Abstract
Soliton microcombs constitute chip-scale optical frequency combs, and have the potential to impact a myriad of applications from frequency synthesis and telecommunications to astronomy. The demonstration of soliton formation via self-injection locking of the pump laser to the microresonator has significantly relaxed the requirement on the external driving lasers. Yet to date, the nonlinear dynamics of this process has not been fully understood. Here, we develop an original theoretical model of the laser self-injection locking to a nonlinear microresonator, i.e., nonlinear self-injection locking, and construct state-of-the-art hybrid integrated soliton microcombs with electronically detectable repetition rate of 30 GHz and 35 GHz, consisting of a DFB laser butt-coupled to a silicon nitride microresonator chip. We reveal that the microresonator’s Kerr nonlinearity significantly modifies the laser diode behavior and the locking dynamics, forcing laser emission frequency to be red-detuned. A novel technique to study the soliton formation dynamics as well as the repetition rate evolution in real-time uncover non-trivial features of the soliton self-injection locking, including soliton generation at both directions of the diode current sweep. Our findings provide the guidelines to build electrically driven integrated microcomb devices that employ full control of the rich dynamics of laser self-injection locking, key for future deployment of microcombs for system applications.
Highlights
Soliton microcombs constitute chip-scale optical frequency combs, and have the potential to impact a myriad of applications from frequency synthesis and telecommunications to astronomy
We develop a technique to experimentally characterize the self-injection locking (SIL) dynamics and study it in a hybridintegrated soliton microcomb device with 30 GHz repetition rate, amenable to the direct electronic detection, using an InGaAsP distributed feedback (DFB) laser self-injection locked to a high-quality-factor (Q0 > 107) integrated Si3N4 microresonator
The generation frequency of the free-running DFB diode is determined by its laser cavity (LC) resonant frequency ωLC and can be tuned by varying the diode injection current Iinj exhibiting practically linear dependence
Summary
Soliton microcombs constitute chip-scale optical frequency combs, and have the potential to impact a myriad of applications from frequency synthesis and telecommunications to astronomy. Laser self-injection locking (SIL) to high-Q crystalline microresonators has been used to demonstrate narrow-linewidth lasers[17,27], ultra-low-noise photonic microwave oscillators[28], and soliton microcomb generation[29], i.e., soliton SIL. 100 mW multi-frequency Fabry–Perot lasers have recently been employed to demonstrate an electrically driven microcomb[33] Another approach was based on a Si3N4 microresonator buttcoupled to a semiconductor optical amplifier (SOA) with on-chip Vernier filters and heaters for soliton initiation and control[34]. We develop a technique to experimentally characterize the SIL dynamics and study it in a hybridintegrated soliton microcomb device with 30 GHz repetition rate, amenable to the direct electronic detection, using an InGaAsP DFB laser self-injection locked to a high-quality-factor (Q0 > 107) integrated Si3N4 microresonator. We demonstrate the presence of the non-trivial dynamics upon diode current sweep, predicted by the theoretical model, and perform the beatnote spectroscopy, i.e., study of the soliton repetition rate evolution under SIL
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