Multimode fiber-optics coupler with low insertion loss.

A fiber-optics coupler is described which is simple to fabricate, is directional, leaves the fiber intact, and has an insertion loss of <0.05 dB. The coupler utilizes the energy contained in the leaky modes of the fiber. The coupling ratio may be varied by changing the type of light source and input conditions. Possible applications are to optical data bus links of up to a few hundred meters or as power or mode monitors.