# Excess noise factor in non-Hermitian mode-locked lasers

## TL;DR

This paper predicts and demonstrates that non-Hermitian modes in actively mode-locked lasers cause an excess noise factor, significantly increasing phase noise and potentially limiting their use in high-precision applications.

## Contribution

It introduces the concept of excess noise due to non-Hermiticity in mode-locked lasers and provides experimental evidence of its impact on phase noise.

## Key findings

- Excess noise factor increases phase noise by over 20 dB.
- Non-Hermitian mode non-orthogonality causes increased spontaneous emission.
- Noise degradation affects applications requiring high spectral purity.

## Abstract

We predict and observe the appearance of an excess noise factor due to the non-Hermiticity of the modes of two actively harmonically mode-locked lasers. The non uniform distribution of gain and losses in the ring cavities of these lasers causes a non-orthogonality of the longitudinal modes, which is responsible for an increase of the rate of spontaneous emission falling in the lasing pulsed mode. This leads to an increase of the phase noise of the pulse train generated by these lasers by more than 20 dB at some offset frequencies. This degradation can be prohibitive for the use of these lasers for several applications such as the generation of high spectral purity RF and low jitter clock signals.