Factors Affecting Polyimide Lightguide Quality

Abstract

Low loss lightguides were fabricated from three commercial polyimides of which one contains one, the others two hexafluoroisopropylidene (6F) groups. The latter utilizes the all para and the all meta isomer of the same diamine, respectively. As the number of 6F groups increases the optical losses of the corresponding lightguides decreases. In thick lightguides of the two 6F groups containing polyimides loss values below 0.1 dB/cm can be realized using optimized conditions. Two mechanisms — ordering with or without charge transfer complex for¬mation and voids or pinholes — are found to be responsible for optical losses. The second type of losses can be reduced by cure optimization. Where ordering is possible annealing leads to increased optical losses. Geometrical restraint of the ordering as in sufficiently thin films, however, leads to loss reduction for otherwise identical conditions. Losses observed in the bulk are always higher than in the top and bottom layers of the polyimide films.