Apart from amplifying the input signal, every optical amplifier also adds some excess noise to the output. This is often quantified with the so-called noise figure.
For phase-insensitive amplifiers (i.e., those where the gain does not depend on the phase of the input), quantum optics dictates some minimum quantum noise level of the added noise. For example, for high gain applied to a shot-noise limited input (a coherent state), the output noise power of the intensity noise must be at least twice that of a hypothetical noiseless amplifier, i.e., the minimum noise figure is 2, corresponding to 3 dB. The phase noise is also increased.
Origins of Amplifier Noise
For laser amplifiers (based on stimulated emission, e.g. fiber amplifiers), the unavoidable excess noise comes 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-degenerate optical 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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See also: noise figure, spontaneous emission, quantum noise, amplified spontaneous emission, amplifiers, laser noise, Spotlight article 2006-07-16
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