# Controlling group and phase velocities in bidirectional mode-locked fiber lasers

## TL;DR

This paper demonstrates a bidirectional mode-locked fiber laser that produces correlated frequency combs, with measurements showing how pulse and phase velocities depend on intensity, revealing the dominant role of gain dynamics over Kerr effects.

## Contribution

It introduces a novel bidirectional mode-locked fiber laser setup and provides the first simultaneous measurements of pulse and phase velocities under varying intensities.

## Key findings

- Phase delay is governed by linear and nonlinear refractive indices.
- Nonlinear Kerr effect has a minor impact on pulse velocity.
- Gain dynamics significantly influence pulse velocity.

## Abstract

A bidirectional mode-locked fiber laser producing two correlated frequency combs of the same repetition rate is demonstrated. The intensity dependence of pulse and phase velocities are measured simultaneously. As expected, the phase delay is determined by the linear and nonlinear indices of refraction. The nonlinear Kerr effect contribution to the pulse velocity is dwarfed by a contribution from saturable gain dynamics.