Ultrasonic wave refractive index

The presence of undissolved gas and of cavitation bubbles affects the transparency and refractive index of a liquid. Thus when a sonicated liquid is irradiated with light, X-rays, y-rays, or even high-frequency ultrasound, the attenuation and (or) refraction of the wave can be used to detect both the cavitation threshold and bubble density, and their variation with time. This is possible even within a very short period of the order of one ultrasonic cycle [138,139]. 

Often an acousto-optic switch is used, for example, for argon lasers and cw dye lasers [648]. Its basic principle is explained in Fig. 6.6. A short ultrasonic pulse with acoustic frequency / and pulse duration T 1 //s is sent nit = to through a fused quartz plate inside the laser resonator. The acoustic wave produces a time-dependent spatially periodic modulation of the refractive index n(t,z), which acts as a Bragg grating with the grating constant A = Cs//, equal to the acoustic wavelength A where Cg is the sound velocity. When an optical wave Eocos((ot — k r) with the... 

The fraction rj depends on the modulation amplitude of the refractive index and thus on the power of the ultrasonic wave. 

Fig. 9 shows the shift in frequency of the Brillouin components as a function of temperature at a constant scattering angle of 90° C in PMMA. The change in the Brillouin lines is due to a change in the velocity of the hypersonic waves with temperature. At 90° C the sound waves or phonons have a wavelength of /I = 30(X) A. Using this value and the value of 1.491 for n, the index of refraction, it was found that at 20° the measured sound frequency of 9.58 x 10 Hz multiplied by A yielded a value of k = 287 x 10 cm/sec which is within 10% of the published ultrasonic values. 

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