The paper proposed a type of all-optical switch based on spatial-filter (AOSF), and a rule for this design. An analysis based on the present techniques used with lasers and photorefractive crystals indicated that there are no significant scientific or technical barriers for fabricating an AOSF. The response time of the proposed switch can reach 1ps, and the extinction ratio of the switch can reach 114:1 or higher. The AOSF can be used for measuring laser pulse contrast, taking a short section from a laser pulse, and improving pulse contrast or generating a short pulse from a long pulse.
Laser ablation of aluminum alloy and stainless steel were investigated experimentally with a picosecond laser (1053 nm, 8.6 ps, 1Hz). We analyzed and compared the damage threshold and morphology of stainless steel and aluminum alloy. The single-pulse ablation threshold of stainless steel is higher than that of aluminum alloy. The ablation morphology are quite different under multi-pulse irradiation, and the micro-scale periodic surface structures are formed on the stainless steel surface but not on the aluminum alloy sample. The influence of metal thermodynamic properties on laser ablation characteristics is analyzed.
Laser induced damage in the final optics assembly (FOA) is one of the bottleneck problems in high power laser systems for the inertial confinement fusion. In the online experiment, a correlation between the transport mirror defects and the optics damage in the final optics assembly has been found. A physical model is built to analyze the influence of defect size and modulation depth on the light intensification in the FOA. Basically, small size defect on the mirror has a tiny influence on the downstream modulation in the FOA. Optical propagation simulation is also carried out with the real defect phase information got from the interferometer. Results show that there could be strong light intensification caused by the upstream mirror defect. So the local high enough intensity is the main reason for the optics damage. The abnormal intensification is mainly caused by the irregular defect contour, which is produced in the laser induced damage process on the mirror. It is best to eliminate this kind of coating defect on the transport mirrors. When this kind of damaged defect on the transport mirror is inevitable, a mitigation strategy for the optics damage in the FOA is proposed. An inverse design of the best distance between the transport mirror and the FOA can be used to reduce the influence of the transport mirror defects on the downstream FOA. The study can provide some reference for the improvement of the damage resistance of final optics assembly in the high power laser facility.
KEYWORDS: Crystals, Laser crystals, High power lasers, Pulsed laser operation, Laser systems engineering, Laser energy, Laser induced damage, Frequency conversion, Crystal optics, Fusion energy
In high power laser system, the 3ω laser fluence can up to 6J/cm2 with triple frequency conversion efficiency up to 75%. Two high fluence laser experiments have been done for proving high efficiency output and damage resistance of KDP crystal. The KDP crystals have different performance in these two experiments due to their characteristics nuances. For the third harmonic crystal in first experiments, centimeter damage occurred on this crystal after about 50 number laser shots, and more than ten thousands micrometer damage points occurred on this crystal. For the second KDP crystal after about 60 number laser shots, most damage size are micrometer, including bulk damage and damage on back surface, micrometer damage doesn’t exist because of its good quality. We classify these damages of crystals to different kinds, observe characteristics of these damages. Observation of laser damage on third-harmonic converter crystals have been done in this paper.
Laser-induced damage is still the key issue to restrict the development of high power laser system for inertial confinement fusion (ICF). Based on a high power laser prototype, laser-induced damage behaviors and performance of large aperture final optics were experimentally studied. And, the damage inducement and morphology were comprehensively analyzed. Probability density functions (PDF) for laser fluence of near field with different resolution were obtained to analyze the influence of optical field distribution on the optical damage. High fluence of the near field was revealed in PDF with higher resolution, and the missing strong modulation fractions in the measured near field was most probably damage the optical elements. The surface damage morphology was observed and the main damage mechanism was discussed. Several kinds of surface damage morphologies with individual characteristics were sorted. The ratio of width and depth and the main contributors of laser damage were discussed.
3ω laser damage of fused silica optics is the bottleneck of high power laser systems for ICF. Excellent beam quality plays an important role in improving the anti-damage capability of final optics system. We have developed a new optical field measurement technology based on computational optical imaging. With the high power laser prototype of SGII-UP facility, damage resistance of final optics was experimentally studied. The near filed of laser beam was measured with a high resolution to study the effects of modulation and propagation on laser damage. The near field improvement of high power laser beam are reported and the influence of near filed quality on damage performance of final optics are discussed. The development of the defect detection techniques of final optics are introduced. Finally, we present the development perspective of final optics system for ICF laser driver. At present, the damage resistance capability of final optics assembly is 6J/cm2 at normal operation, we will continue to improve the ability in the next step of work.
