Microcomb Generation Research Articles

Repetition rate locked single-soliton microcomb generation via rapid frequency sweep and sideband thermal compensation

Dissipative Kerr solitons (DKSs) with mode-locked pulse trains in high- Q optical microresonators possess low-noise and broadband parallelized comb lines, having already found plentiful cutting-edge applications. However, thermal bistability and thermal noise caused by the high microresonator power and large temperature exchange between microresonator and the environment would prevent soliton microcomb formation and deteriorate the phase and frequency noise. Here, a novel method that combines rapid frequency sweep with optical sideband thermal compensation is presented, providing a simple and reliable way to get into the single-soliton state. Meanwhile, it is shown that the phase and frequency noises of the generated soliton are greatly reduced. Moreover, by closing the locking loop, an in-loop repetition rate fractional instability of 5.5 × 10 − 15 at 1 s integration time and a triangular linear repetition rate sweep with 2.5 MHz could be realized. This demonstration provides a means for the generation, locking, and tuning of a soliton microcomb, paving the way for the application of single-soliton microcombs in low-phase-noise microwave generation and laser ranging.

Experimental Demonstration of Self-Oscillation Microcomb in a Mode-Splitting Microresonator

Self-oscillation and bifurcation as many-body dynamics solutions in a high-Q microresonator have induced substantial interest in nonlinear optics and ultrafast science. Strong mode coupling between clockwise (CW) wave and counterclockwise (CCW) wave induces mode-splitting and optical self-oscillation in the optical cavity. This study experimentally demonstrates the self-oscillation microcomb formation in a microresonator with strong backward Rayleigh scattering. When a pump laser sweeps across a resonance, both spontaneous symmetry breaking (SSB) and self-oscillation phenomenon are observed. The breathing soliton and stable soliton state can switch to each other through careful tuning of the pump detuning. Our experiments provide a reliable scheme for breather soliton microcomb generation. Meanwhile, the rich physics process enhances the comprehension of nonlinear optics in a cavity.

Silicon nitride passive and active photonic integrated circuits: trends and prospects

The use of silicon nitride in integrated photonics has rapidly progressed in recent decades. Ultra-low-loss waveguides based on silicon nitride are a favorable platform for the research of nonlinear and microwave photonics and their application to a wide variety of fields, including precision metrology, communications, sensing, imaging, navigation, computation, and quantum physics. In recent years, the integration of Si and III-V materials has enabled new large-scale, advanced silicon nitride-based photonic integrated circuits with versatile functionality. In this perspective article, we review current trends and the state-of-the-art in silicon nitride-based photonic devices and circuits. We highlight the hybrid and heterogeneous integration of III-V with silicon nitride for electrically pumped soliton microcomb generation and ultra-low-noise lasers with fundamental linewidths in the tens of mHz range. We also discuss several ultimate limits and challenges of silicon nitride-based photonic device performance and provide routes and prospects for future development.

Dual-laser self-injection locking to an integrated microresonator.

Diode laser self-injection locking (SIL) to a whispering gallery mode of a high quality factor resonator is a widely used method for laser linewidth narrowing and high-frequency noise suppression. SIL has already been used for the demonstration of ultra-low-noise photonic microwave oscillators and soliton microcomb generation and has a wide range of possible applications. Up to date, SIL was demonstrated only with a single laser. However, multi-frequency and narrow-linewidth laser sources are in high demand for modern telecommunication systems, quantum technologies, and microwave photonics. Here we experimentally demonstrate the dual-laser SIL of two multifrequency laser diodes to different modes of an integrated Si3N4 microresonator. Simultaneous spectrum collapse of both lasers, as well as linewidth narrowing and high-frequency noise suppression , as well as strong nonlinear interaction of the two fields with each other, are observed. Locking both lasers to the same mode results in a simultaneous frequency and phase stabilization and coherent addition of their outputs. Additionally, we provide a comprehensive dual-SIL theory and investigate the influence of lasers on each other caused by nonlinear effects in the microresonator.

