We study, theoretically and experimentally, a new type of a multi-phonon Bragg light scattering in an optically
anisotropic medium. A four-phonon Bragg anomalous light scattering in a tellurium dioxide crystal was observed using a
light-blue optical beam of about 1 W in power from an argon gas-laser. Just the Bragg regime of light scattering was
assured experimentally and a pentet of the light orders was distinguished and investigated.
A few optically matched by each other sub-systems related to an advanced prototype of acousto-optical spectrometer
for radio-astronomy are analyzed jointly. Rather precise control over the incident light polarization should be assured
in the scheme together with a required expanding of the incident light beam. Moreover, the needed light-beam
apodization, suppressing side lobes within registration of each individual resolvable spot and increasing the dynamic
range of spectrometer, has to be taken into account as well. The current stage of analysis related to afore-mentioned
problems as well as the results of trial experiments are presented.
Practical applications to both ground-based and satellite exploitations have demonstrated that acousto-optical spectrum
analyzers of radio-signals represent really reliable signal-processing technique for the millimeter radio-astronomy.
These spectrometers provide sufficiently high efficiency of operation together with the frequency resolution needed for
astronomic observations. The basic component of similar spectrometer is the acousto-optical cell, whose operation is
based on its ability to shape large amount of independent dynamic diffractive gratings. Each of them reproduces the
amplitude, frequency, and phase of only one spectral component from the signal under analysis. A multi-pixel CCD
linear array detects the obtained responses in Fourier plane of the integrating lens. The main peculiarity of this
prototype lies in exploiting a large-aperture tellurium-dioxide crystalline acousto-optical cell, oriented almost along the
[001]- and [110]-axes. This cell allows a two-phonon light scattering providing the improved frequency resolution in
comparison with conventional one-phonon regime. This fact determines technical requirements to the framing sub-systems
and performances of the prototype as a whole. Due to rather high anisotropy of tellurium dioxide, the
efficiency of both one- and two-phonon light scattering depends essentially on the ellipticity of the incident light
polarization, so that high-efficient operation needs the eigen-state elliptic polarization, which is determined by the
incidence angle, light wavelength, and accuracy of the cell's crystallographic orientation. Currently, an advanced
prototype has used a green laser beam at 532 nm with central acoustic frequency about 52 MHz. The first trial
experiments in a two-phonon light scattering regime have shown frequency resolution of about 30 KHz.
We consider rather specific type of a multi-phonon light scattering in both isotropic and anisotropic media. The
analysis performed shows that under some conditions it may be possible to expect realizing various multi-phonon
processes of light scattering in a regime, which is very close to practically used Bragg scattering case. Preliminary
experiment on a four-phonon Bragg anomalous light scattering has been realized in a tellurium dioxide crystal.
Some practical aspects of creating an acousto-optical processor oriented to the calculation of triple auto- and cross-correlations
of low-power short optical pulses in time domain are under preliminary consideration. In so doing, the
shapes of both the triple auto-correlations and the bispectra inherent in the most commonly used pulses are
mathematically expressed and numerically illustrated, and the needed general schematic arrangement for a triple
correlation acousto-optical processor is designed and briefly discussed. Then, in a view of exploiting the one-channel
wide-aperture acousto-optical cells within operating similar processor, the performances of lead-molybdate crystalline
cells are tentatively estimated.
In 1970 - 80s, novel optical spectral devices, electronically tunable acousto-optical filters (AOFs) had been proposed
and developed. During the years gone AOFs have been remarkably progressed, and now they are widely exploited, for
instance, in astrophysical observations. Schematically, AOFs can be separated on collinear and non-collinear filters,
depending on the relative directions of passing the waves through crystalline cell, as well as on sequential and parallel
ones, depending on the algorithm of spectrum analysis. Their features are characterized by the amplitude and spectral
parameters. Here, we consider a few estimations of an advanced collinear AOF based on calcium molybdate single-crystal.
In principle, this new AOF with a 15-microsecond time-aperture operates over all the visible range exhibiting
60%-efficiency at the electric power 1.0 W. Direct square-law dependence for crystal's length and inverse square-law
dependence for its bandwidth on this minimal size make possible optimizing this advanced collinear AOF.
Practical applications confirm that acousto-optical spectrometers of optical and radio signals provide efficient and
accurate processing of data in real time scale. The key component of similar spectrometers is an acousto-optical cell,
which dictates performances of the optical scheme arrangement. Within non-collinear arrangement of spectrometer, the
most compact and simplest scheme of really wide-aperture light beam expander appears with exploiting an even
number of prisms. This arrangement of beam expander does not deflect light beam, provides rather high transmission,
and needs less precision for alignment than as a lens telescope requires. We describe a four-prism beam expander
providing a 35-time magnification and about 65%-transmission at a distance of 10 cm. Within elliptical polarization,
this expander exhibits properties of an amplitude filter being sensitive to the angle of beam incidence. In a view of
obtaining the needed eigen-states of elliptical polarization at the output of expander, the detailed analysis of
polarization features is performed, numerically estimated, and selectively compared with experimental data.
