Position-definable coherent microwave pulse train generation in an actively mode-locked optoelectronic oscillator

An approach to generating position-definable coherent microwave pulse trains is proposed and demonstrated based on an actively mode-locked optoelectronic oscillator (OEO). In this scheme, a low-frequency signal with a customized waveform is employed as the mode-locking signal to periodically manipulate the intracavity gain of the OEO. Once the repetition frequency of the mode-locking signal satisfies the active mode-locking condition, microwave pulses are generated at the time positions corresponding to the gain peaks. The temporal distribution of the microwave pulses within one period is determined by the waveform of the mode-locking signal. Therefore, through designing the waveform of the mode-locking signal, coherent microwave pulse trains with user-defined pulse positions can be obtained. In the experiment, a non-uniformly spaced microwave pulse train, a series of microwave pulse clusters, and a binary-encoded microwave pulse sequence with a "Barker code" format are generated by the actively mode-locked OEO, all of which are with an identical carrier frequency of 10 GHz.