Optical coherence modulation by ultrasonic waves. 2: Application to speckle reduction.

Laser light, which is doubly phase modulated by an ultrasonic wave, can be utilized for speckle reduction. The double phase modulation is found to be more effective for speckle reduction than single phase modulation, since the doubly modulated laser beam for illumination is much degraded in spatial coherence with the same ultrasonic conditions. Average contrast is discussed analytically and experimentally as a measure of image-speckle reduction. Agreement between the measured and calculated results are recognized to the extent that the ultrasonic wave can be regarded as a pure phase grating. The average contrasts for two kinds of diffuser decrease similarly down to a minimum of ~0.4 by increasing the ultrasonic power.