Soliton Microcombs Research Articles

Impact of U-shaped waveguide structures on optical comb generation in microresonator systems

In the research of soliton microcombs, the pump energy conversion efficiency (ECE) of the microcomb is an important parameter. However, in traditional microresonator systems, the ECE is only a few percent. Researchers have enhanced it by introducing additional feedback coupling structures or composite cavity structures into the microresonator system to suppress the output of pump light in the soliton generation region. The additional structures may affect the output comb characteristics of the system, but most articles do not delve into this issue, which hinders the practical application of high-efficiency soliton microcombs. In this paper, we establish rate equations describing the nonlinear dynamics of a microresonator with a U-shaped waveguide feedback coupling structure (coupled-cavity system, CCS), and numerically simulate and conduct steady-state analysis of the simulation results to obtain the influence of the U-shaped waveguide on the CCS’s output comb characteristics. Then we optimize the structural parameters of the CCS reducing the parametric oscillation threshold of the pump by more than 50%, while increasing the ECE to over 25% when the optical field in the microresonator propagates in the form of a single soliton.

Maximizing transmission capacity in optical communication systems utilizing a microresonator comb laser source with adaptive modulation and bandwidth allocation strategies

Traditional optical communication systems employ bulky laser arrays that lack coherence and are prone to severe frequency drift. Dissipative Kerr soliton microcombs offer numerous evenly spaced optical carriers with a high optical signal-to-noise ratio (OSNR) and coherence in chip-scale packages, potentially addressing the limitations of traditional wavelength division multiplexing (WDM) sources. However, soliton microcombs exhibit inhomogeneous OSNR and linewidth distributions across the spectra, leading to variable communication performance under uniform modulation schemes. Here, we demonstrate, for the first time, to our knowledge, the application of adaptive modulation and bandwidth allocation strategies in optical frequency comb (OFC) communication systems to optimize modulation schemes based on OSNR, linewidth, and channel bandwidth, thereby maximizing capacity. Experimental verification demonstrates that the method enhances spectral efficiency from 1.6 to 2.31 bit ⋅ s−1 ⋅ Hz−1, signifying a 44.58% augmentation. Using a single-soliton microcomb as the light source, we achieve a maximum communication capacity of 10.68 Tbps after 40 km of transmission in the C-band, with the maximum single-channel capacity reaching 432 Gbps. The projected combined transmission capacity for the C- and L-bands could surpass 20 Tbps. The proposed strategies demonstrate promising potential of utilizing soliton microcombs as future light sources in next-generation optical communication.

Non-Ambiguous Range Extension by a Frequency Scanning Soliton Microcomb

Non-Ambiguous Range Extension by a Frequency Scanning Soliton Microcomb

Dissipative Kerr soliton formation in dual-mode interaction Si3N4 microresonators

Dissipative Kerr soliton (DKS) microcombs based on multi-mode Si3N4 waveguides turn into an ideal tool that is compact and has precision for optical communication, precision spectroscopy, and frequency metrology. However, spatial waveguide mode interaction leads to local disturbances of dispersion, which may hinder DKS microcombs formation. In this letter, we generate the DKS microcomb in a dual-mode interaction Si3N4 microresonator without suppressing spatial waveguide mode interaction. The spatial waveguide mode interaction is investigated in the dual-mode interaction Si3N4 microresonator with a cross-sectional area of 800 × 1700 nm2. DKS microcomb is deterministically generated in the microresonator using an auxiliary light heating method. Furthermore, an integrated microcomb frequency measurement system is designed based on the DKS microcomb for frequency metrology.

Wavelength-division multiplexing (WDM) FSO communications with soliton microcombs

This study highlights the benefits of soliton microcombs (SMCs) in FSO communication systems. By employing a microring resonator coupled with an add/drop filter, we achieved soliton microcombs with a precise Free Spectral Range (FSR) of 1870 pm and a Full Width at Half Maximum (FWHM) of 60 pm, tailored for Wavelength Division Multiplexing (WDM) channels. The system utilized 100 lines for the WDM channel with an inter-channel spacing of 0.5 nm and PolSK modulation to encode the data. This combination achieved a bit error rate (BER) of 10 − 9 , significantly lower than the 10 − 7 observed with traditional systems. For performance simulation, the channel was characterized using the gamma-gamma model, which assessed the impact of atmospheric turbulence on data transmission. The results show a high channel capacity ( 10 3 Gbit/s) and a low loss probability ( 10 − 8 ), ensuring reliable data transmission even in the presence of atmospheric disturbances. This methodology provides substantial improvements in stability and efficiency for FSO networks.

