Coherence loss of partially mode-locked fiber laser

TL;DR

This paper investigates how nonlinear interactions and modulation instability affect the coherence and mode distribution in partially mode-locked fiber lasers, revealing new insights into complex pattern formation in such systems.

Contribution

It demonstrates that in dissipative fiber lasers, modulation instability can maintain high coherence, and shows how intracavity polarization controls coherence loss and mode redistribution.

Findings

Fiber laser energy redistributes from central to sidebands via parametric instabilities.

Longitudinal modes are populated through cascaded four-wave-mixing.

Partially coherent pulses result from random distribution of modes, intensity, and polarization.

Abstract

Stochastically driven nonlinear processes limit the number of amplified modes in a natural system due to competitive mode interaction, which is accompanied by loss of coherence when increasing the complexity of system. Specifically, we find that modulation instability, which exhibits great fluctuations when it is spontaneously grown from noise in conservative systems, may possess a high degree of coherence in dissipative laser system with gain. Nonlinear mode interactions can be competitive or cooperative: adjusting the intracavity polarization state controls the process of loss of coherence. Single-shot spectra reveal that fiber laser redistributes its energy from center wavelength mode to sidebands through parametric instabilities, and subsequently longitudinal modes are populated via cascaded four-wave-mixing. Parametric frequency conversion populates longitudinal modes with a random distribution of position, intensity and polarization, resulting in partially (rather than highly) coherent pulses. These dynamics unveil a new route towards complex pattern formation in nonlinear laser systems.