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1. Stable Kerr frequency combs excited in the vicinity of strong modal dispersion disruptions

   https://doi.org/10.1117/12.2652926  

   Taheri, Hossein; Savchenkov, Anatoliy; Matsko, Andrey B. (March 2023, Proceedings Volume 12407, Laser Resonators, Microresonators, and Beam Control XXV; 1240709 (2023))

   Armani, Andrea M.; Kudryashov, Alexis V.; Ilchenko, Vladimir S.; Sheldakova, Julia V. (Ed.)

   Optical microresonators possessing Kerr-type nonlinearity have emerged over the past decade as reliable and versatile sources of optical frequency combs, with varied applications including in the generation of low-phasenoise radio frequency (RF) signals, small-footprint precision timekeeping, and LiDAR. One of the key parameters affecting Kerr microcomb generation in different wavelength ranges is cavity modal dispersion. Dispersion effects such as avoided mode crossings (AMCs) have been shown to greatly limit mode-locked microcomb generation, especially when pumping in close proximity to such disruptions. We present numerical modeling and experimental evidence demonstrating that using an auxiliary laser pump can suppress the detrimental impact of near-pump AMCs. We also report, for the first time to our knowledge, the possibility of the breaking of characteristic soliton steps into two stable branches corresponding to different stable pulse trains arising from the interplay of dichromatic pumping and AMCs. These findings bear significance, particularly for the generation of frequency combs in larger resonators or at smaller wavelengths, such as the visible range, where the cavities become overmoded. 

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2. Visible nonlinear photonics via high-order-mode dispersion engineering

   https://doi.org/10.1364/OPTICA.7.000135  

   Zhao, Yun; Ji, Xingchen; Kim, Bok_Young; Donvalkar, Prathamesh_S; Jang, Jae_K; Joshi, Chaitanya; Yu, Mengjie; Joshi, Chaitali; Domeneguetti, Renato_R; Barbosa, Felippe_A_S; et al (January 2020, Optica)

   Over the past decade, remarkable advances have been realized in chip-based nonlinear photonic devices for classical and quantum applications in the near- and mid-infrared regimes. However, few demonstrations have been realized in the visible and near-visible regimes, primarily due to the large normal material group-velocity dispersion (GVD) that makes it challenging to phase match third-order parametric processes. In this paper, we show that exploiting dispersion engineering of higher-order waveguide modes provides waveguide dispersion that allows for small or anomalous GVD in the visible and near-visible regimes and phase matching of four-wave mixing processes. We illustrate the power of this concept by demonstrating in silicon nitride microresonators a near-visible mode-locked Kerr frequency comb and a narrowband photon-pair source compatible with Rb transitions. These realizations extend applications of nonlinear photonics towards the visible and near-visible regimes for applications in time and frequency metrology, spectral calibration, quantum information, and biomedical applications. 

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3. Microcombs in fiber Fabry–Pérot cavities

   https://doi.org/10.1063/5.0177134  

   Musgrave, Jonathan; Huang, Shu-Wei; Nie, Mingming (December 2023, APL Photonics)

   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. 

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4. All normal dispersion fiber laser with bandwidth tunable fiber based spectral filter

   Khanolka, A; Ge, X; Chong, A (January 2020, Conference on Lasers and Electrooptics)

   We have demonstrated a stable ytterbium mode-locked fiber laser with an all fiber, bandwidth tunable spectral filter which can generate mode-locked spectrums of different shapes and bandwidth. 

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5. Multimode nonlinear dynamics in spatiotemporal mode-locked anomalous-dispersion lasers

   https://doi.org/10.1364/OL.471457  

   Wu, Yuhang; Christodoulides, Demetrios_N; Wise, Frank_W (August 2022, Optics Letters)

   Spatiotemporal mode-locking in a laser with anomalous dispersion is investigated. Mode-locked states with varying modal content can be observed, but we find it difficult to observe highly-multimode states. We describe the properties of these mode-locked states and compare them to the results of numerical simulations. Prospects for the generation of highly-multimode states and lasers based on multimode soliton formation are discussed. 

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