Acousto-optical tunable filters (AOTFs) based on interaction of light and ultrasound in uniaxial birefringent crystals are widely used in imaging spectroscopy applications due to random spectral access, narrow controllable bandwidth, compactness, overal ease-of-use and image trasmittance capability. The spectral transmission of AOTF-based spectral imagers is usually characterized for paraxial light beam, but AOTFs inherently have non- uniform spatio-spectral transmission, so the central wavelength of the transmission window varies with the angle of incident light. We demonstrate that the spatio-spectral characteristics of acousto-optic (AO) interaction may be described either by the dependency of ultrasound frequency on the incident light angle for a given wavelength or by the dependency of wavelength on the incident angle for a given sound frequency. These dependencies are derived from the phase matching condition and are determined by the refractive indices, sound phase velocities and the AO diffraction geometry. We experimentally estimate the appearance of this specific spatio-spectral structure on the images acquired by AOTF-based imagers based on both collimating and confocal (telecentric) schemes and show that the variations of central wavelength and spectral bandwidth are noticeable for collimating setup even in the commonly used small field of view. The results of the study may be applied in design of AOTF- based spectral imagers and image processing algorithms.
In this paper, we propose a new design of an imaging optical system combining the advantages of stereoscopy and spectroscopy principles. The quality of spectral stereoscopic image is the key factor influencing the accuracy of threedimensional (3D) measurements and shape reconstruction. The main advantages of acousto-optical tunable filters (AOTF) in combination with stereoscopic imager are shown. We developed a new scheme of an imaging AOTF-based stereospectrometer in order to raise accuracy of 3D-measurements. According to numerical simulation, the scheme has improved image quality compared with existing ones.
We address the problem of single-shot color multiplexed digital holography and propose its new technical realization. To acquire a three-wavelength interference pattern, we propose to implement spectral filtration of wide-band light by means of acousto-optical tunable filter (AOTF) providing three-band operation. We describe digital image processing procedure for amplitude and phase information extraction, and confirm its effectiveness experimentally. Proposed approach may be effective for the detection and classification of structural elements without spectral or mechanical scanning.
We consider the problem of 3D imaging spectroscopy on base of a pair of acousto-optical tunable filters (AOTFs). In such a system, quality of stereoscopic spectral images is the key factor influencing the accuracy of three-dimensional (3D) measurements and shape reconstruction. We analyze the image quality in a dual-channel AOTF-based spectrometer and proposed the optical scheme, which is free of image blur and has only aberrations of image distortion and drift. They can be eliminated by means of geometrical calibration of the imager. The optical characteristics of the system prototype are presented. The theoretical analyses and calculations of image aberrations are confirmed by experiments. The devices based on the dual-channel AOTF-based stereoscopic system may be effectively used for various 3D imaging spectroscopy applications.
Endoscopic imaging systems are widely used for non-invasive diagnostics of inner tissues for biomedical applications as well as for non-destructive testing of various hard-to-reach industrial objects. Conventional white light endoscopy often does not provide contrast visualization of the inspected objects and quantitative characterization of their differences. In these cases, spectral imaging techniques may be helpful to increase the effectiveness of endoscopic inspection. Acoustooptic (AO) filtration of light is a very promising technique for implementing spectral imaging capabilities. In this paper, we discuss the peculiarities and benefits of conjugating AO tunable filters (AOTF) with rigid borescopes, flexible fiberscopes and video endoscopes. We analyze and compare imaging and spectroscopy capabilities of two alternative technical implementations: AOTF-based tunable light sources and AOTF-based add-on imaging modules. The results of this analysis are confirmed by multiple experiments.
Quantitative phase imaging is widely used in biomedical and industrial applications for morphology and dynamics characterization of various unstained samples with nanoscale sensitivity. Registration of phase images in multiple narrow wavelength bands enables analysis of spectral properties as well as extending the dynamic range and increasing the accuracy of quantitative phase measurements. In this paper, we present a new scheme for hyperspectral quantitative phase imaging, based on acousto-optic filtration of light in lens-in-lens common-path interferometer. It may be implemented as a PC-controlled compact add-on module for light microscope, has a robust and vibration insensitive design, and allows a quantitative phase imaging of various samples. Acousto-optic filtration provides fast and arbitrary wavelength tuning within a wide range with high spectral resolution. Choosing proper parameters of the interferometer and acousto-optical filter allows adapting the proposed scheme to many microscopy applications.
