LOW-FREQUENCY BACKWARD ACOUSTO-OPTIC SCATTERING OF BESSEL LIGHT BEAMS

The features of acousto-optic diffraction of a Bessel light beam (BLB) on a Bessel acoustic beam (BAB) in transversely anisotropic crystals are investigated. The scheme of acousto-optic (AO) interaction is considered when the incident TH-polarized BLB propagating along the optical c axis of an optically uniaxial crystal is excited in the crystal of the TE- po-la rized BLB due to the process of anisotropic TH→TE diffraction. The diffraction problem is solved for transversely isotropic crystals, whose cylindrical symmetry is fully consistent with the symmetry of both BLB and BAB propagating along the optical axis and the AO interaction occurs without distortion of the spatial structure of beams. It is shown that in the process of backward acousto-optic scattering, the use of Bessel light beams with a large cone angle makes it possible to significantly reduce the frequency of the acoustic wave necessary for satisfying the longitudinal synchronism condition (values of less than 1 GHz). It is established that the efficiency of the AO interaction of the BLB and the BAB is determined not only by the intensity of the acoustic field, longitudinal wave detuning and the interaction length, but also by the period of transverse oscillations of Bessel beams. These oscillations determine the value of the overlap integrals and, consequently, the effective AO parameters. When the conditions of longitudinal and transverse synchronisms are realized, it is possible to achieve the high diffraction efficiency close to unity. The angular width of transverse synchronism is equal to about 0.5 mrad and increases with increasing acoustic power. It is shown that when the BLB is diffracted, the order of its phase dislocation changes by a value that is equal to the order of BAB phase dislocation. Because of the narrow angular spectrum of a scattered field, the back ward AO diffraction of Bessel light beams is promising for the development of low-frequency AO filters and spectrum analyzers. The property of self-reconstruction of the transverse profile of an optical field is promising for applications of Bessel light beams in defectroscopy.

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