Optical Communications, 2nd Edition

5

COHERENT (HETERODYNE) DETECTION

In the preceding chapter, we studied direct, or incoherent, detection of the received optical field. In such a system we found that the receiver may have to combat the background radiation and the internal receiver thermal noise while attempting to recover the intensity modulation of the transmitted beam. The receiver noise is overcome by achieving shot-noise-limited conditions, and the resulting performance then depends only on an ability to transmit signals whose power level dominates that of any received background. Shot-noise-limited operation is achieved with exceptionally strong received signal fields, or by the use of high-quality photomultipliers. When high-performance photo-multipliers are not available, other means must be used to improve detectability. An alternative method is the use of heterodyning.

In heterodyne detection, the receiver operates by optically adding a locally generated field to the received field prior to photodetection. The prime objective is to use the added local field to improve the detection of the weaker received field in the presence of the receiver thermal noise. The combined field is then photodetected, as if it were a single received optical field. Since the addition of two electromagnetic fields requires spatial alignment of the fields, the use of heterodyne detection is often called (spatial) coherent detection.