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 cal culated 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 t he prism's characteristics and to accurately measure the angles. However, w e 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.