Suppression of AM-PM Conversion in an Optical-RF Phase Locked Loop: Theory and Experiment

A microwave photonic phase detector (MPPD) shows notable advantages in terms of low phase noise, long-term stability, and high power when extracting a microwave signal from a mode-locked laser (MLL). However, the occurrence of amplitude-to-phase (AM-PM) conversion during the optical-to-electrical conversion process poses a challenge. This conversion, where intensity noise or relative intensity noise (RIN) couples to the phase noise of the generated microwave, significantly degrades the purity of the resulting signal. While experimental investigations on suppressing AM-PM conversion in MPPDs have been conducted, a specific theoretical model to AM-PM conversion in an MPPD has not been established. In this study, a comprehensive mathematical model is established to reveal the mechanism of the AM-PM suppression in an MPPD. The proposed model is validated through simulations and experimental verification. The impact of different intensity noise waveforms is also explored for the first time. The results indicate a remarkable suppression of AM-PM conversion, with suppression levels of 113 dB for sine-RIN, 95 dB for sweep-signal-RIN, and 74 dB for white-noise-RIN, representing a significant improvement over previous studies. Such enhanced AM-PM suppression positions a MPPD as a prior candidate for synchronization in advanced scientific instruments. This work not only contributes to the development of the theory of optical-RF phase locking but also provides practical guidance for utilizing MPPDs in scientific instruments.