Origin of thermal modal instabilities in large mode area fiber amplifiers

Abstract

We present a dynamic model of thermal modal instability in large mode area fiber amplifiers. This model allows the pump and signal optical intensity distributions to apply a time-varying heat load distribution within the fiber. This influences the temperature distribution that modifies the optical distributions through the thermo-optic effect thus creating a feedback loop that gives rise to time-dependent modal instability. We describe different regimes of operation for a representative fiber design. We find qualitative agreement between simulation results and experimental results obtained with a different fiber including the time-dependent behavior of the instability and the effects of different cooling configurations on the threshold. We describe the physical processes responsible for the onset of the instability and suggest possible mitigation approaches.

Estimating modal instability threshold for photonic crystal rod fiber amplifiers

“…Recent studies have also showed large dependency between TMI threshold and system parameters such as seed or pump noise as well as cooling conditions [6,19]. Seed noise can contain discrete frequency components possibly larger than the RIN, that if located within the frequency band of the nonlinear coupling constant reduces the TMI threshold.…”

“…It is also assumed in the calculations that the density, heat capacity, thermal conductivity, and thermo-optic coefficient are temperature independent. In experiments, the absolute temperature of the fiber increases significantly at high output power [6] and depends on outer cooling properties, which can lead to variations in these coefficients.…”

“…New complex fiber designs favor large differential gain between the fundamental mode (FM) and higher order mode (HOM); for instance by using resonant structures in the cladding to effectively remove HOMs from the core [2], or delocalizing HOMs at low signal powers [3]. Currently, the onset of transverse modal instability (TMI) degrading the output beam quality sets the upper limit for power scaling [4][5][6]. TMI sets in as the average signal power reaches a certain threshold, typically on the order of a few hundred W. A phase shift between a thermally induced index grating at high power levels and the signal causes power transfer from the FM to a HOM leading to TMI.…”

“…TMI sets in as the average signal power reaches a certain threshold, typically on the order of a few hundred W. A phase shift between a thermally induced index grating at high power levels and the signal causes power transfer from the FM to a HOM leading to TMI. Numerically heavy beam propagation models have been used to study temporal and spectral modal instability properties with impressive results gaining further insight into the phenomenon [4,6,7]. Recently, a numerically simpler model, by Hansen et al [8], was published using a semi-analytic approach to consider thermally induced mode coupling by a coupled-mode model.…”

Numerical modelling of self-heating effects on guiding modes of high-power photonic crystal fibre lasers

“…The results were interesting in the CCF due to thermal lensing that increases the cladding mode overlapping in the core region. Furthermore, one can name other thermal effects such as thermally induced phase mismatching [14], thermal modal instability [15], and depolarization loss [16].…”