Kuixing Zheng

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.

Recent progress of the Integration Test Bed

The Integration Test Bed (ITB) is a large-aperture single-beam Nd:glass laser system, built to demonstrate the key technology and performance of the laser drivers. It uses two multipass slab amplifiers. There are four passes through the main amplifier and three passes through the booster amplifier. The output beam size is 360mm by 360mm, at the level of 1% of the top fluence. The designed output energy of ITB at 1053nm is 15kJ in a 5ns flat-in-time (FIT) pulse, the third harmonic conversion efficiency is higher than 70%. The first phase of the ITB has been completed in July 2013. A series of experiments demonstrated that laser performance meets or exceeds original design requirements. It has achieved maximum energies at 1053nm of 19.6kJ at 5ns and 21.5kJ at 10ns. Based on a pair of split third harmonic generation KDP crystals, the third harmonic conversion efficiency of about 70% and 3ω mean fluences as high as 8.4 J/cm2 have been obtained with 5ns FIT pulse.

Research on the adaptive optical control technology based on DSP

Adaptive optics is a real-time compensation technique using high speed support system for wavefront errors caused by atmospheric turbulence. However, the randomness and instantaneity of atmospheric changing introduce great difficulties to the design of adaptive optical systems. A large number of complex real-time operations lead to large delay, which is an insurmountable problem. To solve this problem, hardware operation and parallel processing strategy are proposed, and a high-speed adaptive optical control system based on DSP is developed. The hardware counter is used to check the system. The results show that the system can complete a closed loop control in 7.1ms, and improve the controlling bandwidth of the adaptive optical system. Using this system, the wavefront measurement and closed loop experiment are carried out, and obtain the good results.

Aperture scalable, high-average power capable, hybrid-electrode Pockels cell

We demonstrate a hybrid-electrode Pockels cell (HEPC) using a thin z-cut deuterated potassium dihydrogen phosphate (DKDP) crystal. The device is a reflective, longitudinally driven, longitudinally cooled Pockels cell, constructed with a plasma chamber providing the incident-side electrode and a silicon-substrate mirror serving the three purposes of a mirror, a back-side electrode, and a heat sink. The mirror and the DKDP are thermally coupled through a sub-millimeter, inert gas-filled gap. A time-multiplex pass-by driving method is proposed to favor low-voltage and fast-response operation. The experimental results support that the HEPC would be a competent device for high-energy and high-repetition-rate lasers.

Process-oriented adaptive optics control method in the multi-pass amplifiers

In this talk, we propose and demonstrate the process-oriented adaptive optics (AO) wavefront control method, for optimizing the beam quality in the multi-pass amplifiers. Different from the conventional target-oriented wavefront control approach, the novel method divides the aberration correction process into several steps, to optimize the wavefront quality in time during the courses of the beams transport and amplification. The experimental results show that the proposed method can effectively prevent the beam quality from worsening and ensure the successful reality of multi-pass amplification, so it has obvious advantages both in efficiency and accuracy over the traditional target-oriented method.

Beam alignment based on two-dimensional power spectral density of a near-field image

Beam alignment is crucial to high-power laser facilities and is used to adjust the laser beams quickly and accurately to meet stringent requirements of pointing and centering. In this paper, a novel alignment method is presented, which employs data processing of the two-dimensional power spectral density (2D-PSD) for a near-field image and resolves the beam pointing error relative to the spatial filter pinhole directly. Combining this with a near-field fiducial mark, the operation of beam alignment is achieved. It is experimentally demonstrated that this scheme realizes a far-field alignment precision of approximately 3% of the pinhole size. This scheme adopts only one near-field camera to construct the alignment system, which provides a simple, efficient, and low-cost way to align lasers.

Laser pulse spatial-temporal inversion technology for ICF laser facility

The laser pulse should be shaped to satisfy the ICF physical requirement and the profile should be flattened to increase the extraction efficiency of the disk amplifiers and to ensure system safety in ICF laser facility. The spatial-temporal distribution of the laser pulse is affected by the gain saturation, uniformity gain profile of the amplifiers, and the frequency conversion process. The pulse spatial-temporal distribution can’t be described by simply analytic expression, so new iteration algorithms are needed. We propose new inversion method and iteration algorithms in this paper. All of these algorithms have been integrated in SG99 software and the validity has been demonstrated. The result could guide the design of the ICF laser facility in the future.

Development of slab amplifiers for integration-test-bed

Integration-Test-Bed(ITB) is Chinas first laser devices with single-beam ten-thousand joules output for Inertial Confinement Fusion (ICF) research. In this paper we describe the development of single-segment slab amplifiers for Integration-Test-Bed (ITB) with 400mm× 400mm aperture. The experiment results shows that the average small signal gain coefficient in 400mm×400mm aperture reach 5.28%/cm with the gain uniformity is about 1.09:1(maximum value/ average value), and up to 1.063:1 (maximum value/ average value) in 360mm×360mm beam-diameter clear aperture. The storage efficiency of system is about 1.47%. The pump-induced wave-front distortion is 5.3λ for the laser beams, which within the correction range of deformable mirror; the thermal recovery time was less than 4 hours. All of this guaranteed the output of 19.6kJ/5ns with wavelength of 1053nm from the Integration-Test-Bed (ITB) device.

Characteristics of Laser-Induced Surface and Bulk Damage of Large-Aperture Deuterated Potassium Dihydrogen Phosphate at 351 nm

Deuterated potassium dihydrogen phosphate damage performance at 351 nm is studied on a large-aperture laser system. Bulk and rear-surface damage are initiated under the 3ω fluences of 6.7 J/cm2 and 3 J/cm2, and show different growth characteristics under multiple laser irradiations with the fluence of 6 J/cm2. The size and number of bulk damage keep unchanged once initiated. However, surface damage size also does not grow, while surface damage number increases linearly with laser shots. Different damage thresholds and growth behaviors suggest different formations of bulk and surface damage precursors. The cause of surface damage is supposed to be near-surface absorbing particles buried under the sol-gel coating.