OPTICAL COHERENCE MODULATION BY 2 ULTRASONIC-WAVES

Some theoretical discussions are provided of a coherence modulation function formulated in the presence of two plane ultrasonic waves. The modulation function is dependent on whether or not a definite phase-relationship is present between the two propagating waves. Some space-invariant conditions are found in association with the modulation of the mutual intensity of the light. The space-invariant modulation functions change with ultrasonic wavelength and the modulation depths can be controlled quantitatively by the electric potential across the ultrasonic transducers. When the two waves are linearly superimposed in the same or opposite direction with no definite relationship in phase, the space-dependent modulation function (SMF) becomes space-invariant and equals the square of the modulation function (OMF) of a progressive ultrasonic wave. In constrast, if they propagate in the same direction with the same phase, the SMF is also space-invariant but is equal to one whose argument is twice as large as the OMF. The modulation function for a standing wave is not space-invariant.