Signal-to-noise ratio and capture effect of microwave photonic links operating under small- and large-signal modulations in random noise

Signal-to-noise power ratio (SNR) and capture effect of intensity-modulation direct-detection (IMDD) microwave photonic links (MWPLs) operating under small- and large-signal modulation in noise have been studied. Arbitrary N sinusoidal signals in random noise are applied at radio frequency (RF) input port of the IMDD MWPL and the output spectrum is derived using a nonlinear analytical approach. General expressions for signal and noise powers at output of the IMDD MWPL are derived. From these expressions SNR and capture effect for all values of input signal power ratios can be determined. It is shown that a) the output noise and signal power for strong and weak input signals can severely be affected by the interaction of input signals with themselves, input signals with noise and input noise with itself due to the effects of nonlinearity of the MWPL combined with sum of several input signals in noise, b) the capture effect of the IMDD MWPL in noise depends on the link input back-off, the power ratio of input signals and on the number of input signals and this dependence is severe in the large-signal regime, c) the capture effect does not depend on the noise and in the presence and absence of noise are the same, d) output SNR for strong and weak input signals depend on input SNR, power ratio of input signals and input back-off. We have shown that when MWPLs are operating in the absence of noise, our theoretical predictions for the capture effect approach the already published results on this case. Besides, we have shown that the general behavior of the ratio of the input SNR to that of the output in small- and large-signal regime, is the same as the case of one input signal in random noise (or noise figure) that have been previously published.