Stimulated Raman scattering induced dark pulse and microcomb generation in the mid-infrared

We demonstrate that strong stimulated Raman scattering in silicon and germanium microresonators can induce stable and breathing dark pulses generation circumventing traditional complex approaches such as pump modulation and mode coupling. Although multi-photon absorption shows a small influence on the detuning value for stable dark pulse excitation, the concomitant free carrier will assist dark pulse excitation and broaden the excitation area of dark pulse thus making it easier to capture stable pulse. Furthermore, dark breather dynamics in Si and Ge are also observed, which shows distinct properties from the dark soliton breathers dominated solely by Kerr effect. Finally, we show that octave spanning mid-infrared (MIR) microcomb can be generated combining with high-order dispersion engineering, which in turn affects the breathing dynamics of dark pulses. Our findings provide another way for the initiation of dark pulses in group IV materials and broadband MIR microcomb generation for spectroscopy applications.

Platicon microcomb generation using laser self-injection locking

The past decade has witnessed major advances in the development and system-level applications of photonic integrated microcombs, that are coherent, broadband optical frequency combs with repetition rates in the millimeter-wave to terahertz domain. Most of these advances are based on harnessing of dissipative Kerr solitons (DKS) in microresonators with anomalous group velocity dispersion (GVD). However, microcombs can also be generated with normal GVD using localized structures that are referred to as dark pulses, switching waves or platicons. Compared with DKS microcombs that require specific designs and fabrication techniques for dispersion engineering, platicon microcombs can be readily built using CMOS-compatible platforms such as thin-film (i.e., thickness below 300 nm) silicon nitride with normal GVD. Here, we use laser self-injection locking to demonstrate a fully integrated platicon microcomb operating at a microwave K-band repetition rate. A distributed feedback (DFB) laser edge-coupled to a Si3N4 chip is self-injection-locked to a high-Q ( > 107) microresonator with high confinement waveguides, and directly excites platicons without sophisticated active control. We demonstrate multi-platicon states and switching, perform optical feedback phase study and characterize the phase noise of the K-band platicon repetition rate and the pump laser. Laser self-injection-locked platicons could facilitate the wide adoption of microcombs as a building block in photonic integrated circuits via commercial foundry service.

Monostable dissipative Kerr solitons.

Kerr microcombs hold the promise of bringing frequency combs onto the chip and into a variety of applications requiring low size, weight, power, and cost. However, reliable Kerr microcomb generation is hindered by the thermal effect and multistability of dissipative Kerr solitons (DKSs). Past approaches toward Kerr microcomb reliability include either deterministic single-soliton generation or self-starting soliton behavior but not both. Here we describe a regime of DKSs that is both deterministic and self-starting, in which only a single soliton can stably exist. We term this new DKS regime "monostable DKSs" (MS-DKSs) as all other optical behaviors, such as continuous-wave-only and multiple solitons, are fundamentally forbidden by the design. We establish a graphical model to describe MS-DKSs and discuss the design principles of MS-DKSs. We numerically demonstrate the MS-DKS behavior in an example periodically poled lithium niobate microring resonator.

Dual-microcomb generation in a synchronously driven waveguide ring resonator.

Microcombs-optical frequency combs generated in coherently driven nonlinear microresonators-have attracted significant attention over the last decade. The ability to generate two such combs in a single resonator device has, in particular, enabled a host of applications from spectroscopy to imaging. Concurrently, novel comb generation techniques such as synchronous pulsed driving have been developed to enhance the efficiency and flexibility of microcomb generation. Here, we report on the first, to the best of our knowledge, experimental demonstration of dual-microcomb generation via synchronous pulsed pumping of a single microresonator. Specifically, we use two electro-optically generated pulse trains derived from a common continuous wave laser to simultaneously drive two orthogonal polarization modes of an integrated silica ring resonator, observing the generation of coherent dissipative Kerr cavity soliton combs on both polarization axes. Thanks to the resonator birefringence, the two soliton combs are associated with different repetition rates, thus realizing a dual-microcomb source. To illustrate the source's application potential, we demonstrate proof-of-concept spectroscopic measurements.

Free carrier induced dark pulse generation in microresonators.

We theoretically demonstrate a novel, to the best of our knowledge, mechanism for dark pulse excitation in normal dispersion microresonators exploiting free carrier dispersion and free carrier absorption effects due to multi-photon absorption. Dark pulses can be generated in the three- and four-photon absorption regimes in the presence or absence of external reverse bias to control the lifetime of free carriers, respectively. Direct generation of dark pulses is proven to be feasible in both regimes with a frequency fixed laser. The dynamics of their temporal and spectral evolution have also been investigated. Our findings establish a reliable path for dark pulse and Kerr microcomb generation in related platforms with simplified controlling and tuning techniques.

