontaneous emission (ASE) source system to simulate the effect of generated backward ASE on stimulated Brillouin scattering (SBS) in master oscillator power amplification (MOPA) system. By injecting the artificial ASE source backward into the Yb-doped MOPA system directly and measuring the evolutions of SBS threshold power with the increasing ASE power level, we can get the changes of SBS process during the amplification. Effects of counterpropagating ASE on SBS threshold decreasing were obvious: When there was no counter-propagating ASE being injected, the backscattered power started to rise very slowly. However, with the added artificial ASE increasing, the backscattered power departed from linear quite early. And the SBS threshold power was reduced to different levels with different power level of additional ASE. As the artificial counter-propagating ASE power increased from 0 to 275.9mW, the SBS threshold power was correspondingly dropped down from 4.75W to 3.35W. The 30% reduction of the SBS threshold can be a siginificant influence in a single-frequency fiber amplifier.
Al target temperature characteristics by laser ablation and its plume expansion
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The thermal coupling between laser and matter has always been a hot topic. The temperature characteristics of aluminum target irradiated by pulsed laser and the target vapor plume ejection were studied. The heat conduction equation was used to describe the temperature change characteristics of Al target irradiated by laser. The mass conservation equation and momentum conservation equation were used to describe the temperature and density of melted fluid. The mass conservation equation was used to describe the density and velocity characteristics of vapor after gasification. The simulation results showed that the target temperature raised and the liquid and vapor phases changed due to the laser irradiation. The model considered the thermal convection effect caused by the liquefaction flow of target material, and the physical model can describe the temperature change of metal target under laser irradiation. The phase change of solid target was obtained by setting the phase indicator, and the process of target steam injection was analyzed. The physical model can be used to analyze the thermal coupling of laser-matter interaction, thus laying a theoretical foundation for the study of laser-matter interaction.
Theoretical study on the evolution of ASE from the seed to the amplifier with different central wavelength and spectrum
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In order to instructing the practice experiment better, it is necessary to predict the inhibiting ability of amplified spontaneous emission(ASE) for ytterbium doped double-clad fiber amplifiers with different central wavelengths. Using the rate equation theory, the evolution of the ASE from different seed wavelength with the same parameters were studied. For the forward pump configuration, the simulation results show that the weak initial ASE component of seed source will generate large increase after high power amplification. When the seed oscillator ASE suppression ratio fixed at 80dB and the central wavelength is in the range of 1050nm-1100nm, the ASE suppression ratios of the amplifier output spectrum increase firstly and then decrease, and the optimized seed wavelength is 1080nm. The higher seed’s ASE suppression ratio, the better ASE inhibition of the amplifier output laser. In order to control ASE suppression ratio above 30 dB in the output spectrum, the ASE suppression ratio of 1075nm,1080nm and 1085nm seed spectrum must be higher than 70.5 dB, 71.4 dB and 79.3 dB respectively.
Damage detection of CFRP laminates by using ultrasonic Lamb wave with a sparse array
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Guided ultrasonic wave array have found many applications in detection and localization of damage in structural health monitoring (SHM) and non-destructive testing (NDT) of plate-like structures. For accurate and reliable monitoring of large structures by array systems, a high number of actuator and sensor elements are often required. In this paper, A minimum redundancy sparse array is adopted to realize high resolution and accurate damage imaging in carbon fiber reinforced polymer (CFRP) laminates, considering the energy skew effect and dispersion of Lamb wave. The Lamb wave wavenumber curve of A0 mode with its geometric relationship is used to calculate the skew angle. In order to avoid the dispersive phenomenon, the array beam-forming and imaging algorithm is presented in frequency domain by applying phase delay to each frequency component. A cross-shaped sparse array is designed according to minimum redundancy linear array. In the experiment, a PZT is used to generate Lamb waves, and a scanning laser doppler vibrometer (SLDV) is used to receive signals by arranging the scanning points in a sparse array. Experimental Results indicated that the proposed sparse array combined with imaging algorithm can locate defects of CFRP laminates with high accuracy while decreasing the processing costs and the number of required transducers. This method can be utilized in critical structures of aerospace where the use of a large number of transducers is not desirable.
Positron generation via ultra-intense laser pulses colliding in a cylinder filled with near-critical-density plasmas
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A new scheme is proposed in which high-flux and high-density positrons are generated via ultra-intense laser pulses colliding in a cylinder filled with near-critical-density plasmas. Comparing with the only cylinder case and the only gas case, positron generation is enhanced with a higher density. When discussing the influence of plasma density in positron generation, an optimal gas density is found around the near-critical density. The scheme will facilitate the realization of positron generation in labs and further applications of positrons.
Effect of 1070nm laser uniformity on temperature distribution and performance of In0.3Ga0.7As solar panel
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In the laser wireless power transmission (LWPT) system, the solar panel plays a decisive role as the receiving end of the energy. As the transmission medium of energy in LWPT system, the energy of laser presents generally Gauss distribution, resulting in uneven energy of the laser received by the solar panel, which may affect the transmission efficiency and capacity of the system. In this paper, a 1070nm continuous fiber laser is used to irradiate the In0.3Ga0.7As solar panel, and the temperature distribution on the back of the panel and the IV characteristics were recorded. The results showed that the temperature distribution on the back side of the solar panel was almost the same under the conditions of the same laser power but different energy distribution. In terms of performance of the panel, due to the increase in beam uniformity, the short-circuit current increased by 33.4%, the maximum output power increased by 18.5%. In addition, the irradiation of different laser power was also studied in this paper. The influence of different laser intensity and different beam uniformity on the efficiency of the panel were given.
LD pumped YAG/Nd:YAG/Cr4+:YAG burst mode laser
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A laser diode end-pumped passively Q-switched YAG/Nd:YAG/Cr4+:YAG burst mode laser at 1064 nm is reported. The maximum burst energy of 13.2 mJ at a repetition rate of 100 Hz is got in a duration of 1 ms. The pulse repetition frequency increases linearly and peaks at 31.1 kHz in the pulse burst, while the pulse width keeps nearly constant around ~2 ns. The highest peak power of 373.7 kW is achieved at 10.2 kHz. The beam quality of passively Qswitched 1064 nm laser is also investigated.
2D HTV image processing in the complex combustion field
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In order to measure the 2-D velocimetry distribution without seed injection in the high-speed flow field of the engine, the 2-D HTV (hydroxyl tagging velocimetry) technique in the complex combustion field has been developed. The image processing method combining difference method with cross correlation method is presented. The background noise is suppressed by the difference method. The variable-template is used to perform cross correlation operation with experimental images, and the obtained correlation images is fitted with two-dimensional polynomial. Not only can the complex background interference be suppressed, but also can realize accurate extraction position of the tagging cross grid center.So the extraction accuracy is better than 0.25 pixels, when the image SNR is higher than 2. At the same time, based on Matlab software, the data processing program is written. The velocimetry distribution is gained by processing the experimental data in the flow field of the scramjet model. The speed calculation error is less than 5%, which meets the requirement of measurement accuracy of the system.
Structured laser illumination planar imaging method for filtering stray light applied to temperature sensitive paints measurement
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Stray light is the main noise source for planar imaging measurement technique, which can affect the accuracy of results directly. A method of Structured Laser Illumination Planar Imaging (SLIPI) was used to solve this issue. The key of SLIPI is periodic modulation of laser spatial intensity and implementation of post filtering algorithm. In this paper, cylindrical micro lens array was used to modulate the spatial intensity of laser periodically, which was compared with Ronchi ruling. The post filtering algorithm adopts phase-locked detection method. The signal results can be separated from the noisy image using only one measurement image by this method. SLIPI method has been used in Temperature Sensitive Paints measurement experiments. A diagnostic optical path combining cylindrical micro lens array and cylindrical mirror was designed for the need of surface light source irradiation. The results show that the method of SLIPI can be applied to most planar imaging measurement techniques, and the accuracy of two-dimensional parameter measurement can be further improved.
Characteristics of WS2 mode-locked Yb-doped fiber laser
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An all-normal-dispersion WS2 mode-locked Yb-doped fiber laser was demonstrated. The saturable absorber (SA) is a piece of WS2-PVA film which is sandwiched between two fiber connectors. The modulation depth and saturation intensity of the WS2-PVA film were 1.78% and 81 MW/cm2 , respectively. When the WS2-PVA film was utilized in the laser cavity, stable mode locking occurred with the pump power of 140 mW. The maximum single pulse energy was estimated to be more than 2.82 nJ. Besides, in order to know more about the influence of the SA on the generation of ultrashort pulses, the dynamic evolution of mode-locked lasers with the parameters of SAs was studied by solving the Ginzburg-Landau equation. Thus the high pulse energy could be reached.
The aging properties of dichroic films used in laser ignition systems
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Two dichroic films were prepared to investigate their aging properties. Results indicate that the age-resistant property of dichroic films can be significantly improved by adopting lower deposition rate and relative high preposition temperature and being treated with stress removing annealing procedure. The correlation of reflective signal to transmission efficiency of igniting laser merely changes after aging for the designed age-resistant dichroic films.
Time-reversal damage imaging based on laser ultrasonic guide waves with temporal filter method
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More attention has been drawn to ultrasonic guided waves (UGW) based damage detection method for its advantages of wide range inspection of large-scale structures. However, complex propagation characteristics of guided waves as well as traditional contact ultrasonic transducers limit its application for the practical damage detection. In this work, A fully noncontact laser ultrasonic detection system is designed by combining YAG pulsed laser with Scanning Laser Doppler Vibrometer (SLDV) to achieve high resolution sensing of UGW field in the structure. A Temporal filter method with multiple frequency bands is proposed to extract the scattering signal without reference signal. The f-k time reversal imaging method is introduced to obtain the reconstructed incident wavefield and scattered wavefield, while the cross-correlation imaging condition is used to identify the damage location in plate structures. Finally, the imaging results of different frequency bands are integrated to achieve accurate imaging with high-precision and high signal-to-noise ratio.
Dynamics character of swirling flame investigated by OH and CH2O planar laser-induced fluorescence
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The model combustor of aircraft engine under fuel-lean condition is characterized by planar laser-induced fluorescence (PLIF) technique. By imaging the fluorescence from OH and CH2O simultaneously under various operation points, the transient structures of the reaction zone and preheat zone have been investigated. By the application of proper orthogonal decomposition (POD) and extended POD (EPOD) methods to the OH PLIF and CH2O PLIF data, the main dynamics modes of the swirling flame are extracted, as well as the CH2O PLIF signal distribution for each POD mode. The experimental results indicate that as the thermal power of the combustor increases, the time-averaged structure and dynamics modes experience notable transitions. At relatively high flow rate, local extinction occurs and unburnt fuel emerges in the external recirculation zone (ERZ).
