PARTIAL COHERENCE CONTROLLED BY A PROGRESSIVE ULTRASONIC WAVE

The mutual coherence function of a light wave modulated by a progressive ultrasonic wave having harmonics is formulated on the basis of Raman-Nath's phase lattice theory. The resultant degree of spatial coherence derived under the assumption that only the fundamental of the ultrasonic wave exists is in good agreement with the results obtained from Young's interference experiments for rather low ultrasonic pressures. These results indicate that the partial coherence condition can be controlled electronically by ultrasonic pressures. Laser speckle reduction is demonstrated as one of its applications.