Control of mode-index modulation of a TIPE grating by multistep proton exchange

Abstract

TIPE gratings intended for integrated optical tapped delay lines have been fabricated in z-cut lithium niobate planar waveguides. The effective mode index modulation of TIPE gratings was enhanced by multi-step proton exchange process. In the first step, the ThLiNbO3 waveguide covered with grating mask was emerged in pure benzoic acid for 6 minutes. Next, the waveguide without grating mask was proton exchanged under the same condition for 4 minutes. This process was done several times until the desired diffraction efficiency was obtained. At the wavelength of 0.63 micron, the diffraction efficienct of a 2 micron period 1 mm X1 mm TIPE rating was about 40% after the first step proton exchange, and was increased to over 80% after the second step proton exchange of 4 minutes. The diffract on efficiency at 1.06 micron wavelength was increased from about 2% to about 80% after 20 minute planar proton exchange. Multiple step proton exchange is an easy, practical, and effective approach to control the diffraction efficiency of TIPE waveguide gratings.