Zhitao Peng

Status of the SG-III solid-state laser facility

SG-III laser facility beam begins with a nanojoule energy laser pulse from the master oscillator and a fiber front-end system that can provide a variety of pulse shapes suitable for a wide range of experiments. The chirped pulse stacking method is used in the front-end system to generate arbitrarily shaped pulse with a rise time less than 50ps. The system stacks a set of 100-ps chirped pulses in fiber time-delay lines to obtain a 5-ns flat-top pulse with a spectral bandwidth of 1.2nm. The pulse is then transported to preamplifier modules under the middle of CSF for amplification and beam shaping. There is a total of 48 preamplifier modules on SG-III, each feeding a single laser beams. The main amplifier column of 4 high by 2 wide has been chosen as a module and the clear optical aperture is 40cm × 40cm. Small PEPC are chosen for system isolation and beam can be rotated by 90 degree in U-turn beam reverser located in the middle of TSF. After main amplifier, beams are subsequently redirected to final optics assembly in switchyard and are focused on the center of the target chamber with the diameter of 6m.

Status of prototype of SG-III high-power solid-state laser

We are currently developing a large aperture neodymium-glass based high-power solid state laser, Shenguang-III (SG-III), which will be used to provide extreme conditions for high-energy-density physical experiments in China. As a baseline design, SG-III will be composed of 48 beams arranged in 6 bundles with each beam aperture of 40cm×40cm. A prototype of SG-III (TIL-Technical Integration experimental Line) was developed from 2000, and completed in 2007. TIL is composed of 8 beams (four in vertical and two in horizontal), with each square aperture of 30cm×30cm. After frequency tripling, TIL has delivered about 10kJ in 0.351 μm at 1 ns pulsewidth. As an operational laser facility, TIL has a beam divergence of 70 μrad (focus length of 2.2m, i.e., 30DL) and pointing accuracy of 30 μm (RMS), and meets the requirements of physical experiments.

Research and construction progress of SG-III laser facility

SG-III laser facility is now the largest under-construction laser driver for inertial confinement fusion (ICF) research in China, whose 48 beams will deliver 180kJ/3ns/3ω energy to target in one shot. Till the summer of 2014, 4 bundle of lasers have finished their engineering installation and testing, and the A1 laser testing is undergoing. A round of physics experiment is planned in Oct. 2014 with 5 bundle of lasers, which means the facility must be prepared for a near-full-capability operation before the last quarter of 2014. This paper will briefly introduce the latest progress of the engineering and research progress of SG-III laser facility.

Research and construction progress of the SG-III laser facility

The under-construction SG-III laser facility is a huge high power solid laser driver, which contains 48 beams and is designed to deliver 180kJ energy at 3ns pulse duration. The testing ending up at September 2012 validated that the first bundle lasers of SG-III facility had achieved all the designed requirements. And shortly later in December 2012, the first round of running-in physics experiment provided a preliminary X-ray diagnostic result. In the testing experiment, detailed analysis of the laser energy, the temporal characteristics, the spatial distribution and the focusing performance was made by using the Beam Integrated Diagnostic System. The 25kJ 3ω energy produced by the first bundle lasers created the new domestic record in China. These great progresses in the laser performance and the physics experiment have already demonstrated that the facility is in excellent accordance with the designs, which establish a solid foundation for completing all the construction goals.

Preliminary experimental results of Shenguang III Technical Integration Experiment Line

We are now constructing a technical integration experiment line (TIL) at CAEP, which is the prototype facility of Shenguang III laser fusion driver. Currently, many important results have been obtained on the first integrated beam line, which established a sound foundation for Shenguang III engineering design.

Research on auto-focusing of high signal to noise ratio image with weak light

For souped-up of Optical Damage Online Inspection system and obtain damage information of the optic exactly, this paper describes an auto focusing method for weak spot which on high signal to noise ratio image in darkroom. We used different calculation strategy to target pixel and adjoining pixel, compute the gray different that on x and y, compared the result of two focusing evaluation function which the gray difference absolute sum function and the gray difference quadratic sum function, select the best calculation strategy and focusing evaluation function. Test results indicate that, the gray difference quadratic sum function can be more exactly in darkroom and under weak light

Experimental research on high-contrast measurement for ns high-power laser shaped pulse

Based on the multiple pulses joining, the cascaded photodetection is experimentally researched on high-contrast measurement of ns high-power laser pulse. The ultrafast photodetectors with the saturation characteristics are used. A joining method for multi-pulse waveforms in the nonlinear region is put forward. The experimental results for ns step-pulse at SG-III laser system show the contrast of ~ 400:1 is achieved, in accord with designed contrast value.

Research of beam conditioning technologies on SG-III laser facility

Multi-FM SSD and CPP was experimentally studied in high fluence and will be equipped on all the beams of SG-III laser facility. The output spectrum of the cascade phase modulators are stable and the residual amplitude modulation is small. FM-to-AM effect caused by free-space propagation after using smoothing by spectral dispersion is theoretically analyzed. Results indicate inserting a dispersion grating in places with larger beam aperture could alleviate the FM-to- AM effect, suggesting minimizing free-space propagation and adopting image relay. Experiments taken on SG-III laser facility indicate when the number of color cycles (Nc) adopts 1, imposing of SSD with 3.3 times diffraction limit (TDL) did not lead to pinhole closure in the spatial filters of the preamplifier and main amplifier with 30-TDL pinhole size. The nonuniformity of the focal spot using Multi-FM SSD and CPP drops to 0.26, comparing to 0.84 only using CPP. The experiments solve some key technical problems using SSD and CPP on SG-III laser facility, and provide a flexible platform for laser-plasma interaction experiments. Combined beam smoothing and polarization smoothing are also analyzed. Simulation results indicate through adjusting dispersion directions of one-dimensional SSD beams in a quad, two-dimensional SSD could be obtained. The near field and far field properties of beams using polarization smoothing were also studied, including birefringent wedge and polarization control plate (PCP). By using PCP, cylindrical vector beams could be obtained. New solutions will be provided to solve the LPI problem encountered in indirect drive laser fusion.

Beam phase-distortion correction in a high power laser based on the stochastic parallel gradient descent technique

The feasibility of the closed-loop adaptive optic system based on laser far-field information was experimentally demonstrated, which can significantly increase the working efficiency of the multi-beamline high power solid laser facility. A stochastic parallel gradient descent algorithm with mean-radius cost function was applied in the adaptive optics that was installed in one chosen beamline in the SG-III laser facility. The far-field diameter from a reference laser was corrected from about 275 to 100??m, very close to the ideal limit of 75??m. Multiple-wavefront correction can be performed with this far-field detection based adaptive optics system, and the experimental result validates the practicability of the proposed system in the multi-beamline high power solid laser facilities.

Recent Progress of the Prototype Laser for Shenguang-III

We have developed the prototype facility of Shenguang III for the laser driven inertia confined fusion. The characteristics and recent progress of the facility was demonstrated. The facility using a new scheme of four-pass amplifiers, all-fiber front-end, compacted PEPC et al has provided primary target-shooting experimental operation and will provide crucial data for the design and construction of the Shenguang III laser fusion driver.