We present an original approach to precise optical spectrum analysis based on combining the nonlinear acousto-optical processing with the cross-disperser technique. This novelty gives us a possibility to overlap all the visible range by the parallel analysis with a gained spectral resolution. For this goal, an advanced rutile-made acousto-optical cell has been designed and tested as dynamic acoustic grating at ultrahigh-frequency to provide the needed slit density to increase the resolving power and the nonlinear apodization to improve the profile of a resolvable spot. Theoretical estimations were developed for low-loss 6-cm rutile-crystal at λ=440 nm, and they show that one can expect the spectral resolution 0.0609 Å, resolving power 72,250, and the number of resolvable spots ∼39,200. The illustrative proof-of-principle experiments with that rutile-crystal-made acousto-optical cell have been carried out. Experimentally we have obtained the spectral resolution ∼0.0815 Å and resolving power ∼54,000.
We develop a new avenue to creating the optical spectrometer for the Guillermo Haro astrophysical observatory (Mexico), which combines specifically progressed prism spectrometer with modern acousto-optical approach in the frame of a joint instrument. This schematic arrangement includes two principal novelties. First, we exploit recently developed acousto-optical nonlinearity of the two-phonon light scattering in crystals with linear acoustic losses, which admits an additional physical degree of freedom. This effect allows us to use nonlinear acousto-optical effect for linear processing of optical signals in parallel regime within all the visible range. Similar effect is based on the possibility for tuning the frequency of elastic waves and admits the nonlinear apodization improving the dynamic range. Secondly, we are using the cross-disperser technique with acousto-optical processing for the first time to our knowledge. Additionally, the acousto-optical spectrometers can provide almost 100% efficiency in the acousto-optical interaction in the optimized regime. In the case of 4% Mg doped LiNbO3 crystal the absorption edge can be shifted down to 370 nm for limited intensity of incoming light. The observation window of optical spectrometer in that observatory is ~ 9 cm, so that the theoretical estimations of maximal performances for a low-loss LiNbO3-crystal for this optical aperture at 405 nm give the spectral resolution 0.0523 Å, resolving power 77,400, and number of spots 57,500. The illustrative proof-of-principle experiments with available for us 6-cm LiNbO3-crystal have been performed and demonstrated the spectral resolution 0.0782 Å at 405 nm and resolving power 51,790.
We describe a potential prototype of modern spectrometer based on acousto-optical technique with three parallel optical arms for analysis of radio-wave signals specific to astronomical observations. Each optical arm exhibits original performances to provide parallel multi-band observations with different scales simultaneously. Similar multi-band instrument is able to realize measurements within various scenarios from planetary atmospheres to attractive objects in the distant Universe. The arrangement under development has two novelties. First, each optical arm represents an individual spectrum analyzer with its individual performances. Such an approach is conditioned by exploiting various materials for acousto-optical cells operating within various regimes, frequency ranges, and light wavelengths from independent light sources. Individually produced beam shapers give both the needed incident light polarization and the required apodization for light beam to increase the dynamic range of the system as a whole. After parallel acousto-optical processing, a few data flows from these optical arms are united by the joint CCD matrix on the stage of the combined extremely high-bit rate electronic data processing that provides the system performances as well. The other novelty consists in the usage of various materials for designing wide-aperture acousto-optical cells exhibiting the best performances within each of optical arms. Here, one can mention specifically selected cuts of tellurium dioxide, bastron, and lithium niobate, which overlap selected areas within the frequency range from 40 MHz to 2.0 GHz. Thus one yields the united versatile instrument for comprehensive studies of astronomical objects simultaneously with precise synchronization in various frequency ranges.
