GAIN-GUIDING EFFECTS IN AN AMPLIFIER WITH FOCUSED GAIN

The non-Hermitian character of a gain-guided amplifier leads naturally to a description of the amplified field in terms of a nonorthogonal m ode 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 proc ess can lead to excess noise in the output of the amplifier. We then r eport the results of two experiments in which the output from a focuse d-gain Raman amplifier was measured. The experimental results are foun d to be in reasonable agreement with the nonorthogonal mode theory.