Acousto-optic Bragg Diffraction Research Articles

Controlled Conversion of Rays with Different Wavelengths Using Acousto-Optic Bragg Diffraction

Controlled Conversion of Rays with Different Wavelengths Using Acousto-Optic Bragg Diffraction

Effect of ellipticity of optical eigenwaves on the enhancement of efficiency of acousto-optic Bragg diffraction. A case of optically active Pb5Ge3O11 crystals

We show that the existence of optical activity in an optical material can lead to essential enhancement of acousto-optic (AO) figure of merit. The reason is that the ellipticity of interacting optical eigenwaves approaches unity near the optical axis and so additional elasto-optic (EO) tensor components become involved in the effective EO coefficient. We demonstrate on lead germanate crystals that the increase in the efficiency of AO diffraction manifests itself for all types of AO interactions whenever the incident optical wave propagates close to the optical axis. In the particular case of diffraction in the interaction plane XZ of Pb5Ge3O11 crystals, the maximal enhancement of the AO figure of merit takes place under conditions of the isotropic diffraction of ordinary and extraordinary optical waves with a pure transverse acoustic wave (AW) polarized parallel to the Y axis (a so-called AW PT2), with the AO figure of merit increasing from zero to 7.5 × 10−17 s3/kg, and the types of anisotropic diffraction on the AWs QT1 and PT2 when the AO figure of merit increases twice (e.g., from 7.6 × 10−17 to 1.5 × 10−16 s3/kg). The maximal AO efficiency in the interaction plane XZ is reached at types I and II of the isotropic AO interactions of ordinary and extraordinary optical waves with a quasi-longitudinal AW; in these cases, the AO figure of merit increases from 6.8 × 10−15 to 12.4 × 10−15 s3/kg and from 5.6 × 10−15 to 12.4 × 10−15 s3/kg, respectively.

Acousto-Optic Devices Based on Multibeam Diffraction

The multibeam acousto-optical Bragg diffraction of laser radiation is considered. This is splitting of an initial beam into several independently controlled beams (channels) without substantial light-power losses. Practically significant relationships that determine the conditions for implementing the multibeam diffraction and its main parameters have been obtained. It is shown that the necessary condition is the form of control radio signal that is close to a frequency (phase) modulated signal. Experimental studies were conducted on a polarization-insensitive acousto-optic deflector based on a paratellurite crystal. Practical applications of the multibeam diffraction are shown: laser image deposition, multichannel transmission (switching) of optical information, and the formation of the laser-beam profile.

Processing of 2D Images Using the Bragg Diffraction

Application of the acousto-optic (AO) Bragg diffraction in the processing of 2D images is analyzed. It is shown that conventional Bragg diffraction can be used for 2D processing provided that the zero-order diffraction is supplemented with the diffraction to an additional (side) order. A 2D contour of an optical image is experimentally detected using the first-order diffraction of radiation with a wavelength of 0.63 × 10–4 cm. An AO cell made from ТеО2 single crystal is used as an AO filter of spatial frequencies. Diffraction orders are formed in such an AO cell due to AO interaction with an acoustic wave with a frequency of 26 MHz.

Acoustoptic Diffraction of Two-Color Radiation at the Limit Frequency of an Acoustic Wave

A method is proposed that allows one to provide high-performance acousto-optic (AO) Bragg diffraction of two-color optical radiation at the highest possible frequency of a sound wave. The method is based on the use of AO cells together with a prism, with which two-color radiation is diluted at the required angle. The method is demonstrated based on the example of AO diffraction of a two-color Ar-laser radiation, which diffracts in a paratellurite crystal at the maximum possible frequency of a transverse “slow” sound wave, equal to ~200 MHz.

Generation of an optical vortex array in the course of acousto-optic diffraction.

Generation of an array of optical vortices (OVs) with fractional charges under the condition of acousto-optic Bragg diffraction is revealed experimentally. The OVs emerge in a wide diffracted optical beam due to reflection from a thick acousto-optic grating that contains bifurcated fringes of acoustic waves (AWs). Changes in the AW frequency lead to deflection of the wide optical beam containing the OV array. The OVs preserve their spatial positions only in the narrow AW frequency range, while larger changes in the frequency impose their movement out from the beam aperture, with appearance of the other OVs in the other places of the beam cross section.

Selection of a two-dimensional image edge using polarisation-independent acousto-optic diffraction

A method for filtering two-dimensional distributions of light-wave spatial frequencies, based on the use of polarisation-independent acousto-optic (AO) Bragg diffraction, is proposed. This method provides a maximally possible transmission bandwidth of spatial frequencies and a minimum size of the resolved AO element. A selection of a two-dimensional edge in the zero Bragg order is demonstrated. The main theoretical conclusions are experimentally confirmed using a paratellurite AO filter.

