Supermode noise suppression with polarization- self-stabilization dual-loop for harmonic active mode-locking optoelectronic oscillator

A harmonic active mode-locked optoelectronic oscillator (HAML-OEO) with a polarization-self-stabilization dual- loop architecture is proposed to generate microwave pulse train with ultra-low supermode noise. Unlike traditional dual-loop HAML-OEOs, the input light in each loop passes through a 45 degrees Faraday Rotator (FR) and different lengths of single mode fiber after reflecting off a 45 degrees Faraday Rotator Mirror (FRM), the polarization state of the output and input optical signals is rotated by 180 degrees, ensuring it is unaffected by external environmental disturbances. Meanwhile, the required fiber length is cut in half due to the dual loop is a reflective structure. Through using the polarization self-stabilization technique and the "Vernier effect", the unwanted longitudinal modes are weakened at the early stage of the oscillation, which is beneficial for suppressing the supermode noise generated by phase-locking between unwanted modes in harmonic mode-locked. In the experiment, microwave pulse train with repetition rates of 506 kHz and 1.012 MHz are generated through 5th- and 10th-order harmonic mode locking, where the supermode noise suppression rations are measured to be beyond 63.09 dB and 55.11 dB, respectively.