In the case of optical waveguides, there are some typical problems, like incoupling of the laser light and optical losses in the waveguide. There are great differences also in these meanings between the fibbers and planar waveguides. In the case of optical waveguide lightmode spectroscopy very thin, nm scale transparent oxide layers are used as planar waveguides. In our case the incoupling of the light is realised using a grating on the surface of the layer. On the other hand, the optical losses can be significant in thin layers, which can remarkably decrease the sensibility and remarkably increase the noise in the measurements performed on these sensors. This effect is emphasised in this case, since such waveguide sensors are primarily sensitive to the surface contaminations.

The thickness of the sensitive oxide layers is about 200nm and/or 10-20nm.The overall transparency measurements performed perpendicular to the surface are not sufficiently sensitive to characterise losses in the layer under investigation.

In this work efforts made to characterize or visualise losses of the incoupled light propagating in the plane of the waveguide layer. An experimental setup was built for such measurements. The light is incoupled via the grating and the light scattered by various centres and scattered out from the layer is studied along the layer. First results are presented in this paper.

The first observations showed already that not the losses caused by density, refractive index or similar inhomogeneities play dominating role, in agreement also with some literature data. Scattering or refractive effect of the surface and interface roughness plays decisive role in the case of such thin planar waveguides. This is important not only in the optical losses, but also in increase of noise level of the measurements. The scattered light, the position of the scattering centres, etc. are observed by a CCD camera and the corresponding software allows also some numerical evaluation.

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