We consider the collinear backward light scattering as one of potential physical mechanisms for creating an advanced collinear acousto-optical filter with significantly improved spectral resolution. Within co-directional crystalline collinear acousto-optical filters, the resolution is inversely proportional to the material’s birefringence, which is ~0.1 in the best case of LiNbO3-crystal. For the backward light scattering this parameter is replaced by the twice refractive index, so that the gain achieves > 45 times in LiNbO3-crystal. Thus, one can expect the resolution ~0.002 Å instead of the value ~0.1 – 0.2 Å recently obtained by us experimentally for co-directional geometry in that crystal. Moreover, the collinear backward light scattering can be implemented in an extended amount of available materials. However, the main limitation for applying the phenomenon is the acoustic attenuation in ultra-high frequency range peculiar to the materials for acousto-optical filters. Linear acoustic attenuation reduces the length of acousto-optical interaction and limits the spectral resolution. At an initial stage, our studies are concentrated on physical aspects of this phenomenon as well as on possibilities of its application to creating the acousto-optical filters of a high resolution. In particular, the frequency band-shapes of the scattered light intensity exhibit a tendency to specific acousto-optical nonlinearity, i.e. to more and more rectangular profile as the acoustic power density grows. Results of this analysis confirm the above-noted estimations for a triplet of trigonal acousto-optical crystals, having possibly lower acoustic attenuation.
We develop a multi-band spectrometer with a few spatially parallel optical arms for the combined processing of their
data flow. Such multi-band capability has various applications in astrophysical scenarios at different scales: from objects
in the distant universe to planetary atmospheres in the Solar system. Each optical arm exhibits original performances to
provide parallel multi-band observations with different scales simultaneously. Similar possibility is based on designing
each optical arm individually via exploiting different materials for acousto-optical cells operating within various
regimes, frequency ranges and light wavelengths from independent light sources. Individual beam shapers provide both
the needed incident light polarization and the required apodization to increase the dynamic range of a system. After
parallel acousto-optical processing, data flows are united by the joint CCD matrix on the stage of the combined
electronic data processing. At the moment, the prototype combines still three bands, i.e. includes three spatial optical
arms. The first low-frequency arm operates at the central frequencies ~60-80 MHz with frequency bandwidth ~40 MHz.
The second arm is oriented to middle-frequencies ~350-500 MHz with frequency bandwidth ~200-300 MHz. The third
arm is intended for ultra-high-frequency radio-wave signals about 1.0-1.5 GHz with frequency bandwidth <300 MHz.
To-day, this spectrometer has the following preliminary performances. The first arm exhibits frequency resolution ~20
KHz; while the second and third arms give the resolution ~150-200 KHz. The numbers of resolvable spots are 1500-
2000 depending on the regime of operation. The fourth optical arm at the frequency range ~3.5 GHz is currently under
construction.
Regime of effective non-collinear acousto-optical interaction with tangential phase matching had been identified and
previously observed only in two limiting cases: in tellurium dioxide (TeO2) at low acoustic frequencies (~60 MHz)
and in rutile (TiO2) at ultra-high frequencies (~5 GHz). Both these limits are motivated by optical properties of the
chosen materials. Low frequencies in TeO2 admit designing a wide-aperture acousto-optical cell, but limit the
frequency bandwidth. While an acousto-optical cell made of TiO2 has very small aperture and exhibits low spectral
resolution due to the effect of linear acoustic attenuation. Instead of those limits, we propose an advanced regime of
the anomalous acousto-optical interaction with tangential phase matching, which allows us varying the frequency
range and optimizing all the performances (for instance, the spectral resolution) of a wide-aperture acousto-optical
cell made of the chosen crystal, as the case requires. Recently, we had suggested and successfully tested
experimentally the revealed additional degree of freedom, i.e. the action of the tilt angle within the refractive indices
ellipsoids to manipulate by the performances of crystalline acousto-optical cells. Now, we consider an opportunity
of refining this additional degree of freedom within those ellipsoids of crystalline acousto-optical cell through some
declination of the acoustic beam. For our investigations, the lithium niobate (LiNbO3) and rutile (TiO2) crystals of
about 5 cm length, operating with the slow-shear acoustic mode along the acoustic axes had been selected. The
needed theoretical analysis, numerical estimations, and 3D-vector diagrams have been developed to reveal potential
benefits of the proposed technique.
