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.
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.
The technique under proposal for 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 solid state cell. The technical advantage of this approach lies in providing a direct multichannel parallel processing of ultra-high-frequency radio-wave signals with essentially improved frequency resolution. This technique imposes specific requirements on the cell's material. 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 can be exploited. With the acoustic velocity of about 1.92 × 105 cm/s and attenuation of ∼10 dB/(cm GHz2), a similar cell is capable of providing an optical aperture of ∼5.0 cm and one of the highest figures of acousto-optical merit in solid states in the visible range. Such a cell is rather desirable for the application to direct 5000-channel parallel spectrum analysis with an improved up to 10−5 relative frequency resolution.
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.
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