Study of OH radicals’ spontaneous radiation of counterflow diffusion flame under non-equilibrium plasma
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Plasma assisting technique has shown great prospect in various combustion environments. Especially in terrible combustion problems, plasma brings heat effects, chemical kinetic effects, or transport effects to influence combustion. Fundamental research of non-equilibrium plasma assisting combustion is prerequisite for its engineering application. Counterflow burner provides an ideal and favourable platform to investigate aforementioned problems. This article integrated non-equilibrium plasma discharge system and counterflow burner to investigate influence of coaxial double air gap dielectric barrier discharge on counterflow diffusion flame. OH radicals generation and radiation were observed to reveal basic chemical kinetic mechanisms of plasma. Ultraviolet intensified ICCD camera was used to record OH radicals’ spontaneous radiation in flame sheet. Radiation images show that plasma discharge would increase intensity of OH radicals’ radiation and combustion of flat flame when discharge voltage was under certain value; when discharge voltage exceeded the certain value, airflow in burner would be inevitably affected by discharge, and uniformity of OH radicals’ radiation declined, while in the central zone, flow speed increased, thermal loss decreased and combustion-assistant effects weakened.
Analytical solution of steady temperature field distribution in cylindrical medium irradiated by laser
Liang Liu,
Shanshan Wang,
Hongyan Xu,
et al.
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Thermal effect severely limits the output power and beam quality in high-power solid-state lasers systems. In solid-state lasers, most optical elements are cylindrical, such as laser medium, mirrors, and so on. Normally, thermal analysis is deduced based on the hypothesis that just one surface is cooled while the others are adiabatic. This paper presents semi-analytical formula of steady-state temperature field distribution, based on the hypothesis that both side surface and one end surface are cooled with different coefficients of heat transfer. Simulation results with Ansys confirmed the correctness of the above formula. The results are helpful to the study of thermal effect in disc laser, rod laser and reflective mirrors in high-power laser system.
EMC design of high power TEA CO2 laser
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High power TEA CO2 laser belongs to the gas laser with high-voltage (HV) pulse excitation. The strong electromagnetic interference (EMI) are generated mainly from the discharge circuit loop, the pulse spark switch and the HV supply when the laser works. It has a strong interference and destructive effect on the electronic equipments inside and outside the laser system. The mechanism analysis and experimental measurement were carried out in this paper. The shielding design on the HV supply, the main discharge circuit loop and the main control unit restrained the transmission of EMI effectively. The mains filters were designed to restrain the conducing EMI propagation path. As a sensitive device to EMI, the control system was shielded, isolated and mains filtered on hardware, anti-interference on software was designed to improve the ability of noise reduction. Experimental results demonstrated that reducing EMI intensity, shielding EMI, improving the hardware ability on noise suppression are the primary methods to retrain EMI and keep the hardware of laser control system from being destroyed, the anti-interference on software is a support and complement of hardware noise suppression, which improves the reliability of the laser system.
Effect of ply scheme on ablation characteristics of carbon fiber composites under CW laser irradiation
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The irradiation effects are studied, of continuous wave (CW) laser on five types of laminated carbon fiber composites (CFRP), [0/90/0/90], [0/90/90/0], [0/45/90/135], 1.5K-Textile ([1.5K/0°/90°/1.5K]) and 3K-Textile ([3K/0°/3K]). The ply scheme of CFRP is especially considered in the proposed 3D model. The numeirical results show that the structure of CFRP has significant influences on the temperature evolution and the heat affect zone (HAZ). The non-monotonic temperature rising at the material surface exposed to the laser radiation are associated with the heat flux in the laminated structure. The HAZ in the adjacent layers is discontinuous. The pyrolysis inside the CFRP is more serious than the one on the laser irradiation surface. The numerical predictions indicate that the material with [0/45/90/135] has the optimum thermal insulation performance. This kind of ply scheme could promote lateral heat transfer and weaken longitudinal heat transfer. 3K-Textile has optimal ablation resistance. It is associated with the 3K-Textile layer having the maximum longitudinal thermal conductivity.
Influence of spectral linewidth and wavelength selection of laser display
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Laser display technology has developed rapidly in the past decade because it has advantages of high color saturation, high conversion efficiency, long life span and the capability to show real objects accurately. Many new laser technologies such as ALPD (Advanced Laser Phosphor Display) solution, Two Primary and three Primary Laser Display solutions have been studied for handling wide-gamut standard of Rec.2020 to reproduce the natural object colors faithfully. Different kinds of laser source including LD (laser diodes), diode-pumped solid-state laser, LD pumped Phosphor, VCSEL (Vertical-cavity surface-emitting laser) and fiber laser have been utilized in these solution. They have different center wavelengths and different linewidths which influence the color gamut and the speckle contrast of different display systems. In this paper, we calculate the color solid volume of laser display systems, and systematically investigate the color gamut conditions among display systems with different spectral linewidth selecting and different combination of three primaries. The color gamut of Rec.2020 is set as an object of comparison, in which the center wavelength of three primaries is set as Red=630nm, Green=532nm and Blue=467nm with spectral line width=1 nm. White balanced point is set as D65(x=0.3127, y=0.3291) and curve of color gamut as a function of spectral linewidth is calculated in CIELAB space. Our results would give researchers working on laser display a reference to choose suitable laser source with center wavelength and linewidth to keep a balance between laser speckle and color saturation.
Characterization of graphite produced by laser irradiated glass fiber reinforced epoxy resin
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Pyrolysis, carbonization and other reactions would occur in laser irradiated Glass fiber reinforced epoxy resin composite (GFREC). Different laser irradiation time experiments were carried out to get some ablation samples. With the help of X-ray diffraction (XRD) and Raman spectroscopy(RS, graphite microcrystals were found in the severe ablation GFREC. The temperature of laser irradiated GFREC was calculated by using energy conservation equation and pyrolytic decomposition equation, and we found the temperature of laser induced graphite production was about 800°C. And then the information of graphite microcrystals was characterized by 5th order Gaussian function curve-fitting analysis. Results showed that the sizes of graphite microcrystals were several nanometers.
Numerical simulation of dazzling effects on CCD induced by a 532nm pulsed laser
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The CCD is widely used to detect laser signal in many industrial vision and automation systems. When the charge coupled device(CCD) is irradiated by intense laser, the image quality may decrease and reversible dazzling effects occur such as single pixel saturation, crosstalk, and full saturation. Understanding the laser dazzling phenomena, on the one hand, it can help to choose the suitable laser sources for the measuring systems, on the other hand, it can optimize the designs of the CCD structures. In order to effectively utilize the dazzling effects, the laser saturation threshold of the CCD must be known. Due to the different internal structure and working mode of CCD, the corresponding laser saturation threshold is also different. In this paper, the dazzling effects on the array CCD induced by a 532nm wavelength pulsed laser is investigated by finite element method. A physical model is established base on drift-diffusion equation according to the working process of CCD and the principle of laser dazzling effects. The working state of CCD under different laser power, charge density, electron concentration and potential curve are presented. This model is found to be useful for the analysis of laser saturation threshold.
Simulation and analysis of distortion correction of supercontinuum wavefront based on SPGD algorithm
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Supercontinuum laser, as a new type of laser source, compared with monochromatic or quasi-monochromatic light, has incandescent wide spectra and the brightness of the laser. However,the beam quality in high power is usually bad and greatly affected by atmospheric turbulence in the transmission process. Therefore, how to improve the beam quality to ensure the energy transmission and image quality has become a popular topic. According to the measurement results of the near field wavefront characteristics of the supercontinuum laser in different wavelength, we know that the wavefront phase of the light source is relatively stable and change slowly over time. The types of wavefront distortion at each wavelength are basically the same, mainly made up of defocus and astigmatism. In order to conform to the wavefront characteristics of supercontinuum laser, the input of simulation is low-order static distortion wavefront generated by using Zernike polynomia with defocus and astigmatism. Stochastic Parallel Gradient Descent (SPGD) algorithm with the thorlabs DMP40 deformable mirror and CCD camera as the imaging sensor, is used to construct a simulation model for wavefront correction of low-order static distortion aberrations. Since B-SPGD algorithm is superior to U-SPGD in terms of convergence speed and correction effect, as well as adaptability to the near field wavefront distortion [1], we use B-SPGD algorithm to construct the simulation program for wavefront correction simulation research. The ability and convergence of SPGD algorithm to correct wavefront distortion are studied, mainly about the random disturbance amplitude and influence coefficient of the core parameters. In order to get close to the evaluation standard of laser in practical application, Strehl Ratio(SR) is selected as the performance evaluation function. According to the effective diameter of deformable mirror and the distance between electrodes, the influence function of deformable mirror is normalized to the unit circle to meet the requirements of simulation calculation. In order to improve the stability of SPGD algorithm and reduce the possibility of SPGD algorithm falling into local extreme values, we use Hessian matrix to optimize SPGD algorithm. After simulation and analysis of distortion correction of Supercontinuum laser wavefront, deformable mirror controlled by SPGD algorithm has good correction capability for low-order static aberration.
Laser amplification by four-wave mixing in plasmas
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In this paper, a scheme is proposed to amplify a seed pulse via non-degenerate four-wave mixing mechanism in plasma. Electron plasma wave excitation and four-wave mixing transmission amplification are examined by two-dimensional particle-in-cell simulations, and the physical factors that affect laser amplification are studied. It is shown that four-wave mixing develops rapidly in dozens of laser cycles and lead to energy transfer between laser beams in the interaction of multiple laser beams and plasma.
2.8kW bidirectonal side-pumped fiber amplifiers
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In this paper, we demonstrated a 2815-W bidirectonal side-pumped DSCCP fiber master oscillator power amplifier (MOPA) configuration within the 250 μm cladding fiber, with the slope efficiency of 77.8%. And the output power is limited by the pump power. As we know, it is very difficult to get multi kW output power in 250μm general double cladding fiber. If a two cascaded amplifiers is employed, a 6 kW output power can be expected.
Mid-infrared optical parametric oscillator pumped by dual-wavelength fiber laser based on SRS effect
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In this paper, a dual-wavelength(DW) mid-infrared optical parametric oscillator(OPO) pumped by a DW fiber laser based on the stimulated Raman scattering(SRS) effect, is demonstrated. When the pump power satisfied the threshold condition, obvious SRS effect was observed and a DW fiber laser with center wavelengths at 1060 nm and 1113 nm was obtained. The DW fiber laser was injected into a single-period crystal, and the whole process went through four stages. In the first three stages, 1060 nm pump laser achieved parametric oscillation and generated 1602 nm signal laser and 3138 nm idler laser and the conversion efficiency was seriously affected by SRS effect. In the fourth stage, two independent parametric processes were realized, which generated two mid-infrared output at 3131nm and 3580nm with powers of 2.46W and 40mW, respectively. The efficiency characteristics in the four stages were also discussed separately.
Study on hydrodynamic effects in longitudinal forced convection for high power laser amplifier
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Thermal effect becomes more prominent in the laser gain medium, to overcome this problem, the forced convective heat transfer with reliability and durability is widely used. The hydro-structures of dimensions of the flow channel affect the thermal performance immediately and efficiently. In this paper, with proposed cooling configuration based on longitudinal forced convective heat transfer, the factors of flow rate, state of flow field and surface roughness are investigated. The results reveal that fully developed flow state, higher flow rate and rougher surface lead to a better cooling capability. In the simulation results with 30 L/min flow rate, the calculated averaged convective heat transfer coefficient is as high as 104 W/m2 ·K, and with slightly fluctuations in fully developed flow period.