We study the potentials of a wide-aperture crystalline calomel-made acousto-optical cell. Characterizing this cell is nontrivial due to the chosen regime based on an advanced noncollinear two-phonon light scattering. Recently revealed important features of this phenomenon are essentially exploited in the cell and are investigated in more detail. These features can be observed more easily and simply in tetragonal crystals, e.g., calomel, exhibiting specific acousto-optical nonlinearity caused by the acoustic waves of finite amplitude. This parametric nonlinearity manifests itself at low acoustic powers in calomel possessing linear acoustic attenuation. The formerly identified additional degree of freedom, unique to this regime, is exploited for designing the cell with an eye to doubling the resolution due to two-phonon processes. We clarify the role of varying the central acoustic frequency and acoustic attenuation using that degree of freedom. Then the efficiency of calomel is exploited to expand the cell’s bandwidth with a cost of its efficiency. Proof-of-principle experiments confirm the developed approaches and illustrate their applicability to innovative techniques of optical spectrum analysis with the improved resolution. The achieved spectral resolution of 0.205 Å at 405 nm and the resolving power 19,800 are the best for acousto-optical spectrometers dedicated to space or airborne operations to date as far as we know.
Performances of any system for data processing based on acousto-optical technique are mainly determined by parameters of the acousto-optical cell (AOC) exploited within the schematic arrangement. Here, basic properties of the AOC, involved into a novel processor for precise optical spectrum analysis dedicated to modern astrophysical applications, are considered. Because potential applications of this processor will be focused on investigations in extra-galactic astronomy as well as studies of extra-solar planets, an advanced regime of the non-collinear two-phonon light scattering has been elaborated for spectrum analysis with significantly improved spectral resolution. Under similar uprated requirements, the AOC, based on that specific regime in the calomel (Hg2Cl2) crystal, had been chosen, and its parameters were analyzed theoretically and verified experimentally. Then, the adequate approach to estimating the frequency/spectral bandwidth and spectral resolution had been developed. The bandwidth was calculated and experimentally realized with the additionally involved tilt angle of light incidence, allowing variations for acoustic frequencies. The resolution was characterized taking into account its doubling peculiar to the nonlinear two-phonon mechanism of light scattering. Proof-of-principle experiments were performed with the calomel AOC of 52 mm optical aperture, providing ~94% efficiency in the transmitted light due to the slow-shear acoustic mode of finite amplitude (the acoustic power density ~150 mW/mm2) with the velocity of 0.347×105 cm/s at the radio-wave acoustic frequency ~71 MHz. As a result, we have obtained the spectral resolution <0.235 Å within the spectral bandwidth <290 Å that looks as the best one can mention at the moment in acousto-optics.
Principally new features of the non-collinear two-phonon light scattering governed by elastic waves of finite amplitude in birefringent bulk crystals are detected and observed. The main goals of our investigations are to reveal novel important details inherent in the nonlinearity of this effect and to study properties of similar parametric nonlinearity both theoretically and experimentally in wide-aperture crystals with moderate linear acoustic attenuation. An additional degree of freedom represented by the dispersive birefringence factor, which can be distinguished within this nonlinear phenomenon, is characterized. This physical degree of freedom gives us a one-of-a-kind opportunity to apply the strongly non-linear two-phonon light scattering in practice for the first time. The local unit-level maxima in the distribution of light scattered into the second order appear periodically as the acoustic power density grows. It makes possible to identify a few transfer function profiles peculiar to these maxima in the isolated planes of angular-frequency mismatches. These maxima give us an opportunity to choose the desirable profile for the transfer function at the fixed angle of incidence for the incoming light beam with a wide spectrum .The needed theoretical analysis is developed and proof-of-principle experiments, performed with a specially designed wide-aperture acousto-optical cell made of the calomel (α-Hg2Cl2) crystal, are presented. The obtained spectral resolution ~0.235 Å at 405 nm (i.e. the resolving power ~17,200) can be compared with the most advanced acousto-optical spectrometers for space/airborne operations. Evidently, our results with the calomel-based acousto-optical cell look like the best we can mention at the moment.
