Acoustic Light Modulators Using Optical Heterodyne Mixing

Detection of optical radiation using heterodyne mixing was pioneered in the classic experiments of Forrester, Gudmundsen, and Johnson.1 They successfully detected the microwave beat between two Zeeman components of a mercury arc. With coherent light sources the technique was utilized initially in the investigation of the mode structure and frequency stability of the helium-neon laser.2 Subsequently, optical heterodyne mixing has been used as a sensitive, high-resolution detector of frequency shifts in the study of Brillouin scattering3 and of the frequency broadening of Rayleigh scattered light.4 It is well known that under the correct circumstances an optical beam, passing through a transparent material containing a traveling acoustic wave, has part of its energy diffracted by the refractive index variations associated with the acoustic wave. In the proper range of parameters, known as the Bragg region, the diffracted light is con367 368 T H E BELL S Y S T E M TECHNICAL JOURNAL, FEBRUARY