Evaluation of the Acousto-Optic Figure of Merit and the Maximum Value of the Elasto-Optic Constant of Liquids.

The elasto-optic properties of liquids on the basis of the first principles of acousto-optics were theoretically investigated. A relationship for calculating the elasto-optic constant of liquids using only the refractive index was obtained. The refractive index values corresponding to the maximum elasto-optic constant for polar and nonpolar liquids were determined. Calculations for about 100 liquids were performed and compared with known experimental data. This study significantly extends our understanding of the acousto-optic effect and has practical applications for predicting the elasto-optic constant of a liquid and estimating its wavelength dispersion.

Evaluation of Acoustic Waves in Acousto-Optical Devices by Ultrasonic Imaging.

The structure of the acoustic field defines the key parameters of acousto-optical (AO) devices. To confirm their compliance with the expected values in the presence of multiple real factors, AO crystalline cells require accurate experimental investigation of the acoustic field after being totally assembled. For this purpose, we propose to detect and quantify all the acoustic waves propagating in AO cells using an impulse acoustic microscopy technique. To validate this approach, we have analyzed both theoretically and experimentally the modes, amplitudes, propagation trajectories, and other features of the ultrasonic waves generated inside an AO modulator made of fused quartz. Good correspondence between theoretical and experimental data confirms the effectiveness of the proposed technique.

Computational technique for field-of-view expansion in AOTF-based imagers.

A rather narrow field of view (FOV) has always been considered as an essential limitation of spectral imagers based on acousto-optical tunable filters (AOTFs). We demonstrate a computational technique to overcome this constraint. It is based on preliminary precise spectral-angular characterization of beam transformation caused by light diffraction on an acoustic wave and consequent correction of acquired stack of spectral images. This technique is applicable for any geometry of acousto-optic interaction and opens the way for the development of AOTFs with significantly expanded FOV.

Spectral Imaging Experiments with Various Optical Schemes Based on the Same AOTF.

Spectral image filtration by means of acousto-optical tunable filters (AOTFs) has multiple applications. For its implementation, a few different optical schemes are in use. They differ in image quality, number of coupling components, dimensions and alignment complexity. To choose the optical system of AOTF-based spectral imager properly, many factors have to be considered. Though various schemes of acousto-optic (AO) filtration have been tested and discussed, their comparative analysis has not been reported up to now. In this study, we assembled the four most popular schemes (confocal, collimating, tandem and double-path) using the same AO cells and experimentally compared their main features. Depending on the application, each scheme may be the basis of compact cost-effective spectral imaging devices.

Polarizer-Free AOTF-Based SWIR Hyperspectral Imaging for Biomedical Applications.

Optical biomedical imaging in short wave infrared (SWIR) range within 0.9-1.7 µm is a rapidly developing technique. For this reason, there is an increasing interest in cost-effective and robust hardware for hyperspectral imaging data acquisition in this range. Tunable-filter-based solutions are of particular interest as they provide image processing flexibility and effectiveness in terms of collected data volume. Acousto-optical tunable filters (AOTFs) provide a unique set of features necessary for high-quality SWIR hyperspectral imaging. In this paper, we discuss a polarizer-free configuration of an imaging AOTF that provides a compact and easy-to-integrate design of the whole imager. We have carried out image quality analysis of this system, assembled it and validated its efficiency through multiple experiments. The developed system can be helpful in many hyperspectral applications including biomedical analyses.

Single-volume dual-channel acousto-optical tunable filter.

In this paper, we present a novel approach to spectral stereoscopic imaging. It is based on simultaneous spectral filtration of two light beams with a tunable acousto-optical filter (AOTF) of original design. It does not require large crystals and complicated optical relay systems, because two beams diffract in the same volume of the crystal medium but at different angles. We show that this geometry can be composed of a common-type AO cell and two triangular prisms of the same material. We derive equations, which specify the prism angles ensuring the necessary orientation of beams trajectories inside the crystal medium as well as parallel propagation of input and output beams. Some angles were additionally optimized for aberrations minimization by means of ray-tracing simulation. Experimental testing demonstrates rather high quality of spectral images, which is necessary for stereoscopic reconstruction procedure. The proposed approach makes possible development of spectral stereo-imaging components based on different types of previously developed AOTFs.

