Guangwei Zheng

Spectral beam combining using superimposed reflective volume Bragg grating

It is an effective approach to obtain high-power laser output by spectral beam combining (SBC) technologies. The power of the output beam increases with the increasing of the number of wavelength channels. However, that makes the traditional SBC system very huge for achieving high power laser output. In this paper, a simple SBC system based on the superimposed Reflective Volume Bragg Grating (RVBG) is proposed to reduce the scale of the SBC system. Two structure models of the superimposed RVBG - the superimposed RVBG with the same period and the superimposed RVBG with different period are analyzed and compared by using the rigorous coupled wave analysis, and their applications for SBC system are discussed. Numerical results show that the superimposed RVBG are easy to be fabricated and have the potential to combine multiple lasers with one volume. The superimposed RVBG with different period can achieve high diffraction efficiency simultaneously for each grating with small slant angle divergence, and is convenience to combine two beams with small spectral separation.

Radially polarized laser beam generation by reflecting volume phase grating with high converting efficiency

A simple setup for radially polarized beam generation based on the reflecting volume phase grating is proposed to enhance the converting efficiency. The radially polarized beam is obtained by coherently combining the two orthogonally polarized Hermite-Gaussian HG0 1 and HG 10 beams. By using the plane wave decomposition and the three-dimensional rigorous coupled wave analysis approaches, the intensity distribution of the diffracted beam is compared with the respective incident beam. The intensity and the polarization angle divergence of the combined beam are analyzed. The results show that the intensity profile of the diffracted beam is not changed remarkably compared to that of the input beam. The radially polarized beam can be obtained with a converting efficiency of 99.93% and the degree of radial polarization approaches a unit at a given polarization angle divergence value of 1 deg when the central alignment divergence is smaller than 0.025 mm.

Experimental research on spatial filtering of deformed laser beam by transmitting volume Bragg grating

Using transmitting volume Bragg gratings (TVBG) as a basis, an experiment on one-dimensional spatial filtering of a deformed laser beam is designed. The deformed laser beam results from a He-Ne laser beam modulated by an amplitude modulation plate with a spatial frequency of 7.2 mm ?1. Results show that when the central wave vector of the deformed beam satisfies the Bragg law of TVBG, the spatial profile of the -1st forward-diffracted order is similar to that of the undeformed He-Ne laser beam due to the TVBG with a spatial frequency selective bandwidth of less than 5.0 mm?1. The higher frequency components of the deformed beam are filtered out in the optical near field. Thus, the TVBG cleanup of the spatiallydeformed laser beam is realized experimentally.

Spatial filter with volume gratings for high-peak-power multistage laser amplifiers

The regular spatial filters comprised of lens and pinhole are essential component in high power laser systems, such as lasers for inertial confinement fusion, nonlinear optical technology and directed-energy weapon. On the other hand the pinhole is treated as a bottleneck of high power laser due to harmful plasma created by the focusing beam. In this paper we present a spatial filter based on angular selectivity of Bragg diffraction grating to avoid the harmful focusing effect in the traditional pinhole filter. A spatial filter consisted of volume phase gratings in two-pass amplifier cavity were reported. Two-dimensional filter was proposed by using single Pi-phase-shifted Bragg grating, numerical simulation results shown that its angular spectrum bandwidth can be less than 160urad. The angular selectivity of photo-thermorefractive glass and RUGATE film filters, construction stability, thermal stability and the effects of misalignments of gratings on the diffraction efficiencies under high-pulse-energy laser operating condition are discussed.

Effect of polarization on efficiency of volume Bragg grating filter

Diffraction efficiency of volume Bragg grating, whose period is in the same order as the incident wavelength, is related to the polarization direction of the incident linear polarized beam. When two linearly polarized recording beams with the same polarization direction are used for recording volume Bragg gratings in a photopolymer with diffusion amplification, the azimuth of polarization of the reconstruction beam influences the diffraction efficiency of the grating. When the probe beam is linearly polarized and oriented orthogonally to the grating vectors, the ?-order diffraction beams are also linearly polarized with polarization direction parallel to that of the probe beam. According to the results, a two-dimensional nonspatial optical filter consisting of the volume Bragg gratings would achieve significantly higher efficiency.

Spatial filtering for high power laser beam by volume Bragg grating

Nowadays, the most widespread used space filters are pinhole filters, consisted of a lens with a pinhole in the focal plane, requiring for matching spatially a focused laser beam to a small hole. Experimentally, the initial alignment of spatial filters is difficult, if the pinhole position changes, a laser can damage the pinhole when the power is increased or even permanent damage due to heating. In contrast, non-spatial filtering, a holographic filter element, which made of volume Bragg grating, is inserted in the laser beam path to selectively diffract light propagating at a particular angle, without a lens or a pinhole. A volume Bragg grating is operated directly on the laser beam propagation angle without focus, made use of the gratings angular selectivity, which alignment is easier than pinhole filter, and endures a high-power laser. In this thesis, a volume Bragg grating was fabricated in a 40μm-thick photopolymer, with period of 911.3nm, preparing for a low-pass non-spatial filtering. It achieves an angular selectivity of 35mrad; diffraction efficiency about 95%. Nevertheless, the results of the experiment can be verified with the theory, but not suitable for high-power application. In that case, the photopolymers grating should be replaced with a photo-thermo-refractive glass.