Microcombs provide a path to broad-bandwidth integrated frequency combs with low power consumption, which are compatible with wafer-scale fabrication. Yet, electrically-driven, photonic chip-based microcombs are inhibited by the required high threshold power and the frequency agility of the laser for soliton initiation. Here we demonstrate an electrically-driven soliton microcomb by coupling a III–V-material-based (indium phosphide) multiple-longitudinal-mode laser diode chip to a high-Q silicon nitride microresonator fabricated using the photonic Damascene process. The laser diode is self-injection locked to the microresonator, which is accompanied by the narrowing of the laser linewidth, and the simultaneous formation of dissipative Kerr solitons. By tuning the laser diode current, we observe transitions from modulation instability, breather solitons, to single-soliton states. The system operating at an electronically-detectable sub-100-GHz mode spacing requires less than 1 Watt of electrical power, can fit in a volume of ca. 1 cm3, and does not require on-chip filters and heaters, thus simplifying the integrated microcomb.

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Narrow linewidth lasers and optical frequency combs generated with mode-locked lasers have revolutionised optical frequency metrology. The advent of soliton Kerr frequency combs in compact crystalline or integrated ring optical microresonators has opened new horizons in academic research and industrial applications. These combs, as was naturally assumed, however, require narrow linewidth single-frequency pump lasers. We demonstrate that an ordinary costeffective broadband hundreds milliwatts level Fabry-Perot (FP) laser diode, self-injection locked to a microresonator, can be efficiently transformed to a powerful single-frequency ultranarrow linewidth light source with further transformation to a coherent soliton comb oscillator. Our findings pave the way to the most compact and inexpensive highly coherent lasers, frequency comb sources, and comb-based devices for mass production.Kerr optical frequency combs in high-Q optical microresonators 1 are attracting growing interest in recent years 2,3 , especially since the mode-locking via generation of dissipative Kerr solitons (DKS) has been demonstrated on a variety of platforms 4-10 . DKS have enabled compact and broadband low-noise frequency combs with repetition rates in the multi-GHz to the THz domain. DKS have been applied for dual comb spectroscopy 11-13 , coherent communication 14,15 , ultra-fast ranging 16,17 , low-noise microwave master oscillators 8 , calibration of astronomical spectrometers 18,19 and imaging of soliton dynamics 20,21 .Integration of high-Q microresonators suitable for soliton generation has advanced significantly 22-24 . A recent breakthrough was the assembly of an integrated photonics based optical frequency synthesiser 25 pumped with an external III-V/silicon based laser 26 . Yet, most of these demonstrations used single frequency lasers with amplifiers and modulators for soliton generation restricting commercialisation, e.g. highly sensitive wearable spectrometers and ranging sensors.A straightforward approach to obtain DKS in microresonators uses single frequency narrow linewidth tunable lasers for pumping. The frequency of the laser, having a * mg@rqc.ru

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.