Acousto-Optic Diffraction in Paratellurite by a Slow Acoustic Mode. Increase of Diffraction Efficiency of Divergent Light

Peculiarities of anisotropic acousto-optic Bragg diffraction in paratellurite by a slow acoustic wave have been considered. The ranges of frequency-angular dependences have been found, in which effective diffraction occurs, and their practical relevance has been formulated. The numerical values for light radiation with a wavelength of 1.06 μm have been obtained. We have proposed the method of enhancement of a diffraction efficiency of divergent optical radiation, which in turn leads to the increase of the response time of an acousto-optical modulator. The method is based on a multifrequency electric control of the acousto-optical modulator. We have shown experimentally that, as distinct from the single-frequency control, a considerable intensity suppression of the zero diffraction order from 30 to 8% is achieved.

Peculiarities of the acousto-optical field formed during diffraction of dual-mode radiation in uniaxial crystals

This paper discusses acousto-optic Bragg diffraction in uniaxial crystals when two intrinsic optical modes of the crystal participate in the diffraction. The calculations are based on two crystals widely used in acousto-optics: lithium niobate and paratellurite. It is shown that the character of the acousto-optical field formed in the diffraction orders when optical rays propagate close to the optic axis while a sound wave propagates orthogonally to the optic axis makes it possible to use the resulting field for two-dimensional processing of optical images. The image can be filtered in this case by using both the zeroth and the first diffraction orders. Acousto-optic filters are fabricated from a paratellurite crystal and are used to experimentally show that a two-dimensional contour is discriminated when the image is subjected to optical Fourier processing (fast Fourier transform), using the zeroth and first Bragg orders.

Acousto-optic Bragg diffraction in paratellurite by the sidelobes of the spatial radiation spectrum of an acoustic transducer

Acousto-optic Bragg diffraction in paratellurite is investigated within the two first diffraction orders for the case of diffraction by the sidelobes of the spatial radiation spectrum of an acoustic transducer. One of the diffraction orders is due to anisotropic diffraction, and the other, to isotropic diffraction. Such a diffraction regime is achieved when the diffraction plane is inclined toward the optical axis of the crystal. For light with a wavelength of 0.63 × 10–4 cm diffracted by a “slow” sound wave with a frequency of 26 MHz, the effect manifests itself when the angle between the acousto-optic diffraction plane and the optical axis of paratellurite is ~3°. The effect is experimentally verified. The diffraction efficiency is 20% for each of the diffraction orders for a microwave signal of 8 V at the transducer.

Acousto-optical modulation of light at a doubled sound frequency

A method of acousto-optical (AO) Bragg diffraction is proposed that provides the amplitude modulation of optical radiation at a doubled acoustic frequency. The method is based on the double transmission of the light through the AO modulator made of a gyrotropic crystal and is experimentally tested by the example of the modulation of light with a wavelength of , controlled by the paratellurite AO cell.

Acousto-optic diffraction of multicolour Ar-laser radiation in crystalline quartz

We have studied acousto-optic Bragg diffraction of multicolour radiation, generated by an Ar laser in the blue-green region of the spectrum, on an acoustic wave propagating in crystalline quartz. It is shown that crystalline quartz significantly exceeds commonly used paratellurite in terms of phase matching of optical beams with a single acoustic wave. We have performed experiments on pulse modulation of Ar-laser radiation. It is shown that distortions introduced into optical pulses are substantially less when use is made of a quartz crystal rather than paratellurite.

Diffraction of three-colour radiation on an acoustic wave

We study acousto-optic Bragg diffraction of three-colour radiation having wavelengths of 488, 514 and on a single acoustic wave propagating in a crystal. A technique is developed that allows one to find diffraction regimes with a proportional change in the intensity of all radiations by varying the acoustic power. According to the technique, radiation with a maximum wavelength has to be in strict Bragg synchronism with the acoustic wave, while other radiations diffract during the synchronism detuning. The results obtained using this technique are experimentally confirmed.

Numerical analysis of first-order acousto-optic Bragg diffraction of profiled optical beams using open-loop transfer functions

In standard acousto-optic Bragg analysis, the incident light and sound beams are assumed to be uniform plane waves (with constant profiles) leading to the results based on standard weak interaction theory. As a follow-up to earlier work dealing with nonuniform incident optical beams, we revisit the problem of Bragg diffraction under nonuniform profiles, and include Gaussian, third-order Hermite–Gaussian, and zeroth-order Bessel profiles in our investigation, along with a few others. The first-order diffracted beam is examined (using a transfer function formalism based on angular spectra) under several parametric limits [such as the Klein–Cook parameter Q , the effective profile width, and the optical phase-shift parameter ( α ^ 0 ) in the sound cell]. Wherever feasible, the numerical results are compared with analytic theory. The scattered first-order profile output versus the optical phase-shift appears to maintain behavior similar to the known first-order characteristics ( sin 2 in intensity) encountered for the uniform incident beam case. It is observed, however, that such conformity exists seemingly only at relatively small values of Q (typically about 20 to 50). At higher Q s, on the other hand (where one would otherwise expect behavior closer to standard Bragg theory based on large Q s), it is found that the first-order intensity deviates substantially from the expected sin 2 - (or related) pattern. This deviation actually becomes more severe at even higher Q s. Additionally, the output profiles at higher Q s are also found to be distorted relative to the incident profiles. These results, though anomalous, are nevertheless generally compatible with earlier studies. Based on the transfer function theory, it is also known that for very large optical phase shifts (i.e., when α ^ 0 goes to infinity), the scattered first-order output for a Gaussian profile undergoes an axial (spatial) shift past the output plane of the sound cell. This predicted result is corroborated in our numerical simulation for both the Gaussian and third-order Hermite–Gaussian profiles, but not the zeroth-order Bessel or Airy profiles. These results provide both confirmation of some expected behavior for profiled beam scattering (including likely affirmation of the well known and unique diffractionless properties of certain Bessel beams), but also some insight into unexpected and anomalous photon-phonon interaction behavior.

