Abstract

The fused tapered biconical optical fiber coupler has become the key type of fiber coupler for its low loss, better thermal stability, and easiness of fabrication.¹ When appropriately designed, such a coupler can be used for wavelength-multiplexing and polarization beam splitting. Unlike the polished-type coupler in which the structure maintains weakly guiding, in the neck region of the tapered coupler the light is guided by the boundary between the (reduced) fiber cladding and the external medium which is air unless some other potting material is employed. In the theoretical modelling of couplers, the coupling coefficient, defined as half of the difference between the propagation constants of the symmetric (even) and the antisymmetric (odd) modes on the composite waveguide, is the key quantity. By integrating the coupling coefficient over the interaction region along the propagation direction, the coupling characteristics can be predicted.

© 1992 Optical Society of America