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Noise figure of microwave photonic links operating under large-signal modulation and its application to optoelectronic oscillators.

The noise performance of intensity-modulation direct-detection microwave photonic links (MWPL) operating under large-signal conditions has been studied in this paper. A sinusoidal signal plus narrowband white Gaussian noise is applied at the radio frequency input of the link, and the output spectrum is derived using a nonlinear analytical approach. We show that the output SNR can be severely affected by the interaction of signal and noise due to the nonlinearity of the MWPL combined with the large input modulating signal. It is shown that the large-signal noise figure (NF) of an MWPL depends on the input power, a dependence that is not readily apparent under small-signal conditions, due to two unavoidable issues appearing in the large-signal conditions: (1) the link power gain is a function of its input power, and (2) the link power gain is not the same for the signal and noise due to the capture effect. We also have observed that if shot noise or laser relative intensity noise (RIN) is the dominant source of noise, link large-signal NF increases as the input signal power increases. We have shown that, when the MWPL is operating in the linear regime, our theoretical predictions approach the already published results on small-signal NF, which are verified by experimental data. We have shown that large-signal NF affects the noise performance of optoelectronic oscillators because they contain MWPLs at saturation.