UV laser damage is still the key issue of high power nanosecond laser systems. The operation performance of the final optics in SGII-UP facility is first reviewed. Based on a high power laser prototype, laser-induced damage of large aperture final optics at 351nm was experimentally studied, including damage initiation, growth and morphologies. The near filed of 351nm laser beam was precisely measured with a high resolution by using the precision diagnostics system (PDS) to study the effects of laser modulation and propagation on laser damage. The damage behaviors were comprehensively analyzed and the main contributors to laser damage were discussed. The development perspective of final optics system for high power laser system is briefly introduced.
In high power laser system, the upstream flaw could induce light intensification in the downstream, thus damaging the optical component. In most of the research, the shape of the defect model is ideal, for example, Gaussian shape. However, the defect in the real system is non-ideal with different shapes. In this paper, the light intensification effect caused by defects with different shapes are compared by numerical simulation. Results show the shape dependence of downstream light intensification caused by flaws. When only the linear effect is considered, the change of defect shape could change the maximum light intensification factor and the downstream location for the maximum intensity. When the nonlinear effect is also considered, the light intensification effect will be more sensitive to the shape of defects. This research can provide some reference for the beam quality control and defect management in the high power laser systems.
The latest progress on high power laser facilities in NLHPLP was reported. Based on a high power laser prototype, damage behavior of 3ω optics was experimentally tested, and the key influencing factors contributed to laser-induced damage in optics were deeply analyzed. The latest experimental results of advanced precision measurement for optical quality applied in the high power laser facility were introduced. At last, based on the accumulated works of 3ω elements damage behavior status in our laboratory, beam expanding scheme was presented to increase the total maximum output 3ω energy properly and decrease the laser induced damage risking of ω optics simultaneously.
Laser damage performance of large aperture optical components has been study under fourth harmonic of 1053nm Nd:glass laser irradiation (263nm).The threshold of optical components is very low under 263nm laser irradiation ,due to conversion of beam to higher energy photons of the quadrupled frequency (4ω), and is relative to material characteristic. A preliminary test of laser induced damage in fused silica (SiO2) and CaF2under 263nm laser is reported in this article. Thresholds of these two materials are obtained. Laser damage threshold of SiO2 is found about 2 J/cm2 by 1-on-1 method using pulsed 263nm laser, lower than CaF2 whose threshold.
KEYWORDS: Sensors, Photodetectors, Silicon, Superposition, Interference (communication), Data acquisition, Environmental sensing, Temperature metrology, Prisms, Signal to noise ratio
To accurately measure the linearity of photodetectors in near-infrared waveband, based on the beam superposition method, a new design idea which use the tow-beam path and correlation methods was proposed. Using the 1053nm laser, and the Si photodetector as the experimental subject, a linearity measurement system of highly accurate photodetectors was designed. This system has over seven orders of magnitude dynamic range. The joint uncertainty is superior to 0.08%. Meanwhile, the linear factor of four different conditions which include the different size of incident beam spots, incident angles, positions and the environment temperature have been measured and analyzed. The experiment shows that the linearity of Si photodetector is ideal when the size of beam spots are bigger, the incident angles are smaller and the environment temperature is lower, moreover, the linearity of margin area is unsatisfactory.
Transverse stimulated Raman scattering (TSRS) gain coefficient of a KDP sample is measured by improved methods.
The improvements include color separation of TSRS, noise light management and acquisition of valid TSRS temporal
pulse. After extracting TSRS temporal pulse and data analysis, we obtained a TSRS gain coefficient of
0.28±0.03cm/GW for the KDP sample. Our improvements of measurement method include the following three aspects:
First, the separation of TSRS irradiation (362.2nm) from Rayleigh irradiation (351.15nm) is realized by first-order
diffraction of grating for TSRS and Rayleigh. Second, to improve the ratio of TSRS signal and noise light owing to
spurious reflection of pump radiation, we remove noise light by using band-pass filter and absorption traps. Third, by
analyzing the time delay between pump signal and noise signal, we demonstrates the valid TSRS temporal pulse can be
extracted from the noise signal and used to calculate the TSRS gain coefficient of KDP.
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