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Abstract Nonlinear frequency conversion underpins numerous classical and quantum photonics applications but conventionally relies on synchronized femtosecond mode-locked lasers and dispersion-engineered enhancement cavities—an approach that imposes substantial system complexity. To address the challenges, here we report a fundamentally different paradigm: mode-locking of nonlinear frequency conversion enabled by the physics of dissipative quadratic soliton (DQS). We present the operating principle of femtosecond DQS mode-locking and experimentally validate it for the first time in a continuous-wave-pumped doubly resonant second-harmonic generator in free space, yielding bichromatic frequency combs spanning the visible and near-infrared. The observed DQSs exhibit 3 dB optical bandwidths and transform-limited pulse durations of 1.15 THz and 274 fs for the pump and 1.13 THz and 279 fs for the second harmonic. By harnessing phase-matched group-velocity-matched cascaded quadratic nonlinearities, we demonstrate an in-situ tunable effective Kerr nonlinearity that exceeds the intrinsic material response by over three orders of magnitude, enabling femtosecond DQS generation in both free-space and chip-scale cavities across normal and anomalous dispersion regimes. Our results establish a simple, flexible, and scalable approach to nonlinear frequency conversion without the need for synchronized femtosecond mode-locked lasers and expand the reach of soliton-based technologies across diverse cavity platforms and a wide range of challenging wavelengths that are otherwise inaccessible. 

Abstract Dissipative quadratic solitons (DQSs), enabled by nonlinearity engineering through cascaded quadratic processes, have remained a central theoretical prediction in nonlinear optics since their proposal in 1997. Despite their predicted ultralow operational thresholds, remarkable tunability, and potential for spectral extension into unconventional wavelengths, their experimental realization has remained elusive. Here, we demonstrate bright, dual-color DQS generation in the normal dispersion regime, in strong agreement with the proposed theoretical framework. Furthermore, by simply adjusting the nonlinear crystal temperature - without any structural modification - we reverse the sign of the effective nonlinearity and switch from bright DQS to platicon generation in situ. This work not only advances the fundamental understanding of dissipative solitons but also establishes a practical pathway for ultralow-threshold frequency comb generation at unconventional wavelengths, with broad implications for applications such as atomic clocks, optical coherence tomography, and astrocombs. 

The fiber single-cavity dual-comb laser (SCDCL) is an emerging light-source architecture that opens up the possibility for low-complexity dual-comb pump-probe measurements. However, the fundamental trade-off between measurement speed and time resolution remains a hurdle for the widespread use of fiber SCDCLs in dual-comb pump-probe measurements. In this paper, we break this fundamental trade-off by devising an all-optical dynamic repetition rate difference (Δf_rep) modulation technique. We demonstrate the dynamic Δf_repmodulation in a modified version of the recently developed counterpropagating all-normal dispersion (CANDi) fiber laser. We verify that our all-optical dynamic Δf_repmodulation technique does not introduce excessive relative timing jitter. In addition, the dynamic modulation mechanism is studied and validated both theoretically and experimentally. As a proof-of-principle experiment, we apply this so-called dynamic CANDi (DCANDi) fiber laser to measure the relaxation time of a semiconductor saturable absorber mirror, achieving a measurement speed and duty cycle enhancement factor of 143. DCANDi fiber laser is a promising light source for low-complexity, high-speed, high-sensitivity ultrafast dual-comb pump-probe measurements. 

A grating coupler on 700-nm-thick Z-cut lithium-niobate-on-insulator platform with high coupling efficiency, large bandwidth, and high fabrication tolerance is designed and optimized by inverse design method. The optimized grating coupler is fabricated with a single set of e-beam lithography and etching process, and it is experimentally characterized to possess peak coupling efficiency of −3.8 dB at 1574.93 nm, 1 dB bandwidth of 71.7 nm, and 3 dB bandwidth of over 120 nm, respectively. 

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. 

Broadband frequency comb generation through cascaded quadratic nonlinearity remains experimentally untapped in free-space cavities with bulk χ⁽²⁾materials mainly due to the high threshold power and restricted ability of dispersion engineering. Thin-film lithium niobate (LN) is a good platform for nonlinear optics due to the tight mode confinement in a nano-dimensional waveguide, the ease of dispersion engineering, large quadratic nonlinearities, and flexible phase matching via periodic poling. Here we demonstrate broadband frequency comb generation through dispersion engineering in a thin-film LN microresonator. Bandwidths of 150 nm (80 nm) and 25 nm (12 nm) for center wavelengths at 1560 and 780 nm are achieved, respectively, in a cavity-enhanced second-harmonic generation (doubly resonant optical parametric oscillator). Our demonstration paves the way for pure quadratic soliton generation, which is a great complement to dissipative Kerr soliton frequency combs for extended interesting nonlinear applications. 

Abstract Dissipative Kerr soliton (DKS) microcomb has emerged as an enabling technology that revolutionizes a wide range of applications in both basic science and technological innovation. Reliable turnkey operation with sub-optical-cycle and sub-femtosecond timing jitter is key to the success of many intriguing microcomb applications at the intersection of ultrafast optics and microwave electronics. Here we propose an approach and demonstrate the first turnkey Brillouin-DKS frequency comb to the best of our knowledge. Our microresonator-filtered laser design offers essential benefits, including phase insensitivity, self-healing capability, deterministic selection of the DKS state, and access to the ultralow noise comb state. The demonstrated turnkey Brillouin-DKS frequency comb achieves a fundamental comb linewidth of 100 mHz and DKS timing jitter of 1 femtosecond for averaging times up to 56 μs. The approach is universal and generalizable to various device platforms for user-friendly and field-deployable comb devices. 

Abstract Dissipative Kerr soliton (DKS) frequency combs—also known as microcombs—have arguably created a new field in cavity nonlinear photonics, with a strong cross-fertilization between theoretical, experimental, and technological research. Spatiotemporal mode-locking (STML) not only adds new degrees of freedom to ultrafast laser technology, but also provides new insights for implementing analogue computers and heuristic optimizers with photonics. Here, we combine the principles of DKS and STML to demonstrate the STML DKS by developing an unexplored ultrahigh-quality-factor Fabry–Pérot (FP) mesoresonator based on graded index multimode fiber (GRIN-MMF). Complementing the two-step pumping scheme with a cavity stress tuning method, we can selectively excite either the eigenmode DKS or the STML DKS. Furthermore, we demonstrate an ultralow noise microcomb that enhances the photonic flywheel performance in both the fundamental comb linewidth and DKS timing jitter. The demonstrated fundamental comb linewidth of 400 mHz and DKS timing jitter of 500 attosecond (averaging times up to 25 μs) represent improvements of 25× and 2.5×, respectively, from the state-of-the-art. Our results show the potential of GRIN-MMF FP mesoresonators as an ideal testbed for high-dimensional nonlinear cavity dynamics and photonic flywheel with ultrahigh coherence and ultralow timing jitter.