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

Microresonator-based frequency combs enable high coherence, flexible line spacing, and broad bandwidth, supporting applications such as optical frequency synthesis, clocks, lidar, spectroscopy, and communications. Chromatic dispersion critically determines microcomb nonlinear dynamics. In anomalous dispersion (AD) resonators, bright localized dissipative structures (dissipative Kerr solitons, DKSs) arise from a balance between nonlinearity and dispersion, and between gain and loss. In normal dispersion (ND), dark localized structures related to switching waves can form and achieve high conversion efficiency via unique stabilization mechanisms. Recently, attention has focused on near-zero-dispersion regimes where small GVD broadens soliton spectra, while higher-order dispersion (e.g., TOD) dominantly shapes dynamics. Theory and experiments have shown TOD can allow bright solitons in ND and across the zero-dispersion wavelength; such structures have been termed zero-dispersion solitons (ZDS) or, more generally, near-zero-dispersion solitons (NZDS). A natural open question is whether a corresponding NZDS structure exists when pumping on the anomalous-dispersion side, where parametric gain via modulational instability (MI) profoundly alters dynamics. Systematic, especially experimental, studies addressing this have been lacking. This work investigates Kerr microcomb dynamics in a fiber-based Fabry–Perot (F-P) microresonator with ultra-small anomalous GVD under pulsed driving. We demonstrate broadband MI combs and, crucially, observe anomalous-dispersion-based NZDS (AD-NZDS) comprising tightly bound multi-soliton clusters with sub-100 fs constituent pulses and multi-THz local repetition rates. We discuss the distinct advantages of broadband MI (efficiency, accessibility) and AD-NZDS (coherence, low phase noise, high local repetition rate) states for various applications.

Prior work established that in AD microresonators, bright DKSs form through double balance of nonlinearity–dispersion and gain–loss, while in ND regimes dark solitons and switching waves can stabilize and enhance conversion efficiency. Near zero dispersion, diminished GVD lets higher-order dispersion govern evolution. Theoretically, third-order dispersion (TOD) enables bright solitons in ND and across the zero-dispersion wavelength (Parra-Rivas et al.), with experimental confirmations showing single-peak bright solitons spanning across zero dispersion and interlocked switching-wave structures termed ZDS/NZDS. However, whether analogous NZDS structures exist when pumping on the anomalous side remained unverified experimentally due to constraints of dispersion profiles and perturbations. This study addresses that gap.

Simulation: The microresonator dynamics were modeled using the normalized Lugiato–Lefever equation for a Fabry–Perot cavity (FP-LLE). The cavity was set in anomalous dispersion with strong third-order dispersion: normalized parameters d2 = −1 (GVD), d3 = +1 (TOD). The integrated dispersion profile was dint = d2 Ω^2 + d3 Ω^3 (Ω normalized angular frequency). TOD-induced temporal drift was compensated by a drift coefficient d = −2.4, representing a group-velocity offset between intracavity fields and pulsed pump. The driving pulse comprised M+1 = 51 spectral lines with equal phase and power; normalized peak pump F = 12. The laser-cavity detuning ζ0 was scanned linearly from −2 to +8 to traverse states. Temporal and spectral intracavity field evolutions were recorded, identifying primary comb, broadband MI, unstable NZDS, and stable NZDS (multi-soliton bound clusters) states. The number of bound solitons and corresponding coherent spectral envelopes were analyzed as a function of detuning.

Experiment: A highly nonlinear fiber-based Fabry–Perot (F-P) microresonator (~1 cm cavity length) was fabricated and characterized. The free spectral range (FSR) was measured as 10.41506 GHz via a dual-cavity method. A resonance near 1550 nm showed 15 MHz linewidth, corresponding to loaded Q ≈ 1.3 × 10^7. Integrated dispersion Dint was measured using a fiber Mach–Zehnder interferometer method and fitted around 1550 nm, yielding D2/2π = +167 Hz and D3/2π = −1.9 Hz, corresponding (dimensionless) to d2 = −1 and d3 = +1.14. The resonator was driven in the anomalous dispersion regime by a desynchronized pulsed pump generated via electro-optic comb formation: the pump had 49 comb lines within −10 dB and a 2.1 ps pulse duration. Desynchronization δfrep = frep − D1/2π was set to −20 kHz to counteract TOD-induced drift and sustain soliton structures. The pump central wavelength (around 1549.55 nm) was scanned from blue to red detuning at 20 MHz/s with ~23.5 dBm average coupled power. Output was monitored with an optical spectrum analyzer (OSA) and an electronic spectrum analyzer (ESA); a 1580 nm, 0.02 nm bandwidth bandpass filter was used for repetition-rate beatnote measurements. Temporal, spectral, intensity-noise, and phase-noise characteristics were recorded across the observed operating regimes.