Wakefield contraction and high-quality electron bunch generation in transverse nonuniform plasmas driven by intense laser pulses
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The effect of ion density in a plasma channel on the profile of bubble rear is investigated theoretically. The wakefield contraction and the steepness of the bubble rear depend on the ratio of ions in a uniform plasma to those in a channel. With the decreasing of ion density in the channel, the bubble length becomes shorter and its bubble rear is steeper. The electron dynamics show that a typical electron can oscillate between the upper and lower boundaries of the channel, not like the betatron oscillation around the on-axis in homogeneous plasma. These results are verified by two-dimensional particle-in-cell simulations. By optimizing the radius of the hollow plasma channel, the front part of a self-injected electron bunch can be reflected back into the channel by the transverse focusing force, while its rear part will escape from the channel. The electric field is transversely uniform and has longitudinally plateau in the regime of self-injected electron bunch, which allows for monoenergetic acceleration.
Passively Q-swtiched operation of Tm, Ho:YAP laser with a MoS2/WS2 saturable absorber mirror
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Passive Q-switching (PQS) operation of 2 μm a-cut Tm,Ho:YAP laser was experimentally demonstrated with a MoS2 /WS2 saturable absorber (SA) mirror. A MoS2 SA was used in the experiment, the PQS Tm,Ho:YAP laser was the first acquired with an average output power of 3.3 W and per pulse energy of 23.31 μJ, and the beam quality factors of Mx =1.06 and My =1.06 were measured. A WS2 SA was used in the experiment, an average output power of 696 mW and a peak power of 1.79 W were acquired from PQS Tm,Ho:YAP laser with the pump power of 9.26 W, corresponding to pulse widths of 9.1 and 5.6μs and the optical-optical conversion efficiencies of 7.5% and 4.1%. The beam quality factors of Mx =1.05 and My =1.06 were measured.
Linear to radial polarization optical convertor with larger diameter and high damage threshold fabricated by single point diamond turning
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We present a symmetrically semi-spiral phase plate (SSSPP), which possesses a continuous and closed surface, for converting a linear polarization light into radial polarization light. By technique of ultra-precision free-form surface machining, it is possible to achieve a convertor with large aperture and high damage threshold. This kind of SSSPPs, firstly fabricated using MgF2 and CaF2 with Φ50mm by single point diamond turning, is demonstrated and measured in an experiment. Additionally, the available materials also include KDP, LiNbO3, some plastic material and glasses, etc.
Shape dependence of nonlinear optical activities of tungsten oxide nanostructures
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he understanding of shape dependence of nonlinear optical (NLO) behaviors is crucial for designing of an effective component and device. In this work, nanostructured tungsten oxide (WO3) in shapes of nanospheres, nanowires, and nanoplates have been prepared using a facile hydrothermal method by controlling the molar ratio of the precursors. Field emission scanning electron microscopy (FE-SEM) clearly confirmed the morphology characteristic of the three nanostructures. X-ray diffractometry (XRD) identified the nanospheres were well consistent with octahedral WO3·0.33H2O while nanowires and nanoplates can be well indexed to the hexagonal structure of WO3. Meanwhile, the WO3 nanostructures displayed shape dependent linear optical behaviors. The NLO properties were measured at 532 nm using open aperture (OA) Z-scan technique. The two-photon coefficients of WO3 nanospheres, nanowires and nanoplates were calculated to be 9.8×10-11 m/W, 1.44×10-10 m/W, 2.02×10-10 m/W, respectively. The nanoplates exhibited the best NLO performance while the nanospheres present the weakest one. The result implies that the NLO activities can be effectively tailored by morphology control.
Studies on the operating characteristics of anisotropic acousto-optic device with multi-transducers
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The bandwidth and the diffraction efficiency are two important operating parameters of the acousto-optic device, which are incompatible for acousto-optic device designed with single-piece transducer. In order to improve the performance of the acousto-optic device, theoretical studies on the operating characteristics of anisotropic acousto-optic device designed with multi-transducers are carried out. The calculation results show that the contradiction between the bandwidth and the diffraction efficiency of the acousto-optic device can be coordinated efficiently and the octave condition can be satisfied when the number of the transducer reaches 16. Furthermore, the linear relationships between the central frequency and the incident angle as well as detailed operating parameters of anisotropic acousto-optic device with multi-transducers are also determined.
Analysis of laser-induced surface profile variation in silicon glass based on particle swarm optimization algorithm
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To increase the lifetime of components in high power lasers and to study downstream light field the influence of the damaged optical components, numerical model of surface profile of damaged optical components were built with particle swarm optimization algorithm, and the relationship between the damage degree and the parameter of numerical model was analyzed. First laser irradiation experiment was carried out to acquire the damaged optical components. Then surface morphology was measured with Zygo interferometer system. With a typical Gaussian filter. A numerical model of one dimensional lineout of surface profile was established with particle swarm optimization algorithm. Numerical results shows that the model was valid with the particle swarm optimization (PSO) algorithm. The results also shows that there was a relationship between parameter of the model and the damage degree.
Electroluminescence enhancement in GaInP/GaAs/Ge tandem solar cells induced by laser
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In order to study the mechanisms of thermal damage during laser machining in GaInP/GaAs/Ge tandem solar cells (TSCs), the spatial electroluminescence (EL) characterization on sub-cells pre and post laser irradiation was carried out. Results showed that post laser irradiation, the EL of GaAs middle cell increased to saturation in the damage zone, but decreased to zero at the rest part. A theory was put up to explain this phenomenon by using two-unit equivalent circuit model, and then verified through GaInP top cell spatial EL analysis. Conclusion was drawn that current redistribution induced by local shunt resistance decreasing in GaInP top cell was the main cause for the EL enhancement in GaAs middle cell.
Comparison of inversion methods of complex refractive index for rough surface objects by polarization measurement
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Determining index of refraction of rough surface object is important for target identification and classification by polarization measurement, but it is difficult to directly acquire the imaginary part of complex refractive index. Many numerical inverse methods are proposed, which are Vimal-Milo method and improved Vimal-Milo inversion method. The first method is evaluated by P-G model, which is based on scalar bidirectional reflectance distribution function with a mode of “polarization ratio – angle correlation”. But this method is only applied in almost specular reflection and its defects are explicit in application. The second method named improved Vimal-Milo method is proposed based on the mode of ‘relative polarization component – angle correlation’. In this method, the inversion formula for the complex refraction index was semi-empirical deduced and global search algorithms such as genetic algorithm or Hooke – Jeeves search algorithm were used. Although two inversion methods are same based on P-G model, polarization measurement are different of which one is polarization ratio and the other is S1 component. The simulation results showed that (1) roughness parameters (correlation length and height standard variance of microfacet) have little effect to inversion of complex refraction index, and they must be calibrated accurately. (2) stokes values and reflection angle have smaller effect and they can be measured by polarimeter and angulometer. (3) incident angle is strongly correlated with complex refraction index and must be measured largely and precisely. (4) we must try to avoid poor specular reflection condition and improve degree of polarization.
Simultaneous temperature, H2O concentration and pressure measurement in a scramjet combustor with combined operation mode using wavelength modulation spectroscopy
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Wavelength Modulation Spectroscopy (WMS) was utilized to study the exhaust flow of a scramjet combustor with a combined operation mode in this work. 8 optical probes for WMS, 5 vertically with a spacing of 5cm and 3 horizontally with a spacing of 3cm, were mounted on the jet nozzle of the combustor, which was operated in 3 combustion stages, H2 combustion at first, kerosene combustion in oxygen-rich condition and fuel-rich condition, sequentially. The WMS system recorded the complete process of combustion, then simultaneously inferred the temperature, H2O concentration and pressure from each sensor. Measured results showed that the parameters of interest in all the 3 stages, during 5.4s combustion period, had relatively stable values. However, when the stage transited to the next, the combustion parameters leaped rapidly in 20ms. The WMS system had fast enough response speed and successfully caught the leaping. The 2D distributions of temperature and H2O concentration were also reconstructed, which illustrated that the combustion flow was not uniform in each stage.
Broadband programmable radio frequency signal generation based on ultrafast optical pulse shaping
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Broadband radio frequency signal generation based on ultrafast optical pulse shaping, which is a typical microwave photonic technology, has been performed. The ultrafast optical pulse starts from our home-made mode-locked laser which has a repetition rate of ~171 MHz and spans from 1524 nm to 1593 nm. Following the MLL is an optical amplifier which makes up for the loss of the photonic link. A commercial pulse shaper is introduced to flatten and shape the spectrum of the optical pulse. However, it results in a additionally frequency cutoff. Only the C-band (5 THz optical bandwidth) is used efficiently which results in waste of spectrum resources. Then, the shaped pulses travel through a 5km long optical fiber realizing frequency-to-time mapping. The RF signal is acquired from the optical intensity profile by a high-speed photodetector at last. Several different kinds of wideband RF signals are generalized from the same hardware system, such as trigonometric and linear frequency-chirped waveform from 2 GHz up to 3 GHz, These results may be of interest to the radar and communication systems with ultraband RF signals.
K-shell spectra of a laser-produced carbon plasma
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K-shell spectra of a laser-produced carbon plasma have been measured and analyzed. A graphite plate was irradiated by a 2.1ns ultraviolet laser pulse, and the radiated X-rays were measured with a flat-field grating spectrograph. The recorded lines have been identified as hydrogenic Lyman series 1s-np and heliumlike transitions 1s2 -lsnp (n=2-4). The measured wavelength and the spectral resolution of the flat-field grating spectrograph were calibrated by utilizing those lines. Hydrodynamic simulations and calculations of charge-state distributions in the local thermodynamic equilibrium (LTE) regime were carried out to interpret the K-shell spectra.
Impulse coupling characteristics of GAP, PTFE and aluminum with laser ablation
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Laser ablation propulsion and pulsed plasma propulsion are both valuable and effective space propulsion systems for microsatellite, which have the advantages of light weight, small structure, wide thrust range, high specific impulse, high control accuracy and so on. Combing the laser ablation propulsion and pulsed plasma propulsion to be a laserelectromagnetic hybrid pulsed plasma propulsion has been a novel system of propulsion, which has got great attentions around the world these years. It is an important research topic in laser propulsion field to find high performance ablative working material. Energetic polymers such as Glycidyl Azide Polymer (GAP) is an excellent working material, which exhibit excellent performance due to the release of chemical energy during the process of laser ablation. Polytetrafluoroethylene(PTFE, Teflon) is widely applied in the research of pulsed plasma thruster(PPT) because PTFE is easy to ionize under arc discharge and its chemical state is stable in both vacuum environment and normal environment. Aluminum has been proved to have great performance in laser ablation and laser-electromagnetic hybrid pulsed plasma propulsion. A laser ablation system was established to research the impulse coupling characteristics of GAP, PTFE and aluminum, all of which were used as propellant under the irradiation of different laser intensity. To evaluate the propulsion performance of the propellant, a torsion pendulum measuring system was established. Thrust and impulse coupling coefficient were obtained. The results show that GAP, PTFE and aluminum all got better thrust effect while the laser intensity increased. GAP got the best performance in thrust with the help of the release of chemical energy while aluminum did the worst. Impulse coupling characteristics of three working materials were the same trend that with the increase of laser intensity, impulse coupling characteristics were all rising rapidly at the very beginning but went down slowly when the laser intensity increased over some specific point respectively. GAP got the highest impulse coupling coefficient overall and aluminum got the lowest impulse coupling coefficient due to its high laser irradiation threshold. Specific impulse of aluminum were at the range from 3000s to 9500s while GAP and PTFE as polymer got the same performance at 600s~800s mostly. The ablation efficiency of three working materials differ a lot that GAP is around 120% in maximum and aluminum is around 60%. PTFE’s ablation efficiency is around 10%.
