Evaluating polyimides as lightguide materials

Lightguides were fabricated from three commercial polyimides of which one contains one and the others contain two hexafluoroisopropylidene (6F) groups. The latter are isomers using either the para or the meta isomer of the same diamine. 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 formation and voids or pinholes-are seen 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, however, leads to loss reduction in otherwise identical conditions. Losses observed in the bulk are always higher than in the top and bottom layers of the polyimide films.