The refractive index of polyimide can be changed by irradiation with ultraviolet light. The amount of change in refractive index is >10-3. The Kramers–Kronig model can explain the mechanism. Utilizing this phenomenon, Bragg gratings can be fabricated in a polyimide waveguide by irradiation with an ultraviolet laser beam. It has a possibility of application in tunable wavelength filters because polyimide has a birefringent characteristic and a large thermooptical constant.