AbstractA few comprising methods of mode‐conversion type low‐loss optical branching waveguides are proposed anew, and the fundamental properties are clarified by numerical analysis based on the coupled mode theory. For the analytical model, we select a singlemode slab waveguide, and assume that the waveguide possesses mode transducers at the input and output sides, respectively. The mode transducers are designed in the best possible way for mode conversion from the input to the main part of the branch waveguide and also from that main part to the output waveguide, that is, so as to minimize the radiation loss at the output end of the branching waveguide. The waveguide thus designed has a taper at the input side, and the cross‐sectional shape varies in the direction of the transmission axis. For the analysis, first we calculate the coupling coefficients between the coupled modes of the tapered and branch sections of the three‐layered slab waveguide, respectively. Then we substitute the coefficients into the corresponding coupled mode equation whose solutions are obtained numerically by the Runge‐Kutta‐Gill method. The result of numerical simulations shows that the comprising methods proposed in this paper can reduce the mode conversion loss more remarkably than the other methods kno