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° Faraday Rotator (FR) and different lengths of single mode fiber after reflecting off a 45° Faraday Rotator Mirror (FRM), the polarization state of the output and input optical signals is rotated by 180°, 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.