Zunqi Lin

SG-II laser elementary research and precision SG-II program

Abstract Shen Guang-II laser facility, which will be put into formal operation in a few months, is now the largest laser driver in China. During the construction phase several new techniques have been developed. Some of those were adopted and well performed. We present in this paper several elementary research results. The operating stable longitudinal mode oscillator is controlled by the mode loss modulation. The pulse amplitude controller has been demonstrated properly. Laser pulse shaping systems were developed. The time-space-time converter system was well investigated and chosen at the first phase as the facility temporal tailor, whose on-line performance was good. Two sets of 2×2 segment array amplifiers with co-axis double pass structure were operated for the first time. Several features will be briefly introduced. We will also present the features of the novel high quality Nd:glass with non-platinum inclusions, the big KDP crystal growth and the sol-gel coating technology. The precision SG-II project was initiated late in 1996. The energy balance and power balance were normally considered. The environment improvement process has started.

Stress-induced single polarization DFB fiber lasers

Single-frequency and single polarization distributed-feedback (DFB) fiber laser by stress-induced phase-shift birefringence was demonstrated and the corresponding theoretical analysis was presented. The polarization extinction ratio of the DFB fiber laser is about 15 dB.

Characteristics of beam alignment in a high power four-pass laser amplifier

Beam alignment is used to control the pointing, rotation, and position of a beam automatically. We give an overview of the beam alignment system for a four-pass amplifier system. The coupling relationships between dependent quantities and deviations thereof for parameters of key optical elements are analyzed using matrix optics techniques. The rotation characteristics of a near field are discussed. The characteristics of the four-pass adjustment in the cavity spatial filter are shown and the output characteristics are provided. Finally, we develop an adjustment procedure.

Propagation of flat-topped multi-Gaussian beams through a double-lens system with apertures

A general model for different apertures and flat-topped laser beams based on the multi-Gaussian function is developed. The general analytical expression for the propagation of a flat-topped beam through a general double-lens system with apertures is derived using the above model. Then, the propagation characteristics of the flat-topped beam through a spatial filter are investigated by using a simplified analytical expression. Based on the Fluence beam contrast and the Fill factor, the influences of a pinhole size on the propagation of the flat-topped multi-Gaussian beam (FMGB) through the spatial filter are illustrated. An analytical expression for the propagation of the FMGB through the spatial filter with a misaligned pinhole is presented, and the influences of the pinhole offset are evaluated.

Design of ultrabroadband internal reflection gratings with high efficiency

A high-efficiency, ultrabroadband dielectric internal reflection grating with rhombus-shaped grooves is designed by a rigorous coupled-wave analysis, and an effective method for predicting spectral bandwidths of gratings from their efficiency maps is presented. The grating can be fabricated from a single dielectric material, and its reflection diffraction efficiency of the -1st order can reach more than 0.99. More importantly, an ultrabroadband top-hat diffraction spectrum with efficiency exceeding 0.98 over 170 nm wavelength wide is achieved, which makes the gratings suitable for applications associated with broadband illumination, such as ultrashort pulses.

Fractional Fourier transform of flat-topped multi-Gaussian beams

The fractional Fourier transform (FRFT) of the flat-topped multi-Gaussian beam (FMGB) is investigated based on the three kinds of FRFT optical systems: Lohmann I, Lohmann II, and quadratic graded-index systems. The analytical expressions for the FRFT of the FMGB are derived based on the propagation of the FMGB through the three systems. By introducing a hard-edge aperture function, the analytical expressions for the FRFT of the FMGB carried out by the apertured FRFT optical systems are presented. The FRFT characteristics of the FMGB for the three kinds of FRFT optical systems with and without apertures are discussed in detail. Results show that the three types of FRFT optical systems have the same function when the apertures are ignored but that significantly different characteristics are exhibited when the apertures appear.

Improved uniformity of target illumination by combining a lens array and the technique of spectral dispersion

A scheme using a lens array and the technique of spectral dispersion is presented to improve target illumination uniformity in laser produced plasmas. Detailed two-dimensional simulation shows that a quasi-near-field target pattern, of steeper edges and without side lobes, is achieved with a lens array, while interference stripes inside the pattern are smoothed out by the use of the spectral dispersion technique. Moving the target slightly from the exact focal plane of the principal focusing lens can eliminate middle-scale-length intensity fluctuation further. Numerical results indicate that a well-irradiated laser spot with small nonuniformity and great energy efficiency can be obtained in this scheme.

Numerical simulations of the impact of wavefront phase distortions of pump on the beam quality of OPA

A numerical model of optical parametric amplification (OPA) is introduced to investigate the impact of wavefront phase distortion of pump on the beam quality of signal. Numerical results show that the unidentical walk-off directions of the pump and the idler waves are the main factors leading to the transfer of wavefront phase distortions of the pump to the signal, and by reducing the angle between the two directions, the beam quality factor (M2) can be greatly decreased and hence the good beam quality of the signal can be maintained.

SG-II solid state laser ICF system

The 8 beam SG-II laser facility with beam size of 245 mm is reported. Single-longitudinal mode oscillator including the temporal-spatial-transform pulse shaping, and a switch free coaxial double pass disk amplifier technology will be described. CCD beam quality diagnostic system is discussed. The two-beam co-line focus and two series coupling targets are in x-ray experiment is also introduced. The goal of the facility is to produce total pulse energy of 2.4 KJ at tripling frequency.

Optimization of quadrature frequency conversion with type-II KDP for second harmonic generation of the nanosecond chirp pulse at 1053 nm

A second harmonic generation system with two type II KDP crystals in quadrature is optimized for the nanosecond chirp pulse. The acceptance bandwidth of this optimizing scheme is close to 10 nm by using two crystals with slightly opposite angular detuning from phase matching and the conversion efficiency can reach 70% for top-hat chirp pulse at ~2 GW/cm2 in theory. The preliminary experimental results are obtained on the 9th beam of Shen Guang II SGII laser system, and the performance of optimization is partially verified.