Fiber-waveguide insertion loss - a subtle, mode sensitive effect.

We discuss a subtle, heretofore unreported effect of polarization on fiber-waveguide insertion loss and we report on the first measurements of waveguide mode position as a function of polarization. We find that, in general, the TE and TM modes of a Ti:LiNbO sub 3 waveguide peak at different positions with respect to the crystal surface. This results in different optimum positions for fiber-waveguide coupling and thus to an unavoidable access loss when attaching fibers to a polarization independent device. For our standard diffusion conditions (660Angstroms Ti, 1050C, 6 hours) we predict a minimum excess loss of 0.25 dB/interface.