Theoretical analysis of light diffraction on a periodic structure by using exact analytical solutions of the equations and numerical computer calculations has been conducted. The influence of different apodization functions on diffraction curves of reflection and transmission is studied and their comparative analysis is performed. Complete elimination of oscillating sidelobes and a significant suppression of the “tails” of a diffraction reflection curve are achieved for the specific functions of spatial apodization. The maximum values of the reflectivity of the medium with periodic dielectric permittivity are determined. The effects of light absorption in the crystal and moving gratings on the spectral resolution are evaluated. The Fabry-Perot resonator formed by two spatially-distributed “mirrors” is also studied. A significant increase in spectral resolution of filter due to the spatial variation of the refractive index of the medium is shown. It is shown that the high purity crystals are needed to observe the considered effects. Current technologies allow to obtain the glasses with the absorption coefficient α=10-7 cm-1 at which the spectral resolution Δλ ≈10-2 pm could be reached. This is three orders of magnitude narrower than the resolution of existing acoustooptic AO filters with the same crystal thickness. The considered system (a Fabry-Perot spectral filter combining a Bragg grating mirrors with apodization) can significantly increase the resolution of optical diffraction filters. Complete elimination of sidelobes and a significant suppression of the “tails” of a diffraction reflection curve are demonstrated. These results open perspectives to significantly enhance the resolution of the spectrometers, improve the parameters of mobile devices and communication channels. The results are of practical interest and can be used in the development of new diffraction acousto-optic modulators, AO filters and spectrometers.
The possibility of efficiently using metamaterials in acousto-optics has been demonstrated. Diffraction of light in heterogeneous medium with non-uniform spatial distribution of dielectric nanoparticles taking into account absorption of light is investigated. It is shown that by changing the concentration of dielectric nanoparticles in the medium, complete elimination of side oscillations and suppression of the “tails” of the diffraction reflection curve can be achieved. The possibility of controlling the hardware function of acousto-optic devices by changing the material, concentration, size, shape and spatial orientation of the inclusions, as well as the polarization of the incident radiation is shown. It is shown, that extremely large electric field enhancement can be observed in an anisotropic crystal in the presence of spatial apodization of the amplitude or abrupt change in the phase of acoustic wave.
Diffraction of light in heterogeneous medium with non-uniform spatial distribution of dielectric nanoparticles taking into account absorption of light is investigated. Influence of dimensional (geometric) effects of nanoparticles on the reflection and transmission curves is shown. In particular, by changing the concentration of dielectric nanoparticles in the medium, complete elimination of side oscillations and suppression of the "tails" of the diffraction reflection curve is achieved. The possibility of controlling the hardware function of acousto-optic devices by changing the material, concentration, size, shape and spatial orientation of the inclusions, as well as the polarization of the incident radiation is shown.
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.
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
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.
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 method for photodetectors quantum efficiency determination based upon a photocurrent fluctuations analysis at a priori defined radiation field statistics is presented. The value of quantum efficiency of silicon photodiode at the radiation wavelengths 0.67 and 0.95 micrometers is obtained experimentally.