Power transfer in fused biconical-taper single-mode fiber couplers: dependence on external refractive index

It is demonstrated that through control of the refractive index surrounding a fused biconical-taper single-mode coupler the coupling coefficient can be changed from 0% to 96%. Data are also presented that show the coupling mechanism cannot be attributed to power transfer between the modal fields guided by the fiber cores in the taper-waist region. The results support the model of the coupling mechanism as a beat phenomenon between modes guided by a glass core with a dumbbell cross section and an external refractive index.