Angular Filtering And Beam Control With Volume Bragg Gratings

The angular spatial filtering (ASF) is proposed and achieved with Volume Bragg Gratings (VBGs) recorded in Photo-Thermo-Refractive (PTR) Glass. The ASF can be directly used in laser systems to realize high quality near field filtering, which is helpful in overcome the inhebit drawbacks of traditional spatial filter used in the laser systems, and may be a potential candidate element for improving near field laser beam quality in advanced laser systems. The main results and contributions of this dissertation are as follows:(1) Two-dimensional angular filtering on the basis of volume Bragg gratings (VBGs) in series connection for cleaning up a laser beam was proposed and performed. The dependence of near field beam distribution and ctoff frequency with selectivity of VBGs is studied. The results show that the medium and high frequency components of laser beam are effectively suppressed and the near field beam distribution improved.(2) The design method of angular filter is initially established on the basis of coupled wave theory. The influence of angular selectivity and peak diffraction efficiency of VBGs with different gratings parameter, such as thickness, period, tilt angle of grating vector and refractive index modulation, is analyzed. The angular filter with diffraction efficiency of 99% and the cutoff frequency of 1mm-1 is achieved. The Error Tolerance shows that the characteristics of VBGs are mainly affected by the thickness and refractive index modulation of the gratings.(3) A hybrid filter based on the combination of transmitting volume Bragg grating and traditional spatial filter and a non-sidelobes angular filter with two VBGs in series connection are proposed and performed. The results shows that the sidelobes can be suppressed with non-sidelobes angular filter and the angular selectivity can also be improved. The cutoff frequency can be effectively improved with hybrid filter. Besides, the sidelobes can also be suppressed with hybrid filter.(4) A evaluation method for angular filtering is proposed. The relative peak diffraction efficiency and Full Width at First Zero (FWFZ) are used to evaluate characteristics of angular filtering, The near field modulation, contrast ratio and Power Spectral Density (PSD) is used to evaluate the filtered near field beam quality. Besids, the evaluation methods and standards are described in detail. (5) The polarization characteristics of angular filters with VBG in PTR glass are studied. The peak diffraction efficiency, FWFZ and output polarization of the beam through VBGs are experimenally studied. The results show that diffraction efficiency and FWFZ of the diffracted beams are not affected by the polarization state of the incident beams, and the polarization state of the diffracted beams keeps almost the same as that of the incident beams, which has potential applications in high efficiency and high reliability angular filtering in laser facilities.