On the physical origin of the 3-dB noise figure limit in laser and parametric optical amplifiers

We integrate into a unifed model the fundamental conclusions of previous theoretical works concerning the quantum theory of linear optical amplifiers. We analyze the statistical processes contributing to noise in both laser and parametric amplification. We show that the so-called "signal/amplified spontaneous emission" beat noise directly originates from thermal statistics in stimulated emission, which causes unavoidable gain fluctuations. Such noise, which is in no way related to amplified spontaneous emission, is at the origin of the amplifier noise figure limit of 3 dB at high gains. The additional contribution of spontaneous emission noise (or more generally "amplified vacuum noise"), with one or two degrees of freedom (polarization), results in other "shot" and "beat" noise components, as discussed. This unified presentation thus provides an accurate and simple conceptual view of the different noise components in optical amplifiers. (C) 1999 Academic Press.