Scaling laws of optical components during laser induced thermal damage process
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In order to study the scaling laws of optical components, we set up a model based on the heat conduction theory and thermodynamic theory. Then the similarity theory was used to the model analyzation. Finally, we demonstrate three conclusions which are related to the practical engineering application. The first one is that thermal damage behaviors of different scale optical components are similar when the linear power density of irradiated laser are the same. In other words, we should use the linear power density to represent the resistance of damage tolerance for optical components The second one is the judgement standard of scram time. We find the scram time of large-aperture system is certain times as much as the scram time of small-aperture system. The third one is about how to design the scaled experiment can we make two different scale laser systems obey the similar thermal damage behaviors. This study is of great help for the damage prevention of the optical components.
Evaluation of pressurized shell damage induced by high energy laser irradiation
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In the paper, evaluation of pressurized shell damage induced by laser irradiation has been done by the numerical simulation method. Basing on the finite element method, a general process of simulation of pressurized shell damage induced by laser irradiation has been given. Then, basing on the distribution of parameters, a damage evaluation has been done for cylindrical shell of steel material by Latin hypercube sampling and Monte Carlo method. The results indicate that higher inner pressure could cause the shell damage more quickly; when inner pressure is low, the shell cannot burst but pressure-releasing failure.
A real-time measuring system for temperature and components of the detonation field
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Based on the Black-body radiation theory, a multi-wavelength photoelectric detection system for the on-line measurement of temperature and components of the detonation field is designed. In this system, the detonation spectrum is split by bifurcated optical fiber and filters, then, received by the photoelectric-detectors and convert to electrical signal. Finally, the temperature evolution process is derived through regression algorithm. Besides, concentrations of certain components can also be revealed through real-time detection of corresponding spectral lines. The spectral response coefficients and the whole system are calibrated with a standard tungsten lamp and a standard radiation source respectively. With this system, experiments are carried out, and, the variations of denotation temperature and concentrations of some key components are recorded.
Study on system status evaluation method
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To solve the problem of system-level thermal effect and condition assessment, the dynamic reliability assessment model of the structural and functional components is established. The continuous process simulation is combined with discrete event simulation. The temperature field, deformation field and stress field is calculated by continuous process simulation, and the reliability index, such as the health status is calculated by discrete event simulation. Meanwhile the efficient and collaborative algorithm of the model is studied. On the basis of the above analysis method, the multiple component system is established and the cycles directed energy boundary conditions is applied. The state of the system is obtained by simulation. The analysis results show that the health state of the system decreases faster with the increase of power density. When the power density increases from 2W/cm2 to 8W/cm2 , the decline rate of health state of the system increases by 1 times. This analysis method can provide a technical basis for the study of system irradiation effect.
A non-linear temperature calibration equation for filter selection in 2-D LIF thermometry approach
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Due to non-interruption of laser intensity and dye content, two-colour Laser Induced Fluorescence (LIF) ratio thermometry approach is widely used in the studies of fluid. Ratio of temperature sensitive dye Photo Luminescence (PL) intensity at two wave bands with different temperature sensitivity can efficiently remove interruption of laser intensity and dye content in time and space. To achieve high temperature sensitivity and Signal to Noise Ratio (SNR) in these technique, selection of two wave bands’ peak wavelengths and band widths should be carefully considered. In this work, influences of peak wavelengths, band widths and SNR to temperature sensitivity of this two-colour LIF ratio thermometry approach are discussed. Temperature property of a traditional temperature sensitive dye (rhodamine B) aqueous solution is studied in a wide temperature range from -10°C to 90°C by spectroscopic method. A non-linear fitting method based on Arrhenius equation is present to accurate describe rhodamine B PL intensity decay along with increasing of temperature, achieving significant improved fitting accuracy compared with traditional linear fitting model. Based on this non-linear fitting method, influences of filters’ center wavelengths and band widths to temperature sensitivity are analyzed. These results give very important information of filter’s selection to ensure sufficient temperature sensitivity and SNR in two-colour LIF ratio thermometry approach.
Pyrolysis gas diffusion model in kevlar/nomex honeycomb structure on laser irradiating
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Kevlar/nomex honeycomb sandwich structures are widely used by many apparatuses and vehicles in many domains. Since there are large quantities of epoxy resins in the structure, it is considerable to study the process that the structure is heated and produces pyrolysis gases which diffuse among the honeycomb. In this paper, the process of a laser beam irradiating a kevlar/nomex honeycomb sandwich is studied for building a mathematical model. The process is divided into two parts. One part focuses on the pyrolysis gas producing, the other one focuses on the gas diffusing among the honeycomb. The pyrolysis gas producing model is built according to experiment analysis, as a Boltzmann formula. The gas diffusion model is also built in the form of ODE equations. Validation experiment is carried out, demonstrating the model correct and accurate. Finally, the two models are combined together. By comparing with experiment, the laser irradiating and pyrolysis gas diffusing model is demonstrated to be appropriate to the case that kevlar laminas are bonded to the nomex honeycomb.
Numerical study of molten pool dynamics and thus bur formation in laser drilling of metals
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A 2D numerical model is developed to investigate the transient dynamics of molten pool and thus the bur formation mechanism in millisecond pulsed laser drilling process. The model features the utilization of the sharp interface method for accurate consideration of the complex boundary conditions on the hole wall and a comprehensive hydrodynamic calculation for both the gaseous and liquid phases. The model gives good prediction of the bur phenomena, and more importantly, provides detailed information regarding the multi-phase interaction and its effects on hole dynamics, also on the bur formation. It is shown that different substrate can cause different coupling effects between the vapor plume and molten pool and hence produce holes of different shapes. The model gives a basic study of the bur formation during the ablation process of different metals and shows a virtual technique for the visual of the movement of melt and vapor. And the obtained results show good qualitative correspondence with experimental data.
A method of mechanical properties temperature dependence test by two-sided laser irradiation heating
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For mechanical properties temperature dependence test in high temperature rising rate, a method of heating specimen using two-sided laser irradiation is proposed. Simulation about different materials in laser heating about different laser power density and different laser duration were conducted. The result shows that two-sided laser irradiation can obviously improve the temperature field of the specimen in normal laser irradiating. The simulation shows that this method is better for metal than composite materials because of the heating pyrolisys may have a bad influence on the thermal balance. Based on this method, the tensile strength temperature dependence of the T700 carbon fiber laminate was obtained. The result is close to the parameter obtained in electrical mechanical machine by furnace heating. The research can make a reference for the mechanical properties test of composite materials or metal in high temperature with high heating rate.
Supercontinuum generation in a random fiber laser structure
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A novel supercontinuum (SC) generation structure based on a random fiber laser is demonstrated in this paper. The effect of the passive fiber length for spectrum boarding is analyzed and discussed. A near-infrared SC with 20 dB bandwidth of more than 500 nm can be generated with 1 km length of passive fiber under 11.13 W pump power. The experimental results show that shorter passive fiber length is beneficial for the SC broadening to long wavelength region. The long wavelength edge of the output SC with 200 m and 1930 m passive fiber are 1680 nm and 1375 nm respectively when the pump power is 11.13W. The results proved that random fiber lasers can be a novel, simple, low cost, low coherence, and robust near-infrared SC generation source.
Progress on pulse-shaped laser driven ramp compression study
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Laser drive ramp compression is an important way to achieve extremely high pressure but relatively low temperature material state. This article introduces our progresses in recent years. The main progresses contain theoretical researches and experimental researches. In theoretical part, an analytical model for designing driving pressure pulse was set up, and a new characteristic method based on a Murnaghan-form state equation was developed. What’s more, X-ray preheating in the target and laser pulse design in laser-indirect-driven experiments were studied. In the experimental part, laser-direct driven experiments and laser laser-indirect driven experiments were performed on aluminum and iron, results showing the experimental design methods were feasible, and the compressions were in isentropic ways.
Numerical simulation of three laser-induced in-phase bubbles
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The potential applications of laser-induced bubbles are gradually revealed. And the performance of multiple - bubbles also attracted considerable attention. This paper is intended to study the complex interaction among three in-line arranged in-phase bubbles. This work illustrates the interactions of three bubbles oscillation and the shockwave motion after the recombination of the plasma nanosecond laser excited. The Volume of Fluid (VOF) model is used to numerically analyze the behaviour of same equilibrium size, same distance and in-phase multiple-bubbles, which enables tracking of the gas-liquid interface and the field. The numerical results indicate that, in the expanding stage, due to the pressure gradient, bubbles at both ends move towards the centre, while the bubble surfaces facing to other bubbles get flattening. The central bubble has longer oscillation period than the side bubbles. In the shrinking stage, the side bubbles get deformation faster, and then collapse with a jet or get adhere to the central bubble. The initial high-pressure bubble radiates a shockwave at the beginning of the expansion. The shock wave propagates outwards from the point source in a circular pattern, resulting in interference in the overlap of the two shock waves. A curved pressure wave, which is parallel to the in-line array, is produced by the interference of multiple shock waves. The pressure wave decays to an acoustic wave after a certain distance according to its initial intensity. In particular, a significant variable γ=D/(R+R) deserves studying in this progress.
Study on the influence of ultrafast laser power on the entrance morphology of vessel microfluidic chips
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The finite element software was adopted to make a simulation analysis of the temperature field changes of PMMA basis material at micro-channel entrance when the ultrafast laser was set at different powers. In addition, the ultra-field depth digital 3D microscope, scanning electronic microscope (herein referred to as SEM) and white light interferometer were also used to test and analyze the influence of ultrafast laser at different powers on the morphological and structural characteristics of the post-irradiation PMMA basis material.
MOPA-structure, SESAM-based mode-locked Er3+-doped ZBLAN fiber laser at 2.78 μm
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We reported the first erbium-doped ZBLAN fiber MOPA system centered at 2789 nm. The master oscillator was a passively mode-locked ZBLAN fiber laser based on a semiconductor saturable absorber mirror in a linear cavity. The pulse repetition rate was measured as 15.3 MHz with a signal-to-noise ratio of 62 dB, which indicating stable mode-locking operation. Then a one-stage Er3+-doped ZBLAN fiber amplifier was used to boost the average output power after a polarization dependent isolator. The maximum output power of 0.7 W was measured at the end of the amplifier with a slope efficiency of 25%. And the width of 3 dB spectrum was 4.5 nm from 2787.5 nm to 2792 nm.