Our work is devoted to the collinear acousto-optical filter governed by the acoustic waves of finite amplitude. It represents a novel bulk-optical component, namely, the dispersive element for optical spectroscopy. This filter is based on specifically doped lithium niobate single crystal that unexpectedly works in the near ultraviolet range as well as this material usually works in the visible range. We examine the phenomena affecting the filter transmission efficiency and its resolution, i.e. the light-induced absorption and photorefraction. A new nonlinear approach is used to characterize performances of this collinear LiNbO3 acousto-optical filter exploiting our revealed specific acousto-optical nonlinearity. We have carried out the experiments with the collinear filter based on the congruent LiNbO3 crystal of 6.3 cm length at λ = 405 and 440 nm to verify our analysis and estimations. We also explore an opportunity to trade an amount of the efficiency to improve the spectral resolution. The transmission efficiency steeply increases with increasing light wavelength and with decreasing length of the filter, nevertheless the efficiency still remains higher than 30% in the near ultraviolet, if the spectral resolution is limited by δλ = 0.28–0.29 Å. Moreover, we demonstrate the possibility to reach a resolution as high as δλ = 0.12–0.15 Å (R > 24600), preserving at the same time an efficiency higher than 10% over the spectral interval that we considered. It looks like our filter holds the best to our knowledge experimentally confirmed spectral resolution for any collinear acousto-optical spectrometers dedicated to space/airborne operations.
We explore the potentiality of using an advanced collinear LiNbO3 acousto-optical filter as a dispersive element for a high-resolution optical spectrograph. Our analysis is focused on weak optical signals in the blue to near-ultraviolet range accessible to ground-based facilities. We examine the phenomenon affecting the filter transmission efficiency and its spectral resolution, namely, the light-induced absorption and photorefraction. A new nonlinear approach is used to determine the performance of this collinear LiNbO3 filter governed by acoustic waves of finite amplitude. The highest available spectral resolution attains δλ=0.15 Å at λ=370 nm (the resolving power R∼25,000), with an efficiency of 11%, or δλ=0.18 Å at λ=532 nm (R∼30,000), with an efficiency of 33%. A slight decrease in the spectral resolution would imply a significant increase in transmission efficiency. Then, we carried out proof of principle experiments with the collinear filter based on the congruent LiNbO3 crystal of 6.3-cm length at λ=405 and 440 nm to verify our analysis and estimations. Potential applications are tackling many issues in astronomy, from detailed abundance analysis in a variety of targets to precise radial velocity measurement.
This investigation represents a deep and advanced analysis of exploiting lithium niobate (LiNbO3) crystals for the
collinear acousto-optical tunable filter (AOTF) in violet and near ultraviolet ranges. The selection of this material is
motivated by its high birefringence, which is a key parameter for improving the resolution of AOTF. For this matter, we
take into account all the important factors that can deteriorate the resolution in order to find extreme conditions for the
best performances. In concrete, we analyze the well- known photorefraction effect accompanied by the light induced
absorption in those ranges for the LiNbO3 crystals doped by selected materials. The best observed results have been
obtained with magnesium (Mg) dopant in the congruent melt of LiNbO3, which also shifts the absorption edge far into
the middle UV-range. This analysis had made it possible to formulate the physical criterion determining the enlarged
practical limitations of the incident light power density.
Together with previously studied non-uniformity and dispersion of the birefringence along the length of acousto-optical
interaction in a crystal, we exploit the recently discovered and experimentally confirmed acousto-optical nonlinearity,
which can improve the transmission function inherent in the collinear interaction via applying the acoustic waves of
finite amplitude in the AOTF. As a result, the obtained spectral resolution is the best available for any collinear AOTF to
our knowledge.
