Broadband microwave-rate dark pulse microcombs in dissipation-engineered LiNbO$_3$ microresonators

TL;DR

This paper demonstrates the generation of broadband dark pulse microcombs in dissipation-engineered lithium niobate microresonators, overcoming Raman effects to enable high-power, microwave-rate combs for integrated photonics applications.

Contribution

The authors introduce a dissipation engineering approach to realize dark pulse microcombs in LiNbO$_3$ microresonators, addressing Raman response challenges and achieving tunable, high-coherence microwave-rate combs.

Findings

Achieved 25 GHz repetition rate dark pulse microcombs with 200 nm span.

Controlled phase-matching to damp Raman-active resonances.

Demonstrated coherence and tunability of the microcombs.

Abstract

Kerr microcombs generated in optical microresonators provide broadband light sources bridging optical and microwave signals. Their translation to thin-film lithium niobate unlocks second-order nonlinear optical interfaces such as electro-optic modulation and frequency doubling for completing comb functionalities. However, the strong Raman response of LiNbO$_3$ has complicated the formation of Kerr microcombs. Until now, dark pulse microcombs, requiring a double balance between Kerr nonlinearity and normal group velocity dispersion as well as gain and loss, have remained elusive in LiNbO$_3$ microresonators. Here, by incorporating dissipation engineering, we demonstrate dark pulse microcombs with 25 GHz repetition frequency and 200 nm span in a high-$Q$ LiNbO$_3$ microresonator. Resonances near the Raman-active wavelengths are strongly damped by controlling phase-matching conditions of a specially designed pulley coupler. The coherence and tunability of the dark pulse microcombs are also investigated. Our work provides a solution to realize high-power microcombs operating at microwave rates on LiNbO$_3$ chips, promising new opportunities for the monolithic integration of applications spanning communication to microwave photonics.