Zero birefringent polyimide for polymer optical waveguide

A novel zero birefringent and photosensitive polyimide was synthesized. The polymer is soluble in aprotic solvents and contains a chalcone group for photo-crosslinking by UV exposure. The glass transition and decomposition temperature of the polymer were 254 °C and 430°C before cross-linking, respectively. Evolution of the absorption spectra upon UV exposure indicated that the cross-linking reaction is related to the cycloaddition of the double bonds in the chalcone group to form cyclobutane. The photo-crosslinking reaction not only increased the thermal stability, but also induced a refractive index change of the films. The refractive index of the film was reduced upon UV exposure from 1.5862 to 1.5697 for TE mode and from 1.5807 to 1.5697 for TM mode, respectively, resulting in zero birefringence after curing. Loss of π-conjugation in the chalcone group by the crosslinking reaction is supposed to induce the reduction of the refractive indices and orbital change from sp² to sp³ makes the polymer chain be kinked, resulting in decrease of birefringence. The polymer film before photo-crosslinking showed optical loss of 0.14 dB/cm at 1.3 μm and 0.33 dB/ cm at 1.55 μm. Zero birefringence and high thermal stability combined with photo-processibility of the material are making it an excellent candidate for the high performance waveguide materials.