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.
Highlights
A phenomenon known as Mode Instability, or Transverse Mode Instability (TMI), has emerged as one of the greatest limitations on the power scalability of large mode area (LMA) ytterbium-doped fiber amplifiers [1, 2]
This long-period grating (LPG) is induced by mode beating between the light in the fundamental mode (FM) and a small amount of light unintentionally coupled into the higher-order mode (HOM), since the resulting spatial intensity oscillation creates a matching index grating due to the ytterbium doping in the fiber core
Since the refractive index of fused silica depends on temperature, the mode beating between the FM and the HOM again leads to a LPG, which has the correct period to couple the two modes [6]
Summary
A phenomenon known as Mode Instability, or Transverse Mode Instability (TMI), has emerged as one of the greatest limitations on the power scalability of large mode area (LMA) ytterbium-doped fiber amplifiers [1, 2]. An initial attempt to understand the physical mechanism responsible for TMI proposed that a self-induced long-period grating (LPG) could cause a transfer of power from the fundamental mode (FM) to a HOM of the fiber [5] This LPG is induced by mode beating between the light in the FM and a small amount of light unintentionally coupled into the HOM, since the resulting spatial intensity oscillation creates a matching index grating due to the ytterbium doping in the fiber core. The ytterbium ions can cause the required refractive index perturbation either directly, since their contribution to the refractive index of the doped core depends on the population inversion, which again depends on the local intensity, or indirectly though the thermo-optic effect In the latter case, the intensity oscillation of the signal leads to spatially varying stimulated emission, which due to the quantum defect creates a spatially oscillating temperature profile in the fiber.
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