Kerr Microcombs Research Articles

Directly accessing single-soliton state of Kerr microcomb and its universal scaling law

Directly accessing single-soliton state of Kerr microcomb and its universal scaling law

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.

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.

Comparative study on pump frequency tuning and self-injection locking in Kerr microcomb generation

Comparative study on pump frequency tuning and self-injection locking in Kerr microcomb generation

Optical Signal Processor Based on a Kerr Microcomb for Real Time Video Image Processing

Optical Signal Processor Based on a Kerr Microcomb for Real Time Video Image Processing

Photonic signal processor based on a Kerr microcomb for real-time video image processing

Signal processing has become central to many fields, from coherent optical telecommunications, where it is used to compensate signal impairments, to video image processing. Image processing is particularly important for observational astronomy, medical diagnosis, autonomous driving, big data and artificial intelligence. For these applications, signal processing traditionally has mainly been performed electronically. However these, as well as new applications, particularly those involving real time video image processing, are creating unprecedented demand for ultrahigh performance, including high bandwidth and reduced energy consumption. Here, we demonstrate a photonic signal processor operating at 17 Terabits/s and use it to process video image signals in real-time. The system processes 400,000 video signals concurrently, performing 34 functions simultaneously that are key to object edge detection, edge enhancement and motion blur. As compared with spatial-light devices used for image processing, our system is not only ultra-high speed but highly reconfigurable and programable, able to perform many different functions without any change to the physical hardware. Our approach is based on an integrated Kerr soliton crystal microcomb, and opens up new avenues for ultrafast robotic vision and machine learning.

Slot-Waveguide Based All-Optical RF Spectrum Analyzer

All-optical radio-frequency spectrum analyzers (AORFSAs) with ultrabroad bandwidth break the electronic bottleneck and provide an efficient frequency analysis means for ultrafast optical signals in communications, signal generation and processing systems. Here, we propose and experimentally demonstrate an AORFSA built on the cross-phase modulation effect in a 50 cm long CMOS-compatible photonic slot-waveguide. The waveguide has a 100 nm thick thin-film core of fused silica that is sandwiched by two 750 nm thick cladding layers of high-index doped silica, which shows optimized dispersion and comparable nonlinear characteristics. The measured 3 dB bandwidth of the proposed slot-waveguide-based AORFSA has a three-fold increase over the conventional channel waveguide having the same dimension and length. The sensitivity and wavelength- and polarization-dependence properties are investigated, confirming the proposed waveguide as a versatile platform for frequency analysis of ultrafast optical signals, such as Kerr microcombs with hundreds of GHz or even THz mode spacing.

A perspective on nonlinear, microwave, and quantum photonics with Kerr microcombs

Microresonator Kerr optical frequency combs currently constitute a well-established research area in integrated, nonlinear, and quantum photonics. These systems have found a plethora of technological applications, while serving as an excellent platform to investigate fundamental scientific topics such as light–matter interactions, pattern formation in driven-dissipative systems, or entangled twin-photon generation. We here provide a brief overview of the topic, highlight some of the most recent advances, and discuss a few of the main challenges ahead in this field.

Microcombs in fiber Fabry–Pérot cavities

Optical frequency combs, which consist of precisely controlled spectral lines covering a wide range, have played a crucial role in enabling numerous scientific advancements. Beyond the conventional approach that relies on mode-locked lasers, microcombs generated from microresonators pumped at a single frequency have arguably given rise to a new field within cavity nonlinear photonics, which has led to a robust exchange of ideas and research between theoretical, experimental, and technological aspects. Microcombs are extremely attractive in applications requiring a compact footprint, low cost, good energy efficiency, large comb spacing, and access to nonconventional spectral regions. The recently arising microcombs based on fiber Fabry–Pérot microresonators provide unique opportunities for ultralow noise and high-dimensional nonlinear optics. In this review, we comprehensively examine the recent progress of fiber Kerr microcombs and discuss how various phenomena in fibers can be utilized to enhance the microcomb performances that benefit a plethora of applications.

