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Ultralow-power chip-based soliton microcombs for photonic integration
Junqiu Liu, Arslan S. Raja, Maxim Karpov, Bahareh Ghadiani, Martin H. P. Pfeiffer, Nils J. Engelsen, Hairun Guo, Michael Zervas, Tobias J. Kippenberg
TL;DR
Soliton formation in Si3N4 microresonators at electronically detectable, sub-100-GHz repetition rates was limited by power, Q-factor, dispersion, and thermal-access requirements. Using photonic Damascene reflow fabrication, the paper demonstrates low-power single solitons at 99 GHz and accesses them across many resonances with simple laser piezo tuning. These results support Si3N4 microresonators as candidates for integrated low-power soliton microcombs.
Problem
Soliton formation in Si3N4 microresonators at electronically detectable repetition rates was constrained by low Q factors, insufficient anomalous GVD, external amplification, and thermal effects.
Method
The paper uses photonic Damascene reflow fabrication to produce high-Q Si3N4 microresonators and characterizes soliton formation across multiple resonances.
Results
9.8 mW input power generates a single soliton at 99 GHz in a Si3N4 microresonator with Q0 > 15 × 10^6.
Takeaways & Limitations
Sub-100-GHz soliton microcombs can be generated at milliwatt power levels and accessed by simple laser piezo tuning across many resonances.
Abstract
from arXiv · showhide
The generation of dissipative Kerr solitons in optical microresonators has provided a route to compact frequency combs of high repetition rate, which have already been employed for optical frequency synthesizers, ultrafast ranging, coherent telecommunication and dual-comb spectroscopy. Silicon nitride (Si$_3$N$_4$) microresonators are promising for photonic integrated soliton microcombs. Yet to date, soliton formation in Si$_3$N$_4$ microresonators at electronically detectable repetition rates, typically less than 100 GHz, is hindered by the requirement of external power amplifiers, due to the low quality ($Q$) factors, as well as by thermal effects which necessitate the use of frequency agile lasers to access the soliton state. These requirements complicate future photonic integration, heterogeneous or hybrid, of soliton microcomb devices based on Si$_3$N$_4$ microresonators with other active or passive components. Here, using the photonic Damascene reflow process, we demonstrate ultralow-power single soliton formation in high-Q ($Q_0>15\times10^6$) Si$_3$N$_4$ microresonators with 9.8 mW input power (6.2 mW in the waveguide) for devices of electronically detectable, 99 GHz repetition rate. We show that solitons can be accessed via simple, slow laser piezo tuning, in many resonances in the same sample. These power levels are compatible with current silicon-photonics-based lasers for full photonic integration of soliton microcombs, at repetition rates suitable for applications such as ultrafast ranging and coherent communication. Our results show the technological readiness of Si$_3$N$_4$ optical waveguides for future all-on-chip soliton microcomb devices.
I. INTRODUCTION
Si3N4 microcombs offer chip-scale, high-repetition-rate frequency combs and broad photonic integration, but electronically detectable soliton operation remained constrained by power, dispersion, and repetition-rate challenges. The paper addresses these challenges with a photonic Damascene reflow process and reports low-power soliton formation at sub-100-GHz repetition rates.
- Dissipative Kerr solitons provide coherent microcomb states with broad bandwidth and applications including coherent communication, dual-comb spectroscopy, and ultrafast ranging.
- Si3N4 combines a wide transparency window and high material nonlinearity with compatibility for integrating lasers, modulators, and photodetectors on silicon.
- Soliton formation in Si3N4 remained difficult because comparatively low Q factors, insufficient anomalous GVD, and limited access to suitable pumping conditions constrained operation.
- Earlier integrated-laser soliton generation used a 200 GHz repetition rate, which is not electronically detectable and therefore limits application potential.
- The paper demonstrates single solitons at 88 GHz with 48.6 mW input power and at 99 GHz with 9.8 mW input power, while accessing multiple resonances by laser piezo tuning.
II. SAMPLE FABRICATION
The fabrication process uses a reflowed SiO2 preform, LPCVD Si3N4 deposition, polishing, and thermal annealing to reduce roughness, scattering, and hydrogen-related losses. The Damascene geometry also supports wider inverse nanotapers compatible with DUV stepper lithography.
