Optical-amplifier noise-figure measurement by Q-factor analysis

In the semi-classical picture of linear optical amplifiers, the amplifier output is considered as a mixture of an amplified signal and amplified spontaneous emission (ASE). The amplified signal is a coherent field with Poisson statistics, and the ASE can be well described as a thermal field. This simple picture, however, can be fully justified quantum mechanically. It has been shown that the signal-to-noise ratio, and hence the noise figure of a linear optical amplifier, can be determined by measuring the Q-factor of the amplified continuous wave (CW) signal modulated with a low extinction-ratio external modulator.