Harnessing Dispersion in Soliton Microcombs to Mitigate Thermal Noise

TL;DR

This paper demonstrates how dispersion engineering in soliton microcombs can significantly reduce thermal noise, leading to ultra-low phase noise and linewidths, with both simulations and experiments confirming the approach.

Contribution

It introduces a novel dispersion design strategy to control thermal-noise transduction in soliton microcombs, achieving substantial noise suppression and improved stability.

Findings

Suppressed repetition-rate frequency fluctuations by up to 50 dB through dispersion tuning.

Achieved a 15 dB reduction in repetition-rate phase noise experimentally.

Demonstrated carrier-envelope-offset frequency linewidths of 1 MHz and 100 Hz in two microcombs with similar spectra but different dispersion.

Abstract

We explore intrinsic thermal noise in soliton microcombs, revealing thermodynamic correlations induced by nonlinearity and group-velocity dispersion. A suitable dispersion design gives rise to control over thermal-noise transduction from the environment to a soliton microcomb. We present simulations with the Lugiato-Lefever equation (LLE), including temperature as a stochastic variable. By systematically tuning the dispersion, we suppress repetition-rate frequency fluctuations by up to 50 decibels for different LLE soliton solutions. In an experiment, we observe a measurement-system-limited 15-decibel reduction in the repetition-rate phase noise for various settings of the pump-laser frequency, and our measurements agree with a thermal-noise model. Finally, we compare two octave-spanning soliton microcombs with similar optical spectra and offset frequencies, but with designed differences in dispersion. Remarkably, their thermal-noise-limited carrier-envelope-offset frequency linewidths are 1 MHz and 100 Hz, which demonstrates an unprecedented potential to mitigate thermal noise. Our results guide future soliton-microcomb design for low-noise applications, and, more generally, they illuminate emergent properties of nonlinear, multi-mode optical systems subject to intrinsic fluctuations.