We report design of laboratory prototype for a compact infrared acousto-optic imaging spectro-polarimeter, which may be implemented for remote or close-up analysis of planetary surfaces. The prototype concept contains a telecentric optics, apochromatic design over the bandwidth of 0.9–3.4 μm, and simultaneous imaging of two orthogonal linear polarizations of the same scene at a focal plane array (FPA). Two acousto-optic channels, the near-IR (0.9-1.7 μm) the mid-IR (1.5–3.4 μm), were developed with spectral resolution of 100 cm-1 (10 nm at 1 μm) and 25 cm-1 (20 nm at 3 μm) respectively. When imaging samples, the spatial resolution of 0.2 mm at the target distance of one meter was reached. It corresponds to 100 by 100 elements resolved at the FPA for each of the two light polarizations. This type of instruments may be considered as a potential reconnaissance and analysis tool for future planetary or moon landers and rovers to study spectral and polarization properties of the regolith.
We propose a concept of an imaging near-IR spectrometer for sensing of planetary surfaces. This instrument is intended to analyze mineralogical and, in some cases, petrographic composition of the upper surface layer in the planetary regolith; to identify and monitor OH/H2O bearing minerals and water adsorption in this layer. The scheme of the spectrometer was designed on a basis of an acousto-optic tunable filter (AOTF) that allows imaging of samples in two orthogonal polarization planes simultaneously. Images are registered as a light (e.g. solar one) reflected and scattered from an observed target in the near infrared spectral range. The AOTF’s electrical tuning provides fast and flexible spectral scanning of an image through whole the range analyzed – potentially, ten microseconds per a spectral point. Thus, it is possible to explore reflectance spectra of specified areas on a sample and to detect its minerals composition and microstructure variations. In parallel, one can estimate polarization contrast at different wavelengths thanks to the AOTF’s birefringence properties. In this paper we report design and performance of a laboratory prototype for the near-IR spectro-polarimeteric imaging AOTF system operating in the spectral range from 0.8 to 1.75 μm. Reflectance spectra of some minerals were measured with the spectral resolution of 100 cm-1 (passband 10 nm at 1 μm). When imaging samples the spatial resolution as high as 0.5 mm was reached at the target distance of one meter. It corresponds to 100 by 100 resolving elements on the CCD matrix for each of two polarizations of the reflected light. Such a concept is also being designed for the spectral range from 1.7 to 3.5 μm.
Acousto-optic (AO) interaction for a case when an acoustic signal contains three frequency components is theoretically and experimentally investigated. An existence of combinative frequencies at the multifrequency AO interaction is proved. A spatial distribution of transmitted and diffracted light intensities is demonstrated in approximation of three-step scattering process. The functioning of an AO spectrum analyzer is illustrated by an example of the three-frequency AO interaction. A new method for the expansion of the analyzer dynamic range is suggested. The efficiency of the method is experimentally confirmed.