Slow modulation unlocks mode-locking via parametric amplification in photonic molecule

Artificial photonic molecule systems consisted of coupled ring resonators have emerged as a promising platform for simulating programmable discrete energy levels and transitions in quantum optics. This capability has led to diverse applications, such as optical frequency combs, avalanche photodiodes, photonic molecule switches and spectrometers. In this work, we unveil the parametric amplification in a photonic molecule, which generates mode-locked pulses at a significantly low modulation frequency matching the photonic molecule's level spacing. Proof-of-principle experiments are performed using fiber-based cavities. The results show stable mode-locking at a modulation frequency significantly lower than the free-spectral-range (FSR) of the rings, with a ratio reaching up to 1:75, and potential for further reduction through parametric optimization. These findings overcome the strict requirement in active mode-locking where the modulation is usually resonant with the FSR. Our design, in turn, enables the realization of integrated pulsed laser sources within small-footprint resonators under slow and broadband modulation. Additionally, the generated mode-locked pulses from our approach demonstrate burst-mode emission with adjustable powers. This feature allows for the precise customization of optimal pulse fluences, which can be beneficial for various applications, ranging from precision matching to spectroscopy.