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.

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

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The effect of self-injection locking of a laser to a nonlinear microresonator is considered. It is shown that an additional detuning arises in the system, depending on the pump power. This effect can contribute to the generation of optical combs in the pull mode, which will increase the stability of the generated signal.Frequency combs are of great importance for many modern applications in science and technology. Even more interest was attracted to this field, when they were shown to exist in whispering gallery mode (WGM) microresonators [1]. One of the most important objects associated with microresonator-based frequency combs is dissipative Kerr soliton (DKS) [2] representing coherent optical frequency combs with smooth spectral profile. The DKS may only exist if the pump is red detuned from a WGM resonance to compensate Kerr frequency shift. Usually, this can be achieved by tuning the pump laser frequency from the blue to the red slope of the resonance curve.Previous works [2] show that soliton generation happens in a certain range of normalized detunings with lower boundary being the bistability criterion and upper boundary being the soliton stability criterion where is normalized pump, is the detuning of the generation frequency from the microresonator eigenfrequency, normalized to the mode half-linewidth (half loaded decay rate). The obstacle lies in the temperature drop, the resonator experiences when the pump laser transits from the blue detuned (high intracavity power) to the red detuned (lower intracavity power) state. This sudden temperature drop leads to a blue-shift of the resonance frequency and a loss of the soliton state. On the other hand, when the tuning into the soliton state is too quick, the resonator is still cold, and its subsequent heating will again lead to a loss of the soliton state. This problem was initially solved by tuning into the soliton state with an ideal, intermediate speed of frequency scan, such that the resonator reaches the soliton state in a thermal equilibrium, that is, neither too hot nor too cold.A simpler and more efficient method proposed in [3] and implemented in [4] is to selfinjection lock a pump laser onto a high-Q microcavity. The drag effect occurs due to Rayleigh scattering inside the microcavity and the feedback of the wave, which returns to the laser, gets a resonant coupling. It was shown in [3] that the frequency of a self-injection