Both ground-based and satellite applications have demonstrated that acousto-optical spectrometers of radio-signals
represent really reliable signal-processing technique for sub-millimeter and millimeter radio-astronomy due to these
devices exhibit rather well efficiency as well as present sufficiently high frequency resolution in reasonably large
frequency bandwidth. The key component of spectrometer is the acousto-optical cell, which dictates the basic
parameters of signal processing. Its operation is based on the ability of cell to produce a large amount of independent
from one another dynamic acoustic diffractive gratings, so that each of them reproduces the amplitude and frequency
of only one spectral component from the incoming radio-signal. A multi-pixel CCD linear array detects and digitizes
the obtained responses in the Fourier plane of a large-aperture integrating lens. The main peculiarity of this prototype
lies in exploiting a large-aperture tellurium dioxide crystalline acousto-optical cell, which is oriented under a small
angle to the [001]- and [110]-axes, in the regime of anomalous light scattering by extremely slow acoustic waves
providing the improved frequency resolution. This circumstance determines the majority of technical requirements to
the framing sub-systems and performances of prototype as a whole. Due to an extremely high anisotropy of this crystal,
the efficiency of light scattering depends essentially on the incident light polarization, so that high-efficient operation
needs the eigen-state polarization, which is determined by the incidence angle, light wavelength, and accuracy of the
cell's crystallographic orientation. The first trial experiments have shown frequency resolution of about 45 KHz within
about 1500 parallel frequency channels in real time scale.
Dissipative three-wave weakly coupled states, appearing within collinear and non-collinear Bragg light scattering in a
two-mode square-law nonlinear medium with the linear optical losses, are uncovered. The conditions for localizing
these dissipative coupled sates as well as the spatial-frequency distributions of their optical components are studied
theoretically in quasi-stationary regime. Then, a set of estimations related to the realization of similar dissipative three-wave
coupled states have been performed within the acousto-optical experiments in the α-quartz crystalline cells
providing collinear and non-collinear geometries of interactions. The distinguishing feature of these potential
experiments is the fact that the presence of linear optical losses affects both shaping these dissipative weakly coupled
states and the technique for detection and identification of their optical components.
Recently proposed modern technique of a precise spectrum analysis within an algorithm of the collinear wave
heterodyning implies a two-stage integrated processing, namely, the wave heterodyning of a signal in a square-law
nonlinear medium and then the optical processing in the same cell. Technical advantage of this approach is in
providing a direct processing of ultra-high-frequency radio-wave signals with essentially improved frequency
resolution. This algorithm can be realized on a basis of various physical principles, and we consider an opportunity of
involving the potentials of modern acousto-optics for these purposes. From this viewpoint, one needs a large-aperture
effective acousto-optical cell, which operates in the Bragg regime and performs the ultra-high-frequency co-directional
collinear acoustic wave heterodyning. The technique under consideration imposes specific requirements on the cell's
material, namely, a high optical quality of large-size crystalline boules, high-efficient acousto-optical and acoustic
interactions, and low group velocity of acoustic waves together with square-low dispersive acoustic losses. We focus
our attention on the solid solutions of thallium chalcogenides and take the TlBr-TlI (thallium bromine - thallium
iodine) solution, which forms KRS-5 cubic-symmetry crystals with the mass-ratio 58% of TlBr to 42% of TlI.
Analysis shows that the acousto-optical cell made of a KRS-5 crystal oriented along the [111] -axis and the
corresponding longitudinal elastic mode for producing the dynamic diffractive grating in that crystal can be exploited.
With the acoustic velocity of about 1.92 mm/μs and attenuation of approximately 10 dB/(cm GHz2), similar cell is
capable to provide an optical aperture of 50 mm and one of the highest figures of acousto-optical merit in solid states
in the visible range. Such a cell is rather desirable for applications to direct parallel multi-channel optical spectrum
analysis with substantially improved frequency resolution.
The main features of arranging the polarization control in optical scheme of a 1000-channel acousto-optical
spectrometer for the needs of radio-astronomy are analyzed. To realize a high-resolution spectrum analysis rather
specific acousto-optical cell has to be exploited. For this cell a tellurium dioxide single-crystal oriented along the
[001]- and [110]-axes has been chosen. Due to an extremely high anisotropy of this crystal, the efficiency of light
scattering depends essentially on the ellipticity of the incident light polarization and increases when the state of
polarization reaches the eigen-state of elliptic polarization, which is determined by the incidence angle, light
wavelength, and accuracy of the cell's crystallographic orientation. This is why we analyze some peculiarities of
designing the beam-shaping scheme, which includes tunable light polarizers and a multi-prism beam expander and has
to provide the needed pre-assigned states of light polarization on the inputting aperture of acousto-optical cell.