• Demonstration of 2/3-octave-spanning Kerr microcombs in a near-zero anomalous-dispersion F-P microresonator without external broadening, with ~10.4 GHz spacing, spectral span >84 THz (1240–1950 nm), and >8400 comb lines in a broadband MI state.
• Observation and identification (first report) of anomalous-dispersion-based near-zero-dispersion solitons (AD-NZDS), comprising tightly bound multi-soliton clusters: local repetition rate up to 8.6 THz, individual pulse duration <100 fs, spectral span >32 THz, and >3200 comb lines.
• Numerical FP-LLE simulations (d2 = −1, d3 = +1) under pulsed driving (51 lines, F = 12) with detuning scan from −2 to +8 reproduced key states: primary comb → broadband MI → unstable NZDS (jitter step) → stable NZDS (multi-soliton binding), with decreasing number of bound solitons at larger detuning; spectra exhibit prominent comb envelopes near pump.
• Experimental microresonator parameters: FSR = 10.41506 GHz; loaded Q ≈ 1.3 × 10^7; integrated dispersion around 1550 nm with D2/2π = 167 Hz and D3/2π = −1.9 Hz (dimensionless d2 = −1, d3 = 1.14).
• Desynchronized pulse driving (δfrep = −20 kHz) effectively compensates TOD-induced drift and enables stable AD-NZDS steps during wavelength scan; beatnote signals are enhanced on transmission steps corresponding to coherent states.
• Noise characteristics: Broadband MI combs exhibit broadband intensity noise, whereas AD-NZDS states show lower intensity noise and narrower repetition-rate beatnotes with reduced phase noise relative to MI (phase-noise spectra presented, with system/RF references).

The study addresses whether near-zero-dispersion solitons can exist when pumping on the anomalous-dispersion side, where MI provides parametric gain and higher-order dispersion shapes dynamics. By combining desynchronized pulse pumping with a fiber F-P resonator engineered for ultra-small anomalous GVD and finite TOD, the authors demonstrate stable AD-NZDS states comprised of tightly bound multi-soliton clusters. Simulations reveal that MI and TOD jointly enable formation and stabilization: MI seeds broadband comb formation and dense temporal structures, while TOD and desynchronization set the drift compensation and binding conditions that yield coherent, evenly spaced bound solitons with distinct spectral envelopes near the pump. Experimentally, the transition from broadband MI to coherent NZDS states appears as transmission steps with enhanced beatnotes; the NZDS states exhibit reduced phase noise compared to MI, validating their higher coherence. These findings extend the established NZDS concept from the normal-dispersion side to anomalous-dispersion pumping near zero dispersion, enriching the taxonomy of Kerr cavity solitons and offering practical operating modes: MI combs for high power and efficiency over a wide accessible range (aided by thermo-optic negative feedback), versus AD-NZDS for applications requiring coherence, low phase noise, and very high local repetition rates.

This work theoretically and experimentally establishes broadband MI Kerr microcombs and, crucially, anomalous-dispersion-based near-zero-dispersion solitons (AD-NZDS) in a fiber Fabry–Perot microresonator with ultra-small anomalous GVD. The MI state yields 2/3-octave bandwidth and thousands of lines without external broadening, while the AD-NZDS state provides sub-100 fs constituent pulses with multi-THz local repetition rates and lower phase noise. Simulations using FP-LLE elucidate the evolution from primary combs to broadband MI and into bound multi-soliton NZDS states governed by MI and TOD, with desynchronization compensating drift. These results broaden the understanding of Kerr-comb dynamics near zero dispersion and offer a flexible operational strategy tailored to application needs in optical computing, sensing, communications, and spectroscopy. Future work could explore finer dispersion engineering (including controlled TOD and higher orders), extended cavity platforms, integrated implementations, and active control of multi-soliton binding to tailor repetition rates and coherence.