Broadband electro-optic frequency comb generation in an integrated microring resonator

TL;DR

This paper demonstrates an integrated electro-optic frequency comb generator in lithium niobate that produces over 900 lines across the entire telecommunications L-band, with high stability, tunability, and potential for octave-spanning spectra.

Contribution

It introduces a novel integrated EO comb generator with large EO response, low optical loss, and dispersion engineering, enabling wide, stable, and tunable combs in a compact platform.

Findings

Over 900 comb lines spanning the L-band

Frequency spacing controllable from 10 Hz to 100 MHz

Generation of dual frequency combs in a single resonator

Abstract

Optical frequency combs consist of equally spaced discrete optical frequency components and are essential tools for optical communications and for precision metrology, timing and spectroscopy. To date, wide-spanning combs are most often generated by mode-locked lasers or dispersion-engineered resonators with third-order Kerr nonlinearity. An alternative comb generation method uses electro-optic (EO) phase modulation in a resonator with strong second-order nonlinearity, resulting in combs with excellent stability and controllability. Previous EO combs, however, have been limited to narrow widths by a weak EO interaction strength and a lack of dispersion engineering in free-space systems. In this work, we overcome these limitations by realizing an integrated EO comb generator in a thin-film lithium niobate photonic platform that features a large electro-optic response, ultra-low optical loss and highly co-localized microwave and optical felds, while enabling dispersion engineering. Our measured EO frequency comb spans more than the entire telecommunications L-band (over 900 comb lines spaced at ~ 10 GHz), and we show that future dispersion engineering can enable octave-spanning combs. Furthermore, we demonstrate the high tolerance of our comb generator to modulation frequency detuning, with frequency spacing finely controllable over seven orders of magnitude (10 Hz to 100 MHz), and utilize this feature to generate dual frequency combs in a single resonator. Our results show that integrated EO comb generators, capable of generating wide and stable comb spectra, are a powerful complement to integrated Kerr combs, enabling applications ranging from spectroscopy to optical communications.