ANALYSIS OF OPTICAL DIRECTIONAL COUPLER WITH NONPARALLEL TRANSITION SECTIONS

Two optical waveguides comprising a directional coupler, approach each other gradually through curved waveguide sections from the input ends where the two are sufficiently distanced with negligible coupling. Af ter the coupling section where the two waveguides are placed in parall el, the two are separated again through curved sections to the output ends where they are sufficiently distanced with negligible coupling. I n the treatment taking into account the bends of the waveguides, the o ptical waves propagating through the two waveguides are approximated f or simplicity by a superposition of the even and odd local normal mode s. The coupling between the guided modes and the radiation wave at the bends often is neglected. In this paper, the method proposed by Sawa for the analysis of the coupled mode equation taking into account the coupling between the guided modes and the radiation modes in the waveg uide system including the curved sections is used for the analysis of the directional coupler including the curved sections. The coupling le ngth and power transfer rate are computed for the directional coupler with curved sections represented by a linear function, a cosine functi on, and a hyperbolic tangent function. The numerical results indicate that the radiation loss is the smallest and an excellent power transfe r rate can be obtained if the shape of the curved section is a hyperbo lic tangent function.