Shanghai Institute of Optics and Fine Mechanics,Chinese Academy of Sciences
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Numerical investigation of output beam quality in efficient broadband optical parametric chirped pulse amplification
No full textNational Natural Science Foundation of China [61521093, 61378030]We theoretically analyzed output beam quality of broad bandwidth non-collinear optical parametric chirped pulse amplification (NOPCPA) in LiB3O5 (LBO) centered at 800 nm. With a three-dimensional numerical model, the influence of the pump intensity, pump and signal spatial modulations, and the walk-off effect on the OPCPA output beam quality are presented, together with conversion efficiency and the gain spectrum. The pump modulation is a dominant factor that affects the output beam quality. Comparatively, the influence of signal modulation is insignificant. For a low-energy system with small beam sizes, walk-off effect has to be considered. Pump modulation and walk-off effect lead to asymmetric output beam profile with increased modulation. A special pump modulation type is found to optimize output beam quality and efficiency. For a high-energy system with large beam sizes, the walk off effect can be neglected, certain back conversion is beneficial to reduce the output modulation. A trade-off must be made between the output beam quality and the conversion efficiency, especially when the pump modulation is large since. A relatively high conversion efficiency and a low output modulation are both achievable by controlling the pump modulation and intensity. (C) 2016 Elsevier B.V. All rights reserved
Range extension in laser-induced breakdown spectroscopy using femtosecond-nanosecond dual-beam laser system
No full textNational Basic Research Program of China [2014CB921300]; National Natural Science Foundation of China [11674340, 61575211, 11404357, 11604351]; Strategic Priority Research Program of the Chinese Academy of Sciences [XDB16030300]; Shanghai 'Yang Fan' program [14YF1406100]We extend the detection range of laser-induced breakdown spectroscopy by combining high-intensity femtosecond laser pulses with high-energy nanosecond - CO2 laser pulses. The femtosecond laser pulses ionize the molecules and generate filament in air. The free electrons generated in the self-confined plasma channel by the femtosecond laser serve as the seed electrons which cause efficient avalanche ionization in the nanosecond - CO2 laser field. We show that the detection distance has been extended by three times with the assistance of femtosecond laser filamentation
Compensation for the spatial periodic modulation of the near-field beam with an improved iterative weight-based method
No full textAn improved iterative weight-based method is studied to compensate for the spatial periodic modulation (SPM) of the near-field beam, Based on the beam angular spectrum transmit formula (ASTF). the required phase that compensates for the intensity distribution of the incident SPM beam is iterated by this algorithm, which can contribute to improve the uniformity and quality of the output near-field beam. The experimental results show that the similarity values is improved from 0.9878 to 0.9947 and is getting closer to 1. The modulation degree M of the output near-field beam is decreased from 1.3631 to 1.3401 and the contrast degree C is decreased from 0.3018 to 0.2635 by using the liquid-crystal spatial light modulator (SLM). This indicates that the experiment verifies the feasibility of this iterative method to compensate for the SPM of the near-field beam
Temporally Modulated Phase Retrieval Method for Weak Temporal Phase Measurement of Laser Pulses
No full textNational Natural Science Foundation of China [61205103]We propose a simple iterative algorithm based on a temporally movable phase modulation process to retrieve the weak temporal phase of laser pulses. This unambiguous method can be used to achieve a high accuracy and to simultaneously measure the weak temporal phase and temporal profile of pulses, which are almost transform-limited. A detailed analysis shows that this iterative method has valuable potential applications in the characterization of pulses with weak temporal phase
Deuteron-deuteron fusion in laser-driven counter-streaming collisionless plasmas
