The present work suggests a new method of direct measurements of the difference propagation constants Δβ of orthogonal modes (mode beat) of the same order in planar waveguides. The work also presents a method of determining the difference of propagation constants (mode beat) for different refractive indices of the cover. The developed method is particularly adaptable when the difference of propagation constants along the direction of propagation are changed.
Kazimierz Gut, Kazimierz Gut,
"Mode beat measurement of planar optical waveguide", Proc. SPIE 5956, Integrated Optics: Theory and Applications, 59561N (30 September 2005); doi: 10.1117/12.626048; https://doi.org/10.1117/12.626048