Application of planar monomode waveguides to determine parameters of thin active layers used in waveguide sensors

The presence of a thin cover layer on the planar waveguide influences its propagation characteristics. This effect has been taken advantage of in the m-line spectroscopy of the four-layer structure for evaluation of a thickness and refractive index of a thin organic cover layer used as a sensing medium for fiber optic and/or planar sensors. Because due to the small thickness the organic layers do not guide light by themselves, the generalised m-line spectroscopy is a simple and convenient method alternative to conventional measurement of thin layers. The effect of the cover layer is pronounced for low-mode waveguides, especially for single-mode waveguides. But for the latter one can neither solve the dispersion equation nor evaluate the thickness and the refractive index knowing merely a single coupling angle of a structure. An additional measurement of the coupling angle for a beam of different wavelength yields the second equation and assuming that the difference between the waveguide dispersion and substrate dispersion is negligible, results in evaluation of the thickness and refractive index of the layer.