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.