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Unidirectional parametric amplification of an ultrasonic wave propagating in a laser-pumped birefringent medium was observed. Using x-cut quartz pumped by a ruby laser, power gains approaching 10 dB were measured at ultrasonic frequencies in the neighborhood of 75 MHz. The effect was identified as a ``coflow''-type parametric interaction.
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Piltch et al. (1970) studied this question.
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