Octave-spanning Kerr soliton frequency combs in dispersion- and dissipation-engineered lithium niobate microresonators

Dissipative Kerr solitons from optical microresonators, commonly referred to as soliton microcombs, have been developed for a broad range of applications, including precision measurement, optical frequency synthesis, and ultra-stable microwave and millimeter wave generation, all on a chip. An important goal for microcombs is self-referencing, which requires octave-spanning bandwidths to detect and stabilize the comb carrier envelope offset frequency. Further, detection and locking of the comb spacings are often achieved using frequency division by electro-optic modulation. The thin-film lithium niobate photonic platform, with its low loss, strong second- and third-order nonlinearities, as well as large Pockels effect, is ideally suited for these tasks. However, octave-spanning soliton microcombs are challenging to demonstrate on this platform, largely complicated by strong Raman effects hindering reliable fabrication of soliton devices. Here, we demonstrate entirely connected and octave-spanning soliton microcombs on thin-film lithium niobate. With appropriate control over microresonator free spectral range and dissipation spectrum, we show that soliton-inhibiting Raman effects are suppressed, and soliton devices are fabricated with near-unity yield. Our work offers an unambiguous method for soliton generation on strongly Raman-active materials. Further, it anticipates monolithically integrated, self-referenced frequency standards in conjunction with established technologies, such as periodically poled waveguides and electro-optic modulators, on thin-film lithium niobate.

Towards a compact soliton microcomb fully referenced on atomic reference

Towards a compact soliton microcomb fully referenced on atomic reference

Foundry manufacturing of octave-spanning microcombs.

Soliton microcombs provide a chip-based, octave-spanning source for self-referencing and optical metrology. We use a silicon nitride integrated photonics foundry to manufacture 280 single-chip solutions of octave-spanning microcombs on a wafer. By group-velocity dispersion (GVD) engineering with the waveguide cross section, we shape the soliton spectrum for dispersive-wave spectral enhancements at the frequencies for f-2f self-referencing. Moreover, we demonstrate the other considerations, including models for soliton spectrum design, ultra-broadband resonator external coupling, low-loss edge couplers, and the nonlinear self-interactions of few-cycle solitons. To cover the fabrication tolerance, we systematically scan 336 parameter sets of resonator width and radius, ensuring at least one device on each chip can yield an octave-spanning comb with an electronically detectable carrier-envelope offset frequency, which has been supported by our experiment. Our design and testing process permit highly repeatable creation of single-chip solutions of soliton microcombs optimized for pump operation ∼100 mW and high comb mode power for f-2f detection, which is the central component of a compact microsystem for optical metrology.

Polarization-diverse soliton transitions and deterministic switching dynamics in strongly-coupled and self-stabilized microresonator frequency combs

Dissipative Kerr soliton microcombs in microresonators have enabled fundamental advances in chip-scale precision metrology, communication, spectroscopy, and parallel signal processing. Here we demonstrate polarization-diverse soliton transitions and deterministic switching dynamics of a self-stabilized microcomb in a strongly-coupled dispersion-managed microresonator driven with a single pump laser. The switching dynamics are induced by the differential thermorefractivity between coupled transverse-magnetic and transverse-electric supermodes during the forward-backward pump detunings. The achieved large soliton existence range and deterministic transitions benefit from the switching dynamics, leading to the cross-polarized soliton microcomb formation when driven in the transverse-magnetic supermode of the single resonator. Secondly, we demonstrate two distinct polarization-diverse soliton formation routes – arising from chaotic or periodically-modulated waveforms via pump power selection. Thirdly, to observe the cross-polarized supermode transition dynamics, we develop a parametric temporal magnifier with picosecond resolution, MHz frame rate and sub-ns temporal windows. We construct picosecond temporal transition portraits in 100-ns recording length of the strongly-coupled solitons, mapping the transitions from multiple soliton molecular states to singlet solitons. This study underpins polarization-diverse soliton microcombs for chip-scale ultrashort pulse generation, supporting applications in frequency and precision metrology, communications, spectroscopy and information processing.

