The spatial structure of a gain-guided amplifier is naturally describe
d by a set of nonorthogonal modes. We present an analysis of the mode
structure of an amplifier with a focused Gaussian gain. It is found th
at the modes are spatially narrower and have a different radius of cur
vature than the corresponding free-space modes. A calculation of the s
ignal-to-noise ratio shows that the spatial structure of the input fie
ld has a significant effect on the amplifier noise performance.