In high power laser system, the wavefront quality of large optic elements in the mid-frequency region plays a critical role in the system performance and safe operation. A simple and efficient measurement method for mid-frequency wavefront error is used, which employs the extended ptychographical iterative engine algorithm and has simple structure, low environment requirement and flexible adjustable frequency ranges. This method has been successfully implemented for the wedge focused lens to achieve accurate mid-frequency measurement. Further it can be extended to a wide range of large optical components, especially for which the wavefronts are not easy to be measured using interferometers.
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/cm<sup>2</sup> at normal operation, we will continue to improve the ability in the next step of work.
We propose a single shot dual-wavelength phase retrieval method based on low rank mixed-state and phase modulation imaging. Both the amplitude and phase of each wavelength can be reconstructed simultaneously from the recorded diffraction pattern. Proof-of-principle experiment was demonstrated to show its capability in solving the crucial problem of measuring the high energy laser beam of multi-wavelength inside the target cabin. Because of the extremely high energy and limited inner space of high vacuum, no other techniques can realize this kind of measurement by now. This novel method provides an efficient way for fundamental and third harmonic wave diagnostics in high power laser systems, including near-field phase and far-field intensity distribution, power distribution, frequency conversion efficiency etc. Because this method only needs a phase plate and a CCD, it can be integrated into a compact device with convenient use.
Coherent modulation imaging providing fast convergence speed and high resolution with single diffraction pattern is a promising technique to satisfy the urgent demands for on-line multiple parameter diagnostics with single setup in high power laser facilities (HPLF). However, the influence of noise on the final calculated parameters concerned has not been investigated yet. According to a series of simulations with twenty different sampling beams generated based on the practical parameters and performance of HPLF, the quantitative analysis based on statistical results was first investigated after considering five different error sources. We found the background noise of detector and high quantization error will seriously affect the final accuracy and different parameters have different sensitivity to different noise sources. The simulation results and the corresponding analysis provide the potential directions to further improve the final accuracy of parameter diagnostics which is critically important to its formal applications in the daily routines of HPLF.
The commonly used interferometer and Hartmann-Shack sensor are not ideally qualified for the phase or wavefront measurement in the field of high power laser because the optical elements always have large aperture, the steep phase gradient and very irregular surface profile. The ptychography, which is a newly developed coherent diffraction method for the imaging with short wavelength, can be a perfect alternative to traditional technologies due to its outstanding advantages. The complex transmittance of the optical element can be obtained by measuring its transmitted and incident fields with ptychography and calculating their phase difference. Since ptychography can realize measurement with a resolution comparable to that of interferometry, it can find lots of applications in the field of high power laser such as the measurement of the complex transmittance of large optical element, the thermal distortion of the gas-cooled Nd:glass amplifier, and the focal length of the lens array etc.