FORMATION OF A GRADED-INDEX WAVE-GUIDE IN UV EXPOSED POLYIMIDE

We report on the formation of a graded-index waveguide in a photocross linkable fluorinated polyimide upon its photodarkening with the spectr al line 313 nm of a mercury are lamp at exposures exceeding 5 J/cm(2). Photodarkening prevents the UV radiation from penetration deep into t he polymer and finally sets the region with increased refractive index near the top surface. A theoretical model based on this concept recon structs the refractive index profile of the waveguide in agreement wit h the phase integral calculations based on waveguide mode spectrum. (C ) 1997 American Institute of Physics.