A rubber composition comprising 0. The cross-section of the said standard fiber is smaller than the cross-section of the polarization maintaining fiber in the area of fusion of the coupler. Further, the areas opposite to the connecting lines in the sealed area may be formed into a concave shape. A mold defines a cavity, including first and second cavity sections on the respective surfaces of the substrate, in such a manner that the first and second cavities communicates with each other by means of the resin transmitting port. However, the two substrates are mounted on a common carrier so that the set-top box appears to be a single chip configuration to the user.