Theory of spontaneous emission factor and noise figure of optical parametric amplifiers

This study develops a theory of noise in optical parametric amplifiers (OPAs) based on the quantum-mechanical Langevin equations. We derive theoretical expressions for the spontaneous emission factor of OPAs for the first time. This enables analyzing the effect of internal loss on the noise characteristics. In addition, we analyze the noise figures of the laser amplifier, phase-insensitive OPA, and phase-sensitive OPA.