Polarization characteristics of LiNbO3 channel waveguide directional couplers

Measurements are described on Z-LiNbO3, single-mode, channel waveguide directional couplers for the TE- and TM-mode polarizations at 0.83- and 1.3-μm wavelengths. The coupling characteristics for each polarization are strongly dependent on the mode confinement, and, hence, on the mode dispersion, and also vary differently with changes in mode dispersion. With particular waveguide fabrication parameters, the TE- and TM-mode coupling coefficients are equal for any waveguide spacing; for other fabrication conditions, the coupling coefficients are always unequal or equal at only one spacing. Theoretical calculations, based on experimentally determined diffusion parameters, follow the trend of measured values in most cases and correctly predict the effect on the coupling characteristics of changes in waveguide fabrication.