Self-Suppressed Quantum-Limited Timing Jitter and Fundamental Noise Limit of Soliton Microcombs.

Quantum-limited timing jitter of soliton microcombs has long been recognized as their fundamental noise limit. Here, we surpass such limit by utilizing dispersive wave dynamics in multimode microresonators. Through the viscous force provided by these dispersive waves, the quantum-limited timing jitter can be suppressed to a much lower level that forms the ultimate fundamental noise limit of soliton microcombs. Our findings enable coherence engineering of soliton microcombs in the quantum regime, providing critical guidelines for using soliton microcombs to synthesize ultralow-noise microwave and optical signals.

Spectral-interferometry-based diff-iteration for high-precision micro-dispersion measurement

Precise measurement of micro-dispersion for optical devices (optical fiber, lenses, etc.) holds paramount significance across domains such as optical fiber communication and dispersion interference ranging. However, due to its complex system, complicated process, and low reliability, the traditional dispersion measurement methods (interference, phase shift, or time delay methods) are not suitable for the accurate measurement of micro-dispersion in a wide spectral range. Here, we propose a spectral-interferometry-based diff-iteration (SiDi) method for achieving accurate wide-band micro-dispersion measurements. Using an optical frequency comb, based on the phase demodulation of the dispersion interference spectrum, we employ the carefully designed SiDi method to solve the dispersion curve at any position and any order. Our approach is proficient in precisely measuring micro-dispersion across a broadband spectrum, without the need for cumbersome wavelength scanning processes or reliance on complex high-repetition-rate combs, while enabling adjustable resolution. The efficacy of the proposed method is validated through simulations and experiments. We employed a chip-scaled soliton microcomb (SMC) to compute the dispersion curves of a 14 m single-mode fiber (SMF) and a 0.05 m glass. Compared to a laser interferometer or the theoretical value given by manufacturers, the average relative error of refractive index measurement for single-mode fiber (SMF) reaches 2.8×10−6 and for glass reaches 3.8×10−6. The approach ensures high precision, while maintaining a simple system structure, with realizing adjustable resolution, thereby propelling the practical implementation of precise measurement and control-dispersion.

Lithium tantalate photonic integrated circuits for volume manufacturing

Electro-optical photonic integrated circuits (PICs) based on lithium niobate (LiNbO3) have demonstrated the vast capabilities of materials with a high Pockels coefficient1,2. They enable linear and high-speed modulators operating at complementary metal–oxide–semiconductor voltage levels3 to be used in applications including data-centre communications4, high-performance computing and photonic accelerators for AI5. However, industrial use of this technology is hindered by the high cost per wafer and the limited wafer size. The high cost results from the lack of existing high-volume applications in other domains of the sort that accelerated the adoption of silicon-on-insulator (SOI) photonics, which was driven by vast investment in microelectronics. Here we report low-loss PICs made of lithium tantalate (LiTaO3), a material that has already been adopted commercially for 5G radiofrequency filters6 and therefore enables scalable manufacturing at low cost, and it has equal, and in some cases superior, properties to LiNbO3. We show that LiTaO3 can be etched to create low-loss (5.6 dB m−1) PICs using a deep ultraviolet (DUV) stepper-based manufacturing process7. We demonstrate a LiTaO3 Mach–Zehnder modulator (MZM) with a half-wave voltage–length product of 1.9 V cm and an electro-optic bandwidth of up to 40 GHz. In comparison with LiNbO3, LiTaO3 exhibits a much lower birefringence, enabling high-density circuits and broadband operation over all telecommunication bands. Moreover, the platform supports the generation of soliton microcombs. Our work paves the way for the scalable manufacture of low-cost and large-volume next-generation electro-optical PICs.

