A Bragg grating on LiNbO₃ waveguide for velocity-matching of electrooptic modulators

Abstract

An experimental study of the fabrication of a surface grating in a TiO₂ film on a LiNbO₃ substrate is described. Such a grating can be used to achieve velocity-matching between an optical wave and a RF signal in a slow-wave modulator structure. A 1.07 []m period grating was realized on a single mode Ti:LiNbO₃ waveguide using a holographic process and reactive ion etching (RIE). A TiO₂ etching rate of about 100 [A]/min is achieved by RIE with a CHF₃, Ar and He gas mixture. A 1.07 []m period, 0.23[]m deep Bragg grating etched directly into the LiNbO₃ waveguide was also realized. A LiNbO₃ etching rate of about 30 [A}/min by RIE was achieved. The grating was characterized in terms of insertion loss and the diffraction angle and diffracted power of the light coupled out of the waveguide by the grating. The effect of grating depth on the diffraction performance is analyzed.