Direct Measurement of the Guided Modes in LiNbO_3 Waveguides

Mode profiles of the light coming out of Ti diffused waveguides formed in LiNbO3 crystals was imaged using a near field optical microscope (NSOM). We find that the transverse magnetic mode has a smaller spatial dimension and is closer to the surface of the waveguide than the transverse electric mode. By incorporating the NSOM in an interferometer, we measure directly the phase and intensity distribution of the guided optical modes. The effective index (neff) is determined from the spacings of phase fronts, which equals l/neff . Experimental results of mode profiles and neff are in good agreement with theoretical simulations. However, an unexpected non-uniform intensity distribution was observed, which could be linked to materials defects caused by the Ti diffusion.