Quantum Theory of Spectral Modulation
Recent experiments of Delisle and Brochu, in which spectral modulation at a large path difference was observed, are analyzed in quantum mechanical terms. Starting with a stationary density operator for thermal light, a two beam superposition such as taking place in a Michelson interferometer, is investigated. It is shown that a simple interference law holds separately for each mode of radiation. Consequently, a coherent superposition of light should be observed for a time delay arbitrarily large compared to the coherence time. The latter determines merely the range of intensity modulation. Analysis of the Michelson interferometer accounting for the energy returned to the source is also presented.