Frequency Microcombs Research Articles

Spatio-temporal breather dynamics in microcomb soliton crystals

Solitons, the distinct balance between nonlinearity and dispersion, provide a route toward ultrafast electromagnetic pulse shaping, high-harmonic generation, real-time image processing, and RF photonic communications. Here we uniquely explore and observe the spatio-temporal breather dynamics of optical soliton crystals in frequency microcombs, examining spatial breathers, chaos transitions, and dynamical deterministic switching – in nonlinear measurements and theory. To understand the breather solitons, we describe their dynamical routes and two example transitional maps of the ensemble spatial breathers, with and without chaos initiation. We elucidate the physical mechanisms of the breather dynamics in the soliton crystal microcombs, in the interaction plane limit cycles and in the domain-wall understanding with parity symmetry breaking from third-order dispersion. We present maps of the accessible nonlinear regions, the breather frequency dependences on third-order dispersion and avoided-mode crossing strengths, and the transition between the collective breather spatio-temporal states. Our range of measurements matches well with our first-principles theory and nonlinear modeling. To image these soliton ensembles and their breathers, we further constructed panoramic temporal imaging for simultaneous fast- and slow-axis two-dimensional mapping of the breathers. In the phase-differential sampling, we present two-dimensional evolution maps of soliton crystal breathers, including with defects, in both stable breathers and breathers with drift. Our fundamental studies contribute to the understanding of nonlinear dynamics in soliton crystal complexes, their spatio-temporal dependences, and their stability-existence zones.

60 Gbps real-time wireless communications at 300 GHz carrier using a Kerr microcomb-based source

Future wireless communication infrastructure will rely on terahertz systems that can support an increasing demand for large-bandwidth, ultra-fast wireless data transfer. In order to satisfy this demand, compact, low-power, and low noise sources of terahertz radiation are being developed. A promising route to achieving this goal is combining photonic-integrated optical frequency combs with fast photodiodes for difference frequency generation in the THz. Here, we demonstrate wireless communications using a 300 GHz carrier wave generated via photomixing of two optical tones originating from diode lasers that are injection locked to a dissipative Kerr soliton frequency microcomb. We achieve transfer rates of 80 Gbps using homodyne detection and 60 Gbps transmitting simultaneously both data and clock signals in a dual-path wireless link. This experimental demonstration paves a path toward low-noise and integrated photonic millimeter-wave transceivers for future wireless communication systems.

Proposal for an active whispering-gallery microclock

Optical atomic clocks with compact size, reduced weight and low power consumption have broad out-of-the-lab applications such as satellite-based geo-positioning and communication engineering. Here, we propose an active optical microclock based on the lattice-trapped atoms evanescently interacting with a whispering-gallery-mode microcavity. Unlike the conventional passive clock scheme, the active operation directly produces the optical frequency standard without the need of extra laser stabilization, substantially simplifying the clock configuration. The numerical simulation illustrates that the microclock’s frequency stability reaches at 1 s of averaging, over one order of magnitude better than the recently demonstrated chip-scale optical clock that is built upon rubidium vapor cell and also more stable than current cesium fountain clocks and hydrogen masers. Our work extends the chip-scale clocks to the active fashion, paving the way towards the on-chip quantum micro-metrology, for example, the optical frequency comparison and synchronization between multiple microclocks through frequency microcombs.

Applications of Optical Frequency Microcomb Chips

Applications of Optical Frequency Microcomb Chips

Stimulated generation of deterministic platicon frequency microcombs

Dissipative Kerr soliton generation in chip-scale nonlinear resonators has recently observed remarkable advances, spanning from massively parallel communications, to self-referenced oscillators, and to dual-comb spectroscopy. Often working in the anomalous dispersion regime, unique driving protocols and dispersion in these nonlinear resonators have been examined to achieve the soliton and soliton-like temporal pulse shapes and coherent frequency comb generation. The normal dispersion regime provides a complementary approach to bridge the nonlinear dynamical studies, including the possibility of square pulse formation with flattop plateaus, or platicons. Here we report observations of square pulse formation in chip-scale frequency combs through stimulated pumping at one free spectral range and in silicon nitride rings with + 55 fs 2 / mm normal group velocity dispersion. Tuning of the platicon frequency comb via a varied sideband modulation frequency is examined in both spectral and temporal measurements. Determined by second-harmonic autocorrelation and cross correlation, we observe bright square platicon pulse of 17 ps pulse width on a 19 GHz flat frequency comb. With auxiliary-laser-assisted thermal stabilization, we surpass the thermal bistable dragging and extend the mode-locking access to narrower 2 ps platicon pulse states, supported by nonlinear dynamical modeling and boundary limit discussions.

