Microwave pulse generation with independently tunable pulse width and repetition rate in an actively mode-locked optoelectronic oscillator driven by an amplitude-modulated signal.

An approach to generating microwave pulse trains with independently-tunable pulse width and repetition rate is proposed and demonstrated based on an actively mode-locked optoelectronic oscillator (AML-OEO). In this scheme, an external amplitude-modulated signal, consisting of a carrier frequency and an envelope frequency, is injected into a dual-drive Mach-Zehnder modulator (DDMZM) acting as both the modulation device and the mode-locking device in the cavity. Active mode-locking is established by setting the envelope frequency of the amplitude-modulated signal identical to an integer time of the free spectral range (FSR) in the OEO. Microwave pulses are generated at the temporal positions where the amplitude-modulated signal has the maximum absolute amplitudes. In that regard, a higher carrier frequency equal to an integer multiple of the envelope frequency corresponds to a narrower pulse width. Hence, the pulse width and the repetition rate of the microwave pulse train obtained by AML-OEO can be independently tuned through setting the carrier frequency and the envelope frequency of the amplitude-modulated signal. Both numerical simulation and experiment have been accomplished to verify the feasibility of this method. Experimental results show that microwave pulse trains with a fixed repetition rate of 179.61 kHz and tunable pulse widths in the range of 365 ns to 105 ns are generated. In addition, microwave pulse trains with an identical pulse width of 165 ns and tunable repetition rates in the range of 179.61 kHz and 359.22 kHz are generated. Furthermore, an ultra-short microwave pulse train with a pulse width of 15.7 ns and a repetition rate of 475.468 kHz is obtained, where the pulse width is only 3.94% of that from an AML-OEO driven by a sinusoidal signal under an identical repetition rate. The proposed scheme provides a path to independently control the pulse width and the repetition rate of the microwave pulse train and obtain ultra-short microwave pulse trains in an AML-OEO.