On-chip self-referencing using integrated lithium niobate waveguides

TL;DR

This paper demonstrates an integrated lithium niobate waveguide system capable of on-chip self-referencing for optical frequency combs, achieving efficient $f_{CEO}$ stabilization with low pulse energy and broad wavelength adaptability.

Contribution

The authors present a novel integrated lithium niobate waveguide platform that enables efficient on-chip $f_{CEO}$ stabilization using a simplified, adaptable $f$-$2f$ interferometer with low pulse energy requirements.

Findings

Achieved $f_{CEO}$ stabilization with only 107 pJ pulse energy.

Demonstrated $f$-$2f$ interferometry in a single integrated waveguide.

Theoretical modeling confirms the nonlinear dynamics responsible for $f_{CEO}$ generation.

Abstract

The measurement and stabilization of the carrier-envelope offset frequency $f_{\textrm{CEO}}$ via self-referencing is paramount for optical frequency comb generation which has revolutionized precision frequency metrology, spectroscopy, and optical clocks. Over the past decade, the development of chip-scale platforms has enabled compact integrated waveguides for supercontinuum generation. However, there is a critical need for an on-chip self-referencing system that is adaptive to different pump wavelengths, requires low pulse energy, and does not require complicated processing. Here, we demonstrate efficient carrier-envelope offset frequency $f_{\textrm{CEO}}$ stabilization of a modelocked laser with only 107 pJ of pulse energy via self-referencing in an integrated lithium niobate waveguide. We realize an $f$-$2f$ interferometer through second-harmonic generation and subsequent supercontinuum generation in a single dispersion-engineered waveguide with a stabilization performance equivalent to a conventional off-chip module. The $f_{\textrm{CEO}}$ beatnote is measured over a pump wavelength range of 70 nm. We theoretically investigate our system using a single nonlinear envelope equation with contributions from both second- and third-order nonlinearities. Our modeling reveals rich ultrabroadband nonlinear dynamics and confirms that the initial second harmonic generation followed by supercontinuum generation with the remaining pump is responsible for the generation of a strong $f_{\textrm{CEO}}$ signal as compared to a traditional $f$-$2f$ interferometer. Our technology provides a highly-simplified system that is robust, low cost, and adaptable for precision metrology for use outside a research laboratory.