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Abstract |
Some Features of a Laser Beam
Diffraction at Nonlinear Acoustic Waves
S.M.Kolomiets (Scientific &
Production Association "Typhoon", Obninsk, Russia)
e-mail:
kolomiets@typhoon.obninsk.org
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) [1]. 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.
Section
: 9