As the increase of environment and conservation consciousness in recent years, green-lighting concept begins attracting much attention of the people in our country; particularly, the exploration of daylight lighting system is an obvious example. However, in this nature-light lighting system, versatile optical couplers are required to guide and dissipate sunlight into different indoor spaces to produce assistant illumination, and their coupling efficiencies directly affect the whole efficiency of the lighting system. Thus, they play an important role in the daylight lighting system. To obtain high efficient optical couplers, this thesis, other than investigating various used Y-branch couplers, proposes another new type of coupler, which has a positive- or negative-arc Y-branch structure to split a light-beam into two beams or to combine two light-beams into one beam with high efficient light output. From optical simulation results, it can be seen that the output efficiency of this symmetrical Y-branch coupler with positive or negative arc can reach above 90%, no matter it is used as a combiner or splitter. Furthermore, this thesis also goes through an investigation of the Y-branch output field distribution and an improved arc design; the coupling efficiency between the coupler and an externally connected optical fiber can be promoted.