Hybrid Fourier-domain mode-locked laser for ultra-wideband linearly chirped microwave waveform generation

We show the generation of a tunable linearly chirped microwave waveform (LCMW) with an ultra-large time-bandwidth product (TBWP) based on a hybrid Fourier-domain mode-locked (FDML) laser. The key device in the hybrid FDML laser is a silicon photonic integrated micro-disk resonator (MDR) which functions as an optical bandpass filter, to have strong wavelength selectivity and fast frequency tunability. By incorporating the integrated MDR in the fiber-based ring cavity to perform frequency-domain mode locking, an FDML laser is realized and a broadband frequency-chirped optical pulse is generated. By beating the frequency-chirped optical pulse with an optical carrier from a laser diode (LD) at a photodetector (PD), an LCMW is generated. The bandwidth of the LCMW is over 50 GHz and the temporal duration is over 30 µs, with an ultra-large TBWP of 1.5 × 10⁶. Thanks to the strong tunability of the MDR in the FDML laser, the generated LCMW is fully tunable in terms of bandwidth, temporal duration, chirp rate, and center frequency.