Acousto-optic Bragg diffraction is an electronically controllable angular-selective type of laser beam scattering. A variety of transfer functions of acousto-optic Bragg diffraction in crystals is observed from the point of view of laser beam shaping. Those include one-dimensional isotropic filtering, and two-dimensional anisotropic X- and O-type transfer functions. Those types of transfer functions are useful for obtaining flat-top laser beam shaping, focusing, and wavefront sensing. Dynamic generation of ultrasonic waveforms is discussed as a method for adaptive control of the transfer function shape.
The paper represents a brief overview on theoretical and experimental investigations of feedback acousto-optic systems carried out in Moscow State University in recent years. These systems are of great interest by reason of their complicated and intriguing behavior as well as in view of new applications in optical information processing. Various regimes of operation of acousto-optic systems with amplitude and frequency feedback are analyzed. The application of these systems for optical beam intensity and direction stabilization are considered. An original scheme of acousto-optic generator based on the effect of optical heterodyning is proposed and examined theoretically and experimentally.
The acousto-optic method of phase object visualization is generalized to the case when an optical wave as well as being spatially phase-modulated, has also amplitude spatial modulation. A special geometry of acousto-optic interaction is proposed for separation of the phase modulation and visualization of the optical wavefront. Basic characteristics of the visualizing system are calculated. It is shown that in the optimal case the distribution of light intensity in the visualized image is proportional to the phase gradient in the acousto-optic interaction plane. The resolution and the contrast of the visualized image are determined primarily by the divergence angle of the acoustic beam. Some examples of computer simulation are presented for illustration of potentialities of this method.
Results are presented of a theoretical study of optical image processing based on two-dimensional acousto-optic filtration of the spatial spectrum of images. Most attention is concentrated on the analysis on the analysis of the acousto-optic transfer function form and its dependence on crystal cut, geometry of acousto-optic interaction and ultrasound frequency. Results of computer simulation of acousto-optic spatial filtering are illustrated by the example of an object in the form of an amplitude grating. For this object, effects of image differentiation and integration are demonstrated.
A method of phase object visualization based on selective properties of acousto-optic diffraction is described. It is shown that in the optimal case the distribution of light intensity in the visualized image is proportional to the phase gradient in the acousto-optic interaction plane. The resolution and the contract of the visualized image are determined primarily by the divergence angle of the acoustic beam. Some experimental results as well as examples of computer simulation are presented for illustration of potentialities of the method.
Acousto-optic (AO) interaction for a case when an acoustic signal contains three frequency components is theoretically and experimentally investigated. An existence of combinative frequencies at the multifrequency AO interaction is proved. A spatial distribution of transmitted and diffracted light intensities is demonstrated in approximation of three-step scattering process. The functioning of an AO spectrum analyzer is illustrated by an example of the three-frequency AO interaction. A new method for the expansion of the analyzer dynamic range is suggested. The efficiency of the method is experimentally confirmed.
An acousto-optic generator based on the effect of optical heterodyning is investigated theoretically and experimentally. The condition for self-excitation of oscillations in this system is determined, its dependence on parameters of acousto- optic interaction and feedback circuit is analyzed. The possibility of single-mode and multimode regimes of operation is shown. Preliminary results of experimental investigations of the generator with a paratellurite acousto-optic cell are presented. Experiments on active mode locking by means of an external harmonic signal are described as well.
An acoustooptic system with optoelectronic feedback providing laser beam direction stabilization is studied. Different modification of the system based on isotropic and anisotropic diffraction of light are considered. Influence of the acoustooptic selectivity on the operating angular range and the stabilization coefficient is analyzed in detail. Results of experimental investigation of the system based on a paratellurite crystal cell are presented as well.
A number of problems of light diffraction in a nonstationary acoustic field are considered in the paper with using the spectral method. Transient processes arising in the acoustooptic cell during an abrupt change of acoustic amplitude, frequency or phase are analyzed in detail. The effects of diffracted mode concurrence and modulation nonreciprocity which are typical for multifrequency diffraction in the strong acoustooptic interaction regime are studied both theoretically and experimentally.
Angular characteristics of the second and third diffraction orders are theoretically analyzed and experimentally tested for the intermediate regime of acoustooptic interaction. It is established that there is for each order a number of optimal incidence angles of light that provide more effective scattering of light. In case of isotropic diffraction the optimal angles depend on acoustic frequency, whereas at anisotropic diffraction the angles are defined not only by the frequency, but also be optical anisotropy of the interaction medium.
Results on investigation of thermal effects which appear during operation of an acousto-optic deflector are presented in the paper. Influence of the effects on characteristics of the scanning device is evaluated. For a case of the tellurium dioxide deflector, it is proved that the most principal thermal effect consists in an undesirable spatial shift of a scanning optical beam. The shift leads to the optical beam positioning error which in a typical deflector may be of dozens of resolvable spots. A method of the thermal shifts compensation based on acoustic frequency tuning is proposed. For various regimes of the deflector operation, laws of the frequency tuning are found thus providing the compensation of the shifts with the required precision.
Polarization effects that arise during diffraction of an arbitrarily polarized light by an acoustic wave in an anisotropic medium are studies both theoretically and experimentally. Peculiarities of these effects in the cases of isotropic and anisotropic scattering of the light are considered in detail. Results of polarization state measurements performed with a paratellurite acousto-optic cell in the zeroth order of diffraction are presented as well.
An acousto-optic system with a feedback which controls frequency of acoustic waves excited in an acousto-optic cell is investigated theoretically and experimentally. In the system an amplitude transparent situated in front of a photodetector defines a nonlinear dependence of light intensity registered by the photodetector on the ultrasound frequency. Conditions for appearance of multistable states, which differ by amplitude, frequency and diffracted beam direction, are found. Applications of the system for information channels switch in optical communication and for stabilization of laser beam direction are discussed as well.
Some results on theoretical and experimental investigation of acousto-optic deflectors containing an optoelectronic feedback of a hybrid type are presented. Two modifications of these devices in which the feedback signal controls amplitude or frequency of acoustic waves are considered. A possibility of realization of high order multistable regimes is shown. Different applications of such deflectors for optical information processing are discussed.
Results of the theoretical investigation of light polarization changes during Bragg acousto-optic interaction are presented. It is shown that a linearly polarized light wave, entering an acoustic field, becomes elliptically polarized because of an additional phase shift effect. The polarization state can be controlled by varying power or frequency of the ultrasound. An effect of polarization nonreciprocity for two optical waves passing through the acoustic field in opposite directions is also studied.
Results of the theoretical and experimental investigation of an additional phase shift effect, which arises during propagation of a light beam through an acoustic field, are presented. Some consequences of the effect, namely, a light polarization change and a polarization nonreciprocity are considered as well.
An acousto-optic system containing opto-electronic feedback through diffraction maxima of different orders is investigated theoretically and experimentally. Taking into account nonlinear characteristics of generalized view, the existence of bistable state are found in the system. Peculiarities of three regimes of system operation with switching by electrical or optical impulses or by frequency change are studied. A possibility of realizing a multistable regime is demonstrated.