OPTICAL TRANSMISSION LINE

PURPOSE:To uniformize the optical signal level at each branching end and each end by selecting a branching ratio of an optical branching unit closest to the end as 1/2 or below and reducing the branching ratio toward the optical branching unit near the start point. CONSTITUTION:In sending information from a start point A to branching ends B, C and an end D through optical demultiplexers 2a, 2b inserted to a branch part via an optical fiber 1 constituting an optical transmission line, if a branch ing unit branching the optical power into 1/2 each is used, the power level of the optical signal at each branching end has a large difference Thus, branch ing ratios N1, N2 of the optical branching units 3a, 3b are set so that losses LA-B, LA-C from the start point A to the ends B, C are nearly the same as the loss LA-D from the point A to the end D. For example, in case of the losses L1, L2, L3 with 2dB each, the ratio N1 of the branching unit 3a is 1/4 and the ratio N2 of the branching unit 3b is selected to be 1/3, then the optical signal level of each end is nearly arranged.