Normal-Dispersion Kerr Microcombs

• Normal-dispersion Kerr microcombs are optical frequency combs produced via Kerr nonlinearities in resonators with engineered dispersion, achieving high conversion efficiency and tunable comb generation.
• They rely on controlled mechanisms such as avoided mode crossings, pulsed pumping, and χ(2)-assisted mixing to initiate comb formation in regimes where standard modulational instability is suppressed.
• The resulting states include dark pulses, platicons, and hybrid Kerr-Raman configurations, offering diverse spectral and temporal characteristics for applications in communications, spectroscopy, and metrology.

Normal-dispersion Kerr microcombs are optical frequency combs generated by Kerr four-wave mixing in resonators whose pumped modes lie in the normal group-velocity-dispersion regime, commonly written as $\beta_2>0$ or $D_2<0$ depending on notation. In a uniformly CW-driven cavity, this regime suppresses the standard modulational-instability pathway that underpins anomalous-dispersion comb initiation, so comb generation typically requires an additional phase-matching or state-selection mechanism. Reported solutions include controllable avoided mode crossings, synchronous pulsed driving, higher-order-dispersion-mediated dispersive waves, $\chi^{(2)}$-assisted four-wave mixing, Raman- or Brillouin-assisted switching-wave dynamics, and wavelength-selective dissipation engineering (Kim et al., 2019, Xu et al., 2020, Xue et al., 2016, Lv et al., 2024, Bunel et al., 5 Feb 2025). The resulting states include dark pulses, platicons, phase-locked low-noise combs, interleaved Kerr-Raman combs, and, in some cavity models, bright localized structures beyond the usual mean-field picture (Shitikov et al., 7 Nov 2025, Song et al., 28 Jul 2025, Seidel et al., 2024).

1. Emergence of the field and defining experimental milestones

An early experimental anchor was the generation of a coherent near-infrared comb in a monolithic MgF$_2$ whispering-gallery resonator with $9.9\,\mathrm{GHz}$ free spectral range, pumped at $780\,\mathrm{nm}$ or $795\,\mathrm{nm}$. The comb exhibited on the order of 80 lines spanning some $700\,\mathrm{GHz}$, a $9.9\,\mathrm{GHz}$ RF beat note with $>50\,\mathrm{dB}$ signal-to-noise ratio, and fixed relative phases, while numerical reconstruction showed that the intracavity waveform corresponded to dark pulses rather than short bright pulses (Liang et al., 2014). In parallel, few-moded normal-dispersion Si$D_2<0$0N$D_2<0$1 resonators revealed that one initial comb sideband can remain pinned near a mode-crossing frequency, and that coherent, bandwidth-limited pulses can be generated directly at repetition rates down to $D_2<0$2 without first passing through a chaotic state (Liu et al., 2014).

Programmable control appeared in dual coupled Si$D_2<0$3N$D_2<0$4 microrings, where a microheater on the auxiliary ring was used to introduce and tune mode interactions, enabling microcomb generation, repetition-rate selection from $D_2<0$5 to $D_2<0$6, and mode locking in the normal-dispersion region (Xue et al., 2015). Later work moved from proof-of-principle to operationally robust sources: a coupled-ring SiN platform demonstrated automated comb generation with a fixed-frequency pump, pump-to-comb conversion efficiency of $D_2<0$7, and mutual coherence $D_2<0$8 across all lines (Kim et al., 2019). Thin-film lithium niobate then extended the regime toward microwave-rate dark pulses and high-efficiency integrated sources, including $D_2<0$9 dark-pulse microcombs with $\chi^{(2)}$0 span and X-cut TFLN combs with $\chi^{(2)}$1 efficiency and $\chi^{(2)}$2 span (Lv et al., 2024, Song et al., 28 Jul 2025).

These milestones collectively established that normal-dispersion microcombs are not a marginal exception to anomalous-dispersion soliton physics. They define a parallel design space in which dispersion, modal structure, dissipation, and auxiliary nonlinearities are used as explicit control parameters rather than as parasitic perturbations.

