Electrically pumped photonic integrated soliton microcomb

Abstract

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.

Fig. 1

Principle of an ultra-compact, laser injection-locked soliton Kerr frequency comb. a Close-range photo of the experimental setup, in which the laser diode chip is butt-coupled to a Si₃N₄ photonic chip, which contains several microresonators. b Schematic representation of the laser injection-locked soliton Kerr frequency comb. An InP multi-frequency laser diode chip is directly butt-coupled to a Si₃N₄ photonic chip with a microresonator. c An optical image of the InP laser diode chip showing the magnified view. d Sketch of the experimental setup. The microresonator device output is characterized both in the optical domain using an optical spectral analyzer and in the radio frequency (RF) domain using an electrical-signal spectral analyzer. In addition, to assess the coherence of the frequency comb, we employ a heterodyne beatnote measurement to a selected comb tooth with a narrow linewidth reference laser. TC: temperature control module; CC: current control module; AFG: arbitrary function generator; OSA: optical spectral analyzer; OSC: oscilloscope; ESA: electrical-signal spectral analyzer. e False-colored scanning electron micrograph (SEM) image of the waveguide cross-section. The Si₃N₄ waveguide (blue) has no top SiO₂ cladding but only side and bottom SiO₂ cladding (red)

Fig. 2

Electrically pumped soliton microcomb via laser injection-locked soliton formation. a Transmission spectrum of a Si₃N₄ microresonator of 1.02 THz free spectral range (FSR), featuring two sets of resonances: the fundamental transverse electric (TE) mode family (marked by red circles) and one high-order TE mode family. b The laser spectrum of the multi-frequency laser diode chip used in this experiment, corresponding to state i in g. c Measured and fitted heterodyne beat signal between the free running laser diode and a narrow linewidth reference laser (Toptica CTL1550, short-time linewidth ~10 kHz), showing 60 MHz full-width at half-maximum (FWHM) of Voigt profile. d (state ii in g): Spectra of single longitudinal mode that is injection locked to a selected resonance of the microresonator. f (state iii in g): Spectrum of the Kerr frequency comb that stems from the laser injection locking. Inset: One resonance of the fundamental TE mode showing mode splitting due to backscattering, with the estimated 118 MHz coupling strength $\left(\frac{\gamma }{2\pi }\right)$ between the forward and backward propagating modes. e Heterodyne beat signal between the injection-locked laser and a narrow linewidth reference laser. The measured beat signal is fitted with Voigt profile with FWHM ~186 kHz (cf. Methods). RBW: Resolution bandwidth. g Typical transmitted power trace measured at the chip output facet, by current modulation imposed on the laser diode, in which different states are marked: (i) noisy, multi-frequency lasing without injection locking; (ii) laser injection locking to a microresonator resonance, and simultaneous formation of low-noise single-longitudinal-mode lasing (the orange region); (iii) formation of Kerr frequency comb (the green region)

Fig. 3

Soliton comb generation with self-injection locking. Evolution of Kerr frequency comb in the regime of laser self-injection locking, from noisy state in the operation regime of modulation instability (MI) (a) to breathing state (b), and eventually to a low-noise state (c) showing the formation of a dissipative Kerr soliton (DKS) in the microresonator, where the spectrum is a hyperbolic secant envelope (green-solid line showing the fitting of the spectral envelope). Each inset shows the low-frequency radio frequency (RF) spectrum corresponding to each state. The current imposed to the diode is initially set ~300 mA and the increase to evoke the transitions is within 1 mA. The Si₃N₄ microresonator in this measurement has a free spectral range (FSR) of 149 GHz

Fig. 4

Laser injection-locked multiple breathing and dissipative Kerr solitons. a Measured and fitted dispersion landscape in a Si₃N₄ microresonator (cross-section 1.58 × 0.75 μm²) (cf. Methods), which has the FSR = 92.4 GHz, and the second-order dispersion element indicating the anomalous group velocity, D₂/2π ≈ 1.56 MHz. b Histogram of resonance linewidths that are ~110 MHz, corresponding to a loaded Q-factor ~1.8 × 10⁶. c Heterodyne beat signal between the sideband of soliton Kerr frequency comb and the narrow linewidth reference laser. The measured beat signal is fitted with Voigt profile with full-width at half-maximum (FWHM) ~201 kHz (cf. Methods). RBW: Resolution bandwidth. d, e Showcase of multiple dissipative solitons formed in Si₃N₄ microresonators, in the breathing state (d) as well as in the low-noise stable soliton state (e), the fitting of the spectrum envelope (green-solid lines) further shows the relative position of solitons circulating in the micro-ring cavity (schematic insets). The low-frequency RF spectra corresponding to breather solitons are also shown as insets. Spectra in d and e are generated in Si₃N₄ microresonators with a free spectral range (FSR) of ~88 and ~92 GHz, respectively