Optical branch device and optical coupler module

An optical branch device, comprising:

• a waveguide in a basic portion;
• a branch portion having a first branch waveguide extended linearly and continuously from the waveguide in the basic portion, and second and third branch waveguides which are branched from the basic waveguide in such a manner that the first branch waveguide is interposed therebetween and which are each extended in the directions farther from the first branch waveguide;
• a parallel portion composed of the first, second and third branch waveguides extended continuously from the branch portion, in which the second and third branch waveguides are positioned in parallel to the first branch waveguide;
• and
• a divergent portion composed of the first, second and third branch waveguides extended continuously from the parallel portion, in which the second and third branch waveguides are extended gradually farther from the first branch waveguide;
• and
• wherein the first, said second and third branch waveguides are each composed of a core having a relatively high refractive index and a clad arranged around the core and having a relatively low refractive index, and
• the interval pitch P (μm), in the transverse direction, between the first, second and third branch waveguides in the parallel portion and the refractive index difference (Δn) between the core and the clad are set in such a manner that C satisfies a relationship of C=0.5±0.2 wherein C is calculated by substituting P and Δn for an equation of C=Δn×P2.