2. Enabling mechanisms for comb initiation in normal dispersion

In integrated microresonators, the most established route is local dispersion tailoring by avoided mode crossings. In the turn-key SiN device, two oxide-clad rings with individual platinum heaters, waveguide cross section $\chi^{(2)}$3, and slightly mismatched FSRs ($\chi^{(2)}$4 and $\chi^{(2)}$5) create a Vernier condition in which only one pair of modes overlaps within $\chi^{(2)}$6. At that degeneracy the modes hybridize into a split doublet, and heater tuning moves the degeneracy across a fixed pump wavelength so that a local region of effectively anomalous dispersion appears inside an otherwise normal-GVD spectrum (Kim et al., 2019). A related dual-ring SiN architecture formalized the same idea as controllable mode interactions: by selectively splitting a chosen sideband mode, the effective frequency mismatch for four-wave mixing becomes positive and modulational instability can be programmed at the desired comb spacing (Xue et al., 2015).

The signature of this mechanism is often spectral pinning. In normal-dispersion SiN resonators, one of the initial sidebands remained pinned near a mode-crossing frequency while the pump was tuned over many resonances, providing direct experimental evidence that the local mode interaction fixes the initial gain peak (Liu et al., 2014). Ultra-high-Q SiO$\chi^{(2)}$7N$\chi^{(2)}$8 microtoroids provide another variant: their global dispersion around $\chi^{(2)}$9 is normal, with $_2$0 to $_2$1, yet avoided mode crossings enabled type-I and type-II combs and a measured hyperparametric threshold $_2$2 (Chen et al., 2019).

Avoided crossings are not the only route. Synchronous pulsed pumping lifts the need for localized dispersion perturbations by generating switching waves on the leading and trailing edges of the intracavity pump pulses; in a fiber mini-resonator with $_2$3, this produced spectrally flat $_2$4 combs and center-frequency tuning by more than $_2$5 through the pump-cavity desynchronization parameter $_2$6 (Xu et al., 2020). Weak $_2$7 coupling offers a different phase-matching channel: simultaneous second-harmonic generation and Kerr dynamics can modify the sideband phase mismatch so that comb formation occurs in a strongly normal-dispersion SiN ring under conditions where a pure Kerr cavity would not oscillate (Xue et al., 2016). In lithium niobate and silica-based cavities, delayed Raman response can either obstruct or enable normal-dispersion comb formation, depending on design. Reported strategies include Raman suppression by wavelength-selective loss in a pulley-coupled LiNbO$_2$8 resonator (Lv et al., 2024), Raman-assisted locking of switching waves into Kerr-Raman solitons in a fiber Fabry-Perot resonator (Li et al., 2023), Raman-induced platicon formation and Stokes microcombs in Si$_2$9N$9.9\,\mathrm{GHz}$0 (Shitikov et al., 7 Nov 2025), and coherent Brillouin-Kerr triggering in normal-dispersion fiber Fabry-Perot cavities (Bunel et al., 5 Feb 2025).

3. Mean-field models, generalized equations, and state taxonomy

The common theoretical language is the driven-damped Lugiato-Lefever equation. A representative form used for CW-pumped normal-GVD microresonators is

$9.9\,\mathrm{GHz}$1

with total loss rate $9.9\,\mathrm{GHz}$2, detuning $9.9\,\mathrm{GHz}$3, Kerr coefficient $9.9\,\mathrm{GHz}$4, and pump amplitude $9.9\,\mathrm{GHz}$5 (Kim et al., 2019). In modal language, the cold-cavity resonances are expanded as $9.9\,\mathrm{GHz}$6, or equivalently through the integrated dispersion $9.9\,\mathrm{GHz}$7, which is the central object for diagnosing whether normal GVD is global, locally perturbed, folded, or reconfigured by hybridization (Song et al., 28 Jul 2025).

Normal-dispersion comb physics is largely encoded in the extensions to this mean-field model. Third-order dispersion adds a $9.9\,\mathrm{GHz}$8 term that phase-matches dispersive waves emitted by dark cavity solitons and can suppress breathing instabilities, stabilizing the comb (Wang et al., 2015). A periodic modulation of the group-velocity dispersion folds the integrated dispersion into Floquet bands, allowing switching-wave spectra to coexist with Faraday-instability-induced satellite microcombs far from the pump (Anderson et al., 2022). Pulled away from the CW-driven limit, pulsed pumping introduces a desynchronization term $9.9\,\mathrm{GHz}$9 into the phase-matching relation for dispersive waves and thereby makes the comb center frequency a control variable (Xu et al., 2020). Coupled mean-field equations for the fundamental and second harmonic capture $780\,\mathrm{nm}$0-assisted four-wave mixing (Xue et al., 2016), while Raman convolution terms model delayed nonlinear response in LiNbO$780\,\mathrm{nm}$1, silica, and Si$780\,\mathrm{nm}$2N$780\,\mathrm{nm}$3 cavities (Lv et al., 2024).