Chalcogenide Chip-Based Frequency Combs for Advanced Laser Spectroscopy

Optical frequency combs have revolutionized spectral measurement technologies, such as dual-comb spectrometers, for fast and accurate spectroscopic measurements over broad spectral ranges. Moreover, chip-scale frequency combs based on microresonators are revolutionizing laser spectroscopy due to their compact size, low cost, and low power consumption, which offer a promising solution for integrated optical sensing applications by replacing costly bulk optical sensing components. Developing new attractive photonic materials for integrated microcomb is critical to realizing chip-based microcombs with high efficiency, broad bandwidth, and low pumping power. In this tutorial, we present novel chalcogenide chip-based microcombs for broadband laser spectroscopy. The integrated chalcogenide microresonators on silicon wafers are achieved using a home-developed chalcogenide glass (Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">25</sub> Sb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">65</sub> ) with ultra-wide transmittance window, high Kerr nonlinearity, and low thermo-optic coefficient. Two fabrication processes of high quality-factor integrated chalcogenide microresonators with different core-cladding structures operating in the telecom and mid-infrared (MIR) regions, respectively, are introduced. Combined high nonlinearity with lithographically controlled flexible dispersion engineering of the chalcogenide microresonators, we have realized the versatile Kerr microcombs generation and physical mechanism, including bright soliton microcomb, dark-pulse microcomb, Raman-Kerr microcomb, broadband Kerr microcomb and the future opportunity of the MIR Kerr comb. In addition, we review the most potent examples of dual-comb spectroscopy applications for molecular characterization and laser ranging with ultra-high sensitivity and high accuracy and discuss the challenges of realizing MIR microcombs with high performance and ultracompact footprint for laser spectroscopy.

Vernier microcombs for high-frequency carrier envelope offset and repetition rate detection

Recent developments in Kerr microcombs may pave the way to a future with fully stabilized ultralow size, weight, and power consumption (SWaP) frequency combs. Nevertheless, Kerr microcombs are still hindered by a bandwidth/repetition rate trade-off. That is, the octave bandwidth needed for self-referencing is typically realized only with ∼THz repetition rates beyond the range of standard commercial photodetectors. The carrier envelope offset frequency is often likewise too high for detection. Dual-comb techniques for the measurement of THz repetition rates have made exciting progress, but the fCEO detection problem remains largely unaddressed. In this work, utilizing a Vernier dual-comb configuration, we demonstrate simultaneous detection of the electronically divided repetition rate and fCEO carrier envelope offset frequency of an octave-spanning microcomb. This, in turn, could help usher optical atomic clocks, low-noise microwave generators, and optical frequency synthesizers into various real-world applications.

Near-zero-dispersion soliton and broadband modulational instability Kerr microcombs in anomalous dispersion

The developing advances of microresonator-based Kerr cavity solitons have enabled versatile applications ranging from communication, signal processing to high-precision measurements. Resonator dispersion is the key factor determining the Kerr comb dynamics. Near the zero group-velocity-dispersion (GVD) regime, low-noise and broadband microcomb sources are achievable, which is crucial to the application of the Kerr soliton. When the GVD is almost vanished, higher-order dispersion can significantly affect the Kerr comb dynamics. Although many studies have investigated the Kerr comb dynamics near the zero-dispersion regime in microresonator or fiber ring system, limited by dispersion profiles and dispersion perturbations, the near-zero-dispersion soliton structure pumped in the anomalous dispersion side is still elusive so far. Here, we theoretically and experimentally investigate the microcomb dynamics in fiber-based Fabry-Perot microresonator with ultra-small anomalous GVD. We obtain 2/3-octave-spaning microcombs with ~10 GHz spacing, >84 THz span, and >8400 comb lines in the modulational instability (MI) state, without any external nonlinear spectral broadening. Such widely-spanned MI combs are also able to enter the soliton state. Moreover, we report the first observation of anomalous-dispersion based near-zero-dispersion solitons, which exhibits a local repetition rate up to 8.6 THz, an individual pulse duration <100 fs, a span >32 THz and >3200 comb lines. These two distinct comb states have their own advantages. The broadband MI combs possess high conversion efficiency and wide existing range, while the near-zero-dispersion soliton exhibits relatively low phase noise and ultra-high local repetition rate. This work complements the dynamics of Kerr cavity soliton near the zero-dispersion regime, and may stimulate cross-disciplinary inspirations ranging from dispersion-controlled microresonators to broadband coherent comb devices.

