Editing Optical amplifier (section)

====Noise====
The principal source of noise in DFAs is [[Amplified Spontaneous Emission|amplified spontaneous emission]] (ASE), which has a spectrum approximately the same as the gain spectrum of the amplifier. [[Noise figure]] in an ideal DFA is 3&nbsp;dB, while practical amplifiers can have noise figure as large as 6–8&nbsp;dB.

As well as decaying via stimulated emission, electrons in the upper energy level can also decay by spontaneous emission, which occurs at random, depending upon the glass structure and inversion level. Photons are emitted spontaneously in all directions, but a proportion of those will be emitted in a direction that falls within the [[numerical aperture]] of the fiber and are thus captured and guided by the fiber. Those photons captured may then interact with other dopant ions, and are thus amplified by stimulated emission. The initial spontaneous emission is therefore amplified in the same manner as the signals, hence the term a''mplified spontaneous emission''. ASE is emitted by the amplifier in both the forward and reverse directions, but only the forward ASE is a direct concern to system performance since that noise will co-propagate with the signal to the receiver where it degrades system performance. Counter-propagating ASE can, however, lead to degradation of the amplifier's performance since the ASE can deplete the inversion level and thereby reduce the gain of the amplifier and increase the noise produced relative to the desired signal gain.
 
Noise figure can be analyzed in both the optical domain and in the electrical domain.<ref>Baney, Douglas, M., Gallion, Philippe, Tucker, Rodney S., ”Theory and Measurement Techniques for the Noise Figure of Optical Amplifiers”, Optical Fiber Technology 6, 122 pp. 122-154 (2000)</ref> In the optical domain, measurement of the ASE, the optical signal gain, and signal wavelength using an optical spectrum analyzer permits calculation of the noise figure. For the electrical measurement method, the detected photocurrent noise is evaluated with a low-noise electrical spectrum analyzer, which along with measurement of the amplifier gain permits a noise figure measurement. Generally, the optical technique provides a more simple method, though it is not inclusive of excess noise effects captured by the electrical method such multi-path interference (MPI) noise generation. In both methods, attention to effects such as the spontaneous emission accompanying the input signal are critical to accurate measurement of noise figure.