Gain-guiding effects in an amplifier with focused gain

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Abstract
The non-Hermitian character of a gain-guided amplifier leads naturally to a description of the amplified field in terms of a nonorthogonal mode basis. We present a theoretical analysis, based on a nonorthogonal mode expansion, for the output of an amplifier with focused gain. The theory predicts that the gain-guided nature of the amplification process can lead to excess noise in the output of the amplifier. We then report the results of two experiments in which the output from a focused-gain Raman amplifier was measured. The experimental results are found to be in reasonable agreement with the nonorthogonal mode theory.