Low-threshold mode instability in Yb^3+-doped few-mode fiber amplifiers

Abstract

Spatio-temporal instability of the fundamental mode in Yb(3+)-doped few-mode PM fiber amplifiers with a core diameter of 8.5 μm was registered at 2-30 Watts pump power. Both experimental and theoretical analysis revealed the nonlinear power transformation of the LP(01) fundamental mode into high-order modes. Numerical simulation revealed self-consistent growth of the higher-order mode and traveling electronic index grating accompanying the population grating induced by the mode interference field (due to different polarizability of the excited and unexcited Yb(3+) ions). Experimental results and numerical calculations showed the increase of the instability threshold along with an increase of the signal frequency bandwidth.

Transverse mode instability analysis in fiber amplifiers

“…In this case, the combined core thermal load due to pump/signal quantum defect and photodarkening is considered to be the TMI driving force. However, there are big discrepancies in the quoted thermal load values at the TMI threshold, varying from 34W/m [7], down to 15W/m [8] and even 0.3-3W/m [9], [10] in the case of narrow linewidth operation. This clearly indicates that there are other contributions to the observed effects.…”

“…This clearly indicates that there are other contributions to the observed effects. In another study, it has been argued that under narrow- linewidth and lower power operation, the inversion related refractive index changes are responsible for the observed TMI effects [9].…”

Effects of perturbation relative phase on transverse mode instability gain

“…In high power operation, the origin of this effect is predominantly thermal and a number of theoretical models have been introduced to explore the nature and investigate the characteristics of TMI. Theoretical models so far consider the interaction of two propagating modes and study the power exchange between the modes and TMI threshold either though STRS static or dynamic mode interactions [2]- [8]. In an alternative approach, TMI has been studied by considering the stability of the amplified fiber fundamental mode (FM) in the presence of transverse perturbations [10], [11].…”

Мощный одномодовый непрерывный узкополосный линейно поляризованный полностью волоконный Yb лазер с порогом модовой нестабильности более 100 Вт

“…Однако спектральная ширина линии подобных лазеров в полностью волоконном исполнении довольно большая (> 1 нм). До сих пор генерация относительно небольших оптических мощностей (сотни ватт) с малой спектральной шириной линии менее 0.1 нм непосредственно в маломодовом активном Yb волокне затруднена из-за эффекта модовой нестабильности (МН) [2].…”

“…Согласно работе [2], порог МН в Yb усилителе находится на уровне примерно 5 Вт в случае узкополосного (0.1 нм) сигнала на входе. В работе [3] был предложен подход с гибридной накачкой на двух длинах волн, позволяющий поднять порог модовой нестабильности.…”