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T-shaped network configurations cannot be used for optical data bus systems intended for passive communication between more than about ten terminals because of balanced power distribution and the severe requirements that would have to be met with respect to the dynamic range of the detectors. Star network configurations, however, allow passive communication between several hundreds of terminals using state-of-the-art fibers, transmitters, and detectors. The planar mixers used in such star couplers are investigated from both theoretical and experimental aspects with regard to their influence on the uniformity of optical power distribution and insertion losses. Measurements on a star coupler with twelve fibers of 200-microm core diam and 15-microm cladding thickness showed an optical power variation of ~+/-12% along the entire width of the mixer accompanied by an insertion loss of ~3dB.