  Theory of monochromatic wave field amplification in a waveguide array based
on expansion of the wave field in terms of guided array modes is developed.
Equations for the expansion coefficients include cross-modal gain, which
completely changes behavior of the amplified wave field. Analysis of two-mode
amplification reveals new unusual features in characteristics of two-mode
amplifier. Instead of unlimited growth of both modes for incoherent fields, one
of the modes grows with no limit and suppresses the lower-power mode. Effects
associated with the cross-modal gain are illustrated analytically on a system
of two thin parallel planar waveguides. Conditions are found when a mode with
lower gain can become the dominant one at the output of amplifier.
