Some Features of a Laser Beam Diffraction at Nonlinear Acoustic Waves
S.M.Kolomiets (Scientific & Production Association "Typhoon", Obninsk, Russia)
Laser beam diffraction at ultrasonic waves, which wavelength is comparable to a beam diameter (Raman - Nat diffraction), is of interest for some practical problems of acoustooptics. At the beam diameter comparable to the acoustic wavelength, the diffraction maxima are modulated in intensity. There are acoustic wave harmonics in the modulation signal (output optical signal) . The nonlinear wave on itself may be presented as a number of harmonics of some initial frequency, and in general case the levels of various harmonics vary in space (in a direction of wave propagation). In this case, frequency spectrum of an output signal (light beam intensity) may essentially differ from a spectrum of a signal relevant to a .monochromatic. acoustic wave. Generally level of the first harmonic of a signal depends on a level only of first harmonic of a wave; the level of a second harmonic of a signal depends on a level both first and second harmonics of a wave, etc. However, for some angles of observation the situation may be other. So, for an angle of observation in a direction of an axis of a light beam the level of a second harmonic of a signal depends on a level only of first harmonic of a wave. Some opportunities of determination of acoustic wave nonlinearity parameters based on a relation of levels of various harmonics are considered. The features of Fraunhofer diffraction and Fresnel diffraction are analyzed. The special attention is given opportunities of determination of a phase velocity of various harmonics of a wave on amplitude-phase responses of an output signal. The comparative simplicity of similar examinations allows to realize non-contact measurements in varies "points" of medium under study, separated in a direction of wave propagation. References 1. S.M. Kolomiets. Object displacements measured by acoustooptical methods // Physics of vibration, 1999. Volume 7, Number 2, 123 - 129.
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