Xiaomin Zhang

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.

Comparison of efficient third-harmonic generation between phase modulated broadband and narrowband lasers

Based on the existing design in our prototype laser facility, the third-harmonic generation of the phase modulated broadband of 0.3 nm and narrowband lasers are theoretically analyzed and experimentally studied in this paper. The output third-harmonic generation (THG) laser characteristics such as the conversion efficiency, the spectrum distribution and the intensity modulation are compared in detail. It is found that the THG conversion efficiencies for the narrowband and such broadband lasers are fundamentally identical without spectrum narrowing. In addition, the interrelationship between THG conversion efficiency and incident fundamental light intensity for both lasers is experimentally demonstrated and the experimental results agree with the theoretical analysis. The broadband THG spectrum with ideal repetition and without loss of spectrum is obtained in the experiment.

286-TW Ti:sapphire laser at CAEP

We have built a three-stage Ti:sapphire laser system at CAEP which could deliver 5-TW, 30-TW and 286-TW pulses to the corresponding target chambers for diverse applications with innovative high-power Ti:sapphire crystal amplifiers. Pulse durations of 30fs have been obtained by installing an acousto-optic programmable dispersive filter (AOPDF) before the stretcher to compensate for the spectral gain narrowing. By taking a number of advanced measures for spatial beam control, near-diffraction limited focal spots (FWHM) have been obtained which, to our knowledge, are the best far fields ever measured for the existing high-power Ti:sapphire laser systems without deformable mirror correction. Focused laser intensity is about 1021W/cm2 measured with an f/1.7 OAP. The laser system has the potential to operate at 500TW and even higher and laser intensities of 1022W/cm2 are expected with deformable mirror for wavefront correction and small f-number fine OAP for tighter focus added to the system in the near future.

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.

Progress on developing a PW ultrashort laser facility with ns, ps, and fs outputting pulses

A petawatt laser facility with three beams for fast ignition research and strong-field physics applications has been designed and is being constructed. The first beam (referred as SILEX-I) is a Ti:sapphire femto-second laser which pulse width is 30 fs, and till now, output power has reached to 330 TW. The other two beams are Nd3+:glass lasers which output energy are larger than 1kJ and pulse width are about 1ps and 1ns respectively. By using the technology of OPA pumped by 800nm femtosecond laser and seeded by super-continuum spectrum white light, the three beams are synchronized with each other without jitter time. By using the seeds from OPA pumped by femtosecond laser, and by using the pre-amplification stage of OPCPA, the signal to noise ratio of the Nd3+:glass petawatt laser will reach to 108. Active methods are taken to control the gain narrowing effect of the Nd3+:glass amplifiers, giving the option to compress the chirped pulse to ultrashort pulse with width less than 400fs. Tiled multilayer dielectric coating gratings are used for the compressor of the PW beam, which has been successfully demonstrated on a 100J picosecond Nd3+:glass laser system.

Demonstration of broadband frequency tripling based on cascaded partially deuterated KDP crystals

The cascaded KDP crystal scheme with different deuteration levels for enhancing the bandwidth of third-harmonic generation (THG) is experimentally demonstrated in this paper. The deuteration level of a partially deuterated KDP crystal is used as a degree of freedom to alter the phase-matching (PM) wavelength. The experimental results show that this scheme can increase the bandwidth acceptance in broadband frequency tripling. In addition, the spacing between the two cascaded crystals has significant influence on the THG spectrum as well as THG conversion efficiency, and a relatively ideal THG spectrum can be obtained by the optimized parameters. Meanwhile, nonlinear-coupled wave equations are used to theoretically analyze the experimental results, and the experimental results are consistent with the numerical simulation.

Improved uniform irradiation by combining periodic chirped pulse, sinusoidal phase modulation light and lens array

A new method is proposed for obtaining two-dimensional beam smoothing. The periodic chirped pulse generated at the front end of the laser driver is dispersed in one direction and the bulk frequency modulation is used to generate another broad band. Then the beam is dispersed in the other direction. The pattern of the far-field is simulated. The result shows this method can reduce the high influence of the four outer corners. By combining a lens array, an improved uniform pattern on the target can be obtained.

Design and performances of prototype laser amplifiers for technical-integration-line facility

We present work on the design and physical performances for a large-aperture multisegment amplifier, which is an engineering prototype amplifier for our technical-integration-line facility. The amplifier consists of 4×2 apertures with each clear aperture about 300×300 mm. Each amplifier module consists of eight laser slabs, arranged four high, two wide, and one long, and twenty high-pulse power flashlamps with an input electric energy of about 23 kJ per lamp. These twenty flashlamps are arranged in a 6-8-6 scheme. The central eight flashlamps will irradiate their energy to the laser slabs in two directions to increase the pumping transfer efficiency. Experimental results indicated that the energy stored in the upper laser level is about 0.24 J/cm 3 , which corresponds to an energy storage efficiency of about 3.0%. Finally, with an 1.053-μm laser probe we obtained a small-signal gain coefficient of 5.0% cm –1 .

Experimental research on the influences of smoothing by spectral dispersion on the Technical Integration Line

We describe one-dimensional smoothing by spectral dispersion (SSD) in high fluence on the Technical Integration Line. The experimental results indicate that SSD did not influence the load capacity of the laser facility. The near- and far-field analysis prove that adopting SSD could smooth the high-frequency modulations in the near field and dramatically suppress 10 μm-100 μm spatial modulations in the far field. The focal spot contrast decreases from 2.58 to 0.80 after using SSD and adaptive optics. Adopting a 0.31 nm bandwidth frequency-modulated laser pulse and a 1200 l/mm dispersion grating, experimental results proved that 97% 3ω energy passed through the laser entrance hole using a 4 m focal length wedged lens and gold foil target with an 1100 μm hole.

Introduction of SILEX-I Femto-second Ti:sapphire laser Facility

We have built a Ti:sapphire laser system, referred to as SILEX-I, with a peak power of 286TW for a pulse duration of 30fs using chirped-pulse amplification technique. A number of spectral and spatio-temporal beam control measures have been taken and near-diffraction limited focal spots have been obtained which, to our knowledge, are the best far fields ever measured for any existing high-power Ti:sapphire laser system without deformable mirror corrections.