Generation of phase-stabilized microwave frequency combs based on an actively mode-locked optoelectronic oscillator

A novel approach to generating a microwave frequency comb (MFC) with a stabilized carrier-envelope phase is proposed and experimentally demonstrated. The MFC is generated using an actively mode-locked optoelectronic oscillator (AML-OEO) incorporating an octave filter to enable second harmonic injection locking (SHIL), which enables phase-stabilized MFC generation. The second harmonic component, arising from the nonlinearity of the electrical amplifier (EA), is employed as an injection signal for the AML-OEO to eliminate the carrier-envelope offset (CEO). In the experiment, octave-spanning combs with repetition rates of 44.511 MHz, 89.022 MHz, and 133.533 MHz are generated through the fundamental, second-, and third-harmonic active mode-locking (AML), respectively, showing stabilized carrier-envelope phases and ultralow frequency drifts of less than 62 Hz in one hour, which are improved by two orders of magnitude as compared with existing AML-OEOs without using active feedback loop control. The joint operation of AML and SHIL enables the generation of a microwave pulse train with a tunable repetition rate, a stabilized carrier-envelope phase, and an ultralow frequency drift, making it a promising technique for future instrumentation and metrology systems.