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/cm2. 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.
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Sarkisov et al. (1997) studied this question.
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