Single-branch Er:fiber frequency comb for optical synthesis at the $10^{-18}$ level

TL;DR

This paper presents a single-branch erbium-fiber laser frequency comb capable of optical synthesis at the $10^{-18}$ level, improving stability and simplifying the setup for optical clock comparisons.

Contribution

The authors develop a novel single-branch erbium-fiber frequency comb that maintains high stability by integrating all fiber paths within the feedback loop, enabling ultra-precise optical synthesis.

Findings

Achieved fractional frequency stability of 3 x 10^{-18} at 1 second.

Supported optical frequency synthesis across 650 nm to 2100 nm.

Reduced complexity and enhanced robustness compared to traditional multi-branch systems.

Abstract

Laser frequency combs based on erbium-doped fiber mode-locked lasers have shown great potential for compact, robust and efficient optical clock comparisons. However, to simultaneously compare multiple optical clock species, fiber laser frequency combs typically require multiple amplifiers and fiber optic paths that reduce the achievable frequency stability near 1 part in $10^{16}$ at 1s. In this paper we describe an erbium-fiber laser frequency comb that overcomes these conventional challenges and supports optical frequency synthesis at the millihertz level, or fractionally $3 x 10^{-18}$ $tau^{-1/2}$ by ensuring that all critical fiber paths are within the servo-controlled feedback loop. We demonstrate the application of this frequency comb as a synthesizer for optical clocks operating across a wavelength range from 650 nm to 2100 nm.