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Power Scaling Fiber Amplifiers Using Very-Large-Mode-Area Fibers

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Technical Report,24 Jul 2014,23 Feb 2016

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University of Rochester, Institute of Optics Rochester United States

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DEW-class fiber lasers are limited to below 1kW due to limited mode size and thermal issues, particularly thermal mode instability TMI. Two comprehensive models were developed for scaling the mode area of fiber amplifiers while inclusively addressing all relevant spatial and dynamic effects. The beam propagation model was found to be too computationally intensive for design purposes. A rate-equationkinetics model was developed and validated for accurately modeling very large- mode-area fiber amplifiers while simultaneously including thermal lensing and TMI. This model was applied to investigate scaling to an effective 50m core diameter. Mode-area scaling using conventional large-mode-area fibers was found to be simply not possible since the TMI threshold decreases with increased core area. Modifications in pump geometry were found to provide a modest benefit, particularly when flattening out the temperature profile along the fiber. Confined gain was studied and found to provide exceptional resilience to TMI at least 2x increase in TMI threshold, as was originally proposed. Two new promising fiber types were developed, and both were found to exhibit features with modeled and expected resilience to TMI.

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