Soliton optical frequency comb generation from polygon modes in weakly perturbed lithium niobate microdisks

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.