For laser amplifiers (based on stimulated emission, e.g. fiber amplifiers), the unavoidable excess noise can be regarded as arising from spontaneous emission of the gain medium into the amplified mode.
For a four-level gain medium with a low-noise pump, the excess noise can approach the minimum quantum-mechanically allowed level.
(Note that the gain medium acts as an energy reservoir, effectively damping the influence of high-frequency pump noise.)
The use of quasi-three-level gain media leads to additional excess noise, because for a given gain the upper-state population (and thus the spontaneous emission) has to be higher to compensate for the signal reabsorption.
The additional increase in noise in such situations can be quantified with a spontaneous emission factor, which decreases for increasing excitation levels.
In a non-degenerateoptical parametric amplifier, the excess noise comes from vacuum fluctuations entering the idler port, and possibly also from the pump wave.
A degenerate parametric amplifier does not need to add excess noise (it has no idler!), but its amplification is phase-sensitive.
In addition to quantum noise, classical fluctuations of the pump source can also cause excess noise.
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