On the Treatment of Optical Amplifier Noise in Amplified Analog Photonic Links

A new theoretical model is developed to describe signal-spontaneous noise in optically-amplified externally modulated analog photonic links. The model is applied to an optically-amplified phase-modulated link with an unbalanced Mach-Zehnder interferometer (MZI). Theoretically calculated results obtained with the model are compared to measured data to confirm their validity. Theory and measurement both show that a phase-modulated link employing optical amplification and an MZI cannot cancel signal-spontaneous noise to achieve the shot noise limit. To attain the shot noise limit, a high-power low-noise laser is used instead of optical amplification. For this phase-modulated link, a minimum RF noise figure of 7.6 dB is achieved experimentally. Finally, the theoretical model is applied to an optically-amplified dual-output intensity-modulated link to show that signal-spontaneous noise cancels as expected when the modulator is biased at quadrature.