The temporal states supported by these equations are correspondingly diverse. In the standard normal-GVD bistable picture, the localized objects are dark temporal cavity solitons or platicons, namely localized intensity dips or flat-top structures on a high-power background (Shitikov et al., 7 Nov 2025). In dissipation-engineered LiNbO$780\,\mathrm{nm}$4, numerical reconstruction yielded top-hat pulses with a central dip whose width varied from $780\,\mathrm{nm}$5 to $780\,\mathrm{nm}$6 as detuning increased (Lv et al., 2024). However, normal dispersion is not confined to dark states. With sufficiently strong third-order dispersion, stable dark and bright solitons can coexist in the same resonator model (Parra-Rivas et al., 2016). In a different theoretical limit, injected Kerr-Gires-Tournois interferometers were predicted to support bright pulses in normal dispersion beyond the mean-field limit and out of the bistable region, with peak intensities beyond that of the upper steady state (Seidel et al., 2024). Hybrid Kerr-Raman states add further temporal complexity: a $780\,\mathrm{nm}$7 TFLN resonator showed a quasi-rectangular pulse carrying $780\,\mathrm{nm}$8 oscillations on its plateau, while fiber Fabry-Perot Kerr-Raman solitons exhibited strong $780\,\mathrm{nm}$9 oscillations on top of a flat-top pulse (Song et al., 28 Jul 2025, Li et al., 2023).

4. Representative platforms and reported operating regimes

Platform / mechanism	Operating scale	Reported result
MgF$795\,\mathrm{nm}$0 WGM resonator, CW pump	FSR $795\,\mathrm{nm}$1	On the order of 80 lines spanning some $795\,\mathrm{nm}$2; phase-locked $795\,\mathrm{nm}$3 beat note with $795\,\mathrm{nm}$4 SNR (Liang et al., 2014)
SiO$795\,\mathrm{nm}$5N$795\,\mathrm{nm}$6 microtoroid, avoided crossings	FSR $795\,\mathrm{nm}$7	$795\,\mathrm{nm}$8; $795\,\mathrm{nm}$9 span type-I comb at $700\,\mathrm{GHz}$0 (Chen et al., 2019)
Coupled-ring SiN, thermo-optic tuning	Line spacing $700\,\mathrm{GHz}$1	$700\,\mathrm{GHz}$2 efficiency; usable span $700\,\mathrm{GHz}$3; $700\,\mathrm{GHz}$4 (Kim et al., 2019)
Fiber mini-resonator, pulsed pump	Spacing selectable $700\,\mathrm{GHz}$5–$700\,\mathrm{GHz}$6	$700\,\mathrm{GHz}$7 comb; center frequency tuned by $700\,\mathrm{GHz}$8 (Xu et al., 2020)
Normal-GVD LiNbO$700\,\mathrm{GHz}$9 / TFLN	$9.9\,\mathrm{GHz}$0 or $9.9\,\mathrm{GHz}$1	$9.9\,\mathrm{GHz}$2 dark-pulse span; $9.9\,\mathrm{GHz}$3 efficiency and $9.9\,\mathrm{GHz}$4 pure-Kerr span; $9.9\,\mathrm{GHz}$5 hybrid Kerr-Raman span (Lv et al., 2024, Song et al., 28 Jul 2025)

The spread of reported parameters indicates that normal-dispersion comb generation is not tied to a single FSR, material system, or initiation route. Ultra-high-Q toroids emphasize low threshold, coupled SiN rings emphasize programmable and automated operation, and LiNbO$9.9\,\mathrm{GHz}$6-class devices emphasize dense microwave-rate line spacing and integration with electro-optic functionality. A related LN microdisk route used weak perturbation by a silica tapered fiber to form polygon modes and produced a robust soliton comb from $9.9\,\mathrm{GHz}$7 to $9.9\,\mathrm{GHz}$8 at $9.9\,\mathrm{GHz}$9 while eliminating discrete Raman lines and sub-comb structures (Fu et al., 2023).

5. Conversion efficiency, coherence, and spectral-temporal signatures

One of the main motivations for normal-dispersion microcombs is conversion efficiency. Pump-to-comb conversion is commonly defined as

$>50\,\mathrm{dB}$0

In the coupled-ring SiN study, anomalous-GVD dissipative Kerr solitons were described there as typically yielding $>50\,\mathrm{dB}$1 pump-to-comb conversion, whereas the normal-GVD source reached $>50\,\mathrm{dB}$2 and exhibited more than 50 comb lines with at least $>50\,\mathrm{dB}$3 per line (Kim et al., 2019).

