Materials for imaging acousto-optic tunable filters

Abstract

Research and development of robust compact hyperspectral imagers that can acquire both spectral and spatial features from a scene of interest is of utmost importance for standoff detection of targets as well as chemical and biological agents and backgrounds. Hyperspectral imagers can acquire images with a large number of narrow spectral bands and take advantage of the characteristic spectral signatures of different materials making up the scene. At the Army Research Laboratory (ARL), we are developing hyperspectral imagers based on acousto-optic tunable filters (AOTFs) that can provide adaptive no-moving-parts imagers from the ultraviolet (UV) to the long wave infrared (LWIR) to acquire a two-dimensional spectral image and build up a two-dimensional image cube as a function of time instead of using traditional grating or prism based approach that requires relative motion between sensor and scene. Here, we will review the development of different imaging AOTFs operating from the UV to the LWIR based on a variety of birefringent materials and include the spectral imaging carried out with these filters including both with single and double piezoelectric transducers. We will also include the theoretical background needed to carry out the filter design and discuss development of mercurous halide crystals that can be used to develop AOTFs operating over a wide spectral region from the visible to the LWIR.

Acousto-optic interaction model with mercury halides (Hg2Cl2 and Hg2Br2) as AOTF cristals

“…Although Hg2I2 and Hg2Br2 crystals have really high qualities for AO devices, some development research works are still ongoing to be able to grow longer robust crystals 26,41 . At the present time research is ongoing in finding the most suitable bonding media with mercury based crystals which will be stable over a long time 42,43 .…”

Analysis of Phase Mismatch for Mercurous Bromide-Based Non-Collinear AOTF Design in Spectral Imaging Applications

“…However, the AOTF field is still actively seeking better-performing crystals, especially those that can transmit in the mid-to long-wave infrared spectrum. The mercurous bromide (Hg 2 Br 2 ) crystal proved to be outstanding at producing broadband spectral devices due to its advantages of a broad transparent range with high transmittance from visible to long-wave infrared bands (0.42-30 µm) [20]; high acousto-optic figure of merit of 2600 (×1.5 × 10 −18 s 3 /g), which characterizes the diffraction efficiency of acousto-optic devices, while the commonly used TeO 2 has an acousto-optic figure of merit of 800 (×1.5 × 10 −18 s 3 /g) [21,22]; and large birefringence, which characterizes the angular aperture of acousto-optic devices [23,24]. The challenge in Hg 2 Br 2 crystal AOTF research is the availability of high-quality AOTF, resulting from difficulties in the growth of high-quality, large-size crystals and the instabilities in the bonding of piezoelectric transducers.…”

Calibration of Acousto-Optic Interaction Geometry Based on the Analysis of AOTF Angular Performance

“…Noncollinear AOTFs have some advantages, as a larger angular aperture and more materials with a large acousto-optic figure of merit can be chosen. Among them, TeO 2 crystals are very suitable for noncollinear AOTFs that cover the spectral range of 350–4500 nm [ 14 , 15 ].…”