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Integrated turnkey soliton microcombs operated at CMOS frequencies
Boqiang Shen, Lin Chang, Junqiu Liu, Heming Wang, Qi-Fan Yang, Chao Xiang, Rui Ning Wang, Jijun He, Tianyi Liu, Weiqiang Xie, Joel Guo, Dave Kinghorn, Lue Wu, Qing-Xin Ji, Tobias J. Kippenberg, Kerry Vahala, John E. Bowers
TL;DR
Soliton microcombs ordinarily require complex tuning, feedback, and difficult-to-integrate components, while CMOS-compatible repetition rates remain challenging because of power requirements. This paper develops and explains a turnkey injection-locking regime in integrated high-Q Si3N4 microcombs, where binary pump operation generates solitons and supports CMOS-compatible devices down to 15 GHz.
Problem
Complex startup and feedback protocols hinder soliton microcomb integration, while CMOS-rate devices face excessive power consumption from enlarged optical mode volumes.
Method
The paper combines injection-locking theory with integrated high-Q Si3N4 resonators and numerical simulations of conventional, feedback-locked, and injection-locked soliton generation.
Results
15 GHz CMOS-compatible turnkey soliton microcombs are demonstrated, with simulations and experiments showing direct multi-soliton or single-soliton generation under suitable feedback conditions.
Takeaways & Limitations
Binary pump turn-on and turn-off can generate turnkey solitons without complex triggering or tuning schemes, while feedback phase enables control over soliton states.
Abstract
from arXiv · showhide
While soliton microcombs offer the potential for integration of powerful frequency metrology and precision spectroscopy systems, their operation requires complex startup and feedback protocols that necessitate difficult-to-integrate optical and electrical components. Moreover, CMOS-rate microcombs, required in nearly all comb systems, have resisted integration because of their power requirements. Here, a regime for turnkey operation of soliton microcombs co-integrated with a pump laser is demonstrated and theoretically explained. Significantly, a new operating point is shown to appear from which solitons are generated through binary turn-on and turn-off of the pump laser, thereby eliminating all photonic/electronic control circuitry. These features are combined with high-Q $Si_3N_4$ resonators to fully integrate into a butterfly package microcombs with CMOS frequencies as low as 15 GHz, offering compelling advantages for high-volume production.
Appendix A: Silicon nitride chip fabrication
The Si3N4 devices use a fabrication process designed for low-loss waveguides and smooth chip facets, supporting butt coupling to the laser chip.
- Deep-UV stepper lithography patterns the waveguides and stress-release structures to prevent Si3N4 film cracking during LPCVD.
- Deep reactive ion etching defines 5 × 5 mm2 chip facets with surface quality suited to butt coupling.
Appendix B: DFB laser characterization
The DFB pump laser provides substantial optical output with high wall-plug efficiency, while its lasing wavelength shifts with bias current.
- 120 mW maximum output power is delivered by the DFB laser, with a slope efficiency around 0.24 mW/mA.
- Peak wall-plug efficiency exceeds 20%, and the lasing wavelength shifts from ∼1554.5 nm to ∼1556.5 nm as bias current increases.
- The laser has a 50 mA threshold current and an estimated wavelength-current shift of ∼-1.4 GHz/mA.
Appendix C: Experimental details
The experiments characterize pump frequency noise through homodyne beat analysis and monitor soliton stability using a down-mixed repetition-rate beatnote.
- Laser linewidth is inferred from electrical beatnote noise after one beam traverses a long fiber delay and is frequency shifted before recombination.
- Soliton beatnote evolution is recorded by down-mixing with a local oscillator and extracting its power spectrum by oscilloscope FFT.
- The soliton spectrograph uses a 20 µs time window and 50 kHz resolution bandwidth.
- Relative feedback phases are estimated from the laser-to-waveguide facet gap, adjusted with an open-loop piezo micro-stepping motor.
Appendix D: Theory of turnkey soliton generation
The theory models injection locking by coupling soliton, backscattering, and laser fields, then derives nonlinear locking equilibria and verifies turnkey soliton generation numerically. Feedback phase and pump power determine operating points that can directly produce multi- or single-soliton states.
- Model formulation: The injection-locking model contains soliton, backscattering, and laser fields, with normalized variables for detuning, time, pump, and intracavity power.
- Model formulation: The analysis assumes weak backscattering, negligible backscattering-to-soliton coupling, constant propagation phase, and sufficiently strong gain saturation.
- Locking equilibria: The locking response function is set to zero in the infinite-locking-bandwidth approximation, yielding stable solutions selected by ∂χ/∂α < 0.
- Locking equilibria: Nonlinear intracavity-power changes shift the locking equilibrium because self- and cross-phase modulation red-shift the resonances.
- Locking equilibria: The equilibrium detuning separates into feedback-phase and averaged nonlinear-shift contributions, corresponding to the dimensional main-text equation.
- Soliton formation: Including dispersion produces MI combs, and the soliton existence region guarantees generation because the system bypasses the chaotic-comb region before MI onset.
- Numerical verification: Numerical simulations contrast conventional frequency-swept soliton formation with turnkey growth from ripples under feedback or injection locking.
- Soliton formation: Turnkey dynamics tend toward multiple solitons, but phase tuning and MI-gain control can produce a single soliton.
1. Different types of microcombs in the injection locking system
The injection-locking system supports multiple microcomb states, including breather solitons, chaotic combs, and soliton crystals. Their optical spectra and electrical beatnotes distinguish these operating states.
- Breather solitons have time-oscillating shapes, while soliton crystals form equidistant pulse trains through self-organization.
- The chaotic comb corresponds to unstable Turing patterns or soliton states reached as pump power increases.
- The system can operate as breather solitons, a chaotic comb, or a soliton crystal under suitable feedback phase and laser-driving frequency.These states are experimentally observed in Fig. S4.
- A noisy RF spectrum indicates that the chaotic comb is not mode-locked.The figure caption identifies electrical beatnote signals as insets for the breather-soliton and chaotic-comb spectra.
2. Tuning of turnkey soliton microcomb system
Linear pump-frequency scans probe the robustness and accessibility of turnkey soliton generation. Feedback locking produces soliton-number-dependent power steps and permits turn-on from either scan direction.
- A linear current scan drives the pump frequency at approximately 0.36 GHz/ms across the resonance.The scan speed is estimated from the wavelength-current response of the free-running laser.
- Feedback locking pulls the pump frequency toward the resonance until the driving frequency leaves the locking band.
- Power steps during scanning indicate access to soliton states with different soliton numbers.
- The soliton microcomb can power on during a scan from the red-detuned side, unlike conventional pumping in most cases.The passage notes an exception for systems with an effectively negative thermo-optic response.