OPTICAL FIBER TRANSMISSION LINE

PURPOSE:To reduce the quantity of light incident to an optical fiber cable for a light emission part during demultiplexing and to reduce transmission loss by branching an optical fiber cable for a light emission part which has a small- diameter core part from the vicinity of both end parts of an optical fiber cable for a photodetection part. CONSTITUTION:Both end parts of the optical fiber cable 1 are made into directional coupling parts 3a and 3b, optical fiber cables 4a and 4b are branched, and a core part 5 has a less diameter than the core part 2 of the optical fiber 1. Light from a light emitting element 7a enters the light-emission part optical fiber 4a of the directional coupling part 3a, and light entering the photodetection part optical fiber 1b of the directional coupling part 3b through the optical fiber 1 reaches a photodetecting element 6b. The core parts 5 of the optical fibers 4a and 4b for the light emission parts have diameters smaller than those of the core parts 2 of optical fibers 1a and 1b for photodetection parts, so the quantity of light distributed from the optical fiber 1 to the optical fibers 1a and 1b is reduced extremely.