Experimental study of the theory of optical superheterodyne reception

A theoretical expression for the power signal/noise ratio obtained with an optical superheterodyne receiver is presented, and practical departures from it are discussed. A modification to this expression, proposed by an author using photon-fluctuation theory, is considered. A detailed description is given of the experimental equipment used in the study of the theoretical predictions. A helium-neon gas laser, stabilised in amplitude and frequency to give single-axial-mode operation, was used both as a source and a local oscillator, a moving prism giving the required frequency shift. A photomultiplier was used as the photomixer. The experimental results are in very close agreement with theory.