Acoustooptics. Acoustooptic effect

Acousto-optic is a border area between physics and technology, in which the interaction of electromagnetic waves with sound waves is studied and the fundamentals of applying these phenomena in technology are being developed. The interaction of light with sound is used in modern optics, optoelectronics, laser technology to control coherent light radiation. Acousto-optic devices allow you to control the amplitude, frequency, polarization, spectral composition of the light signal and the direction of propagation of the light beam. An important area of ​​practical application of acousto-optic effects are information processing systems, where acousto-optical devices are used to process microwave signals in real time.

The acousto-optic effect, also known in scientific literature as acousto-optic interaction or diffraction of light by acoustic waves, was first predicted by Brillouin in 1921 and then experimentally discovered by Luke, Bicard and Debye, Sears in 1932. When considering the diffraction of light on a monochromatic acoustic wave, first of all, two limiting regimes are distinguished: Raman-NATO and Bragg. The Raman-Nath mode corresponds to relatively low acoustic frequencies f and a small acousto-optic interaction length l (usually f <10 MHz and l <1 cm). This type of diffraction is observed at arbitrary angles of incidence of light on an acousto-optic cell (Fig. 1, a), and the diffraction pattern may contain many diffraction maxima with a symmetrical distribution of the light intensity. In contrast, the Bragg mode is observed at high frequencies of ultrasound, usually exceeding 100 MHz. The diffraction pattern, even at high acoustic power, consists, as a rule, of only two diffraction maxima

zero and first orders. However, even these maxima appear only at certain angles of incidence of light near the so-called Bragg angle (Fig. 1, b). There is no clearly defined boundary between the two described diffraction modes. With increasing ultrasound frequency, the angular selectivity of the acousto-optic interaction increases, and the number of observed diffraction maxima gradually decreases. Traditionally, the Raman-NATO and Bragg modes are determined by the conditions Q << 1 and Q >> 1, respectively, where Q is the Klein-Cook parameter. Since only one diffraction maximum is used in acousto-optic devices (as a rule, the first order), the Bragg mode is preferable because of small light losses. But on the other hand, the acousto-optic selectivity inherent in the Bragg mode limits the frequency range of acousto-optic interaction and, as a result, the speed of acousto-optic devices and their information capacity.

If the acousto-optic medium is optically isotropic, then we have the ratio

In an anisotropic medium, there are two options for acousto-optic interaction. If in the process of acousto-optic interaction the type of the optical mode does not change, then (scattering species ) or (scattering species ), and then the Bragg angle is determined by the expression (1). This variant of acousto-optic interaction is known as isotropic diffraction. In another embodiment, known as anisotropic diffraction, the type of optical mode is transformed in the process of acousto-optic interaction (scattering or ). therefore and addiction it becomes much more difficult. From the point of view of practical application, all the advantages of anisotropic diffraction are a consequence of the more complex dependence of the Bragg angle on the ultrasound frequency. For example, it was shown that the best characteristics of acousto-optic deflectors are obtained in the region where . Similarly, the optimal areas for modulators and filters are areas where and .

Analytical solution of the problem of acousto-optic interaction can be obtained only for the limiting regimes of Raman-Natawa and Bragg diffraction. In the latter case, if we additionally assume that the light falls on the cell at the Bragg angle, we obtain the following expression for the diffraction efficiency:

where l * b is the cross section of the acoustic beam. The parameter M defined by the formula

where p is the density of the medium, is called acousto-optic quality. This is the main parameter by which the suitability of the material for acousto-optic applications is evaluated, since the higher the acousto-optical quality, the less acoustic power is required to obtain the necessary diffraction efficiency. Under the action of mechanical deformations carried by a sound wave, spatial modulation of the optical properties of the medium occurs, due to the elastic-optical, or photoelastic, effect. The optical properties of the medium change in time with the frequency of the sound wave, i.e., much slower both in comparison with the period of electromagnetic oscillations in the light wave and in comparison with the time of passage of the light beam through the sound beam. Depending on the ratio between the transverse size of the incident optical beam d and the length of the sound wave, the propagation of light in such a medium is accompanied by phenomena of either acousto-optic refraction or diffraction of light by ultrasound. The diffraction of light occurs not only on the sound wave introduced from outside, but also on collective excitations of the medium — acoustic phonons, resulting in light scattering with a frequency shift up and down by the frequency of the phonon (Mandelstam-Brillouin scattering). In the spectrum of the scattered radiation, pairs of the Mandel'shtam-Brillouin components shifted in frequency appear, corresponding to the scattering of light by longitudinal and transverse acoustic phonons.