# Hybrid fiber links for accurate optical frequency comparisons

## TL;DR

This paper demonstrates a robust hybrid fiber scheme for accurate optical frequency comparison over 43 km urban fiber, achieving extremely low instability levels without measurement synchronization.

## Contribution

It introduces a novel hybrid fiber setup combining local two-way comparison with active noise compensation, enabling high-precision frequency transfer over urban networks.

## Key findings

- Achieved fractional frequency instability of 1×10⁻²⁰ at 10,000 s.
- Validated the robustness of the hybrid scheme in urban environments.
- Analyzed phase noise contributions from interferometers and laser sources.

## Abstract

We present the first experimental demonstration over a 43-km-long urban fiber network of a local two-way optical frequency comparison, which does not require any synchronization of the measurements. It was combined with a regular active-noise compensation on another parallel fiber leading to a very reliable and robust frequency transfer. This hybrid scheme enables us to investigate the major limiting factors of the local two-way comparison. We analyze the contribution to the phase noise of the recovered signal by the interferometers at local and remote places. By using the ability of this set up to be injected by a single laser or two independent lasers, we measure the contribution to the long-term instability by the demodulated laser instabilities. We show that a fractional frequency instability level of $1\times10^{-20}$ at 10 000 s can be obtained with this simple setup after 43-km-long propagation in an urban area.

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Source: https://tomesphere.com/paper/1702.01687