Diode-end-pumped actively Q-switched YVO4 Raman laser at 2291nm
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This paper demonstrate an intracavity Raman laser at 2291 nm by using YVO4 crystal as Raman crystal to achieve efficient Raman conversion in a compact diode-end-pumped actively Q-switched Tm:YLF (Tm3+-doped lithium yttrium fluoride) laser with an L-shaped cavity. With a pump source of fiber-coupled continuous-wave diode laser operating at 793 nm, the Tm:YLF crystal served as gain material in the Raman laser. We made experimental research on the characteristics of first-Stokes Raman laser output the different pulse repetition rates (PRFs). The maximum average output power, pulse width and the corresponding peak power were 175.2 mW, 21.41 ns and 1.63 kW respectively at a pluse repetition frequency of 5 kHz and an incident pump power of 21W.
Numerical simulation of tensile deformation and failure of aluminium alloys exposed to laser heating
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A 3D mathematical model was established for the investigation of the thermomechanical behavior of aluminum alloys (Al-7075) under the combined action of tensile loading and laser irradiations. The transient temperature fields and stress-strain field was obtained by using the finite element method. The Johnson-Cook’s constitutive equation is implemented in the FE model to study the failure behaviour of alloy. The effects of various pre-loading and laser power densities on the failure time, temperature distribution and the deformation behavior of aluminum alloys are analyzed. The results indicate the significant reduction in failure time for higher laser power densities and for high preloading values, which implies that preloading may contribute a significant role in the failure of the material at elevated temperature. The numerical result agrees well with our previous experiment results, concluding that the numerical model is reasonable.
Study on the influence of Fs-laser power on the entrance morphology of vessel microfluidic chips
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The law of energy conservation and the heat conduction equation were used to study the influence characteristics of the fs-laser power density on the entrance morphology of vessel micro-channels. Meanwhile, PMMA basis material of fs-laser irradiation was studied in this research, and the theoretical analysis model was established to deduce the relational expression between laser power and the melting temperature of PMMA basis material. The finite element software was adopted to make a simulation analysis of the temperature field changes of PMMA basis material at micro-channel entrance when the fs-laser was set at different powers. In addition, the ultra-field depth digital 3D microscope, scanning electronic microscope (herein referred to as SEM) and white light interferometer were also used to test and analyze the influence of fs-laser at different powers on the morphological and structural characteristics of the post-irradiation PMMA basis material. Test results showed that: PMMA melting conditions could be satisfied but the thermal conditions could not be met at the laser power (1W) and the spot radius (1mm). The processed micro-channel entrance was round and good in surface quality.
Development of laser humidity sensor based on tunable diode laser absorption spectroscopy
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The sinusoidal root mean square (RMS) normalization method is proposed to measure humidity, and the ratio of the second harmonic peak-peak value and the sinusoidal RMS value is used as the system output, which can effectively eliminate the influence such as windows pollution and optical intensity fluctuation. The laser humidity sensor is developed, and reflective open gas cell is used. The structure of the optical system is simple, which is easy to be adjusted. Humidity calibration tests are carried out in the north china sea standard and metrology center of state oceanic administration, and the humidity measurement deviation is less than ±0.6% in the range of 30%RH ~ 80%RH. The comparison experiment was completed between the laser humidity sensor and the traditional humidity sensor in the national meteorological basic station. Experimental results show that the consistency of the humidity data is good, which proves the validity of sinusoidal RMS normalization method for measuring environmental humidity.
Numerical simulations of thermal lens effect in K9 glass induced by millisecond laser irradiation
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Non-uniform temperature field in optical transparent glasses induced by high power Gaussian laser irradiation would introduce temperature gradient which will cause thermal lens effects inside materials. Such thermal lens influences on long pulse laser processing are usually omitted in previous studies. However, thermally-induced refractive index change in situ could have a relatively large impact on subsequent incident laser especially under the action of high power laser. In order to obtain a quantitatively transient results of thermal lens effects caused by long pulse laser, we carried out numerical simulations of temperature rise, refractive index change and their influences on light trail in K9 glass heated by a millisecond pulse laser. Different laser energy and glass thickness were applied to reveal the dependence of thermal lens and laser and glass parameters. Results show that the thermal lens effects would increase refractive index in K9 glass and reconverge subsequent laser beam consequently whose amplitude is proportional to incident laser power and glass thickness. The simulation results are compared with the previous experimental results and are consistent in magnitude of refractive index change. But there is a certain gap in refractive index field distribution in the direction of laser radius due to the neglect of influence of refractive index raise to susequent lasers in temperature simulations. This work would provide a reference in the field of millisecond pulse laser processing optical transparent materials.
Optical fiber coupling of high-energy density pulsed lasers for laser ignition
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In order to improve coupling efficiency and maximal laser transmission energy density, lasers with Gaussian distribution and flat-top distribution were utilized to couple to optical fibers with different surface roughness. The coupling efficiency and damage threshold of the fibers were studied. The experiment results showed that for Gaussian distribution laser beam, optical fibers with higher degree of smoothness would get better coupling efficiency, but would suffer more damage inside the fiber. But for flat-top distribution laser beam, optical fibers with higher degree of smoothness would get both higher coupling energy and efficiency.
Optimization of erbium-doped fiber length based on variable gain coefficient in femtosecond pulse amplification system
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In order to solve the problem that the length of gain fiber is difficult to be determined, and implement the femtosecond pulse features of high average power and narrow pulse width in optical measurement, the effect of erbium-doped fiber length on the pulse features is studied. In the simulation analysis, combined the rate equation of two-level system with the nonlinear Schrodinger equation describing the ultrafast pulse propagation, the variation trend of average power and pulse width with erbium-doped fiber length is numerically studied. According to the two-level model, a variable gain coefficient curve with the fiber length is obtained and applied to ultrafast pulse propagation model by polynomial curvefitting method to improve the simulation accuracy. In the experiment, a 980nm pump source is used to build a singlestage forward amplification system, and the fiber truncation method is employed to verify the numerical simulation. The results show that the simulation is consistent with the experimental data. When the average power of femtosecond pulse signal light is 50mW and the pump light is 1000mW, the optimum fiber length is 55~100cm. Compared with traditional simulation method of using fixed gain coefficient, the introduction of variable gain coefficient has an impact on simulation parameters p and d, which present the gain effect and dispersion effect in transmission model.
Design of optical attenuator for high energy laser beam measurement system
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In this paper, we designed an optical attenuator based on optical scatting. Which consist of an optical scatting material and cylindrical attenuation structure. The diffusing characteristics of material and attenuation properties of the device have been simulated by ray-tracing, and the simulated results agree very well with the experimental results. The attenuator has been successfully used in high energy laser beam intensity profile measurement system.
Experimental study of target depolarization characteristics based on BRDF
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The diffusion reflection and depolarization of material surfaces is closely relevant to the physical properties of materials. To investigate the connection between BRDF (binomial reflection distribution function) and depolarized capability of the typical surfaces (e.g., aluminium, epoxy, and F4 board), a multi-angular measurement device is established to quantify these two parameters. With an incident laser beanm of 650 nm, a series of incident and detector view angles is measured and the results show the significant variance in the parameters for different targets. A coincidence is proved between the evolution of the two parameters, which is also valuable in modeling pBRDF (polarized BRDF).
Measurement and analysis of reflectivity accuracy of hyperspectral reflectometer
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The principle of self-developed hyperspectral reflectometer is introduced. The instrument has long-term automatic observation capability, and its spectral range covers the visible-short-wave infrared band. The functions of instrument include ground spectral reflectance observation, sky diffuse irradiance, total irradiance, diffuse total ratio measurement, atmospheric optical thickness measurement and long-term automatic observation. Among these the observation of the spectral reflectance of the ground is the core function of the instrument. According to the principle of ratiometric radiation measurement, the uncertainty of the reflectance measurement of the instrument depends on the uncertainty of irradiance calibration and radiance calibration. In order to verify the measurement accuracy of the instrument, the uncertainty of the instrument irradiance calibration and radiance calibration is quantitatively analyzed, and the uncertainty of the hyperspectral field reflectometer is calculated to be less than 1.74%, which meets the technical index requirements of less than 2%.
The measurement metric for power in the bucket
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Power in the Bucket (PIB) is a key index to describe the quality of Laser. Actually it is hard to precisely measure it. In this work, the numerical simulation on ideal Gauss beam and flat beam with different apertures were carried out. The results show that the problem can be overcomed by guaranteeing the transmission of the aperture. And 97% is a suggested value according to the analysis.
Nonlinear energy deposition of femtosecond laser filamentation in the laser-induced snowfall formation
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Characteristics of energy deposition during filamentation are numerically investigated to understand the formation of different mass of snow with femtosecond laser pulses in a cloud chamber. The results show that the evolution of maximum airflow velocity and mass of snow is closely related to the amount of deposited energy. For the same input energy, the deposited energy is higher for laser pulse externally focused by an f=30 cm lens than an f=50 cm lens, which is in accordance with the higher airflow velocity and mass of snow in experiments with the former lens.
Laser ablation characteristics of liquid energetic polymers with micro flow feed system
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Laser ablation micro propulsion is a novel space propulsion scheme with minimum impulse bit, high specific impulse, less pollution, simple constructer of thruster and less weight compared to other micro propulsion, with great potential application on precise attitude control and orbit control of spacecraft and formation flying of micro satellites. Because of the splashing of the liquid working fluid drastically reducing the efficiency of laser ablation, it is essential to feed the liquid by micro flow approximately 50μg per pulse. Currently the main difficulty is the micro flow feed and ablation of the liquid energetic polymers. A micro flow system, utilizing piezoelectric valve controlled by pulse signal and piston cylinder, was designed and set up to satisfy the requirement of the continuous ignition and ablation of the liquid GAP. The dynamic characteristics of ablation, impulse, plume and splashing were observed and analyzed. The ablation behavior and splashing is detected by shadowgraphy experiments and the impulse bit is measured by torsional pendulum in vacuum. The experiment demonstrates the stability of the feed system, the jam status of the jet after the laser ablation, and the feasibility of the laser ablation propulsion using liquid energetic polymers in engineering application.
1kHz, 532nm top-hat intensity profile high peak power picosecond laser amplification system
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A single pulse energy 6.77mJ, repetition rate of 1kHz, pulse width of 11ps at 532nm wavelength with the near top-hat intensity profile in the near filed picosecond laser amplification system is realized using a semiconductor laser side pumped Nd:YAG crystal. The seed pulse is generated in a home-built Nd:YVO4 oscillator, pumped with a 808 nm CW diode laser. The oscillator provides 2.8nJ, 11ps pulse width, 86 MHz repetition rate at 1064 nm wavelength. Pulse from the Nd:YVO4 oscillator is first amplified to 1.5mJ by a diode side-pump Nd:YAG regenerative amplifier. Then the pulse, increased in size by a negative lens, sequentially passed through a circular aperture and a spatial filter-image relaying system to produce a top-hat intensity profile in the modules, and an 8th order super-Gaussian beam is obtained, and total transmission of beam shaping set-up is about 30%. The beam, passed through a double-pass preamplifier of single rod and a double-pass main amplifier of single rod, is amplified up to 17.3mJ, corresponding peak power is 1.57 GW. A 4F relay-imaging system is used in the amplification stages to preserve the top-hat intensity profile and compensate the thermally induced birefringence of Nd:YAG rod. The amplified output beam leaving the double-pass Nd:YAG module is decreased in size and imaged on a 5×5×13 m^3 second-harmonic generation (SHG) crystal-LBO by a 4F relay-imaging system, finally a 532nm approximate top-hat intensity profile in the near filed, which single pulse energy is 6.77mJ, is obtained after doublefrequency. The second-harmonic generation efficiency is over 51%.
