Theoretical analysis of mode instability in high-power fiber amplifiers

Abstract

We present a simple theoretical model of transverse mode instability in high-power rare-earth doped fiber amplifiers. The model shows that efficient power transfer between the fundamental and higher-order modes of the fiber can be induced by a nonlinear interaction mediated through the thermo-optic effect, leading to transverse mode instability. The temporal and spectral characteristics of the instability dynamics are investigated, and it is shown that the instability can be seeded by both quantum noise and signal intensity noise, while pure phase noise of the signal does not induce instability. It is also shown that the presence of a small harmonic amplitude modulation of the signal can lead to generation of higher harmonics in the output intensity when operating near the instability threshold.

Estimating modal instability threshold for photonic crystal rod fiber amplifiers

“…The semi-analytic model by Hansen et al [8,9], describing TMI in fiber amplifiers is combined with FEM calculations to estimate the TMI threshold for PCF fibers. The mode power transfer is determined by a nonlinear coupling constant χ that depends on the frequency difference Ω between the FM and HOM, the bulk gain coefficient g, and the transverse core overlap ratio Г of the FM and HOM determined from the mode calculations.…”

“…The mode power transfer is determined by a nonlinear coupling constant χ that depends on the frequency difference Ω between the FM and HOM, the bulk gain coefficient g, and the transverse core overlap ratio Г of the FM and HOM determined from the mode calculations. The nonlinear coupling constant χ is given by [8,9] [ ]…”

“…We consider TMI seeded by relative intensity noise (RIN) R N from the seed laser. The approximate expression for the fraction of HOM power content ξ out as a function of extracted average output power P signal is given by [9] ( ) where P seed is the seed power and Ω p is Ω for maximum χ, and ξ in is the fraction of power that is initially coupled to the HOM. A seed laser is characterized by its RIN, which can be on the order of −120 dBc/Hz to −100 dBc/Hz corresponding to a laser with low and high RIN.…”

“…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. In [9], the authors extended the model to include temporal and spectral evolution of TMI and successfully described fiber amplifier characteristics at and above TMI threshold.…”

“…The semi-analytic approach in [8,9] is extended and the TMI threshold is estimated. The previous limitation of constant mode profiles in the semi-analytic model is addressed by using mode profiles that depend on thermal load.…”