Optical gas detection with polyimide lightguides

Thin films of polyimide are prepared by spin coating on glass substrates. They are used and characterized as planar optical lightguides. Index matched guides for the TE₂ and the TM₀ modes were prepared. The optical anisotropy of the polyimide lightguide is influenced by the surrounding gas atmosphere. Pure and mixed atmospheres containing H₂O, C_nH_2n+2 for n=1,2,3 and the alkanes heptane, iso-octane and n-octane were prepared and measured with the optical waveguide. The dynamic measurement of the waveguide anisotropy as a function of time yields 2 results, the saturation value which is a good measure for the partial gas concentration and the relaxation, which is a measure for the type of molecule. The pore size of the polyimide material may also be used to distinguish between special molecules of different size