Development of a Mercury Bromide Birefringence Measurement System Based on Brewster's Angle.

Mercury bromide (Hg2Br2) has been used to develop acousto-optic tunable filters (AOTFs) because it has several advantages, including a high refractive index, a broad optical bandwidth, and a relatively high figure of merit. Therefore, the measurement of its birefringence is a highly important factor for ensuring AOTF quality. However, for single crystals, it is difficult (at the millimeter scale) to quantify the birefringence using an ellipsometer, as this equipment is only designed to conduct measurements on thin films. In this study, a simple birefringence measurement system for Hg2Br2 was developed based on Brewster's angle at the millimeter scale. The planar distributions of the Hg2Br2 crystal along the (100), (010), and (001) planes were used in the experiments. The developed measurement system can measure the reflected light intensity of the Hg2Br2 crystal depending on the incidence angles (rotations at 0.01125° steps) and can calculate the ordinary and extraordinary refractive indices and birefringence. The calculated birefringence of the Hg2Br2 crystal was 0.8548; this value exhibits an error of 0.64% compared with a value of 0.86 reported in the literature. The developed measurement system demonstrates the ability to be used to evaluate the quality of birefringent single crystals.

Implementation of the wide-angle acousto-optical interaction geometry in a mercury bromide single crystal.

A number of acousto-optic figures of merit and elasto-optic constants of a mercury bromide (Hg2Br2) crystal were measured. The key characteristics of the wide-angular acousto-optic tunable filter based on a Hg2Br2 crystal in the (11¯0) plane were calculated from the measurements carried out and previously known data. It is demonstrated that the crystal makes it possible to create effective devices operating in the 5-25 µm wavelength range. In addition, we proposed the design of a filter operating in the long-wavelength infrared range of 8-12 µm, providing spectral resolution up to R=300 and a field of view outside the crystal up to 10°. Its diffraction efficiency can reach up to 2% per 1 W of continuous driving power.