System combining the best features of heterodyne and direct detection receivers.

A system is presented and analyzed that combines the best features of heterodyne and direct detection systems. It provides the noise suppression characteristics of a heterodyne system; at the same time, it has the advantages of a direct detection system in being able to process equally efficiently received fields that are spatially incoherent or coherent, and in being able to use wide field-of-view receiver optics. Moreover, in the presence of strong background noise and detector thermal noise its performance can be even better than that of a direct detection system for which the only receiver noise present is the signal shot noise. When receiving a spatially incoherent field, its performance could be better than a heterodyne system having the same collector area but requiring a spatially coherent received field. The system uses a cascade combination of a wide-band and narrow-band quantum amplifier in the receiver. In contrast to a previously presented system of this type, which used amplitude modulation, the system now described and analyzed permits almost any type of modulation.