Abstract

Low-loss single-mode-fiber optical couplers can be readily fabricated in fused-biconical-taper device geometry.1 The mechanism for light coupling in these couplers had been identified tentatively to be evanescent-wave coupling between modes of the two fiber cores in the taper-waist region. However, new experimental data supported by recent theoretical calculations have shown that evanescent-wave interactions contribute little to coupling action in couplers made from typical single-mode fiber strands. Coupling occurs only when light scattering is present at the glass-air interface in the taper-waist region of the device. This observation supports the studies of Bures et al.2 who calculated that the coefficient for evanescent-wave coupling in coupler structures is very small and successfully modeled the light-coupling mechanism as a beat phenomenon between two normal modes of the glass-air inter- face waveguide formed at the taper waist. An Important result of this new understanding of the coupling mechanism in fused-biconical-taper couplers is the realization that the coupling ratio can be made to depend strongly on the refractive index of the medium contiguous to the taper-waist region of the coupler, essentially without incurring the penalty of an increase in the excess loss of the coupler.

© 1984 Optical Society of America

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References

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