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.
Theory of collinear acousto-optic interaction in anisotropic media has been considered. On the base of the statement that anisotropic diffraction of light by ultrasound originates from the acoustically induced changes of optical indicatrix in an anisotropic medium, the qualitative and quantitative analysis of the collinear AO interaction has been carried out. The collinear diffraction has been examined in the medium subdivided into a number of thin layers. In each of these layers, the axes of the indicatrix are rotated relatively to the neighboring ones. It has been shown that propagation of light through such system may be considered as propagation through a number of birefringent plates, rotated relatively to each other. The analytical expressions for the intensities of the incident and diffracted light have been derived. It has been shown that the diffraction efficiency depends on the angle of rotation of the axes and on birefringence of the crystal. In the case of a big number of the layers, the calculations have been carried out with the help of Jones calculus and the efficiencies of the collinear diffraction in the most commonly used acousto-optic materials such as paratellurite, α-quartz and tellurium crystals, have been calculated.
Theoretical and experimental investigation of collinear diffraction of divergent optic beams by ultrasonic waves near by the direction  of paratellurite crystal has been carried out. The diffraction has been examined in direction  forbidden in TeO2 for acousto-optic interaction in the case of plane waves. It has been shown that the collinear diffraction along this direction exists only for divergent light and sound waves while efficiency of the diffraction increase with the growth of the divergence. The effect has been examined at the acoustic frequency f≈ 149 MHz and the wavelength of the non-collimated light λ=633 nm with the optical spreading up to 4°. A collinear acousto-optic filter on paratellurite applying the diffraction has the length of interaction l=2.7 cm. It has been proved the acoustic attenuation causes a decrease of efficiency of the diffraction and a broadening of a bandwidth of the device. The spectral resolution value as high as R~3000 has been obtained a λ=633 nm. The diffraction efficiency I1/I0≈0.8 and the wide angle aperture Δφ≈4° give perspectives to use the collinear diffraction in filters on the base of TeO2 crystals.