Cover story: Fu BT, Gao RH, Yao N et al. Soliton microcomb generation by cavity polygon modes. Opto-Electron Adv 7, 240061 (2024).

Soliton microcombs, which require the hosting cavity to operate in an anomalous dispersion regime, are essential to integrate photonic systems ranging from precision spectroscopy and data processing. In the past, soliton microcombs were generated on cavity whispering gallery modes (WGM), and the cavity made by normal dispersion material is required to operate in an anomalous dispersion regime through structural dispersion engineering, which inevitably leading to degraded Q-factors and higher pump threshold. Recently, Professor Ya Cheng at East China Normal University and Professor Jintian Lin at Shanghai Institute of Optics and Fine Mechanics reported a soliton microcomb generation through excitation of high-Q cavity polygon modes. Excited by weak perturbations of a tapered fiber, cavity polygon modes are formed through the coherent combination of quasi-degenerate WGMs. Unlike the normal-dispersion WGM counterpart, these modes are protected by the classical orbit and in turn experience a lower dispersion, displaying anomalous dispersion in the telecom band. Meanwhile, light in polygon modes is mostly away from the rough cavity edge and light coupling element, resulting in optical Q factors exceeding 4´106. Moreover, the spatial distribution characteristics of polygon modes have small modal overlaps with other mode families, which further suppresses Raman effects and mode crossings. Consequently, a soliton comb spanning from 1450 nm to 1620 nm with a record low pump power of 11.1 mW is demonstrated on integrated LN platform, allowing soliton microcomb generated with low pump power.