Harnessing sub-comb dynamics in a graphene-sensitized microresonator for gas detection

Since their inception, frequency combs generated in microresonators, known as microcombs, have sparked significant scientific interests. Among the various applications leveraging microcombs, soliton microcombs are often preferred due to their inherent mode-locking capability. However, this choice introduces additional system complexity because an initialization process is required. Meanwhile, despite the theoretical understanding of the dynamics of other comb states, their practical potential, particularly in applications like sensing where simplicity is valued, remains largely untapped. Here, we demonstrate controllable generation of sub-combs that bypasses the need for accessing bistable regime. And in a graphene-sensitized microresonator, the sub-comb heterodynes produce stable, accurate microwave signals for high-precision gas detection. By exploring the formation dynamics of sub-combs, we achieved 2 MHz harmonic comb-to-comb beat notes with a signal-to-noise ratio (SNR) greater than 50 dB and phase noise as low as − 82 dBc/Hz at 1 MHz offset. The graphene sensitization on the intracavity probes results in exceptional frequency responsiveness to the adsorption of gas molecules on the graphene of microcavity surface, enabling detect limits down to the parts per billion (ppb) level. This synergy between graphene and sub-comb formation dynamics in a microcavity structure showcases the feasibility of utilizing microcombs in an incoherent state prior to soliton locking. It may mark a significant step toward the development of easy-to-operate, systemically simple, compact, and high-performance photonic sensors.Graphical

Parallel wavelength-division-multiplexed signal transmission and dispersion compensation enabled by soliton microcombs and microrings

The proliferation of computation-intensive technologies has led to a significant rise in the number of datacenters, posing challenges for high-speed and power-efficient datacenter interconnects (DCIs). Although inter-DCIs based on intensity modulation and direct detection (IM-DD) along with wavelength-division multiplexing technologies exhibit power-efficient and large-capacity properties, the requirement of multiple laser sources leads to high costs and limited scalability, and the chromatic dispersion (CD) restricts the transmission length of optical signals. Here we propose a scalable on-chip parallel IM-DD data transmission system enabled by a single-soliton Kerr microcomb and a reconfigurable microring resonator-based CD compensator. We experimentally demonstrate an aggregate line rate of 1.68 Tbit/s over a 20-km-long SMF. The extrapolated energy consumption for CD compensation of 40-km-SMFs is ~0.3 pJ/bit, which is calculated as being around 6 times less than that of the commercial 400G-ZR coherent transceivers. Our approach holds significant promise for achieving data rates exceeding 10 terabits.

Mechanically Actuated Kerr Soliton Microcombs

AbstractMode‐locked ultrashort pulse sources with a repetition rate of up to several tens of gigahertz greatly facilitate versatile photonic applications such as frequency synthesis, metrology, radar, and optical communications. Dissipative Kerr soliton microcombs provide an attractive solution as a broadband, high‐repetition‐rate compact laser system in this context. However, its operation usually requires sophisticated pump laser control to initiate and stabilize the soliton microcombs, particularly in millimeter‐sized ultrahigh‐Q whispering‐gallery resonators. Here, a mechanically actuated soliton microcomb oscillator is realized with a microwave repetition rate of 15 GHz. This enables direct initiation, subsequent long‐term stabilization, and fine tuning of soliton combs without pump laser tunability that is generally required for soliton microcomb operation. The prospects for using this method with a wide range of applications are revealed that will benefit from mechanical soliton actuation such as optical clocks, spectral extension, and dual‐comb spectroscopy.

Dissipative Kerr soliton generation at 2μm in a silicon nitride microresonator

Chip-scale optical frequency combs enable the generation of highly-coherent pulsed light at gigahertz-level repetition rates, with potential technological impact ranging from telecommunications to sensing and spectroscopy. In combination with techniques such as dual-comb spectroscopy, their utilization would be particularly beneficial for sensing of molecular species in the mid-infrared spectrum, in an integrated fashion. However, few demonstrations of direct microcomb generation within this spectral region have been showcased so far. In this work, we report the generation of Kerr soliton microcombs in silicon nitride integrated photonics. Leveraging a high-Q silicon nitride microresonator, our device achieves soliton generation under milliwatt-level pumping at 1.97 µm, with a generated spectrum encompassing a 422 nm bandwidth and extending up to 2.25 µm. The use of a dual pumping scheme allows reliable access to several comb states, including primary combs, modulation instability combs, as well as multi- and single-soliton states, the latter exhibiting high stability and low phase noise. Our work extends the domain of silicon nitride based Kerr microcombs towards the mid-infrared using accessible factory-grade technology and lays the foundations for the realization of fully integrated mid-infrared comb sources.

