Optical Sources, Detectors, and Systems

Chapter 6

Heterodyne or Coherent Detection and Optical Amplification

Until this point, we have discussed optical detection in terms of the measurement of the incident power. With the advent of the laser, however, it has become possible to measure the amplitude and phase of the incident optical field through the process of heterodyne detection. This technique, used universally in modern communications and radar systems at radio and microwave frequencies, utilizes the “beating” between a single-frequency or monochromatic “local” oscillator and the received signal. In a radio receiver the “mixing” or “beating” process is carried out in a highly nonlinear rectifying element. The resultant output current at the intermediate frequency (i.f.) is amplified ...