Temperature-dependent absorptivity evolution of gray epoxy painted iron under CW laser irradiation
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The energy deposition efficiency of high energy laser irradiation on painted metal was studied. Experiments of uniform 1 070 nm continuous wave (CW) laser beams irradiating an entire 10 mm quadrate surface of gray epoxy painted iron were performed to investigate the laser-heating-induced changes of the surface absorption behavior. Laser power densities were set as 54-233 W/cm2 and temperature rising processes of the painted iron specimens were monitored. By means of heat transfer inversion calculation, the absorptivity data during laser irradiation were obtained, of which the good accuracy was further proven by optical measurements. It is found that the absorptivity showed strong temperature-dependence properties in spite of the different laser power densities. During temperature increasing from 20 to 1 300°C, the temperature-dependent absorptivity experienced four stages, i.e. slowly decline, rapidly decline, slowly increase, and rapidly increase.The visualized color of gray coating layer changing to white at 510°C and physical broken at 1 050°C were the main reasons for absorptivity rapid changes.
Emission spectroscopy characterization of plasma flow in inductively coupled plasma spheroidization
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Gas temperature are crucial parameters for inductively coupled plasma spheroidization. Spatial resolved temperature measurement of the high-temperature flow field in the plasma reactor provide quantitatively basis and evidence for theoretical study of plasma spheroidization and industrial methodology optimization. It leads to research gap in flow diagnostics for high-temperature inductively plasma flow owing to the inapplicability of conventional diagnostic techniques. This paper presents in-situ and non-intrusive diagnostics for argon plasma flow in the inductively coupled plasma spheroidization based on optical emission spectroscopy and electric scanning technique. Two-dimensional spatial temperature in the radial direction was measured at a cross section under the powder feed gun. Gas temperature in the center was 10120 K±240K, while the maximum temperature zone was positioned close to the core with specific values of 10500 K±240K and 10620 K±240K, respectively
Piecewise linear calibration of the spectral responsivity of FTIR based on the high temperature blackbody
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The spectral responsivity of Fourier Transform Infrared Spectrometer (FTIR) measurement system of high temperature blackbody infrared radiation characteristics is calibrated via ThermoGage HT9500 high temperature reference blackbody furnace from National Institute of Metrology, China (NIM). A calculation model of the spectral responsivity calibration of FTIR measurement system is established. The infrared spectrum of the blackbody radiation source is measured in the temperature range from 1273 K and 1973 K on the wavelength range from 1 μm to 14 μm. Calibration is carried out within the temperature range from 1373 K to 1873 K on the wavelength range between 1 μm and 13 μm. The infrared spectral radiation characteristics of ThermoGage HT9500 high temperature reference blackbody furnace are represented. The results indicated that the method of piecewise linear calibration was practicable. The measured infrared spectrum in the temperature range from 1373 K to 1873 K on the wavelength range between 1 μm and 13 μm was compared with the calculation which showed the signal divergence less than 1%. And the calculated temperature obtained by inverse calculation in this temperature region was compared with the actual temperature which showed the temperature divergence less than 0.45%.
Irradiation effect on Ag-dispersed amorphous silicon thin films by femtosecond laser
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The structural and optical properties of amorphous silicon (a-Si) and Ag-dispersed amorphous silicon (a-Si:Ag) thin films irradiated by femtosecond (fs) laser at various energy densities are investigated comparatively in this article. It is found that at a lower energy density of 100 mg/cm2 , the film microstructure evolves from a completely amorphous phase to an intermediate one containing both amorphous and polycrystalline silicon. During laser irradiation, the formation of nanocrystals in a-Si films begins at lower energy density, but the existing Ag nanoparticles inhibits somehow the crystallization of a-Si in a-Si:Ag films at the same energy density. As the energy density is increased to a moderate value of 200 mj/cm2 , the surface of a-Si:Ag films featuring a vertically aligned pillar-shaped structure is emerging. Both the crystallinity and the root mean square of surface roughness exhibit a monotonic increase with the increase of energy density. The Ag nanoparticles are dispersed uniformly in a silicon matrix, resulting in a resonant light absorption due to so-called localized surface plasmon. The localized surface plasmon resonance (LSPR) wavelengthes of the irradiated aSi:Ag films are increased significantly from 600 nm to about 820 nm, and the bandwidth of the measured absorptance is enhanced in the range of 600~1600 nm. The nanocrystallization mechanism, the formation of pillar-shaped structures and the light absorption enhancement are explained regarding the high electron density and the plasma-surface interactions.
Room temperature Fe2+:ZnSe laser pumped by non-chain pulsed HF laser
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An experimental setup of mid-infrared Fe:ZnSe laser operating at room temperature has been established, which was end-pumped by a non-chain pulsed HF laser. The temperature has significant influence on the level lifetime of Fe:ZnSe laser. As the crystal temperature changes from 85 to 295 K, the level lifetime of Fe ions changes from 57 to 0.35 μs, it is important for matching the pump pulse width and the level lifetime. Electronic excitation HF laser with short pulsed width is a good pump source for Fe2+:ZnSe laser at room temperature. For the Fe2+:ZnSe crystal with size of 20×20mm, when the pumping spot diameters is lower than 9.2mm, the phenomenon of transversal parasitic oscillation could been suppressed effectively. At room temperature, the output energy of Fe2+:ZnSe reaches 294mJ, the slope efficiency is about 36%, and the optical to optical efficiency respecting to the pump energy is 34%.
Study on stimulated Raman scattering in fiber amplifier employing tapered Yb3+ doped fiber
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Tapered fiber is expected to effectively suppress nonlinear effects while ensuring excellent beam quality, and may achieve higher power output by breaking through the power limit of ordinary uniform size fiber. Base on the rate equations and the proposed three types of tapered fibers, a nonlinear theoretical model including spectrum, power and stimulated Raman scattering (SRS) is established. Using this model, the SRS characteristics of the three tapered fiber amplifiers are numerically simulated in the condition of keeping same total absorption coefficient of different fibers guaranteed. The simulation results show that the type I fiber has the worst suppression effect on SRS, and the convex type III fiber has the best suppression effect on SRS. When using the backward pumping method, the convex type II fiber can reach almost consistent suppression effect of the convex type III fiber. Theoretical studies show that the use of tapered ytterbium-doped fiber to build fiber amplifiers has a certain suppression effect on SRS in fiber amplifiers, and provides a reference for the selection of fibers in high-power fiber amplifiers.
Study on cross-sections between the fine-structure levels of atomic rubidium
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The diode-pumped alkali laser (DPAL) is a new type of high-powered laser sources which has been paid much attention in recent years. The fluorescent spectra can be used to investigate how the collisions between atomic rubidium and various buffer gases are affected when a sealed rubidium vapor cell is pumped by a LD. In this study, the cross sections between the fine-structure levels of atomic rubidium in a vapor cell were first theoretically deduced by using a gas kinetic procedure. And then, the sensitized fluorescence was experimentally obtained by means of a series of spectral measurements. Finally, the influence of the temperature on the cross section between the fine -structure levels of atomic rubidium was studied with the systematical analyses. The results are thought to be helpful for deeply understanding the theoretical characteristics of a DPAL at the atomic physics level.
High-stable Er-doped all-fiber pulse laser based on free standing black phosphorus
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A free-standing black phosphorus saturable absorber (BP-SA) fabricated by a modified electrochemical delamination strategy exfoliation process was inserted inside a Er-doped ring laser cavity. Based on the saturable absorber of BP, a stable pulse laser could be achieved. When the pump power increased to 30 mW, the pulse laser began to initiate. The maximum output power was 4.8 mW. And the repetition rate could vary from 23.24 kHz to 69.65 kHz. The obtained minimum pulse duration is 1.67 μs. These demonstrations indicate that BP could apply in pulse laser field and benefit industrial community in the future.
500 Hz, 8 mJ Nd:YAG regenerative amplifier for laser-induced plasma
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A high-energy quasi-continuous-wave (QCW) laser diode-pumped regenerative amplifier was demonstrated for using as a radiation source of laser-induced plasma. The seed source was an all-fiber amplifier, provided pulse width of 454 ps and single pulse energy of 7.7 nJ at a repetition rate of 24.17 MHz and a central wavelength of 1063.9 nm. The solid-state regenerative amplifier used a Nd:YAG crystal was side-pumped by QCW diode bars. With this system, high stability and high energy was generated at wavelength of 1064.1 nm, with pulse width of 392.1 ps and output average power was 4.04 W. The single pulse energy and peak power was 8.08 mJ and 20.6 MW, respectively. The M2 factor was about 1.48. The laser system will use as a picosecond radiation source for the following laser ablation and laser-induced plasma spectrum analysis. In the previous work, the process of laser-induced plasma was simulated by fluid dynamics. The temporal and spatial distribution of electron density and temperature was successfully simulated, corresponding the process of laser irradiation on target were recorded for set of materials (Si, Al, Cu). Finally, the mechanism and evolution process of the picosecond laser irradiation target were obtained.
Investigations of mode instability in large-mode-area fiber amplifier pumped by 915/976nm LD sources
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Detailed experimental investigations are presented on investigating mode instability and stimulated Raman scattering in 30μm core diameter fiber amplifier by varying the pumping light wavelength using 915/976nm LD sources. Results reveal that for 976nm pumping light, the highest power can reach to 3570W; and for 915nm pumping light, the highest power can reach to 3771W. Physical characteristics of mode instability do not perform in a traditional form both in output power and time domain traces, such as roll-over point in output power curve. However, the beam quality of output laser clearly reveals there are some more high-order modes which have an effect in mode instability characters. Compared to 976nm LD pumping source, laser of amplifier using 915nm LD pumping source shows stronger non-linear effect. When the output power is 3512W, the Raman light is 13dB below the signal light. By shortening the passive fiber before the endcap, when the output power is 3520W, the Raman light is 20dB.
Construction of beam quality factor prediction model based on support vector machine
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The effectiveness of the laser can be evaluated by the beam quality factor, the beam quality factor is related to the initial parameters of the outgoing laser and various atmospheric parameters on the transmission path. Therefore, the prediction of the beam quality factor can be regarded as a statistical identification problem. Support vector machine has unique advantages in dealing with small sample, nonlinear and high dimensional problems, and can be used to deal with such statistical identification problems. This paper simulates the transmission process of Gaussian laser in the marine atmosphere based on the numerical simulation model of multi-layer phase screen, obtaining simulation data of initial laser radius, initial power, transmission distance, refractive index structure constant, atmospheric visibility, and spot radius, beam drift, and energy circle rate at the receiving surface; then constructing the prediction models of these beam quality factors by support vector machine, the optimal parameter model is obtained by adjusting the kernel function of the model, the insensitive loss coefficient and the penalty factor; finally, the prediction error and accuracy of the model are analyzed. The research results show that the support vector machine can fit the multiple regression relationship between input and output well, and the prediction accuracy of the model is high. The research results can provide a feasible basis for the application of support vector machine in the evaluation of the effectiveness of the laser in the marine atmosphere.
