Integrated Triply Resonant Electro-Optic Frequency Comb in Lithium Tantalate

TL;DR

This paper presents a novel integrated triply resonant electro-optic frequency comb in lithium tantalate, achieving broad spectral coverage, reduced power consumption, and compact size, advancing chip-scale spectrometry and millimeter wave synthesis.

Contribution

It introduces a triply resonant architecture combining monolithic microwave circuits with thin-film lithium tantalate PICs, overcoming spectral and power limitations of previous integrated EO combs.

Findings

450nm (60THz) comb span with >2000 lines

Four-fold spectral extension and 16-fold power reduction

Compact 1cm^2 footprint with detuning-agnostic operation

Abstract

Integrated frequency comb generators based on Kerr parametric oscillation have led to chip-scale, gigahertz-spaced combs with new applications spanning hyperscale telecommunications, low-noise microwave synthesis, LiDAR, and astrophysical spectrometer calibration. Recent progress in lithium niobate (LN) photonic integrated circuits (PICs) has resulted in chip-scale electro-optic (EO) frequency combs, offering precise comb-line positioning and simple operation without relying on the formation of dissipative Kerr solitons. However, current integrated EO combs face limited spectral coverage due to the large microwave power required to drive the non-resonant capacitive electrodes and the strong intrinsic birefringence of Lithium Niobate. Here, we overcome both challenges with an integrated triply resonant architecture, combining monolithic microwave integrated circuits (MMICs) with PICs based on the recently emerged thin-film lithium tantalate. With resonantly enhanced EO interaction and reduced birefringence in Lithium Tantalate, we achieve a four-fold comb span extension and a 16-fold power reduction compared to the conventional non-resonant microwave design. Driven by a hybrid-integrated laser diode, the comb spans over 450nm (60THz) with >2000 lines, and the generator fits within a compact 1cm^2 footprint. We additionally observe that the strong EO coupling leads to an increased comb existence range approaching the full free spectral range of the optical microresonator. The ultra-broadband comb generator, combined with detuning-agnostic operation, could advance chip-scale spectrometry and ultra-low-noise millimeter wave synthesis and unlock octave-spanning EO combs. The methodology of co-designing microwave and optical resonators can be extended to a wide range of integrated electro-optics applications.