Formation of a graded-index waveguide in UV exposed polyimide

We report on the formation of a graded-index waveguide in a photocrosslinkable fluorinated polyimide upon its photodarkening with the spectral line 313 nm of a mercury arc lamp at exposures exceeding 5 J/cm<SUP>2</SUP>. Photodarkening prevents the UV radiation from penetration deep into the polymer and finally sets the region with increased refractive index near the top surface. A theoretical model based on this concept reconstructs the refractive index profile of the waveguide in agreement with the phase integral calculations based on waveguide mode spectrum