Abstract

The optical branching waveguide is analyzed, using the interaction between guided and radiation modes of the main (stem) waveguide. These modes are known to be orthogonal and form a complete set. The electric field in the branching waveguide is therefore assumed to be a weighted sum of these modes and they are coupled through the perturbed boundaries (i.e., the branches). The generated radiation modes are not only a mechanism for radiation loss but also act as a transport medium for coupling between the guided modes of the main waveguide and the branches. The coupling of light between the stem and the branches can then be viewed as a two-step process: (i) the coupling between the guided mode of the stem and the radiation modes, followed by (ii) the coupling between the radiation modes and the guided modes of the branches. This analysis leads to a system of coupled-mode equations involving the amplitudes of the modes. This system of equations is then simplified and solved analytically. Data obtained from the optical measurements on branching waveguides devices fabricated by Ti-diffused into LiNbO₃ channel waveguides are also provided to confirm the result of the analysis.

© 1991 Optical Society of America