Statistical Nature of the Infrared Heterodyne Signal ' ' )

1 HETERODYNE detection is a process which has been used in the radiowave, microwave, 1 and optical regions of the electromagnetic spectrum. Recently, the technique has been extended to the middle infrared as well [I-31. This paper discusses theoretical and experimental considerations related to the signal-to-noise ratio and the statistical nature of the coherently detected signal at the 10.6 pm C02 laser wavelength. Coherent detection differs in several significant respects from direct detection, or simple photon counting. In particular, the increased sensitivity available through its use in the infrared allows the detection of far weaker signals than by means of other techniques. In the submillimeter region, an improvement in sensitivity with heterodyne operation has been demonstrated for InSb, pyroelectric, and Golay cell detectors. Using techniques similar to those described here, coherent detection experiments have been previously reported in the visible and the near infrared with photoemissive devices, photodiodes, and photoconductors. The operation of the heterodyne receiver is made possible by the "nonlinear" response of the photodetector to the incident total radiation field. Two electromagnetic waves of different frequencies (0.1, and 0.1,) mix at the photodevice and produce an electrical signal at the difference frequency (0.1,-0.1,). When one of these beams is made to be strong (if it is locally produced, it is then called the local oscillator or LO), the sensitivity for the process is considerably increased over the straight detection (video) case because of the high conversion gain between power at the input and at the difference frequencies. In . addition to this high conversion gain, the heterodyne detector exhibits both strong directivity and frequency selectivity. It is the frequency selectivity of the coherent detection