We investigate the components of dissipative multi-wave solitons in the form of three-wave weakly coupled states
originating within the collinear acousto-optical interaction due to acoustic waves of finite amplitude. This investigation
is carried out in a square-law nonlinear birefringent medium with linear optical losses, theoretically and experimentally.
Theoretically, we study the three-wave collinear acousto-optical interaction using several acoustic pulse profiles, with
the cases of infinite support (when the acoustic pulse envelope is gradually vanishing on the boundaries) and compact
support (when acoustic pulse envelope is cut down on the boundaries), and consider the appropriate boundary conditions
in a quasi-stationary regime with the phase mismatch. As a theoretical result, one has found that the system can be
described; in particular, by the cnoidal Jacobi elliptic functions whose limiting cases lead to hyperbolic and
trigonometric solutions.
The experiments dedicated to examine these theoretical results have been done with a X-ray irradiated α-quartz
crystalline cell enabling the collinear acousto-optical interaction. The cell used the pure longitudinal acoustic wave with
the frequency mismatch along the 6 cm interaction length. Two types of acoustic pulses had been generated, namely,
hyperbolic-secant pulse (infinite support) and a bounded rectangular pulse (compact support). During these experiments
one had observed the optical components peculiar to the mismatched weakly coupled states. Rather well agreement
between the theoretical model, simulated numerically, and the obtained data of measurements for the frequency
detuning, acoustic power density, and efficiency of the coupled states localization have been achieved.
The Guillermo Haro astrophysical observatory (Mexico) realizes investigations in the visible and near-infrared range . Actually, the classical grating spectrometer with mechanically removable optical filters and diffraction gratings is exploited there. Unfortunately, the regular process of changing the diffraction gratings with different resolutions and recalibrating the spectrometer is inconvenient and wastes expensive time for observations. This is why exploiting an acousto-optical cell as a dynamic dispersive element is practically desirable, because potentially it realizes tuning the spectral resolution and the range of observation electronically and excludes filters. Some aspects of inserting just one dynamic acousto-optical diffraction grating instead of a set of the traditional static diffraction gratings are preliminarily considered.
Optical spectrometer of the Guillermo Haro astrophysical observatory (Mexico) exploits mechanically removable
traditional static diffraction gratings as dispersive elements. There is a set of the static gratings with the slit-density 50 –600 lines/mm and optical apertures 9 cm x 9 cm that provide the first order spectral resolution from 9.6 to 0.8 A/pixel, respectively, in the range 400 – 1000 nm. However, the needed mechanical manipulations, namely, replacing the static
diffraction gratings with various resolutions and following recalibration of spectrometer within studying even the same
object are inconvenient and lead to losing rather expensive observation time. We suggest exploiting an acousto-optical
cell, i.e. the dynamic diffraction grating tunable electronically, as dispersive element in that spectrometer. Involving the
acousto-optical technique, which can potentially provide electronic control over the spectral resolution and the range of
observations, leads to possible eliminating the above-mentioned demerits and to improving the efficiency of analysis.
An opportunity to exploit specific mechanisms of the acousto-optic nonlinearity to regulate performances of the collinear acousto-optical filter, realizing the sequential spectrum analysis of optical signals, is considered. This possibility is theoretically analyzed and experimentally confirmed with an advanced filter based on calcium molybdate (CaMoO 4 ) single-crystal with a 15-μs time-aperture. It is able to operate over red and near-infrared light at relatively low radio-wave frequencies providing almost lossless regime for controlling acoustic waves of the finite amplitude. Under certain conditions, the transmission function of electronically tunable filter exhibits a marked dependence on the applied acoustic power density, and as a result, one can significantly squeeze the transmission function, i.e., improve the spectral resolution of this filter at the cost of decreasing the efficiency of the device partially. The identified and observed non-linear effect makes possible varying the performance data of similar advanced collinear acousto-optical filter governed by external signals of the finite amplitude.
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