The key characteristic of any acousto-optical device, which describes its angular and spectral selectivity, is a transfer function, which depends on the light wavelength, acoustic power, configuration and length of acousto-optic interaction, refractive indices of the media, etc. The shape of transfer functions was earlier analyzed qualitatively for the most commonly used configurations and several topological types of transfer function were described in uniaxial acoustooptical crystals. For spectral imaging applications, however, this analysis is not sufficient and the detailed quantitative investigation is necessary for any particular diffraction geometry. In this study, we first time demonstrate results of accurate calculation of the transfer function for any incident light direction and any reasonable ultrasound wave angle with account of crystal geometry and parameters. All the topographical types of transfer function are described. These results are important for the development of imaging acousto-optical tunable filters with aberrations correction or with required homogeneity of the transmitted image intensity.
A compact and cost-effective lens-in-lens common-path interferometer for quantitative phase imaging of objects in white light as well as in narrow spectral bands is proposed. The optical system of the interferometer includes three lenses. The first component of the interferometer consists of two lenses with the same focal length, but different diameters. The lens with the smaller diameter is installed inside the hole in the bigger diameter lens. The light wave is thus divided into two beams. The second component is a single lens. A pinhole is placed between the first and the second components in their joint focal plane. The pinhole serves as a spatial filter to form a reference wavefront from one of the beams passed through the first component. The reference and the object beams are collected by the second component, and the digital holographic pattern resulting from their interference is registered by the camera. The performance of the system is demonstrated by quantitative phase imaging of red blood cells, onion cells and nylon fiber with use of green and red laser light sources.
An original concept of optical sensing is presented. It assumes detection of differential spectral characteristics of the analyzed object, in particular the spectral derivative. This approach provides direct discrimination of local spectral features (lines of emission, absorption Raman scattering) from the smooth background. The concept is realized with original spectral instrument based on acousto-optical tunable filter operating in modulation mode. Rapid modulation of ultrasonic wave provides inhomogeneous oscillating Bragg grating. It is demonstrated that the component of the periodical photodetector signal selected by phase-sensitive technique at the modulation frequency is proportional to the spectral derivative value at the wavelength of the filter. This optical sensing method does not require spectral recording and further derivative calculation. Therefore, it is promising in spectral imaging applications for real-time acquisition of images rectified from the background.
Two methods of AOTF bandpass broadening are compared: multi-frequency generation and frequency-modulated ultrasound. The principal goal is to increase the optical throughput and signal-to-noise ratio of AOTF-based spectrometers. We present a simplified model of linear frequency-modulated mode, which make easy qualitative analysis of that operation model. Using the model we derived exact formulas for AOTF field-of-view and the total optical throughput raise with bandpass broadening and we found that AOTF field-of-view is much larger in the linear frequency modulation (LFM) mode. We conclude that for imaging applications of AOTF-based spectral elements, LFM mode of bandwidth control is more efficient than multi-frequency mode.
Spectrally tunable illumination is widely used for colorimetry, spectroscopy, spectral-domain optical coherence tomography and other applications. Most of the swept light sources suffer either from a limited number of spectral channels or from a spatial noise and speckles. In this paper, we discuss an approach based on acousto-optic filtration of wide-band illumination. We show experimentally that acousto-optical tunable filter (AOTF) provides a unique collection of features: arbitrary spectral tuning, multi-bandpass mode and acoustic frequency modulation. It allows assigning a number, positions and width of the spectral channels. Results of multiple experiments show that the proposed technique may become the basis of the swept light sources.
