m-lines technique:: prism coupling measurement and discussion of accuracy for homogeneous waveguides

A method is proposed to measure the thickness of the air layer between the prism and the waveguide in a totally reflecting prism coupler. The coupling efficiency of a Gaussian beam from the prism into the waveguide can be calculated when the air-layer thickness (ALT) is known. To perform measurements of the indices and thicknesses of planar waveguides using the m-lines technique, it is necessary to have a good knowledge of the prism's characteristics and to accurately measure the angles. However, we show by means of an example that the small distance between the prism and the guide (i.e. the ALT) should be taken into account in order to achieve accurate measurements.