Mitigating fast thermal instability by engineered laser sweep in AlN soliton microcomb generation

Abstract

Transient thermal instability represents a significant challenge in generating soliton microcombs. Fast laser sweep can be an efficient method to mitigate thermal instability, but it requires an ultrahigh laser sweep rate for crystalline microresonators with fast thermal relaxation. Here, we engineer a laser sweep waveform to generate AlN-on-sapphire soliton microcombs with an intermediate sweep speed (<30 GHz/μs). Two laser sweep methods with backward plus forward tuning or two-step backward tuning added after the fast forward laser sweep were demonstrated to stabilize solitons. Reducing the soliton number is found to be useful to stabilize solitons in fast laser sweep. The effectiveness of the methods was numerically verified. Our measurements and simulations also reveal the impacts of different thermal relaxation processes occurring at quite different time scales on thermal instability. The requirement of the laser sweep protocols is discussed.