Coherence loss of partially mode-locked fibre laser

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 the system. Specifically, we find that modulation instability, which exhibits great
fluctuations when it spontaneously grows 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, first, the fibre laser redistributes its energy from the center
wavelength mode into sidebands through parametric instabilities. 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, and they may be also beneficial for the understanding of supercontinuum,
Kerr-combs phenomena, and optical rogue waves.