Theory of spatial mode competition in a fiber amplifier

Abstract

A theory is developed of monochromatic wave field amplification in a waveguide array based on expansion of the wave field in terms of guided array modes. The equations for the expansion coefficients include cross-modal gain, which completely changes the behavior of the amplified wave field. Analysis of the two-mode amplification reveals unusual features in its characteristics. 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 where the mode with lower gain can become the dominant one at the output of the amplifier.