No full textNational Basic Research Program of China [2013CBA01501, 2013CB834401]; National Science Foundation of China [11135012, 11135005, 11375114]; Science Challenge Project [TZ2016005]; Global R&D Networking Program - Republic of Korea's Ministry of Science, ICT and Future Planning [NRF-2012-0004839]; Science and Technology Commission of Shanghai Municipality [11DZ2260700]Nuclear fusion reactions are the most important processes in nature to power stars and produce new elements, and lie at the center of the understanding of nucleosynthesis in the universe. It is critically important to study the reactions in full plasma environments that are close to true astrophysical conditions. By using laser-driven counter-streaming collisionless plasmas, the fusion d + d -> He-3 + n is studied in a Gamow-like window around 27 keV. The results give hints that astrophysical nuclear reaction yields can be modulated significantly by the self-generated electromagnetic fields and the collective motion of the plasma. This plasma-version minicollider may provide a novel tool for studies of astrophysics-interested nuclear reactions, as well as a useful tool to constrain the models of plasma colliding dynamic
Performance of smoothing by spectral dispersion with consideration of the gain characteristic of Nd:glass amplifier
No full text采用数值方法研究了钕玻璃放大器的增益特性对高功率激光系统中光谱色散匀滑单元性能的影响.分析结果表明,入射光中心波长与放大器增益曲线中心波长不一致时,焦斑强度分布会受到一定的影响,且该影响随放大倍数增大趋于明显,而两个波长一致时,强度分布变化较小.靶面焦斑整体的辐照均匀性则主要取决于经过相位调制后的激光束的带宽,放大器的增益特性对其空间功率谱及均匀性无明显的影响.所得结论为光谱色散匀滑单元在激光系统中的实际应用提供了重要的理论参考.National Natural Science Foundation of China [11204043, 11374067]; Open Fund of Key Laboratory for High Power Laser Physics of Chinese Academy of Sciences [SG-001103]A key issue in developing a high-power laser driver, which can be used for inertial confinement fusion and laser produced plasma experiments, is to obtain uniform irradiation on the target surface, thus a number of spatial or temporal techniques have been proposed for laser beam smoothing. A scheme combining a lens array with the technique of smoothing by spectral dispersion (SSD) is being explored in the SG-II Laser Facility located in Shanghai Institute of Optics and Fine Mechanics. As the laser system involves a variety of optical elements, their influences have to be considered in the implementation of such a scheme. The Nd: glass amplifier is one of the most important parts of the system, and the phase-modulated laser beam will propagate through it along the long light path when SSD is employed. In this paper, the performance of uniform irradiation of the target pattern is studied based on two-dimensional simulations when the gain characteristic of the amplifier is taken into account. The major factors, such as the small signal gain profile of the amplifier, the amplification factor, the bandwidth of the phase-modulated laser beam and the difference between the central wavelength of the laser and the central wavelength of the amplifier gain curve, are analyzed in detail. The numerical results show that when the central wavelength of the incident beam is different from the central wavelength of the amplifier gain curve, intensity distribution of the target pattern will be affected to a degree depending on the amplification factor; while these two wavelengths are very close to or identical with each other, variation in the intensity distribution is trivial. The symmetry of the phase-modulated laser spectrum will be destroyed due to the gain characteristic of the amplifier, especially when the bandwidth is relatively wide. However, the slight asymmetry does not result in significant influence on the spatial power spectrum nor uniformity of the target pattern, even in the case where the central wavelength of the incident beam is different from that of the amplifier gain curve. The reasons would be 1) the gain curve of the amplifier is actually quite flat within the laser bandwidth, and 2) with the technique of SSD, all spectral components contribute to the target intensity distribution within an average time. The analysis indicates that the performance of uniform irradiation of the target pattern depends mainly on the bandwidth of the phase-modulated laser beam. A wider bandwidth can always generate better irradiation when it is within a certain range, say no more than 0.3 nm, but beyond this range, the nonuniformity tends to remain at a level about 0.25-0.3. Multistage Nd: glass amplifiers will be employed in the practical laser driver, and the case investigated in this paper involves only one stage for simplicity. The conclusion obtained in this paper is important for implementing the technique of SSD in the laser system
