Mode indices measurement of a special ti-diffused LiNbO ₃ waveguide structure: A strip waveguide array embedded in a planar waveguide

We demonstrate the feasibility of using a commercial Metricon 2010 prism coupler to characterize the mode indices (at 1.5-μm wavelength) of a special Ti-diffused LiNbO ₃ waveguide structure where a single-mode strip waveguide is embedded in a multimode planar waveguide. To achieve it, a structure of a strip waveguide array embedded in a planar waveguide was fabricated. The characterization involves simultaneously coupling light from a prism into an array of identical strip waveguides and a planar waveguide. As a result, the modes of the strip waveguide and planar waveguides simultaneously appear on the light reflection pattern measured in the case where the strip waveguide axis is parallel to the prism axis. The mode belonging to the strip waveguide is assigned by comparing the pattern with that measured either from the clear planar waveguide area or also from the strip waveguide array area but in the case where the strip waveguide axis is perpendicular to the prism axis. The direct observation for the near-field mode pattern of the strip waveguide and the numerical results of both the strip and planar waveguides provide evidences for the mode assignment.