Noise Figure of Amplifiers
The noise figure of an amplifier (e.g. a fiber amplifier) is often said to simply be the amount by which the signal-to-noise ratio of an input signal is degraded. Then, however, it is not understandable how the noise figure of an ideal high-gain four-level amplifier can always be 3 dB. In particular, how about two such amplifiers in series? If the signal-to-noise ratio is degraded twice, the noise figure should be 6 dB. But considering the two amplifiers as a single amplifier (made as an amplifier chain), it should be 3 dB. It is not conceivable why e.g. two fiber amplifiers in series should be worse concerning amplifier noise than a single longer amplifier.
The confusion arises from the frequently encountered omission of an important condition: the noise figure is the amount by which the signal-to-noise ratio of an input signal is degraded, if the input signal is shot-noise limited. In a chain made of two high-gain amplifiers, the first device may have a shot-noise limited input. Its output will then have strongly amplified quantum noise, plus some excess noise according to the noise figure. This means that the input of the second amplifier has strong excess noise (i.e., noise at a level high above the shot noise level), so that this amplifier won't degrade the noise performance further if it also has a noise figure of e.g. 3 dB.
The exact analog of this occurs with electronic amplifiers, only that quantum noise has to be replaced with thermal noise. Obviously, the apparent mystery as described above is not one of these quantum mysteries, and is relatively simple to resolve. And the understanding of such issues is obviously quite important when considering amplifier noise, e.g. in photodetection and particularly in optical fiber communications.
This article is a posting of the Photonics Spotlight, authored by Dr. Rüdiger Paschotta. You may link to this page and cite it, because its location is permanent. See also the RP Photonics Encyclopedia.
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