- A 1250°C preform reflow uses SiO2 surface tension to smooth etched sidewalls before Si3N4 deposition.
- The Damascene flow deposits LPCVD Si3N4 into preform trenches, then uses CMP to remove excess material and create an ultrasmooth top surface.
- Thermal annealing at 1200°C in nitrogen reduces residual hydrogen introduced during LPCVD processing, while SiO2 top cladding remains optional.
- The Damascene process enables inverse nanotapers wider than 400 nm, unlike the sub-100-nm widths required by the standard subtractive process.
III. SOLITON COMB OF 88 GHZ REPETITION RATE
The 88-GHz devices achieve single-soliton operation in high-Q Si3N4 microresonators, with resonance characterization, calibrated dispersion measurements, and transmission-based soliton identification. Operation requires 48.6 mW at the chip input, corresponding to 30.6 mW in the bus waveguide.
- 88 GHz is the measured free-spectral range of the microresonators used for the first single-soliton demonstration.
- A calibrated femtosecond frequency comb extracts resonance frequencies, free-spectral range, anomalous GVD, intrinsic linewidth, loaded linewidth, and coupling strength.
- Q0 > 8.2 × 10^6 is obtained at λ = 1558.0 nm with loaded linewidth κ/2π = 30.3 MHz and intrinsic loss κ0/2π ∼23.3 MHz.
- The soliton state is identified by a transmission step during laser tuning and verified for the TE00 spectrum using a vector network analyzer.
- 48.6 mW input power produces a single soliton, corresponding to 30.6 mW in the bus waveguide after 63% per-facet coupling.
- 20 µW per comb line remains sufficient for the same goals as prior sub-100-GHz soliton applications that used input power exceeding 1 W.
IV. SOLITON COMB OF 99 GHZ REPETITION RATE
The study demonstrates single-soliton formation in 99-GHz-FSR silicon-nitride microresonators at milliwatt input powers and accesses solitons across many resonances using simple laser piezo tuning.
- Hydrogen absorption losses were reduced through repeated annealing and a thick SiO2 top cladding.These process changes targeted losses that had prevented lower-power soliton generation.
- 9.8 mW input power (6.2 mW in the bus waveguide) generated a 99-GHz single soliton without an EDFA.The resonator had strong anomalous GVD of approximately 1.23 MHz.
- The soliton state remained accessible for several hundred microseconds to milliseconds during diode-laser scans.This enabled access through simple laser piezo tuning rather than frequency-agile tuning.
- Single solitons formed reproducibly below 10 mW in resonances close to avoided mode crossings.The authors suggest spatial mode interactions may assist formation at lower power.
- Single solitons were accessed in twenty selected resonances, including consecutive resonances in the telecom L- and C-bands.All were accessed with simple laser piezo tuning at around 22 mW laser output power.
- The broad resonance coverage offers flexibility for studying avoided mode crossings, dark pulses in normal-GVD regions, and breathing solitons.
V. CONCLUSION
The work demonstrates ultralow-power single-soliton generation in integrated high-Q silicon-nitride microresonators below 100 GHz. The demonstrated milliwatt power levels support applications requiring low-power photonic integration.
- 48.6 mW input power produced a soliton in an 88-GHz-FSR microresonator.
- 9.8 mW input power (6.2 mW in the waveguide) produced a 99-GHz single soliton with Q0 > 15 × 10^6.
- The 99-GHz soliton was accessed through simple laser piezo tuning and reproduced across twenty selected resonances.
- Milliwatt-level soliton generation is central for low-power applications including photonic chip-based microwave generators, integrated frequency synthesizers, OCT, and dual-comb spectroscopy.
- The study's code and plotting data are available on Zenodo, while other data are available from the corresponding authors upon reasonable request.
FUNDING INFORMATION
The work received support from DARPA’s Defense Sciences Office through Contract HR0011-15-C-0055 (DODOS) and from the Swiss National Science Foundation through grant agreement No. 161573 (precoR).
- DARPA’s Defense Sciences Office supported the work through Contract HR0011-15-C-0055 (DODOS).
- The document identifies DODOS and precoR among the work’s supporting grants and contracts.
- The Swiss National Science Foundation supported the work through grant agreement No. 161573 (precoR).