Aluminum nitride nanophotonics for beyond-octave soliton microcomb generation and self-referencing

Frequency microcombs, alternative to mode-locked laser and fiber combs, enable miniature rulers of light for applications including precision metrology, molecular fingerprinting and exoplanet discoveries. To enable frequency ruling functions, microcombs must be stabilized by locking their carrier-envelope offset frequency. So far, the microcomb stabilization remains compounded by the elaborate optics external to the chip, thus evading its scaling benefit. To address this challenge, here we demonstrate a nanophotonic chip solution based on aluminum nitride thin films, which simultaneously offer optical Kerr nonlinearity for generating octave soliton combs and quadratic nonlinearity for enabling heterodyne detection of the offset frequency. The agile dispersion control of crystalline aluminum nitride photonics permits high-fidelity generation of solitons with features including 1.5-octave spectral span, dual dispersive waves, and sub-terahertz repetition rates down to 220 gigahertz. These attractive characteristics, aided by on-chip phase-matched aluminum nitride waveguides, allow the full determination of the offset frequency. Our proof-of-principle demonstration represents an important milestone towards fully integrated self-locked microcombs for portable optical atomic clocks and frequency synthesizers.

Frequency microcomb stabilization via dual-microwave control

Optical frequency comb technology has been the cornerstone for scientific breakthroughs in precision metrology. In particular, the unique phase-coherent link between microwave and optical frequencies solves the long-standing puzzle of precision optical frequency synthesis. While the current bulk mode-locked laser frequency comb has had great success in extending the scientific frontier, its use in real-world applications beyond the laboratory setting remains an unsolved challenge due to the relatively large size, weight and power consumption. Recently microresonator-based frequency combs have emerged as a candidate solution with chip-scale implementation and scalability. The wider-system precision control and stabilization approaches for frequency microcombs, however, requires external nonlinear processes and multiple peripherals which constrain their application space. Here we demonstrate an internal phase-stabilized frequency microcomb that does not require nonlinear second-third harmonic generation nor optical external frequency references. We demonstrate that the optical frequency can be stabilized by control of two internally accessible parameters: an intrinsic comb offset ξ and the comb spacing frep. Both parameters are phase-locked to microwave references, with phase noise residuals of 55 and 20 mrad respectively, and the resulting comb-to-comb optical frequency uncertainty is 80 mHz or less. Out-of-loop measurements confirm good coherence and stability across the comb, with measured optical frequency instability of 2 × 10−11 at 20-second gate time. Our measurements are supported by analytical theory including the cavity-induced modulation instability. We further describe an application of our technique in the generation of low noise microwaves and demonstrate noise suppression of the repetition rate below the microwave stabilization limit achieved.

Nanometric Precision Distance Metrology via Hybrid Spectrally Resolved and Homodyne Interferometry in a Single Soliton Frequency Microcomb.

Laser interferometry serves a fundamental role in science and technology, assisting precision metrology and dimensional length measurement. During the past decade, laser frequency combs-a coherent optical-microwave frequency ruler over a broad spectral range with traceability to time-frequency standards-have contributed pivotal roles in laser dimensional metrology with ever-growing demands in measurement precision. Here we report spectrally resolved laser dimensional metrology via a free-running soliton frequency microcomb, with nanometric-scale precision. Spectral interferometry provides information on the optical time-of-flight signature, and the large free-spectral range and high coherence of the microcomb enable tooth-resolved and high-visibility interferograms that can be directly read out with optical spectrum instrumentation. We employ a hybrid timing signal from comb-line homodyne, microcomb, and background amplified spontaneous emission spectrally resolved interferometry-all from the same spectral interferogram. Our combined soliton and homodyne architecture demonstrates a 3-nm repeatability over a 23-mm nonambiguity range achieved via homodyne interferometry and over 1000-s stability in the long-term precision metrology at the white noise limits.

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.