The manuscript addresses the advantages and possible applications of acousto-optic image spectral filtration in lowcoherence interferometry. In particular, an effective operation of acousto-optical tunable filters in combination with Michelson-type interferometers is shown. The results of original experiments are presented. It is demonstrated that amplitude and phase spatial distributions of light waves reflected from or transmitted through the object can be fast determined in contactless manner for any spectral intervals with use of the presented techniques.
Multi-spectral imaging techniques are widely used in microscopy for many applications. One of the most widespread spectral elements for this purpose is an imaging acousto-optic tunable filter (AOTF). AOTF-based contrast visualization is especially effective when used together with other imaging techniques. Simultaneous utilization of two or more imagers requires optical coupling to provide point-to-point matching of the obtained images. Small linear and angular aperture of AOTF additionally hampers the development of multi-sensor imaging systems for microscopy. In this paper, we present a compact optical system which allows to integrate AOTF-based spectral imager into the schemes of conventional microscopes and provide high-quality spectral image of the same scale as in another, for example, wideband channel. The effectiveness of the proposed approach is demonstrated experimentally.
Endoscopic instrumentation is widely used for diagnostics and surgery. The imaging systems, which provide the hyperspectral information of the tissues accessible by endoscopes, are particularly interesting and promising for in vivo photoluminescence diagnostics and therapy of tumour and inflammatory diseases. To add the spectral imaging feature to standard video endoscopes, we propose to implement acousto-optical (AO) filtration of wide-band illumination of incandescent-lamp-based light sources. To collect maximum light and direct it to the fiber-optic light guide inside the endoscopic probe, we have developed and tested the optical system for coupling the light source, the acousto-optical tunable filter (AOTF) and the light guide. The system is compact and compatible with the standard endoscopic components.
We report on the calculation method for the development of fiber-coupled acousto-optical (AO) spectral filtration modules compatible with conventional inexpensive wide-band light sources (incandescent lamps). The formulas for main parameters of the optical systems of such modules are given. Two possible schemes (collimating and telecentric confocal) of AO filtration are discussed and compared. Their optical throughput and aberration parameters are analyzed. The proposed approach may be useful for the tunable light filtration in various applications (microscopy, endoscopy, interferometry, etc.).
The airborne hyperspectral (HS) systems for geomonitoring from board of unmanned aerial vehicles (UAV) use special modes of collection, transmission and processing of HS information. We suggest that autonomous mode can be realized on the base of real-time on-board information processing by means of spectral imaging. We demonstrate that such systems based on aberration-compensated pair of acousto-optical tunable filters (AOTF) satisfy most of the requirements for UAV HS system. The operation principle, characteristic features and prospects of the developed HS imaging system are described.
A technique for spectral filtration of interference images in full-field optical coherence tomography is proposed, implemented and tested. It is based on the spectral selection of light in the registration channel of the interferometer by imaging acousto-optic tunable filter. It is demonstrated that the diffraction of two interfering optical beams via the same ultrasound wave does not destruct the coherence. This new technique, which can be named tunable-imaging-filter-based full-field spectral-domain optical coherence tomography (TIF FF SD OCT), is applicable for 3D surface and inner structure visualization of optically inhomogeneous biomedical objects with moderate spectral, lateral and axial resolution, however with rather high speed.
A new analytical technique, named fragmentary spectrum registration (FSR), with use of acousto-optical spectrometers
(AOS’s) is proposed and studied. It is based on the method of differential optical absorption spectroscopy (DOAS) and
the unique AOS feature of fast (10 μs) random spectral access (RSA). The technique is the most efficient for objects
exhibiting sparse optical spectra. The technique permits a substantial (up to 100 times) reduction of detection time in
comparison with the record time of total spectrum and provides the decrease of inaccuracy of quantitative analysis of
multicomponent mixtures containing substances with similar spectral features. The results of numerical simulation with
use of real spectra detected by the trace gas monitoring system GAOS based on AOS are presented and discussed. The
experimental results demonstrate the capabilities of the FSR-technique for the huge reduction of the measurement time
or for the decrease of measurement error (up to 2.5 times) when the total measurement time is fixed while concentrations
being varied from the environment background up to industrial emissions level.
