Amplified noise nonstationarity in a mode-locked laser based on nonlinear polarization rotation

TL;DR

This paper demonstrates that in a mode-locked laser with nonlinear polarization rotation, the amplified spontaneous emission noise remains nonstationary due to small gain modulations, affecting laser stability and noise characteristics.

Contribution

It reveals that amplified spontaneous emission noise in such lasers is nonstationary, influenced by small gain modulations synchronized over cavity round-trips, despite fast mode-locking mechanisms.

Findings

Amplified spontaneous emission noise is nonstationary in mode-locked lasers.

Small gain modulations can significantly affect noise over multiple cavity round-trips.

Nonstationarity persists even with fast mode-locking mechanisms like nonlinear polarization rotation.

Abstract

Beat note measurements between a mode-locked and a continuous-wave laser as well as between two mode-locked sources were used to demonstrate that the sub-threshold, cavity filtered, amplified spontaneous emission is not stationary even when a fast mode-locking mechanism, such as nonlinear polarization rotation, is used to generate short pulses. A relatively small gain modulation of a few percents created by high intensity pulses can produce a significant modulation of the amplified noise once synchronously accumulated over several cavity round-trips, even if the repetition rate is faster than the gain dynamics.