Octave soliton microcombs in lithium niobate microresonators.

Soliton microcombs are regarded as an ideal platform for applications such as optical communications, optical sensing, low-noise microwave sources, optical atomic clocks, and frequency synthesizers. Many of these applications require a broad comb spectrum that covers an octave, essential for implementing the f - 2f self-referencing techniques. In this work, we have successfully generated an octave-spanning soliton microcomb based on a z-cut thin-film lithium niobate (TFLN) microresonator. This achievement is realized under on-chip optical pumping at 340 mW and through extensive research into the broadening of dual dispersive waves (DWs). Furthermore, the repetition rate of the octave soliton microcomb is accurately measured using an electro-optic comb generated by an x-cut TFLN racetrack microresonator. Our results represent a crucial step toward the realization of practical, integrated, and fully stabilized soliton microcomb systems based on TFLN.

Integrated optical frequency division for microwave and mmWave generation.

The generation of ultra-low-noise microwave and mmWave in miniaturized, chip-based platforms can transform communication, radar and sensing systems1-3. Optical frequency division that leverages optical references and optical frequency combs has emerged as a powerful technique to generate microwaves with superior spectral purity than any other approaches4-7. Here we demonstrate a miniaturized optical frequency division system that can potentially transfer the approach to a complementary metal-oxide-semiconductor-compatible integrated photonic platform. Phase stability is provided by a largemode volume, planar-waveguide-based optical reference coil cavity8,9 and is divided down from optical to mmWave frequency by using soliton microcombs generated in a waveguide-coupled microresonator10-12. Besides achieving record-low phase noise for integrated photonic mmWave oscillators, these devices can be heterogeneously integrated with semiconductor lasers, amplifiers and photodiodes, holding the potential of large-volume, low-cost manufacturing for fundamental and mass-market applications13.

Optimizing auxiliary laser heating for Kerr soliton microcomb generation.

Auxiliary laser heating has become a widely adopted method for Kerr soliton frequency comb generation in optical microcavities, thanks to its reliable and easy-to-achieve merits for solving the thermal instability during the formation of dissipative Kerr solitons. Here, we conduct optimization of auxiliary laser heating by leveraging the distinct loss and absorption characteristics of different longitudinal and polarization cavity modes. We show that even if the auxiliary and pump lasers enter orthogonal polarization modes, their mutual photothermal balance can be efficient enough to maintain a cavity thermal equilibrium as the pump laser enters the red-detuning soliton regime, and by choosing the most suitable resonance for the auxiliary and pump lasers, the auxiliary laser power can be reduced to 20% of the pump laser and still be capable of warranting soliton generation. Moreover, we demonstrate soliton comb generation using integrated laser modules with a few milliwatt on-chip pump and auxiliary powers, showcasing the potential for further chip integration of the auxiliary laser heating method.

Photonic bandgap microcombs at 1064 nm.

Microresonator frequency combs and their design versatility have revolutionized research areas from data communication to exoplanet searches. While microcombs in the 1550 nm band are well documented, there is interest in using microcombs in other bands. Here, we demonstrate the formation and spectral control of normal-dispersion dark soliton microcombs at 1064 nm. We generate 200 GHz repetition rate microcombs by inducing a photonic bandgap of the microresonator mode for the pump laser with a photonic crystal. We perform the experiments with normal-dispersion microresonators made from Ta2O5 and explore unique soliton pulse shapes and operating behaviors. By adjusting the resonator dispersion through its nanostructured geometry, we demonstrate control over the spectral bandwidth of these combs, and we employ numerical modeling to understand their existence range. Our results highlight how photonic design enables microcomb spectra tailoring across wide wavelength ranges, offering potential in bioimaging, spectroscopy, and photonic-atomic quantum technologies.