Optical attenuation in planar waveguides of unidirectionally oriented copolyester film

The in‐plane optical loss was correlated with molecular orientation in a unidirectionally stretched poly(ethylene terephthalate‐co‐isophthalate) copolyester film. X‐ray scattering was applied to ascertain the anisotropic and amorphous natures of the stretched film. The film was adhered onto a silicon wafer to fabricate a planar waveguide. Multiple TE and TM propagation modes were obtained on the planar waveguide using a prism coupler at 632.8 nm yielding in‐plane refractive indices of 1.6545 and 1.5464, parallel and transverse to the stretch direction, respectively. For each waveguide mode, a propagation streak was generated and registered on photographic film. The in‐plane linear attenuation coefficient, α∥≡{ln[I(O)/I(x∥)]}/x∥, was derived from the propagation streak. The optical path of each mode was determined and used to normalize α∥. For the TE modes, α∥ increased with the mode number and was highly anisotropic, i.e., α∥ (parallel to the stretch direction)≤α∥ (transverse to the stretch direction). Ho...