Unified framework for isolation analysis in microwave photonic circulators

We present a generalized analytical framework for characterizing the isolation performance of microwave photonic circulators under both small- and large-signal conditions. Validated using a system comprising a laser diode, an electro-optic phase modulator, and a photodetector, the model evaluates isolation by applying radio-frequency signals to both ends of the modulator electrode and analyzing the output power spectrum. Experimental results show that, under small-signal operation (modulation index m = 0.18), the measured isolation agrees with conventional transmit-isolating photonic receiver (TIPRx) theory. More notably, under large-signal operation (m = 3.20), isolation improves by 3 dB. Communication tests further confirm that crosstalk from the transmitted signal is significantly suppressed in the large-signal regime compared to the small-signal case. This versatile model provides a unified tool for analyzing and optimizing microwave photonic circulators across diverse operating conditions.