PRODUCTION OF SINGLE MODE OPTICAL TWO-BRANCHING FILTER

PURPOSE:To obtain a method having high mass-productivity by fusion splicing two multimode fibers having about 0.3% specific refractive index difference to unite them, heating an extending the fused and united fiber like a taper and fusing and connecting a multimode fiber having the same diameter as that of said composite fiber to the composite fiber. CONSTITUTION:The two multimode fibers 1, 2 having about 0.3% specific refrac tive index difference are fused and united and the composite fiber is heated and extended to form the composite fiber like a taper until the composite fiber has the same diameter part as the diameter of one multimode fiber. The multimode fiber 5 having the same diameter as that of the part 3 is fused and spliced to the end surface of the part 3 of the composite fiber 4 to form a multimode optical branching fiber 6 and the periphery of the fused part is heated and extended so that the optical branching fiber 6 has the same diameter as the core diameter of the single mode fiber. The obtained extended part is cut off between A and B to form a single mode fiber material 7 and the secondary clad layer 8 having a refractive index less than that of the primary clad is formed on the material 7 to complete a single mode optical two- branching filter 9.