Factors affecting polyimide lightguide quality

A discussion is presented of how the changes in the structure of polyimides have been used to elucidate the mechanisms leading to optical losses in planar optical waveguides made from such materials. It is shown that the introduction of the CF3 groups into the polyimide repeat unit greatly improves the optical transparency of polyimides. The optical losses drop from very high values in the case of PMDA-ODA over moderate losses in HFDA-ODA to low losses in HFDA-HFDAM. There are two origins for optical losses: Ordering processes for one can lead to refractive index fluctuations producing scattering centers. Furthermore, ordering can favor the formation of charge transfer complexes thus leading to increased absorption. Secondly the evaporation of complexed or trapped casting solvents as well as of water produced during imidization can cause voids or pinholes. Optical losses can be avoided in HFDA-HFDAM films if optimized curing procedures are followed. Annealing to 300°C increases the loss values slightly indicating that ordering processes in these polyimides are mostly suppressed.