Microresonator-based Kerr soliton microcombs are coherent light sources consisting of equally spaced and phase locked discrete optical frequency components, which are essential tools for practical applications in precision spectroscopy and data processing. While anomalous microresonator dispersion is mandatory for Kerr soliton microcomb formation, so far almost all dispersions are susceptible to manufacturing error and cannot be tuned once the microstructure is made. Moreover, microcomb formation in strongly Raman-active mediums like Lithium Niobate (LN) is challenging in the suppression of stimulated Raman scattering and mode crossing due to the existence of densely distributed multiple Whispering Galley Mode (WGM) families. Here, Kerr soliton microcombs were formed in a normal dispersion LN microdisk resonator by mode trimming. Despite that the fundamental WGM family is of normal dispersion and there are densely distributed high-order WGM families within the LN microdisk, high-Q square modes of anomalous dispersion and small mode volume are coherently formed by introducing weak perturbation for mode trimming. Under the optical pump of the square mode of 35-mW power, densely distributed WGM families are avoided to be excited, leading to the suppression of Raman scattering effects and mode crossing, and the formation of soliton microcomb with a spectrum spanning from 1450 nm to 1620 nm.
Mode trimming of suitable dispersion in a high-Q microresonator is of vital importance for lots of photonic applications ranging from Kerr soliton comb generation, single-frequency lasing to nonlinear frequency conversion. However, almost all the dispersion engineering and mode trimming cannot be tuned once the resonator structure is made. To overcome this difficulty, weak perturbation was introduced into a circular lithium niobate (LN) microdisk for the formation of polygon modes by a coupled tapered fiber. Various polygon modes with different spatial field distributions and star mode can be formed by tuning the pump wavelength and the coupling position. Thanks to the small spatial modal overlap between the polygon modes and the whispering gallery modes (WGMs), densely distributed WGM families were avoided to be excited under polygon-mode optical pump. Therefore, single-frequency/dual-wavelength microlasers have been demonstrated with narrow linewidths in weakly-perturbed erbium-doped LN microdisks, and Kerr soliton microcomb has been generated in the telecom band in a normal-dispersion LN microdisk by mode trimming through the excitation of the anomalous-dispersion polygon modes.
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