# Photonic-chip supercontinuum with tailored spectra for precision frequency metrology

## TL;DR

This paper demonstrates dispersion-engineered silicon nitride waveguides that generate customizable, low-noise supercontinuum spectra for precision frequency metrology, achieving high stability in clock comparisons.

## Contribution

The work introduces tailored silicon nitride waveguides for supercontinuum generation that support self-referenced, low-noise combs suitable for high-precision optical measurements.

## Key findings

- Achieved a clock-limited relative frequency instability of 3.8×10⁻¹⁵ at 2 seconds.
- Generated two-octave supercontinuum spectra with tailored dispersion.
- Enabled high-precision clock comparison between laser sources.

## Abstract

Supercontinuum generation using chip-integrated photonic waveguides is a powerful approach for spectrally broadening pulsed laser sources with very low pulse energies and compact form factors. When pumped with a mode-locked laser frequency comb, these waveguides can coherently expand the comb spectrum to more than an octave in bandwidth to enable self-referenced stabilization. However, for applications in frequency metrology and precision spectroscopy, it is desirable to not only support self-referencing, but also to generate low-noise combs with customizable broadband spectra. In this work, we demonstrate dispersion-engineered waveguides based on silicon nitride that are designed to meet these goals and enable precision optical metrology experiments across large wavelength spans. We perform a clock comparison measurement and report a clock-limited relative frequency instability of $3.8\times10^{-15}$ at $\tau = 2$ seconds between a 1550 nm cavity-stabilized reference laser and NIST's calcium atomic clock laser at 657 nm using a two-octave waveguide-supercontinuum comb.