Research on ultrasonic damage imaging based on inverse scattering model in frequency domain
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Total Focusing Method (TFM), as a kind of post-processing imaging method, has attracted researchers’ attention due to its better resolution and high signal-to-noise ratio (SNR) in comparison to traditional imaging techniques. However, without analyzing the properties of damage scattering, the TFM algorithm fails to realize the quantitative evaluation of the damage. On the purpose of improving imaging sensitivity and SNR, an ultrasonic scattering model is developed which takes into account the interaction between the incident ultrasonic fields and the damage, then the reflectivity of the damage surface can be obtained. Finally, the imaging of the reflectivity of the damage is formed by using this inverse scattering model in frequency domain. Because of the advantages of non-contact, non-destructive and couplant free, laser-generated ultrasound is used as an excitation method in the model. In this paper, the finite element models of ultrasonic propagation in damaged structures are carried out. The damage types are circular holes and cracks of different sizes. The simulation results show that the TFM algorithm combined with the inverse scattering model can locate the damages accurately, and the size as well as the orientation of the cracks can also be identified quantitatively. The proposed model obviously enhances the image sensitivity and SNR, which proves its ability of small damage location and characterization.
Laser polarization characteristics from phase-shifted grating inscribed by polarized ultraviolet Argon laser
Haifeng Qi,
Zhenqiang Li,
Zhiqiang Song,
et al.
Show abstract
Phase-shifted grating based distributed feedback fiber lasers with different polarization characteristics were made with Ppolarized and S-polarized 244nm Argon ion laser separately. In this paper, the laser polarization characteristics from the phase-shifted gratings were investigated experimentally. The narrow linewidth laser emitted from phase-shifted grating in Erbium doped fiber showed different polarization and wavelength characteristics for incident laser in different polarization states and in different doses. By high resolution spectrum analysis, the laser always had two close wavelengths with a gap of several pm which were in two orthogonal polarization states and had similar powers, if the phase-shifted grating was made with a P-polarized (which is paralleled to the the axial fiber) incident laser. The laser showed evidently different characteristics when the incident laser is in S-polarization (which is perpendicular to the axial fiber). Here, the laser operated in single wavelength mode when irradiated dose was in a range. However, the laser operated in dual-wavelength mode and dual polarization states when irradiated dose exceeded a threshold. The wavelength gap increased from several pm to more than 10 pm with the increasing irradiated dose. The power ratio of the two wavelengths changed simultaneously.
Visualization of supersonic ethylene jet flames in a hot coflow by OH-PLIF
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Planar laser induced fluorescence (PLIF) is a powerful tool to visualize the flame structure, especially for the turbulent flame. In this paper, we employ OH-PLIF technique to analyze the structure of a supersonic ethylene jet flame on a turbulent burner. This burner consists of a central jet and hot coflow. The Mach numbers of the jet vary from 1.0 to 1.6, corresponding to Reynolds numbers ranging from 40893 to 65455. The flame structures are imaged by OH-PLIF measurement. The measurement results reveal that the OH concentrations decrease with the increase of jet velocity or decrease of the O2 fraction. And the extinction and re-ignition of flame take place when the jet velocity is high or the O2 fraction is small. These measurement results help to understand the interaction between flame and highly turbulent flow.
Absolute optical frequency measurement of stabilized lasers using an Er-doped fiber femtosecond laser comb
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As a bridge connecting microwave frequency and optical frequency, the femtosecond optical frequency comb plays an important role in absolute optical frequency measurement. Compared with the traditional Ti:sapphire femtosecond optical frequency comb, with the advantages of compact structure, strong anti-interference ability and low cost, the fiber femtosecond optical frequency comb has a tendency to replace Ti:sapphire femtosecond optical frequency comb in some applications. Especially, due to the spectrum can be extended to visible light, fiber femtosecond optical frequency comb has a wider application prospect in the field of absolute optical frequency measurement. A frequency measurement system is set up based on an Er-doped femtosecond fiber comb. A hydrogen clock is used as a frequency standard, the optical frequency comb is traced to the hydrogen clock. Then an absolute frequency measurement of an acetylene-stabilized laser is realized by using this highly stable optical frequency comb. In addition, a narrow spectrum with a central wavelength of 633nm is achieved by Raman shifts and frequency doubling. The frequency and stability of the 633nm wavelength secondary standard are measured by beating
Design of laser collimating and focusing system under small working distance
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Laser micro thruster based on semiconductor laser has great advantages in volume. However, the design of optical collimating and focusing system is very challenging under highly integrated conditions. The operating distance from the laser exit to the working ablation target needs to be tens of millimeters. Based on the ray tracing method, a convenient and fast design method of the optical collimating and focusing system is proposed under the small working distance. The method is verified by an example. The design conditions are as follows: laser wavelength is 940 nm, fiber core diameter is 105 microns, and the numerical aperture is 0.22; focusing spot diameter is 50 microns; fiber head and ablation surface distance is not more than 20 mm; only one lens is used. Using the method proposed in this paper, the final design results are as follows: doubly convex lens R1 = 5mm, R2 = 25mm~30mm, lens aperture D = 6mm, central thickness TC = 2mm. According to the energy simulation results, the method proposed in this paper is effective.
Safe criterions for front-surface digs in fused silica optics
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A kind of defects on the incident surfaces of fused silica optics are reported having the potential to initiate the damages on the exit surfaces in the final optical assembly in high power lasers. In this light, the new safe criterions for defects on the incident surfaces are proposed to avoid the detrimental modulation effects in downstream.
Modal decomposition for optical fibers based on the Wigner representation of modal field
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In this paper, a novel Modal decomposition (MD) method based on the Wigner representation of fiber modal field is developed. An important distinction between this work and some others is that it can allow either fully or partially spatial coherent modal contents to be decomposed into guided modes. When the Wigner representation is reconstructed, based on the orthogonal property of guided modes and Moyal identity rule, the modal weights and possible relative phases can be obtained in an exact way. Further, the validity and reliability of the method are demonstrated with the numerical simulations.
The influence of beam energy profile on the flight behavior of a laser-driven flyer
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Various target samples were prepared by adhering 20μm thick Al foils to sapphire substrates. 0.2mm thick steel barrels with Φ0.6mm and Φ0.8mm diameter were attached to the surface of Al foils. A diffractive beam homogenizer was used to modify the laser pulse emitted from a Q-switched Nd:YAG laser. The unmodified beams presented a multi-ringed nonuniform energy profile, while numerous small energy spikes randomly distributed over the homogenized laser beam. The unmodified and homogenized beams at 80mJ were used to ablate the target samples, and the flight trajectories of Al flyers were visualized by a time-resolved shadowgraph technique. The images showed that more debris and fragments were formed at the radiation of the homogenized laser pulse, while a reverse results were obtained when barrels were employed. Moreover, the laser induced crater silhouettes presented differently in sizes and morphologies resulting from the change in the energy profiles pre- and post-homogenization. The flyer velocities were ~1300m/s regardless of the barrel parameters for unmodified laser ablation. While for homogenized lasers, the flyer velocity was up to ~1500m/s for free surface foils, and much lower velocities (~700m/s and ~1000m/s) were observed for Al foils covered with Φ0.6mm and Φ0.8mm barrel.
Influence factors of thermal damage of solar cells irradiated by CW laser
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The effect of single junction GaAs solar cells irradiated by 808nm, 1070nm and 10.6um CW lasers is investigated. The results show that, as long as under the same laser coupling intensity, the damage modes of solar cells under different irradiation conditions are similar. With the increase of laser coupling intensity, the maximum temperature of solar cells rises, and the maximum power of solar cells shows a ‘stair-step’ decline. The multiple irradiation experiments of triple junction GaAs solar cells by 1070nm CW laser are carried out. The results show that when the laser intensity is more than 12.8W/cm2 , the performance degradation of solar cells will show a significant accumulation effect. In addition, the thermal sensitive damage factors are explored and verified. The results show that the maximum temperature and the duration of high temperature are sensitive factors for laser irradiation damage of solar cells.
Performance jump characteristics of CCD image sensor under laser irradiation
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In this paper, a series of effects of CCD interaction with laser are taken together in consideration. These effects divide the light intensity axis into three sections named respectively as ‘normal’, ‘dazzle’ and ‘damage’, along the positive direction. For the effects on first two sections, a general model is proposed to describe them, which reflect the performance jump charateristics of CCD under laser irradiation. In fact, the model contains the jump functions of three performance parameters, which are response efficiency, charge transfer inefficiency and leakage current. Thereinto, the first is used to describe the pixel itself, and the remaining two are used to describe the influence between pixels. The leakage current parameters include a variety of situations, such as the leakage current between pixels, the leakage current between channels and even the leakage current between subarrays in a large array. When all three kinds of parameters don’t jump, the CCD works normal. When anyone of them jumps, the CCD is dazzled by light. Of course, the parameter jump in a dazzled CCD can return to normal when light intensity decreases. However, the damage section on light intensity axis is temporarily not described in this paper. After all, the damaged CCD is not a CCD again.
Analytic error model of beam profile evaluation for detector array method
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In this paper, an analytic model of beam profile error evaluation for detector array method is advanced. The model based on Gaussian beam distribution considering spatial sampling frequency, the non-uniformity of detectors and the high frequency components of integral laser spot. Finally, the analytic error model of laser beam profile evaluation was obtained by derivation and calculation. The model is adapted to calculate the integral energy, the beam centroid, and beam size of Gaussian approximation beam and can be extended to monotonous distribution beam.
Microchannels on aluminosilicate glass fabricated by selective laser etching
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We demonstrate a technique called “In volume Selective Laser Etching” (ISLE) to combine femtosecond laser modification technique with chemical etching for the fabrication of microchannels in the aluminosilicate glass. The influence of laser power on the length-diameter ratio of microchannels is studied. We fabricated tapered aluminosilicate glass microchannels with diameters from 30 to 50 μm. We used the same method to modify surface of aluminosilicate glass, and obtained taper-free microgrooves with a width of 50 μm. The microgrooves had no length limitation. In industrial production, the surface roughness was also improved in this way.
Laser ablation properties of glass fiber reinforced epoxy composite under obliquely impinging air jet
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The purpose of this investigation is to study laser ablation properties of glass fiber composite under obliquely impinging air jet. The continue wave near infrared laser ablation of fiber-reinforced composite materials under air jet with different velocity and inclination was studied experimental. The laser intensity is uniform distribution and the weight of sample was measured before and after experiment. Effective ablation heat was defined as the ratio of laser energy to weight loss. The experimental results showed that the process of the glass fiber reinforced epoxy composite ablation under laser and air jet can be divided into two stages. The first stage is mainly thermal ablation. The effective ablation heat increased as the inclination angle increased under same laser intensity. The effective ablation heat decreased with laser irradiation duration under same inclination angel and laser intensity. the second stage is mainly mechanical damage. the remain materials was broken and removed by air jet impingement, thus the effective ablation heat decreased evidently.
