Some theoretical aspects of the photo-detection process. A tutorial approach

Coherent heterodyne detection in the optical range of the electromagnetic spectrum is a relatively new technique. The usual device for measuring these optical fields essentially involves a quantum mechanical process, namely, photo-electron excitation, for example from the valence to the conduction band in a semiconductor. In a more or less tutorial manner the underlying theoretical aspects of coherent detection are clarified and some remarks are made concerning present day photo-electron counting techniques.