Real-time characterization of FM-AM modulation in a high-power laser facility using an RF-photonics system and a denoising algorithm
No full textFM-AM modulation of high-power lasers significantly affects laser performance. Therefore, precise measurement of the FM-AM modulation depth is necessary. The subsequent FM-AM modulation generated by group velocity dispersion when the laser pulse propagates through a fiber affects the measurement accuracy. In order to eliminate this effect, a waveform-acquisition module is proposed that converts a broad-spectrum pulse of 1053 nm to a narrow-spectrum pulse of 1550 nm, without affecting the waveform. In addition, a signal-processing algorithm based on the orthogonal matching pursuit method is implemented to remove the sampling noise from the wave-form. In this way, the signal-to-noise ratio of the measurement can be readily improved. Both theoretical and experimental results indicate that the proposed FM-AM modulation detection system is effective and economical. It can measure the FM-AM modulation depth precisely, and therefore shows considerable promise for future applications in high-power lasers. (C) 2017 Optical Society of Americ
Charge state and energy transfer investigation of iron-chromium co-doped ZnS polycrystalline prepared by step-temperature diffusion for mid-infrared laser applications
No full textNational Nature Science Founds of China [51502321, 61405221, 61575198, 61378069]; Shanghai City Star Program [14QB1400900]The spectroscopic characterization and energy transfer mechanism of iron-chromium co-doped ZnS polycrystalline (Cr,Fe:ZnS) were reported with dimension of 10 mm x 10 mm x 1.5 mm produced by step-temperature thermal diffusion method. The infrared absorption is characterized by two absorption broad-bands centered at 1.67 mu m and 2.9 mu m which can be attributed to the spin-allowed transition T-5(2) <-> E-5 within the 3 d(4) shell of Cr2+ and Fe2+ ions. The valences of iron and chromium in ZnS were analyzed by X-ray photoelectron spectroscopy. Photoluminescence spectrum shows a relatively strong broad emission band centered at 4 mu m with a width of 800 nm (FWHM) under 1.56 mu m excitation at room temperature and reveals an effective Cr2+ -> Fe2+ energy transfer process. The measured photoluminescence decay was about 5 mu s at room temperature. All the results indicate that Cr, Fe: ZnS could be used as the gain medium operating at 3.5-4.5 mu m via Cr2+ -> Fe2+ energy transfer using a more convenient laser pump source in the near IR region. (C) 2016 Elsevier B.V. All rights reserved
Polarization-independent broadband beam combining grating with over 98% measured diffraction efficiency from 1023 to 1080 nm
No full textNational Natural Science Foundation of China (NSFC) [10704079, 11604352, U1430121, U1630140]; Shanghai Science and Technology Committee Program [16JC1420600]We report a grating solution for achieving broadband and polarization-independent properties that brings a laser combining system to much higher power levels. The grating, with a high-refractive-index-contrast bilayer ridge, was designed and successfully fabricated based on high-power laser coatings, lithography, and ion-beam etching technology. The measured -1st order non-polarized reflective diffraction efficiency of the grating exceeds 98% over the wavelength range of 1.023-1.08 mu m, and the highest value is 99.15%
Polarization-independent broadband dielectric bilayer gratings for spectral beam combining system
No full textNational Natural Science Foundation of China [61308021, 10704079, 11104295]We report on a polarization-independent all-dielectric trapezoidal bilayer grating with broadband and high diffraction efficiency. The bilayer trapezoidal grating ridge on a reflector consists of an HfO2 layer and a SiO2 layer. The theoretical-1st order efficiencies of the grating are more than 95% with wavelength range from 1010 nm to 1080 nm for both TE and TM polarizations. The fabrication tolerances depending on the HfO2 and SiO2 layer grating ridge depths are enough to obtain the designed grating using current craft. The fabricated grating with exceeding 94% efficiency from 1000 nm to 1085 nm measured by a non-polarization laser has been fabricated and applied in a spectral beam combining external cavity to combine eight beams into one beam output with 10.77 kW