3D measurements of swirling flame heat release rate based on CH chemiluminescence
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Heat release rate of the swirl flames is an important parameter for the gas turbine state. It plays an essential role in the study of combustion characteristics, combustion efficiency and chamber protection. Several swirling CH4/air diffusion flames were investigated in a gas turbine model combustor via the spatial flame mode transition. In the combustion mode transition, we utilized three-dimensional computed tomography of chemiluminescence (3D-CTC) technique due to the complexity of swirling combustion flow field. The 3D emissions of CH* were measured and taken as qualitative indicators of the heat release rate under three Reynolds number conditions. This 3D measurement method utilizes 8 multi-directional CH* images as inputs combined with tomographic algorithms to compute the 3D distribution of CH* intensities. In this study, the transitions of heat release area with Reynolds number were analyzed, and the results show that the heat release rate changing more obviously along the nozzle radial direction than the axis direction, and the largest heat release area moves forward significantly.
Beam optimization for tomographic absorption spectroscopy with given probe locations
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Tomographic absorption spectroscopy (TAS) is a promising combustion diagnosis technique. It can simultaneously provide 2D temperature and concentration of the flow field. Some engine combustors, such as scramjet combustors and internal-combustion engines, only allow limited optical access. Only a few beams (ca. 20-40) are available to measure the combustor and beam layout becomes a key factor affecting TAS accuracy. In engineering application, probes are embedded in the wall of the combustor for laser propagation. The probe size should be taken into account when designing beam layout. In this paper, we considered the probe locations into beam optimization. The math formulation of the problem and the solving algorithm (genetic simulated annealing algorithm) is developed.
Interaction of pulsed laser plasma with normal shock In a shock tube
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With the prominent advantage of high peak power density and easy to break down air to form plasma, nanosecond pulsed laser has an important application value in reducing supersonic wave drag. To reveal the mechanisms of drag reduction by nanosecond pulsed laser, bow shock is simply considered as a normal shock in this paper. The phenomenon of the interaction of laser plasma and normal shock is studied by experiment. A high resolved schlieren system is applied to reveal the complex wave structures. A high speed PIV system is built to measure the velocity and vorticity. Time resolution of the PIV system reaches up to 500ns. Firstly, a nanosecond pulsed laser is focused in the experiment section of the shock tube to form laser plasma. Secondly, laser plasma is impacted by a normal shock in the shock tube. The flow features and evolution of the laser plasma during the interaction are revealed. Research results show that: Under the impact of the normal shock, the high temperature and low density region evolves into an upper and lower symmetric double vortex ring structure, and the size of them increases with the laser energy. The entrainment and contra-flow of the vortex can cause the reduction of the surface pressure of the aircraft, and reform the shape of the bow shock. It is the key flow structure that causes the drag reduction of supersonic vehicle.
Laser beam filamentation and filamentation instability in ultrarelativistic laser-plasma interaction: influence of relativistic effect and plasma nonuniformity
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In this work, influence of intense relativistic influence and plasma nonuniformity on ultrarelativistic laser beam filamentation and filamentation instability has been investigated in homogeneous and inhomogeneous plasma. The results are shown that, in ultrarelativistic laser-plasma interaction, relativistic effect plays a crucial role on beam filamentation and self-focusing, and plasma inhomogeneity further increases beam filamentation, while it also strengthens filamentation instability. To avoid filamentation instability, the research results show that it should try to reduce plasma inhomogeneity in laser-plasma interaction.
Numerical simulation of thermal and mechanical damage in CCD detector induced by laser
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CCD is a charge-coupled device, which is a new type of solid-state imaging device. It is an analog integrated circuit chip developed on the basis of large-scale silicon integration process. In the process of photoelectric countermeasures, the CCD detector is the core device of optoelectronic equipment. Due to its sensitivity to light, and the focusing effect of the optical system, the CCD is easily damaged by the laser interference, causing the system to be paralyzed and huge losses. So studies on the damage mechanism of laser to CCD detector and characteristics of different laser damage to CCD become the key technology of CCD protection. The temperature rise of the CCD detector after laser irradiation leads to thermal saturation of the device, changes in internal microstructure, thermal strain and thermal stress, and thermal and mechanical effects are important causes of damage. The CCD detector mainly comprises a multi-layer material such as a Si substrate and a SiO2 oxide layer light-shielding film. When the laser irradiates the surface of the detector, since the SiO2 layer is thin, the laser energy is hardly absorbed, and the laser energy is completely absorbed by the Si substrate material. The laser irradiation on CCD detector can be simplified as the thermal action of the laser on the Si material. Using finite element analysis, combined with the structural characteristics and heat transfer theory of CCD detector, the theoretical model of laser irradiated CCD detector is established. The temperature and stress of the detector after laser irradiation are numerically analyzed, and the Si base of the detector is calculated. The temperature and thermal stress distribution at the interface between the bottom layer and the SiO2 oxide layer and the intersection of the two are discussed. The damage mechanism of the laser irradiation detector is discussed, and the continuous laser and the repetitive pulse laser pair detection are compared under the same average power density. The calculation results show that under the condition of constant average power density, detector irradiated by the repetitive pulsed laser has higher temperature and higher stress; the thermal stress of the SiO2 oxide layer is larger, and the opaque aluminum film layer and the SiO2 layer may separate, while the Si material mainly suffered compressive stress and the stress value is small, and the possibility of damage is small.
Theoretical simulation of laser-supported absorption wave velocity induced by millisecond pulsed laser on aluminum alloy
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In this paper, we established a two-dimensional spatial axisymmetric finite element model to simulate the laser-supported absorption wave(LSAW) induced millisecond pulsed laser on aluminum alloy, obtained the relationship among velocity of LSAW, laser energy density and pulse width. And the finite element analysis software, COMSOL Multiphysics, was utilazed in the research. we simulated the generation and propagation procedure of LSAW based on hydrodynamic theory. All the important physical process were considered in the model which were inverse bremsstrahlung, thermal radiation, heat conduction and thermal convection. We simulated aluminum alloy irradiated by long pulse laser with different energy densities and pulse widths, the results showed that the time when the velocity reached maximum was increased with the growth of laser energy density, after laser irradiation, the velocity of LASW decreased immediately to zero, and the velocity of LSAW become slower by increasing the pulse width while the laser energy density was unchanged, moreover, the velocity of LSAW increased by increasing the laser energy density while the pulse width was unchanged. The results of the study can be applied in the laser propulsion and laser enhancement technology, etc.
High efficient terahertz generation from cryogenic gallium phosphide based on collinear difference frequency
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In this paper, a high efficient terahertz source based on n-type gallium phosphide crystal via cryogenic process is investigated through collinear difference frequency generation pumper by 1064 nm Nd:YAG laser and its OPO system. Absorption coefficient of this crystal at THz range shows a dramatic decrease from ~ 50 cm-1 to 0.5 cm--1 as the temperature decreases from 300 k to 80 k. Four times enhancement of the terahertz emission power and much more broad spectra range (~ 0.2- 3.8 THz) has been achieved in this kind of 0.5 mm length gallium phosphide crystal during the whole varied temperature difference frequency generation from 300 k to 80 k. These results indicate that cooling down the crystal temperature is an effective way to improve the terahertz source property, such as terahertz output power and frequency range.
Efficient phase-locking of a tiled fiber array of 37 fiber lasers using SPGD
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We present the laboratory experiments of phase locking of a 37-channel tiled fiber array using a stochastic parallel gradient descent (SPGD) feedback controller. The experimental setup comprises a hexagonally close-packed array of thirty-seven 23-mm-diameter fiber collimator sub-apertures. The fraction of the output laser power within a solid angle of 1.22λ/D was measured in open loop state and then in closed loop state. The results show that the power in the bucket was increased from 0.028 in open loop to 0.890 in closed loop. When the 37 beams were phase-locked, the residual phase error was λ/23. The power ratio in the bucket (PIB) was 28% that improved 32 times than the open loop state.
Diode laser absorption tomography for swirl flames application
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Tunable Diode Laser Absorption Spectroscopy (TDLAS) has been one of the most powerful techniques for combustion diagnostics in different kinds of burners. Combined with Hyperspectroscopy Tomography (HT), TDLAS can improve its spatial solution. This study reports a TDLAS-tomography system and its application in a swirl burner. The diagnostics system composed of sixteen beams (13X13, 13 parallel beams and 13 vertical beams). Four water vapor absorption lines, 7185.6 cm-1, 7444.3 cm-1, 7466.3 cm-1, and 6807.8 cm-1, were utilized in each beam using time-division-multiplexed (TDM) method at total measuring frequency of 2.5 kHz. A reconstruction routine based on simulated-annealing algorithm was used to deduce distributions of temperature T and water partial pressure PX. Dynamic data was obtained during the ignition of hydrogen fuel at the exit of the scramjet combustor. T and PX distribution of cross section indicate the flame location and its intensity. Successful experiments show great performance of this diagnostic method.
Research on laser damage to typical reconnaissance uav
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Unmanned aerial vehicle (UAV) reconnaissance is very stealthy and can break through enemy air defense system to conduct close-in reconnaissance. In particular, small and micro tactical UAVs are light in structure and have low requirements on operating environment. Besides, they are not afraid of casualties, flexible and highly intelligent in operational styles, and have derived many new combat methods and operational styles. Laser can effectively interfere or blind photoelectric sensor, is an important means to counter reconnaissance UAV. In this paper, laser damage research is carried out for typical reconnaissance UAVs, its weakness is analyzed, and the main methods of laser countermeasure reconnaissance UAVs are summarized.
Laser-induced periodic surface structure in silicon wafer irradiated by continuous laser
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Laser-induced periodic surface structure (LIPSS) is a universal phenomenon which occurs for both continuous laser and pulsed laser. Recently, most studies are focus on LIPSS irradiated by fs laser. However, LIPSS irradiated by continuous laser still need to be carefully studied. Here, We study LIPSS in silicon wafer irradiated by continuous laser for different duration time and power. For the same power, we can observe the evolution process of LIPSS for different time. It is surprising that the evolution process of LIPSS seems to be layered, which occurs for different power. The inner layer occurs at first, then the outer layer occurs. Our study can be used to control the formation of LIPSS.
Research on responsivity of Si-based p-i-n quadrant photodiode detector interact with millisecond pulsed laser
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Si-based p-i-n Quadrant Photodiode Detector(QPD) has be1 en widely used in experimental, military and civilian fields for its spot position detection characteristics. In optoelectronic countermeasure technology, there are defects in the detector after Si-based p-i-n Quadrant Photodiode Detector irradiated with laser, and detectivity is affected, nonlinear changes of dark current and responsivity is produced. Responsivity is an important parameter of photodetector. In this paper, the experimental research of responsivity in the process of interaction between millisecond pulsed laser and Si-based p-i-n Quadrant Photodiode Detector is carried out. It is gotten that the relationship between responsivity with laser energy density and laser pulse width in the process of Si-based p-i-n Quadrant Photodiode Detector irradiated by 1064nm millisecond pulse laser. The result of this research establish the foundation to investigate electrical damage of Si-based p-i-n Quadrant Photodiode Detector interacting with millisecond pulsed laser.