Calculations of the signal/Noise ratio in underwater optical heterodyne detection systems

The development of high intensity lasers operating in the blue - green portion of the spectrum has given new impetus to the application of these lasers to problems of underwater detection and communication. One of the advantages of a laser source over an incandescent source is the spatial coherence property of the laser. Previous work1 has shown that the spatial coherence properties of the laser radiation are not adversely affected upon propagation through water. Therefore, it is of interest to compare the performance of an optical heterodyne detection system, which utilizes the spatial coherence properties of the radiation to be detected, with that of a direct detection system. A comparative analysis of direct detection systems and heterodyne detection systems is presented for operation in typical lake water as characterized by Duntley.2 For both systems, the maximum detection range, defined as the range at which it can be determined whether an infinite extent white target is present, is calculated for both diffusely reflecting and specularly reflecting targets. The criterion for comparing the two systems is the signal-to-noise ratio, and while the maximum range estimates for the two systems should be fairly accurate, it must be pointed out that for an operating system the maximum detection range may vary by several orders of magnitude depending on the water and target parameters.