High efficiency has not precluded coherence. On X-cut TFLN, normal-dispersion operation yielded up to $>50\,\mathrm{dB}$4 efficiency in a $>50\,\mathrm{dB}$5 device and $>50\,\mathrm{dB}$6 over a $>50\,\mathrm{dB}$7 hybrid Kerr-Raman span in a $>50\,\mathrm{dB}$8 device; Lorentzian linewidths of 33 comb lines followed a parabolic mode-number dependence with a minimum linewidth of $>50\,\mathrm{dB}$9 at mode $D_2<0$00, and the $D_2<0$01 repetition rate produced a single clean beat note after EO frequency division (Song et al., 28 Jul 2025). In dispersion-folded Si$D_2<0$02N$D_2<0$03 racetracks, comb-reconstruction measurements yielded narrow $D_2<0$04 linewidths for all lines in both the core and satellite combs (Anderson et al., 2022).

The spectral-temporal signatures differ from the sech$D_2<0$05 bright-soliton archetype. Early MgF$D_2<0$06 normal-GVD combs exhibited multi-peaked optical envelopes and dark-pulse intracavity waveforms (Liang et al., 2014). Dark-pulse LiNbO$D_2<0$07 combs showed a sinc-like modulation with deep spectral notches whose spacing tracks the dark-pulse width, and their $D_2<0$08 RF beat note was resolution-limited at $D_2<0$09 RBW with phase noise of $D_2<0$10 at $D_2<0$11 offset and $D_2<0$12 at $D_2<0$13 offset (Lv et al., 2024). Pulsed normal-dispersion mini-resonators can instead produce spectrally flat central bands with better than $D_2<0$14 variation across $D_2<0$15 and no excess intensity noise on the fundamental RF beat note (Xu et al., 2020).

6. Applications, misconceptions, and active directions

Reported applications follow directly from the combination of high per-line power, flat spectral envelopes, and microwave-to-subterahertz line spacing. The turn-key SiN source was presented as an on-demand high-power comb source for wavelength-division multiplexing, with fixed-frequency operation suitable for dense WDM channels and phase-modulation formats such as DPSK (Kim et al., 2019). Pulsed normal-dispersion cavities were explicitly positioned for broadband spectroscopy, optical communications, frequency synthesis and metrology, LiDAR, and optical coherence tomography (Xu et al., 2020). Thin-film lithium niobate implementations target communications, frequency synthesis, and integrated signal processing, especially where electro-optic and $D_2<0$16 functionalities must coexist on one chip (Song et al., 28 Jul 2025).

A common misconception is that anomalous dispersion is mandatory for Kerr microcombs. The published record is narrower than that: anomalous GVD is the conventional route to bright dissipative Kerr solitons, but normal-dispersion combs have also been realized through controllable mode interactions (Xue et al., 2015), second-harmonic-assisted four-wave mixing (Xue et al., 2016), Raman-assisted switching-wave binding (Li et al., 2023), and Brillouin-Kerr triggering (Bunel et al., 5 Feb 2025). Another misconception is that normal-dispersion combs are necessarily narrowband or intrinsically incoherent. Reported counterexamples include $D_2<0$17 type-I comb span in SiO$D_2<0$18N$D_2<0$19 microtoroids (Chen et al., 2019) and $D_2<0$20 dark-pulse span in LiNbO$D_2<0$21 (Lv et al., 2024).

Current directions emphasize expansion of the accessible state space rather than convergence to a single canonical device. Dispersion-folded cavities suggest that Floquet phase matching can add coherent satellite microcombs tens of THz from the pump (Anderson et al., 2022). Beyond-mean-field Kerr interferometers suggest that bright normal-GVD localized states can exist outside the usual bistable region (Seidel et al., 2024). Post-fabrication modal perturbation in LN microdisks and controllable switching between Raman-dominated and Kerr-dominated Si$D_2<0$22N$D_2<0$23 states suggest a broader design space in which spatial-mode engineering and nonlinear-response engineering are treated jointly (Fu et al., 2023, Shitikov et al., 7 Nov 2025). A plausible implication is that normal-dispersion Kerr microcombs will continue to develop less as anomalous-dispersion solitons with reversed sign conventions than as a distinct class of dissipative states whose utility depends on deliberately engineered interactions among dispersion, coupling, gain, and loss.