Gain-switched semiconductor laser driven soliton microcombs

Abstract: Dissipative Kerr solitons (DKSs) have been generated via injection locking of chipscale microresonators to continuous-wave (CW) III-V lasers. This advance has enabled fully integrated hybrid microcomb systems that operate in turnkey mode and can access microwave repetition rates. Yet, CW-driven DKS exhibits low energy conversion efficiency and high optical power threshold, especially when the repetition rate is within the microwave range that is convenient for direct detection with off-the-shelf electronics. Efficient DKS can be generated by spatiotemporally structured light (i.e., pulsed pumping), which to date however has required complex cascaded modulators for pulse synthesis. Here we demonstrate a photonic integrated approach to pulsed pumping. By actively switching the bias current of injection-locked III-V semiconductor lasers with switching frequencies in the X-band and K-band microwave ranges, we pump a crystalline and integrated microresonators with coherent picosecond laser pulses, achieving DKS generation with stable repetition rates and lowering the required average pumping power by one order of magnitude to a record-setting level of a few milliwatts. In addition, we unveil the critical role of the phase profile of the pumping pulses, and for the first time implement phase engineering on the pulsed pumping scheme by either accessing a multimode lasing regime in the gain-switching mode or applying external chirping to support robust single-soliton generation. Our work leverages the advantages of gain switching technique and pulse pumping technique, and establishes the merits of combining distinct compact frequency comb platforms that enhance the potential of energy-efficient chipscale microcombs.