Study of microcomb threshold power with coupling scaling

We model the generation threshold and conversion efficiency of microcombs by scaling the cavity coupling. With the Lugiato–Lefever equation (LLE), quantitative analysis of threshold is established in the parameter space of pump power and coupling. Considering the large detuning and Kerr-induced phase shift, the threshold power is numerically solved with the minimum at over-coupling, in agreement with that from the traveling wave theory. Furthermore, the coupling dependence on microcomb generation is discussed, providing the accessibility of high-efficient, stable combs (≥ 40%) around the threshold. This work offers universal guidelines for the design of microcombs with low-power and high-efficient operation.

Free-carrier-assisted mid-infrared microcavity soliton generation

Multi-photon absorption (MPA) and free-carrier (FC) effects, usually considered to be detrimental to microcomb generation by introducing strong nonlinear loss, also offer opportunities to overwhelm the thermal-optic effect by modifying the refractive index. Here, we derive the theoretical expression of solitons expanded with MPA and FC effects, accompanied by numerical simulations to reveal the dynamics and mechanism for capturing steady mid-infrared solitons in both Si and Ge microcavities. It is found that with increased FC lifetime, the intensity-dependent MPA underlies non-monotonous variation of nonlinear detuning which enables soliton generation. More interestingly, proper control on the FC lifetime admits bidirectional switching of soliton states (i.e., both decreasing and increasing the number of solitons) as well as self-starting solitons, yielding different technique routes toward microcavity solitons. This research could contribute to a better understanding of nonlinear behaviors influenced by FC effects and find practical applications by releasing the demand on precise control of laser sources, which is especially meaningful for the mid-infrared region.

Hertz-linewidth semiconductor lasers using CMOS-ready ultra-high-Q microresonators

Driven by narrow-linewidth bench-top lasers, coherent optical systems spanning optical communications, metrology and sensing provide unrivalled performance. To transfer these capabilities from the laboratory to the real world, a key missing ingredient is a mass-produced integrated laser with superior coherence. Here, we bridge conventional semiconductor lasers and coherent optical systems using CMOS-foundry-fabricated microresonators with record high $Q$ factor over 260 million and finesse over 42,000. Five orders-of-magnitude noise reduction in the pump laser is demonstrated, and for the first time, fundamental noise below 1 Hz$^2$ Hz$^{-1}$ is achieved in an electrically-pumped integrated laser. Moreover, the same configuration is shown to relieve dispersion requirements for microcomb generation that have handicapped certain nonlinear platforms. The simultaneous realization of record-high $Q$ factor, highly coherent lasers and frequency combs using foundry-based technologies paves the way for volume manufacturing of a wide range of coherent optical systems.

Dirac solitons in optical microresonators

Mode-coupling-induced dispersion has been used to engineer microresonators for soliton generation at the edge of the visible band. Here, we show that the optical soliton formed in this way is analogous to optical Bragg solitons and, more generally, to the Dirac soliton in quantum field theory. This optical Dirac soliton is studied theoretically, and a closed-form solution is derived in the corresponding conservative system. Both analytical and numerical solutions show unusual properties, such as polarization twisting and asymmetrical optical spectra. The closed-form solution is also used to study the repetition rate shift in the soliton. An observation of the asymmetrical spectrum is analysed using theory. The properties of Dirac optical solitons in microresonators are important at a fundamental level and provide a road map for soliton microcomb generation in the visible band.

Polygon Coherent Modes in a Weakly Perturbed Whispering Gallery Microresonator for Efficient Second Harmonic, Optomechanical, and Frequency Comb Generations.

We observe high optical quality factor (Q) polygonal and star coherent optical modes in a lithium niobate microdisk. In contrast to the previous polygon modes achieved by deformed microcavities at lower mechanical and optical Q, we adopt weak perturbation from a tapered fiber for the polygon mode formation. The resulting high intracavity optical power of the polygon modes triggers second harmonic generation at high efficiency. With the combined advantages of a high mechanical Q cavity, we observe optomechanical oscillation in polygon modes for the first time. Finally, we observe frequency microcomb generation from the polygon modes with an ultrastable taper-on-disk coupling mechanism.

Turing patterns in a fiber laser with a nested microresonator: Robust and controllable microcomb generation

The paper study the formation of microcomb Turing patterns in a system comprised of a micro-resonator nested in an amplifying laser cavity. The authors show a method for repetition rate control of these waves over both fine and large scales.

