The detection of atmospheric turbulence using two ultrasonic light modulators and heterodyne signal processing

The use of double diffraction of coherent light, passing through a pair of ultrasonic light modulators (ULM) with slightly different frequencies, has been studied for optical heterodyne detection processing. A beat signal can be generated with a difference or sum sound frequency between the two ULM's when a pair of diffracted light beams of first order, produced after passage through the two ULM's are photomixed with each other. This system has the merit that it requires no optical components except the two ULM's. It is also convenient for heterodyning techniques that the alignment of the two light beams to be photomixed is not critical. As an application of this system the phase fluctuations of the beat caused by atmospheric turbulence have been processed electronically to obtain its statistical properties.