Interleaved difference-frequency generation for microcomb spectral densification in the mid-infrared

With their compact size and semiconductor-chip-based operation, frequency microcombs can be an invaluable light source for gas spectrcoscopy. However, the generation of mid-infrared (mid-IR) frequency combs with gigahertz line spacing as required to resolve many gas spectra represents a significant challenge for these devices. Here, a technique referred to as interleaved difference-frequency generation (iDFG) is introduced that densifies the spectral line spacing upon conversion of near-IR comb light into the mid-IR light. A soliton microcomb is used as both a comb light source and microwave oscillator in a demonstration, and the spectrum of methane is measured to illustrate how the resulting mid-IR comb avoids spectral undersampling. Beyond demonstration of the iDFG technique, this work represents an important feasibility step towards more compact and potentially chip-based mid-IR gas spectroscopy modules.

Coherent satellites in multispectral regenerative frequency microcombs

Multispectral frequency combs provide new architectures for laser spectroscopy, clockwork, and high-capacity communications. Frequency microcombs have demonstrated remarkable impact in frequency metrology and synthesis, albeit with spectral bandwidth bounded by intrinsic second-order dispersion and consequently low-intensities at the spectral edges. Here we report coherent satellite clusters in multispectral regenerative frequency microcombs with enhanced intensities at the octave points and engineered frequency span. Beyond the conventional bandwidth of parametric oscillation, the regenerative satellites are facilitated by higher-order dispersion control, allowing for multiphase-matched parametric processes. The satellite span is deterministically controlled from 34 to 72 THz by pumped at C/L-bands, with coherence preserved with the central comb through the nonlinear parametric process. We further show the mirrored appearance of the satellite transition dynamics simultaneously with the central comb at each comb state. These multispectral satellites extend the scope of parametric-based frequency combs and provide a unique platform for clockwork, spectroscopy and communications.

Efficient electro-optical tuning of an optical frequency microcomb on a monolithically integrated high-Q lithium niobate microdisk.

We demonstrate efficient tuning of a monolithically integrated lithium niobate (LN) microdisk optical frequency microcomb. Utilizing the high optical quality (Q) factor (i.e., Q∼7.1×106) of the microdisk, the microcomb spans over a spectral bandwidth of ∼200nm at a pump power as low as 20.4 mW. Combining the large electro-optic coefficient of LN and the optimum design of the geometry of microelectrodes, we demonstrate electro-optical tuning of the comb with a spectral range of 400 pm and a tuning efficiency of ∼38pm/100V.

A low-phase-noise 18 GHz Kerr frequency microcomb phase-locked over 65 THz.

Laser frequency combs are coherent light sources that simultaneously provide pristine frequency spacings for precision metrology and the fundamental basis for ultrafast and attosecond sciences. Recently, nonlinear parametric conversion in high-Q microresonators has been suggested as an alternative platform for optical frequency combs, though almost all in 100 GHz frequencies or more. Here we report a low-phase-noise on-chip Kerr frequency comb with mode spacing compatible with high-speed silicon optoelectronics. The waveguide cross-section of the silicon nitride spiral resonator is designed to possess small and flattened group velocity dispersion, so that the Kerr frequency comb contains a record-high number of 3,600 phase-locked comb lines. We study the single-sideband phase noise as well as the long-term frequency stability and report the lowest phase noise floor achieved to date with −130 dBc/Hz at 1 MHz offset for the 18 GHz Kerr comb oscillator, along with feedback stabilization to achieve frequency Allan deviations of 7 × 10−11 in 1 s. The reported system is a promising compact platform for achieving self-referenced Kerr frequency combs and also for high-capacity coherent communication architectures.

Stability and Intrinsic Fluctuations of Dissipative Cavity Solitons in Kerr Frequency Microcombs

The generation of dissipative cavity solitons is one of the most intriguing features of microresonator-based Kerr frequency combs, enabling effective mode locking of comb modes and synthesis of ultrafast pulses. With the Lugiato–Lefever model, here, we conduct detailed theoretical investigations on the transient dynamics of dissipative cavity solitons and describe how several intrinsic effects of the Kerr comb disturb the stability of cavity solitons, including soliton breathing, higher order dispersion, dispersive wave emission, and cavity mode coupling. Our results and analysis agree well with recent measurements and provide insight into some as yet unexplained observations.