Repetition-rate multiplicable soliton microcomb generation and stabilization via phase-modulated pumping scheme

We numerically demonstrate repetition rate multiplicable soliton microcomb generation using the phase-modulation pump scheme (PMS). Dissipative Kerr solitons are directly obtained avoiding the primary comb and chaotic modulation-instability stage and the repetition rate can be multiplied by changing the modulation frequency with fixed intensity. The PMS can not only help to stabilize emitted solitons with inhibited temporal drift against the Raman self-frequency shift effect, but also achieve soliton spacing equalization recovered from certain position fluctuations. Such approach can enable delicate manipulation on soliton behaviors, and provide a feasible route to soliton microcombs with even higher repetition rates and improved stabilities.

Generation of stable and breathing flat-top solitons via Raman assisted four wave mixing in microresonators

Flat-top-soliton (or platicon) dynamics in coherently pumped normal dispersion microresonators is important for both fundamental nonlinear physics and microcomb generation in the visible band. Here we numerically investigate the platicon generation that is initiated via Raman assisted four wave mixing instead of mode interaction. To show the possibility of generating coherent combs in the visible band, we design an aluminum nitride (AlN) microresonator with normal dispersion and investigate the comb generation dynamics in simulations. Stable platicon Kerr combs can be generated in this AlN microresonator using a 780-nm pump. Moreover, we also observe a breather platicon dynamics induced by the narrow Raman gain spectrum of crystalline AlN, which shows distinct dynamics from the dark soliton breathers reported in previous work that are dominated by Kerr effect. A phase diagram bearing the influence of the pump detuning and pump power on the breathing dynamics of the breather platicon is also computed. Furthermore, a transition to chaotic breathing is numerically observed.

Low-Threshold 4/5 Octave-Spanning Mid-Infrared Frequency Comb in a LiNbO$_3$ Microresonator

Mid-infrared (MIR) optical frequency combs (OFCs) are ideal light sources for precision metrology and can find extensive applications, especially in molecular spectroscopy and gas detection. Here the generation of mode-locked MIR frequency comb in a LiNbO 3 microring through optimized slot-waveguide design is proposed. Such microcomb can span over four-fifths of an octave (ranging from ~2810 nm to ~4630 nm) with pump power as low as 50 mW, which can be applied to the 2f-3f self-referencing for fully stabilization and helpful to monolithic integration of the whole system. Further investigations on complex dynamical processes for the microcomb generation suggest that the intracavity soliton drifting caused by the third-order dispersion could be compensated by the self-steepening effect as introducing an additional drift through proper dispersion tailoring. This work could facilitate the low-threshold broadband MIR frequency comb generation technique, as well as provide a way to realize more delicate control of soliton microcombs in both temporal and spectral domain.

Soliton microcomb generation at 2 μm in z-cut lithium niobate microring resonators.

Chip-based soliton frequency combs have been demonstrated on various material platforms, offering broadband, mutually coherent, and equally spaced frequency lines desired for many applications. Lithium niobate (LN), possessing both second- and third-order optical nonlinearities, as well as integrability on insulating substrates, has emerged as a novel source for microcomb generation and controlling. Here we demonstrate mode-locked soliton microcombs generated around 2μm in a high-Q z-cut LN microring resonator. The intracavity photorefractive effect is found to be still dominant over the thermal effect in the 2μm region, which facilitates direct accessing soliton states in the red-detuned regime, as reported in the telecom band. We also find that intracavity stimulated Raman scattering is greatly suppressed when moving the pump wavelength from the telecom band to 2μm, thus alleviating Raman-Kerr comb competition. This Letter expands mode-locked LN microcombs to 2μm, and could enable a variety of potential applications based on LN nanophotonic platform.

Microresonator Frequency Combs for Integrated Microwave Photonics

Microresonator-based Kerr frequency combs (microcombs) have many outstanding features, including chip-level integration, high repetition rate, and ultrabroad bandwidth. These merits make microcombs very promising for integrated microwave photonic applications. Here, we provide a brief review on recent progress of microcomb generation and its application in microwave photonics. Both coherent and incoherent examples are discussed, including low-phase-noise microwave and optical frequency synthesis, programmable photonic microwave signal processing, true time delay beamforming, and channelized receiver. An outlook on the future research direction and key problems is also given.