Vitaly B. Voloshinov

Hyperspectral imager, from ultraviolet to visible, with a KDP acousto-optic tunable filter

Hyperspectral imaging in the ultraviolet to visible spectral region has applications in astronomy, biology, chemistry, medical sciences, etc. A novel electronically tunable, random-wavelength access, compact, no-moving-parts, vibration-insensitive, computer-controlled hyperspectral imager operating from 220 to 480 nm with a spectral resolution of 160 cm(-1), e.g., 2 nm at 350 nm, has been developed by use of a KDP acousto-optic tunable filter (AOTF) with an enhanced CCD camera and a pair of crossed calcite Glan-Taylor polarizing prisms. The linear and angular apertures of the AOTF are 1.5 x 1.5 cm2 and 1.2 degrees, respectively. Imager setup and spectral imaging results as well as analyses and discussion of various factors affecting image quality are presented.

Tunable acoustooptic filters and equalizers for WDM applications

We describe a high performance acoustooptic tunable filter for add-drop application and for signal equalization in WDM telecommunication crossconnects. It results from a thorough investigation in TeO/sub 2/ of bulk collinear interaction, the geometry of which, particularly the direction of propagation of the acoustic wave, has been chosen in order to obtain the best compromise between the spectral resolution of the device and the acoustooptic figure of merit. Less than 40 mW of electric power is needed either to deviate 100% of a selected light wavelength /spl lambda/ at resonance, or to induce a 30-dB attenuation of its intensity. The sidelobes practically vanish for this configuration and the resolution is equal to 0.75 nm (or 94 GHz) for /spl lambda/=1.55 /spl mu/m. Polarization splitters combined with half-wave plates allow to completely get rid of polarization sensitivity problems. The use of optical fibers to collect the signal at the filter outputs, actually contributes to the high performance of the device as a whole. Experiments have been performed by multiplexing three signals in the input fiber, separated by 4, 2, and 1 nm. The transmission of the filter has been examined through the bar and cross state.

Close to collinear acousto-optical interaction in paratellurite

First results on the investigation of close to collinear coustooptical interaction in paratellurite single crystals are presented. Anisotropic Bragg diffraction of light on ultrasound in TeO 2 with a length of interaction of up to several centimeters is examined. A peculiarity of the interaction is optical beam propagation in the crystal collinearly with group velocity of ultrasound. It is shown and verified experimentally that the interaction is characterized by an extremely high selectivity of scattering. Some applications of the examined regime of diffraction are discussed.

Improvement in performance of a TeO2 acousto-optic imaging spectrometer

This paper concerns the problem of the optimization of parameters in acousto-optic image processing systems. The optimization was carried out with the purpose of obtaining the best performance and minimum driving power. A choice of optimal geometry of acousto-optic interaction in the crystal is discussed. Longitudinal aberrations in the device caused by dispersion of refractive indices of acousto-optic media are studied. Calculations prove that a specific choice of distances between optical elements in the device can reduce the aberration to a value less than the focal depth of a filtered image. Spectral analysis of radiation reflected from artificial test objects demonstrates operationaal capability of the imager.

Ultraviolet-visible imaging acousto-optic tunable filters in KDP

There is a need to develop large-aperture acousto-optic tunable filters (AOTFs) in the UV region for applications in astronomy, environmental sciences, biology, etc. We have developed a high-quality noncollinear AOTF cell that uses a single crystal of KDP that has nearly a four times larger acousto-optic figure of merit, M2, than quartz. The linear and angular apertures of this cell are 1.5 cm x 1.5 cm and 1.2 degrees, respectively. The spectral range is 220-480 nm, with 160-cm(-1) spectral resolution and high transmission in the UV. We present an analysis of the design and describe the characterization results.

