Optical heterodyne (coherent) detection

The optical heterodyne principle is described and its history traced. Early hopes for optical communication by this technique were frustrated by problems of wavefront degradation and laser frequency instability. However, fiber optics has recently revitalized this as an attractive possibility. In the interim, many other successful applications have been found that exploit different advantages of the heterodyne principle. Several examples are described. A number of simple optical heterodyne experiments utilizing a HeNe laser and electrical spectrum analyzer are presented. Details are furnished concerning experimental requirements and special considerations for optical heterodyne detection.