Abstract
A model of geometrically dependent gain coefficient is applied to small-sized media to explain the amplified spontaneous emission intensity and bandwidth reduction along the light propagation direction. The model is used for two typical experimental examples of photo-pumped pyrromethene 567-doped polymer and BuEH-PPV conjugated polymer waveguides, and excellent consistency with the measurements is obtained. The calculations are performed both analytically and numerically, where the effect of saturation on the gain coefficient for the homogeneously broadened line-shape upon using a 4-level system is considered.