Study On The Broadband Angular Filtering With Volume Bragg Grating

The laser beam control is the key technology in the field of high power laser research. In the high power laser facilities, dust, dirty on the surface of mirrors and optical inhomogeneous will result in amplitude or phase modulation and degrade the beam quality. The classical solution to improve the beam quality is to employ spatial filtering in the facilities. The classical pinhole spatial filtering, however, is large and needs vacuum environment, and suffers from plasma plugging effect and self-focusing. From the end of the 20-th century, compact and simple filtering solutions have been explored, such as the angular filtering. The solution does not need 4f structure and vacuum environment, and operates in the region of near field. The key element in the angular filtering is the volume Bragg grating (VBG), which has good angular selectivity and high diffraction efficiency to filter the medium and high frequencies in the beam and improve the spatial homogeneity. Two problems, however, are required to be solved in the VBG-based solutions. Firstly, beam deflection in the angular filter. In most of large laser facilities, modular design requires the element to be "plug-and-play", which cannot be realized in the classical angular filters. Secondly, lasers with broadband cannot be filtering efficiently due to the spectral selectivity of VBGs. To solve the problems, an angular filtering scheme with broadband and "plug-and-play" properties is proposed and studied. This scheme is realized with combination of surface grating and VBG, with which lasers with bandwidth of several tens of nanometers can be filtered efficiently. The scheme will provide a novel solution to improve the spatial homogenous of broadband lasers. The main contents and results are as follows:(1) The angular selectivity and spectral selectivity of VBGs are analyzed. The above two properties show close relationships with each other, and are related to the diffraction efficiency of VBGs. Increasing the grating thickness and period at the same time, the bandwidth of spectral selectivity will increase and the angular selectivity will remain the same. The application range of VBG-based angular filter is analyzed. For a beam with a certain wavelength, there is a corresponding incident angle to reach high diffraction efficiency, which is the physical foundation to realize the broadband angular filtering with VBGs.(2) A broadband angular filtering with "plug-and-play" property is proposed. With the combination of dispersion of surface grating and angular selectivity of VBG, the broadband angular filtering is constructed. The relationship between the surface grating and VBG in the filter is analyzed with the vector analysis. The results show that any component with the wavelength between 750-850 nm can be diffracted efficiently with the broadband angular filter, while the angular selectivity works independently for all wavelength components.(3) The theoretical model of broadband diffraction through the broadband angular filter is built. With the Fourier analysis and the coupled wave theory, the diffraction and filtering features of broadband beam with finite aperture through the broadband angular filter are studied. The simulation results show that, the filter can filter the medium and high frequencies in the modulated beams completely. In the filtering applications, the angular selectivity of VBGs requires to be optimized, and the spectral selectivity is free to design. The impact of mismatch of the Bragg condition on the diffraction and output beam propagation is analyzed. The mismatch can be compensated with rotating the VBG carefully. The studies will benefit the application of broadband angular filtering.(4) The broadband and angular filtering of the filter are demonstrated. With a blazing surface grating and a PTR glass VBG to constmct the broadband angular filter, the filtering experiment with the white light as the source is studied. The results show that the bandwidth of the filter is up to 65 nm. The grating period mismatch between the surface grating and VBG shows close relationship to the bandwidth. When the mismatch is small, however, rotating the VBG carefully can reach the maximum bandwidth. The filtering ability of the filter is studied both in space and frequency domain. The results show that the filter can filter the frequencies of more than 1 mm’1 completely, and a laser beam with high performance in near field distribution can be obtained.The results obtained in this thesis lay a foundation for the broadband angular filtering, and will activity expand the application of VBGs in the area of high power lasers.