Integrated soliton microcombs beyond the turnkey limit

Abstract

Soliton microcombs generated in optical microresonators are accelerating the transition of optical frequency combs from laboratory instruments to industrial platforms. Self injection locking (SIL) enables direct driving of soliton microcombs by integrated lasers, providing turnkey initiation and improved coherence, but it also pins the pump close to resonance, limiting both spectral span and tuning flexibility. Here we theoretically and experimentally demonstrate that introducing a thermally tunable auxiliary microresonator extends the bandwidth of SIL soliton microcombs. By engineering hybridization of the pumped resonance, we achieve deterministic access to single soliton states and then push operation into a far detuned regime inaccessible to direct initiation. The resulting combs reach a near 200 nm span at a 25 GHz repetition rate, while preserving the SIL-enabled noise suppression throughout. Moreover, the added degree of freedom afforded by the coupled resonator architecture enables orthogonal control of the comb's repetition rate and center frequency. These advances expand the spectral reach and controllability of integrated soliton microcombs for information processing and precision metrology.

Figures (4)

• Figure 1: Dynamics of self-injection locking (SIL) in coupled microresonators.a, Schematic of a coupled-resonator device directly pumped by a distributed-feedback (DFB) laser. The original NR mode is indicated by dashed lines, and the NR (red) and AR (blue) fractions of the hybridized mode are encoded in color. b, Effect of pump-mode hybridization on the NR integrated dispersion (middle) and on the parametric gain spectrum (bottom). Both quantities are normalized to half the NR linewidth, $\kappa_\mathrm{NR}/2$. c, Phase diagram of soliton existence under SIL conditions, plotted as a function of comb detuning and mode shift. Arrows indicate the pathway from turnkey excitation to spectrally broadened soliton microcombs.
• Figure 2: Generation of soliton microcombs.a, Device photographs. From left to right: Si$_3$N$_4$ photonic chip, DFB laser directly butt-coupled to the chip, and the fully packaged assembly in a butterfly package. b, Upper panel: measured transmission spectra at different hybridization levels, obtained by varying the AR heater power. The AR resonance, NR resonance, and hybridized resonances (AR $\times$ NR) are indicated. Lower panel: comb power versus DFB drive current over 100 consecutive scans.
• Figure 3: Tuning and characterization of soliton microcombs.a, Experimental setup. PD: photodetector; ESA: electrical spectrum analyzer; OSA: optical spectrum analyzer; FBG: fiber Bragg grating; PNA: phase-noise analyzer; LO: local oscillator; PC: polarization controller; AOFS: acousto-optic frequency shifters. b, Measured comb electrical beat note (upper panel) and DFB laser frequency (lower panel) as a function of AR heater power. c, Optical spectra of soliton microcombs in the turnkey (left) and extended (right) regimes at the operating points indicated in b. The 3-dB spectral bandwidth is extracted by fitting the spectra with $\mathrm{sech}^2$ envelopes. Insets: corresponding electrical beat notes of the combs. d, Power spectral density (PSD) of the frequency noise of the free-running and SIL DFB lasers. The noise floor at high offset frequencies is indicated. e, Measured 3-dB spectral-envelope bandwidth (left axis) and Lorentzian linewidth (right axis) of the microcomb as a function of AR heater power.
• Figure 4: Orthogonal control of comb properties.a, Repetition rate (red) and pump frequency (blue) via AR tuning. b, Repetition rate (red) and pump frequency (blue) via NR tuning. c, Repetition rate (red) and pump frequency (blue) via combined AR and NR tuning.