The analysis of fused biconical single-mode fiber couplers shows that, as opposed to other types of single-mode fiber coupler, the coupling mechanism is not associated with evanescent waves and that the infinite cladding approximation is not valid. The two original claddings must be considered, after fusion and stretching, to form a single guide whose asymmetric excitation gives rise to the well-known beat phenomenon between two normal modes of the guide. For calculation purposes, this asymmetrically excited single guide is modeled into a pair of equivalent circular guides tangent to each other and having the same longitudinal profile as the real structure. The experimental results are compared with the theory.
© 1983 Optical Society of America
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