Spatial modes and spectral narrowing of the amplified spontaneous emission in dye amplifiers

The propagation of amplified spontaneous emission (ASE) in a planar cell having a width large with respect to the absorption length of the optical pump wavelength in material is investigated. Both the spectral narrowing and the spatial behaviour of the ASE are considered. A model for the spatial inhomogeneity of the index of refraction and the gain across the cell is developed and the results are compared with experiment. It is shown that refraction-index step changes of the order of 5.0 × 10−5 can be produced by thermalization processes within the dye molecules. This, in turn, provides a guiding structure such that low loss field modes can propagate in the cell.