This study summarizes the theory of laser-amplifier noise based on the
master equation, the quantum-mechanical Langevin equation, and the rate
equation. The three approaches are used differently depending on the
application. Laser-amplifier noise in the direct-detection receiver is
described by the master equation. Meanwhile, the quantum-mechanical
Langevin equation is most effective for analyzing noise appearing in
coherent optical communication systems. The rate equation approach
provides a clear understanding of the intensity-noise characteristics of
laser oscillators, including the amplitude squeezing.