DIRECT WRITING OF GRATINGS BY ELECTRON-BEAM IN POLY(METHYL METHACRYLATE) OPTICAL-WAVEGUIDES

Changes in refractive index and in film thickness caused by electron beam irradiation are studied in poly (metyl methacrylate), PMMA. The index at 0.633 µm increases by 8% and the thickness decreases by 12% after 10-3 C/cm2 irradiation. Utilizing these phenomena, gratings with periods of 0.8 and 2 µm are fabricated in PMMA waveguides as Bragg diffraction beam splitters. The effective index change of a guided mode due to the electron beam irradiation is analyzed. An expression for the diffraction efficiency is derived by the coupled wave theory. The dependence of the diffraction efficiency on the grating length is measured, and fairly good agreement is obtained between the experiment and the theory. © 1979 The Japan Society of Applied Physics.