S. N. Mantsevich

Propagation of acoustic beams in a paratellurite crystal

A general solution to the problem of acoustic beam diffraction in an anisotropic medium has been obtained. The effect of acoustic anisotropy of paratellurite crystal on the structure of beams propagating in the XY crystallographic plane has been analyzed. Ray spectra are calculated for different propagation directions of ultrasound. The effects of beam focusing, defocusing, and autocollimation caused by acoustic anisotropy have been analyzed.

Wide-aperture TeO2 AOTF at low temperatures: Operation and survival

The effect of temperature on the performance in a wide-angle paratellurite acousto-optic tunable filter (AOTF) is analyzed on the example of two different AOTF configurations. The present study is a by-product of the AOTF characterization for space-borne applications. The two AOTFs serve as dispersion elements in spectrometers for Moon and Mars space missions. The operation of the AO filters was tested in the range of -50° to+40°C; we have also demonstrated the survival of an AOTF device at -130°C. The phase matching ultrasound frequency varies with temperature within 2.5×10(-5) K(-1) and 6.6×10(-5) K(-1). We link this temperature shift to elastic characteristics of the TeO2, and demonstrate that it is mostly explained by the temperature modification of the slow acoustic wave velocity. We point out the best reference describing experimental results (Silvestrova et al., 1987). A generalization is made for all wide-angle acousto-optic tunable filters based on tellurium dioxide crystal.

Acousto-optic interaction in an inhomogeneous acoustic field

Light diffraction in an inhomogeneous acoustic field of the near-field zone of a piezoelectric transducer has been investigated for two versions of acousto-optic interaction: collinear diffraction (at a Bragg angle of 90°) and quasi-orthogonal diffraction (at small Bragg angles). The amplitude, angular, and spectral characteristics have been numerically calculated for calcium molybdate and paratellurite crystals. It is established that the acoustic-field inhomogeneity reduces the diffraction efficiency, broadens the angular and spectral interaction ranges, and thus negatively affects characteristics of acousto-optic devices.

Collinear diffraction of divergent optical beams in acousto-optic crystals

Peculiarities of collinear acousto-optic interaction of a strongly divergent optical beam are examined theoretically by examples of two crystals widely used in acousto-optics: calcium molybdate (CaMoO(4)) and paratellurite (TeO(2)). These materials demonstrate essentially different diffraction characteristics because of peculiar features of optical and acousto-optic anisotropy in these crystals. The dependence of the integral diffraction efficiency and the transmission band of collinear acousto-optic filters on the optical beam divergence and acoustic power is studied. It is shown that with increasing light divergence these characteristics of the filter are worsened according to the same law, and the product of the relative bandwidth and the diffraction efficiency remains constant and independent of the optical wavelength and the acousto-optic interaction length.

Influence of the Divergence of a Light Beam on the Characteristics of Collinear Diffraction

Two types of anisotropic collinear diffractions of light on a diffraction grating are theoretically investigated, including that where the light is scattered forward in the direction of the incident radiation and that where the light is scattered backward in the direction toward the incident radiation. For both types, two-dimensional transfer functions are calculated, and the character of their transformation upon variation of the light wavelength and the period of the diffraction grating is analyzed. The dependence of the integrated diffraction efficiency and transmission band of diffraction filters on the divergence angle of the light beam is studied.

Influence of acoustic anisotropy in paratellurite on quasicollinear acousto-optic interaction.

The influence of paratellurite acoustic anisotropy on the quasicollinear acousto-optic diffraction characteristics was examined. In the presented case the quasicollinear geometry of acousto-optic diffraction is realized with the use of acoustic beam reflection from one of the crystal surfaces. The simulations were based on the solution of acoustic beams propagation problem for anisotropic media previously presented in Balakshy and Mantsevich (2012). It is shown that media inhomogeneity affects the distribution of the acoustic energy in the ultrasound beam and the shape of wave fronts. The acoustic beam structure influences the characteristics of quasicollinear acousto-optic diffraction causing transformation of acousto-optic device transmission function shape and reducing the diffraction efficiency.

Ray spectra and structure of acoustic beams in crystals

Propagation of acoustic beams in crystals is theoretically investigated. The effect of acoustic anisotropy on the structure of acoustic beams is analyzed based on their ray spectra. Numerical calculations are performed for fast and slow acoustic modes in the XY crystallographic plane of paratellurite single crystal, which is characterized by very strong anisotropy of elastic properties.

Influence of light polarization on characteristics of a collinear acoustooptic diffraction

The collinear acoustooptic diffraction of arbitrarily polarized radiation is studied. It is shown that the spectrum of diffracted light at the exit of an acoustooptic cell generally consists of four components, which have different frequencies and polarizations. Beats of these components lead to the modulation of the light passed through an analyzer installed at the exit of the system. Dependences of the amplitudes of components of the passed radiation on the frequency and power of an acoustic wave are studied for different orientations of the polarizer and analyzer.

Compact acousto-optic imaging spectro-polarimeter for mineralogical investigations in the near infrared

Spectral imaging in the near infrared is a promising method for mineralogy analysis, in particular well-suited for airless celestial objects or those with faint atmospheres. Additional information about structure and composition of minerals can be obtained using spectral polarimetry with high spatial resolution. We report design and performance of laboratory prototype for a compact near infrared acousto-optic imaging spectro-polarimeter, which may be implemented for remote or close-up analysis of planetary surfaces. The prototype features telecentric optics, apochromatic design over the bandwidth of 0.8-1.75 µm, and simultaneous imaging of two orthogonal linear polarizations of the same scene with a single FPA detector. When validating the scheme, reflectance spectra of several minerals were measured with the spectral resolution of 100 cm-1 (10 nm passband at 1 µm). When imaging samples, the spatial resolution of 0.6 mm at the target distance of one meter was reached. It corresponds to 100 by 100 diffraction-limited elements resolved at the focal plane array (FPA) for each of the two light polarizations. A similar prototype is also being designed for the spectral range from 1.7 to 3.5 µm. This type of the spectro-polarimeter is considered as a potential reconnaissance and analysis tool for future planetary or moon landers and rovers.

Development of a mast or robotic arm-mounted infrared AOTF spectrometer for surface Moon and Mars probes

We introduce a pencil-beam infrared AOTF spectrometer for context assessment of the surface mineralogy in the vicinity of a planetary probe or a rover analyzing the reflected solar radiation in the near infrared range. One application is the ISEM (Infrared Spectrometer for ExoMars) instrument to be deployed on the mast of ExoMars Rover planned for launch in 2018. A very similar instrument LIS (Lunar Infrared Spectrometer) is planned to be flown on Russian Luna-25 (Luna Globe Lander) and Luna-27 (Luna Resource Lander) missions in 2018 and 2021 respectively. On the lunar landers the instrument will be mounted at a robotic arm (Luna-25) or at a dedicated mast (Luna-27). The instrument covers the spectral range of 1.15–3.3 μm with the spectral resolution of ~25 cm-1 and is intended to study mineralogical and petrographic composition of the uppermost layer of the regolith. Both the Mars and the Moon instruments target waterbearing minerals, phyllosilicates, sulfates, carbonates in the vicinity of the Mars rover, and H2O ice and hydroxyl in the vicinity of lunar lander. The optical scheme includes entry optics, the TeO2 AOTF, and a Peltier-cooled InAs detector. To cover the extended spectral range the AOTF is equipped with two piezotransducers. At present the qualification prototype of the instrument is being characterized. The requirements, instrument optics, and different aspects of its characterization, including low-temperature survival validation is described.