Numerical examination of the nonlinear dynamics of a hybrid acousto-optic Bragg cell with positive feedback under profiled beam propagation

In standard weak interaction theory, acousto-optic Bragg analysis typically assumes that the incident light and sound beams are uniform plane waves. Acousto-optic Bragg diffraction with nonuniform profiled input beams is numerically examined under open loop via a transfer function formalism. Unexpected deviations in the first-order diffracted beam from the standard theory are observed for high Q values. These deviations are significant because the corresponding closed-loop system is sensitive to input amplitudes and initial conditions, and the overall impact on the dynamical behavior has not been studied previously in standard analyses. To explore the effect of such nonuniform output profiles on the feedback system, the numerically generated scattered output is fed back to the acoustic driver, and the resulting nonlinear dynamics are manipulated to create novel monostable, bistable, multistable, and chaotic regimes. The effects of the nonuniform input on these regimes are examined using the techniques of Lyapunov exponents and bifurcation maps. The orbital behavior is characterized with quadratic maps, which are an intuitive method of predicting the parametric behavior of the system. The latter trajectory-based approach offers yet a third arm in the process of developing a fuller understanding of the profiled output beam under feedback. The results of this work indicate that the nonlinear dynamical thresholds of the hybrid cell are significantly different for the profiled propagation problem than for the uniform case. The mono and bistable regimes do not coincide with the well-known uniform plane wave results, and the chaotic thresholds, which are critical to understanding encryption applications, are altered noticeably.

Guided-wave acoustooptic Bragg diffraction in silica waveguides

A rigorous theoretical analysis of the optical guided-wave, surface acoustic wave and AO Bragg interaction between the optical guided-wave and surface acoustic wave in Si-based silica waveguide is presented. The mechanical displacement, strains and acoustic power of the SAW are studied for various SAW frequencies and ZnO layer thickness. The effects of SAW frequency, ZnO layer thickness, waveguide thickness, acoustic power and acoustic apertures on the AO Bragg interaction are provided as well.

Theory of acousto-optical Bragg diffraction of ultra-short laser pulses in dispersive crystals

A theoretical analysis of collinear acousto-optical diffraction of ultra-short laser pulses for strong interaction is presented. The model simultaneously considers Bragg diffraction and group delay of wave packets. The new theoretical approach reveals transformation of the pulse envelope and pulse spreading even in a medium without group delay dispersion. These results are crucial for precise characterisation of acousto-optical dispersive delay lines.

Anisotropic acousto-optic diffraction as a method to measure and control the angular spectrum of laser radiation

The features of the anisotropic acousto-optic Bragg diffraction in paratellurite related to the measurement and control of the angular spectrum of monochromatic optical radiation are analyzed. We consider the diffraction configuration in which a narrow angular component whose position is determined by the frequency of acoustic wave is deflected from the divergent optical beam. The angular spectrum of optical radiation can be measured using variations in the acoustic frequency. It is demonstrated that such a configuration can be used for the measurement of the angular spectrum of light and for the formation of the desired angular distribution.

Acoustooptic Bragg diffraction without overemodulation with a phased-array transducer

Acoustooptic Bragg diffraction without overemodulation (i.e., with the extended high-efficiency region on the diffraction efficiency dependence vs. sound amplitude) is studied theoretically and experimentally. It is found that this effect appears in the case of a symmetrically nonuniform acoustic field and is due to the equality of additional opposite phase shifts of light beams passing through symmetric regions of the acoustic field. The situation is considered when an acoustic field is excited by a three-section phased-array transducer. The conditions are determined, in which reverse optical power transfer from the diffracted beam to transmitted beam (overemodulation) in the case of a high (close to 100%) diffraction efficiency is considerably suppressed. In the case of a phased array, the effect weakly depends on the frequency of sound and the size of transducer sections, which makes it possible to observe it in a wide range of acoustooptic interaction parameters.

10.1007/s11487-008-3011-1

The influence of acousto-optical Bragg diffraction on image characteristics is investigated theoretically and experimentally. It is shown that the zeroth, first, and second diffraction orders act, respectively, as high-, intermediate-, and low-spatial-frequency filters.