It is discussed the reliability problem of time-optimized method for remote optical spectral analysis of gas-polluted
ambient air. The method based on differential optical absorption spectroscopy (DOAS) enables fragmentary spectrum
registration (FSR) and is suitable for random-spectral-access (RSA) optical spectrometers like acousto-optical (AO)
ones. Here, it is proposed the algorithm based on statistical method of independent component analysis (ICA) for estimation of a correctness of absorption spectral lines selection for FSR-method. Implementations of ICA method for RSA-based real-time adaptive systems are considered. Numerical simulations are presented with use of real spectra detected by the trace gas monitoring system GAOS based on AO spectrometer.
An optimized algorithm of quantitative gas analysis for spectrometers based on acousto-optical tunable filters (AOTFs)
is presented. Efficiency of the algorithm is based on unique feature of AOTFs - random spectral access. This property
makes it possible utilization of specialized procedures for on-line processing of spectral information without any
significant loss of time. The procedure of finding optimal set of spectral points has developed and presented. The
optimized algorithm has been tested with use of gas analytical system GAOS comprising movable spectrometer based on
double-stage collinear AOTF. GAOS uses differential optical absorption spectroscopy (DOAS) for measuring gases
abundance. Optimized algorithm improves the accuracy of the results and reduces measurement time compared to
spectral-scanning algorithm. It can be used for such applications as rapid analysis of emissions in emergency, the
analysis of large collection of samples in the laboratory or in the production processes, etc. An optimized calibration
procedure for gas analyzers employing AOTF-based spectrometers is presented. It takes into account the possible
interference of the calibration coefficients of different substances and is insensitive to possible ill-conditioned calibration
matrix. Using the optimized calibration procedure allows to reduce the systematic error.
The basic factors causing the deformations of optical spectrum recorded by the AOTF-based spectrometers are discussed. It is shown that the spectrum restoration being an inverse ill-posed problem can be formulated in correct form. Some approaches are developed for spectra correction (partial restoration) and examples of correction are presented.
AOTF-based Spectral Imaging System for microscopic sample analysis in visible and NIR is described, characteristic features and some potential applications are discussed. Advantages of double AOTF configuration are analyzed
Spectrometer for real-time differential spectroscopy has been created. Spectrometer provides detection of spectrum derivatives with random spectral access. The instrument is based on AOTF with ultrasound phase manipulation.
A series of AOTF-based Imaging Spectrometers is described. Their characteristic feature is double monochromatization of optical radiation, which provides two highly important features: strong suppression of out-of-bandpass radiation, and elimination of image spectral drift. Technical characteristics and spectral images obtained are presented. Potential applications of those instruments are discussed.
Long-path spectral optical AOTF-based gas analyser is described. Comparison to other gas analyser operating on DOAS-principle is presented. Advantages of using AOTF as a spectral slective element are analysed. Results of instrument testing are listed.
The problem of finding the optimum measurement algorithm is formulated for acousto-optical spectrometers (AOS) exhibiting random spectral access. The problem is treated with the example of the measurements of a substance abundance in a two-species mixture. The optimum algorithm depends on the noise spectral distribution and the abundance of the interfered substances. The optimization being a part of the measurement process makes available the adaptation of that process to the analyzed sample. This DOAS technique based on measurements in a few selected spectral points finds an application to UV AOS-based gas analyzers for ambient air monitoring.
This paper reviews the work performed in Russia and the Former Soviet Union in the last twenty year in the development and application of acousto-optic spectrometer technology for Remote Sensing and other applications. A family of spectrometers designed for remote sensing of the Earth surface from a satellite or from an aircraft is described. There are presented the collection of visible range spectra obtained in test experiments and space mission. The collection includes spectra of the oceans, Azov, Barents, Black, Japan, Caspian, Aral seas, Dnieper and some other rivers, and different landscapes.
A mobile optical spectral equipment for monitoring concentrations of 4 gaseous impurities (SO2, NO2, Cl2, H2CO) in the air is described. The used technique is based on registration of absorption of optical radiation by the gases. The technical parameters, specification and modes of operation of the equipment are presented.
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