Development and characterization of two-transducer imaging acousto-optic tunable filters with extended tuning range

We developed two high-quality large-aperture acousto-optic tunable filter cells in TeO2 with more than two octaves spectral coverage for hyperspectral imaging applications from the visible to the midwave infrared: the first cell covers from 0.43 to 2.1 microm and the second from 0.69 to 4.0 microm. The key feature of these cells is a special design of two transducers in tandem with a special bonding technique that results in such a wide spectral coverage with virtually no acoustic and electrical loss due to careful matching of both acoustic and electrical impedances. Each of these cells has high spectral transmission, as well as low power requirement. We discuss the design, characterization, and performance results for these cells.

Hyperspectral imaging performance of a TeO 2 acousto-optic tunable filter in the ultraviolet region

A compact UV hyperspectral imager can be designed with an acousto-optic tunable filter (AOTF) installed in front of an enhanced CCD camera. Tellurium dioxide (TeO2) is the most commonly used crystal for AOTFs from 0.4 to 4 microm. We conducted an experiment to determine the UV spectral response of an imaging TeO2 AOTF in tandem with a known KDP AOTF and obtained good-quality hyperspectral images by using the TeO2 filter down to 365 nm. We compared the performance of the TeO2 filter-based hyperspectral imager with an imager based on a KDP filter because currently the latter is the best available material for the UV, and we found the performance of the TeO2 imager in the range 355-480 nm to be superior to that of a KDP imager. We present our experiments and the results obtained.

Spectral and polarization analysis of optical images by means of acousto-optics

This paper presents results of a theoretical and experimental study of Bragg anisotropic diffraction. The diffraction may be used for filtration of optical radiation and processing of images. Special attention is devoted to the investigation of diffraction, which provides spectral filtration simultaneously with polarization analysis of divergent optical beams. A tunable tellurium dioxide acousto-optical filter for processing of optical images is described in the paper. Some experimental results of the image analysis in visible light and in the near-infrared region of the spectrum are discussed.

Tunable wide-angle acousto-optic filter in single-crystal tellurium

The acoustic, optic and acousto-optic properties of tellurium crystals have been examined in order to develop an acousto-optic tunable filter (AOTF) operating in the long-wave infrared (LWIR) region. The AOTF design is based on the wide-angle regime of light diffraction in the YZ plane of the birefringent crystal operating from 8.4 to 13.6 µm. Device characteristics were obtained from both theoretical and experimental investigations. Experiments were carried out using both a 10.6 µm pulsed CO2 laser as well as a tunable CO2 laser operating in a continuous wave mode from 9.2 to 10.7 µm. The AOTF was tuned over the acoustic frequency range of 81.5–94.7 MHz. The filtering performance in the tellurium device was provided by a pure shear elastic wave propagating at a 95.8° angle with respect to the positive direction of the optic axis, while an ordinary polarized optical beam was incident at the Bragg angle of 6.0° relative to the acoustic wavefront. At 10.6 µm, the measured spectral bandwidth of the filter was 127 nm and the optical transmission coefficient was around 8.8% with 2.0 W drive power. This paper presents detailed results from both the theoretical as well as experimental device characterization including the spectral images obtained with a 256 × 256 mercury cadmium telluride camera cooled to 77 K.

Acousto-optic investigation of propagation and reflection of acoustic waves in paratellurite crystal

The paper presents results on acousto-optic investigation of unusual acoustic phenomena taking place in crystals possessing strong anisotropy of elastic properties. Advantages of the applied method of analysis are demonstrated by the example of the commonly used acousto-optic material tellurium dioxide. The major goal of the research consists of experimental verification of theoretical conclusions related to peculiar cases of acoustic propagation and reflection recently observed in the crystalline material. In particular, the case of glancing incidence and the following reflection of elastic energy from a free boundary separating the paratellurite crystal and the vacuum is examined in the paper. It is shown in the acousto-optic experiment that, in the case of glancing incidence, energy flow of a reflected acoustic wave may propagate practically in a reverse direction with respect to an incident wave. It is also proved that strong elastic anisotropy of the crystal is responsible for the unusual propagation and reflection of the acoustic waves. The research confirms the conclusion that the examined acoustic effects may be useful in development of new acousto-optic devices.