S. Monneret et al., m-lines technique: prism coupling measurement and discussion of accuracy for homogeneous waveguides, J OPT A-P A, 2(3), 2000, pp. 188-195
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.