Effective thickness of optical waveguides in tunable directional couplers

The power transfer between coupled thin-film optical waveguides is studied with respect to the tuning and the strength of coupling of the guides. The experimental results show that the ray-optic theory is inadequate for the calculation of the spatial beat frequency of power exchange. When the effective-thickness concept based on phase arguments is introduced in the ray theory, a better agreement with observations is obtained, but it is found that only with the effective-thickness formula derived from energy-flow considerations is the theory in satisfactory agreement with the present measurements.