A fiber optic star coupler particularly for use in providing optical interconnects between circuit boards has a light mixer into which input and output fibers are inserted. The mixer has rectangular cross-section passage, an interior surface of which is highly reflecting. The mixer is long enough to promote a degree of internal reflection of light from any one of the input fibers sufficient that the corresponding light intensity reaching the output fibers is substantially uniform. In an optical interconnect arrangement, interconnection between circuit boards in a rack or cabinet is achieved by mounting light emitting and light receiving units on each board and leading fibers to which the units are fixed to the fiber optic star coupler so that light from the emitter units enters one end of the light mixer and fibers leading to the receiver units enter the opposed end of the light mixer. The fiber optic star coupler is substantially cheaper and more easily manufactured than known fiber optic star couplers using a polished glass slab as a light mixer.