Subharmonic instabilities in Kerr microcombs.

We report experimental observation of subharmonic mode excitation in primary Kerr optical frequency combs generated using crystalline whispering-gallery mode resonators. We show that the subcombs can be controlled and span a single or multiple free spectral ranges around the primary comb modes. In the spatial domain, the resulting multiscale combs correspond to an amplitude modulation of intracavity roll patterns. We perform a theoretical analysis based on eigenvalue decomposition that evidences the mechanism leading to the excitation of these combs.

Dispersion-Suppressed Mode Depletion by Exceptional Points for On-Chip Nonlinear Optics

Dispersion-suppressed mode depletion in a resonant cavity is usually challenging due to the entanglement of amplitude and phase response dictated by the Kramers-Kronig relationship. In this study, we examine the dispersion-suppressed mode depletion induced by parity--time- (PT-) symmetric dual resonators by taking advantage of the exotic property at the exceptional point (EP). We show that this technique can modify the linear spectrum of an optical resonator for relaxed phase-matching conditions in nonlinear optics, thus profoundly altering the nonlinear behavior of integrated Kerr combs. Indeed, dissipation-induced modulation instability (MI) is predicted and numerically verified in normal group-velocity dispersion (GVD) microresonators, and the single-soliton-generation probability is greatly enhanced in the anomalous GVD regime. The merging of two vibrant disciplines, i.e., non-Hermitian optics and nonlinear optics, especially Kerr microcombs, can deepen our understanding of non-Hermitian optical frequency combs and opens broad applications ranging from fiber-optic communications, microwave photonics, and ultrafast optics to quantum state manipulations.

Loss modulation assisted solitonic pulse excitation in Kerr resonators with normal group velocity dispersion.

We demonstrate an emergent solitonic pulse generation approach exploiting the externally introduced or intrinsic loss fluctuation effects. Single or multiple pulses are accessible via self-evolution of the system in the red, blue detuning regime or even on resonance with loss perturbation. The potential well caused by the loss profile not only traps the generated pulses, but also helps to suppress the drift regarding high-order dispersion. Breathing dynamics is also observed with high driving force, which can be transferred to stable state by backward tuning the pump detuning. We further investigate the intrinsic free carrier absorption, recognized as unfavored effect traditionally, could be an effective factor for pulse excitation through the time-variant loss fluctuation in normal dispersion microresonators. Pulse excitation dynamics associated with physical parameters are also discussed. These findings could establish a feasible path for stable localized structures and Kerr microcombs generation in potential platforms.

High Bandwidth and Highly Reconfigurable Rf Microwave Photonic Hilbert Transformers Using Kerr Micro-Combs

High Bandwidth and Highly Reconfigurable Rf Microwave Photonic Hilbert Transformers Using Kerr Micro-Combs

Radio Frequency and Microwave Photonic Signal Processing and Generation Based on Optical Kerr Micro-Combs

Radio Frequency and Microwave Photonic Signal Processing and Generation Based on Optical Kerr Micro-Combs

Neuromorphic Photonic Computing Based on Wavelength-Division Multiplexing Methods with Optical Kerr Microcombs

Neuromorphic Photonic Computing Based on Wavelength-Division Multiplexing Methods with Optical Kerr Microcombs

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.

Highly Versatile Broadband RF Photonic Fractional Hilbert Transformer Based on a Kerr Soliton Crystal Microcomb

We demonstrate an RF photonic fractional Hilbert transformer based on an integrated Kerr micro-comb source featuring a record low free spectral range of 48.9 GHz, yielding 75 microcomb lines across the C-band. By programming and shaping the comb lines according to calculated tap weights, we demonstrate that the Hilbert transformer can achieve tunable bandwidths ranging from 1.2 to 15.3 GHz, switchable centre frequencies from baseband to 9.5 GHz, and arbitrary fractional orders. We experimentally characterize the RF amplitude and phase response of the tunable bandpass and lowpass Hilbert transformers with 90 and 45-degree phase shift. The experimental results show good agreement with theory, confirming the effectiveness of our approach as a powerful way to implement the standard as well as fractional Hilbert transformers with broad and switchable processing bandwidths and centre frequencies, together with high reconfigurability and greatly reduced size and complexity.