China's progress in the study of bulk Bragg grating angle amplifier

[ China Instrument Network Instrument Development ] The Thin Film Optics Laboratory and the High Power Laser Unit Technology Laboratory of the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences are jointly optimized for the angled amplifier in the beam large angle deflection system - cascaded multiplexed Bragg grating Designed, three 4-channel multiplexed Bragg gratings with a working wavelength of 1064 nm were successfully fabricated. The angle amplifier obtained by cascading has 12 channels, and the diffraction efficiency of each channel is over 80%, and has good polarization-independent characteristics. . Combining the angle amplifier with a small angle continuous deflection amplifier component such as a liquid crystal optical phased array enables continuous deflection of the beam in a one-dimensional space at a wide angle of ±45°.

Non-mechanical beam deflection technology is widely used in laser radar, free-space optical communication, high-energy laser systems and other fields. Large-angle spatial scanning is easy to implement in the microwave frequency band, while in the near-infrared optical band, due to the limitation of the process level and physical limit, the single device currently used, such as liquid crystal optical phased array (LCOPA) or liquid crystal polarization grating, can only be small. Continuous deflection over an angular range.

In order to realize the continuous deflection of the spatial large-angle beam in the high-energy laser system, a conventional solution is a dual LCOPA addition Bragg grating method: the first layer LCOPA realizes the irradiation of the holographic grating by small angle steering of the laser beam. Selecting, then using the volume Bragg grating (VBG) to achieve discrete large-angle deflection of the laser beam, and finally using LCOPA to achieve precise control and filling of the deflection angle of the outgoing beam. At present, the international research on VBG as an angular amplifier has not been reported in addition to several patented technologies, theoretical research and experimental implementation of in-depth systems.

In this study, the researchers considered the maximum refractive index modulation range, the small absorption loss and the distance of the distance, the crosstalk between multiple channels, and the laser divergence angle to the body grating. After the factors such as the influence of diffraction efficiency, the multi-parameter optimization design of the angle amplifier was carried out. The theoretical analysis results show that the maximum refractive index modulation range of the photothermographic glass and the divergence angle of the laser beam are the main factors limiting the maximum diffraction efficiency of each channel of the bulk grating. Multi-block multi-channel multiplexer raster cascading can effectively solve this problem.

In addition, through the symmetrical grating channel design, each set of 4-channel grating only needs to set up 2 sets of exposure light paths, which greatly improves the preparation efficiency of the multiplexed body grating. Several multiplexer gratings obtained by multiple exposure and single heat treatment are cascaded in sequence, and one-dimensional large-angle discrete deflection of -45°~45° is realized experimentally, and the diffraction efficiency of each channel is more than 80% and exhibits polarization. Irrelevant characteristics. This work provides clear and detailed guidance for the theoretical design and experimental preparation of angle amplifier based on multiplexed body grating. It is of great significance to promote the application of multiplexed body grating in beam large angle continuous deflection system. The foundation of the implementation of a two-dimensional angle amplifier with a volume grating is laid.

(Original title: Progress in the research of the Bragg grating angle amplifier of the Shanghai Opto-mechanical Machine)