Coupling of Modes in Bent Biconically Tapered Single-Mode Fibers

Abstract

Biconically tapered single-mode fibers are fabri- cated by heating a single-mode fiber while applying tension. As a result of this tapering, cladding modes are excited in the tapered region of the fiber where the V parameter of the fiber goes down below 1.0. These cladding modes couple to one another leading to fluctuations of the optical throughput. Since the index difference between the cladding and the external medium is rather high (~ 0.5), the cladding modes will be of the exact type, namely, TE, TM, HE, and EH modes. The coupling of these modes in the tapered region is analyzed using the exact mode formalism when the tapered region is bent. The theoretical results agree very well with the experimental results obtained on bent tapers which show strong fluctuations of the optical power as a function of the bending angle. Bent tapered fibers have been shown to be useful in a number of applications involving fiber-optic sensors. © 1991 IEEE