Soliton microcombs in X-cut LiNbO3 microresonators

TL;DR

This paper demonstrates the generation of soliton microcombs in X-cut lithium niobate microresonators by overcoming Raman nonlinearity, enabling integrated, fast-tunable microcombs for advanced photonic applications.

Contribution

It introduces a method to generate soliton microcombs in X-cut TFLN chips by orienting the resonator to mitigate Raman effects, expanding TFLN photonics capabilities.

Findings

Successful generation of 25 GHz repetition rate soliton microcombs.

Extended microcomb spectra to 350 nm with pulsed pumping.

Mitigation of Raman nonlinearity through resonator orientation.

Abstract

Chip-scale integration of optical frequency combs, particularly soliton microcombs, enables miniaturized instrumentation for timekeeping, ranging, and spectroscopy. Although soliton microcombs have been demonstrated on various material platforms, realizing complete comb functionality on photonic chips requires the co-integration of high-speed modulators and efficient frequency doublers, features that are available in a monolithic form on X-cut thin-film lithium niobate (TFLN). However, the pronounced Raman nonlinearity associated with extraordinary light in this platform has so far precluded soliton microcomb generation. Here, we report the generation of transverse-electric-polarized soliton microcombs with a 25 GHz repetition rate in high-Q microresonators on X-cut TFLN chips. By precisely orienting the racetrack microresonator relative to the optical axis, we mitigate Raman nonlinearity and enable soliton formation under continuous-wave laser pumping. Moreover, the soliton microcomb spectra are extended to 350 nm with pulsed laser pumping. This work expands the capabilities of TFLN photonics and paves the way for the monolithic integration of fast-tunable, self-referenced microcombs.