Frequency-Modulated Microwave Photonic Links with Direct Detection: Review and Theory

This work is a theoretical study of microwave photonic links which use optical frequency modulation (FM) and lter-slope discrimination for demodulation. The high modulation e ciency of optical FM devices is attractive for achieving low noisegure links, but linear demodulation of the signals is also desired. In order to design discriminators which produce low distortion, this paper presents general full-signal and small-signal models of the e ects of arbitrary optical ltering on an FM link, including the interaction between FM and residual IM. The small signal model is used to derive gures of merit for the linearity, noise and dynamic range of the FM links. The results of the models invalidate a common assumption: that linear FM to IM discrimination is possible. Instead, the discrimination must be from FM to amplitude modulation of the electric eld for the link response to be linear. The linearity of links using two di erent sets of FIR discriminator lters, designed using the minimax relative error and maximally linear criteria, are compared in order to evaluate the better design method. The evolution of link linearity with lter order is studied. The analytical models are compared to numerical simulations of the lters, and the models are found to be consistent. Finally, a Monte Carlo simulation is used to analyze the sensivity to errors in fabrication for high-order lters realized in planar lightwave circuits (PLC) with a lattice lter architecture.