Crystal Growth, Fabrication, and Design of Mercurous Bromide Acousto-Optic Tunable Filters

Device-quality single crystals of mercurous bromide were grown by the physical vapor transport method. Crystals transmitted light wavelengths up to 30 µm and did not show any absorption bands. Detailed x-ray Laue and x-ray diffraction studies were used to characterize and orient the crystals. Optical evaluation was performed by fabricating slabs of crystals. A design was developed to fabricate acousto-optic tunable filters with 10-deg off-axis orientation operating in the mid- and long-wavelength regions. An acousto-optic tunable filter (AOTF) was fabricated using a crystal with a 16-mm optical aperture for the 10-deg design. A theoretical tuning curve for a mercurous bromide crystal-based AOTF using this design was also computed for the first time. Experimentally measured data on frequency matching agreed well with the theoretical predictions, and the transducer thickness was suitable for filtering 7.58 µm with the fabricated AOTF.