RGB laser based on an optical parametric oscillator for single-shot color digital holographic microscopy.

We report on the new technical realization of single-shot color digital holographic microscopy. For this application, we propose to use an original three-wavelength red, green, and blue laser based on the Nd:YAG active element with sequential parametric downconversion and upconversion of optical frequencies into red (634 nm), green (532 nm), and blue (451 nm) spectral intervals. This light source provides high-power short (∼10 ns) pulses and enables simultaneous formation of three color interference patterns in a two-path interferometer. Their registration by a color image sensor provides fast acquisition of phase delay distribution induced by the inspected object at three wavelengths without spectral tuning.

Multispectral phase imaging based on acousto-optic filtration of interfering light beams [Invited].

Acousto-optic spectral selection of light is an effective technique for interference imaging at multiple wavelengths. In this paper, we show that, depending on the location of the acousto-optical tunable filter relative to the interferometer, it is possible to enhance important characteristics of the whole system: spectral contrast, insensitivity to ambient light, performance stability, and measurement accuracy. We analyze theoretically and compare experimentally a quantitative phase imaging system based on a Mach-Zehnder interferometer with one and two acousto-optical filters located in the illumination or/and in the output channels. Visibility of the interference patterns and noise root mean square in the calculated phase maps are estimated for all cases at room temperature. It is shown that acousto-optic filtration of interfering light beams in the output channel ensures better contrast of the interference pattern and, therefore, provides better stability and higher precision of the phase measurements.

Acousto-optic tunable spectral filtration of stereoscopic images.

We propose a new technique for three-dimensional (3-D) imaging in arbitrary spectral intervals. It is based on a simultaneous diffraction of two divergent stereoscopic light beams on a single acoustic wave propagating in a uniaxial birefringent crystal. We discuss in detail this configuration of acousto-optic (AO) interaction, derive basic relations, and experimentally demonstrate the applicability of the proposed approach to 3-D spectral imaging. A stereo-imager of this type may be produced as an ultra-compact embeddable optical element, which is promising for many imaging applications.

Aberration analysis of AOTF-based spectral imaging systems.

Image aberrations caused by acousto-optic (AO) anisotropic diffraction in uniaxial crystals are discussed. For their analysis, we propose a simplified ray-tracing model of an AO crystal cell (AOC). With this approach, one can assign any configuration of AO interaction, any material and geometry of the crystal, and then estimate all conventional ray aberrations, such as spherical, coma, astigmatism, distortion, etc. The optimization procedure is demonstrated by the aberration analysis of three principal spectral imaging schemes based on AO tunable filters (AOTFs). The approach developed promises performance improvement of AOTF-based systems for high-quality spectral imaging and image processing.

Acousto-optical tunable filter for combined wideband, spectral, and optical coherence microscopy.

A multimodal technique for inspection of microscopic objects by means of wideband optical microscopy, spectral microscopy, and optical coherence microscopy is described, implemented, and tested. The key feature is the spectral selection of light in the output arm of an interferometer with use of the specialized imaging acousto-optical tunable filter. In this filter, two interfering optical beams are diffracted via the same ultrasound wave without destruction of interference image structure. The basic requirements for the acousto-optical tunable filter are defined, and mathematical formulas for calculation of its parameters are derived. Theoretical estimation of the achievable accuracy of the 3D image reconstruction is presented and experimental proofs are given. It is demonstrated that spectral imaging can also be accompanied by measurement of the quantitative reflectance spectra. Examples of inspection of optically transparent and nontransparent samples demonstrate the applicability of the technique.

Image aberrations caused by light diffraction via ultrasonic waves in uniaxial crystals.

The problem of optical image deformations caused by the phenomenon of light beam diffraction in uniaxial crystals by ultrasonic waves is considered in the paper. A general analytical expression is derived describing a dependence of spatial deformations and transmission coefficients on incidence angles as well as on parameters of the crystal and the ultrasound. The most interesting wide-angle diffraction configurations are analyzed, and all types of spatial distortions and transfer functions are described.