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.
Highlights
Microresonator-based Kerr frequency combs (Kerr microcombs) have provided a route to compact chip-scale optical frequency combs, with broad optical bandwidth and repetition rates in the microwave to terahertz domain (10 GHz−1 THz)[3,4]
Photonic integration of soliton microcombs requires the integration of nonlinear high-Q microresonators on chip and an on-chip solution for the narrow linewidth seed lasers with output power levels that are sufficient for soliton initiation, as well as any laser tuning mechanism used in the soliton excitation process[7,8,18,19]
Photonic integration of high-Q microresonators suitable for the soliton formation has advanced significantly, in particular using Si3N4—a CMOS-compatible material used as a capping layer[20]
Summary
Microresonator-based Kerr frequency combs (Kerr microcombs) have provided a route to compact chip-scale optical frequency combs, with broad optical bandwidth and repetition rates in the microwave to terahertz domain (10 GHz−1 THz)[3,4]. By using high-Q (Q0 > 1 × 107) photonic chip-scale Si3N4 microresonators fabricated using the photonic Damascene reflow process[24,25], in conjunction with a multiple-longitudinal-mode (multi-frequency) Fabry–Pérot InP laser diode chip, we observe self-injection locking[26,27] in a regime where solitons are formed concurrently Such self-injection locking with concurrent soliton formation has recently been demonstrated for bulk ultrahigh-Q crystalline MgF2 resonators[14,28]. Heterodyne measurements demonstrate the low-noise nature of the generated soliton states Such an electrically driven photonic chip-based soliton microcomb demonstrated here provides a solution for integrated, unprecedentedly compact optical comb sources suitable for high-volume applications. In comparison with a concurrent report of integrated soliton microcomb[29], our scheme alleviates the need for on-chip Vernier filters, as well as for thermal heaters for soliton tuning[30], which avoids extra power consumption (30 mW per heater29) and the complexity in both the fabrication process and the process of soliton initiation
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