Polarization coupling in y-cut titanium in-diffused lithium niobate planar waveguides

This paper describes polarization coupling in Ti:LiNbO₃ waveguides: a source of polarization dependent loss and potentially a source of noise in integrated-optical devices. We are developing a methodology for the fabrication and testing of waveguide lenses made by titanium in-diffusion and proton-exchange in y-cut lithium niobate. One unexpected result of this work has been the observation of polarization coupling in Ti:LiNbO₃ slab waveguides. When transverse electric field (TE) polarized light is coupled into waveguide modes propagating along the crystal´s x-axis, transverse magnetic field (TM) radiation modes radiate from the waveguide sample. Since this polarization coupling occurs for a propagation direction that is along the crystal axis, the observed TM radiation can be distinguished from the TM radiation that would result from leaky modes propagating off-axis in an anisotropic waveguide. When the polarization of the input was changed to TM, we did not observe polarization coupling into TE polarized radiation modes