Estimating the effective photo-elastic constants peculiar to a set of partial processes inherent in a one-phonon Bragg
anomalous light scattering of light in tellurium dioxide crystal is progressed. Really high optic and acoustic anisotropy
of this crystal leads to the fact that the efficiency of light scattering is critically conditioned by the ellipticity of the
incident light polarization and details in the geometry of acousto-optical interaction. Using a technique of the eigenvectors
for elliptical states of light polarization in anisotropic medium, we describe analytically the efficiency of a onephonon
Bragg anomalous light scattering in and optimized cell, oriented along the [001] and [110] crystallographic
axes with variously polarized incident light modes. Possible interpretation of the results obtained is briefly discussed.
This work is devoted to the problem of improving the frequency resolution inherent in a parallel acousto-optical
spectrum analysis via involving an additional nonlinear phenomenon into the data processing. In so doing, we examine
possible application of the wave heterodyning to the real-time scale acousto-optical analysis of the frequency spectrum
belonging to various ultra-high-frequency radio-wave signals. The nonlinear process of wave heterodyning is realized
through providing a co-directional collinear mixing of the longitudinal acoustic waves of finite amplitudes. This
process, which is beforehand studied theoretically, allows us either to improve the frequency resolution of spectrum
analysis at a given frequency range or to increase by a few times the current frequencies of radio-wave signals under
processing. The theoretical findings are used in our experimental studies aimed at creating a new type of acoustooptical
cell, which is able to improve the resolution inherent in acousto-optical spectrum analyzer operating over ultra-high-
frequency radio-wave signals. In particular, the possibility of upgrading the frequency resolution through the
acoustic wave heterodyning is experimentally demonstrated using the cell made of lead molybdate crystal. The
obtained results demonstrate practical efficiency of the novel approach presented.
Two optically matched by each other subsystems related to an advanced prototype of acousto-optical spectrometer for
radio-astronomy are analyzed in frames of this work. The main peculiarity of the spectrometer's prototype is exploiting
a large-aperture tellurium dioxide cell in the regime of anomalous light scattering by acoustic waves, so that just this
circumstance determines the majority of technical requirements to both the subsystems under consideration and their
potential performances. This is why the initial section is devoted to describing basic properties inherent in the chosen
regime of acousto-optical interaction. Then, within characterizing a multi-prism beam shaper, we restrict ourselves
here by the case of linear state of the incident light beam polarization. Broadly speaking, such a restriction does not
provide the highest performance data of spectrometer, in particular, the most efficient anomalous light scattering in
tellurium dioxide crystal, but similar restriction is exactly in a line with the to-day's level of our progress. The
characterization of Fourier transform subsystem is directed, of course, to achieving the resolution corresponding as
much as possible to theoretically desirable value, namely, to a pair of the CCD-pixels for each individual resolvable
spot. The obtained theoretical and preliminary experimental results are presented and discussed.
Characterizing the Bragg normal light scattering by the traveling acoustic waves in isotropic medium in with essential
optical dispersion is performed for the first time. It is shown that the scattering process under consideration includes the
main properties peculiar to the anomalous light scattering in optically uniaxial anisotropic media. In particular, an
optimized non-collinear light scattering and collinear interaction become to be unexpectedly possible in just isotropic
media. These opportunities can be exploited in acousto-optical devices to improve their performance data.
The process of co-directional collinear wave heterodyning, considered for the particular case of its realizing through interaction the longitudinal elastic waves of finite amplitudes, is studied theoretically and investigated experimentally via the acousto-optical technique. We examine possible applications of this phenomenon to a real-time scale acoustooptical analysis of the frequency spectrum belonging to ultra-high-frequency radio-wave signals. The first step along this way was connected with experimental modeling of the elastics wave heterodyning process in solids via exploitation of acousto-optical cell on the base of liquids, which allow the simplest realization of a cell with needed performance data. Then, these theoretical findings have been used in our experimental studies aimed at improving the accuracy of acousto-optical devices. In particular, the possibility of creating the acousto-optical data processing with
the acoustic wave heterodyning has been experimentally observed in a cell made of lead molybdate crystal. The obtained results demonstrate efficiency of the approach presented.
We consider two technically important problems related to implementing a new acousto-optical spectrometer for the
analysis of radio-astronomical signals. This project lies in a line with the program of developing the metrological
equipment for the Mexican Large Millimeter Telescope. Here, the main attention is paid to arranging the optical
scheme of such a spectrometer, namely, to designing the prism-made optical beam shaper and to characterizing the
potential resolution, i.e. the number of resolvable spots, inherent in acousto-optical spectrometer exploiting a one-phonon
optimized anomalous light scattering by acoustic phonons in a large-aperture cell made of the specifically
oriented tellurium dioxide single crystal.
Here, we consider one of the most important problems related to optimizing the performance data of a new acoustooptical
spectrometer for the analysis of radio-astronomical signals. The main attention is paid to estimating two factors
governing the dynamic range of that spectrometer. At first, we determine the influence of the acoustic attenuation
along a large-aperture acousto-optical cell on potential levels of lobes in focal plane of the integrating lens and then
describe capabilities of the incident light beam apodization for increasing the dynamic range of spectrometer. These
studies lie in a line with the program of developing metrological equipment for Mexican Large Millimeter Telescope.
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