Fiber tapers have been essential elements in many fiber optic devices such as beam expanders, higher-order mode strippers, fused-type directional couplers. In this work, we demonstrate a novel application of fiber taper technique to realize mode selective polished type couplers.
We constructed a fiber tapering system using a traveling burner and optimized the fabrication conditions such as gas flow rate and tapering speed to minimize the insertion loss of tapers. The insertion loss was less than 0.5% when a matched-cladding, single-mode fiber was tapered from 120㎛ down to 20㎛ in its diameter.
As the first application of fiber taper, we made fused couplers by fusing and tapering a pair of identical single mode fibers at 1.55㎛. Any coupling ratio could be achieved by controlling the pulling length. The insertion loss was less than 2%. We also observed a number of interesting phenomena with highly-tapered couplers, such as the exponential decay of the coupling period as a function of the pulling length, polarization dependent coupling strength and cutoff of the antisymmetric supermode.
As the second application, we demonstrated mode selective polished type couplers which couple the second order mode of a two mode fiber (TMF) and the fundamental mode of a single mode fiber (SMF). The matched SMF was easily prepared by tapering a given TMF and thereby changing its optical mode property. With this novel method, we could avoid the difficulty of getting a proper SMF. The optimum diameter of the tapered SMF was predicted by the optical waveguide theory and the coupled-mode theory. A mode selective coupler was made from a given TMF at 1.55㎛ with a 100㎛ diameter and its taper with 52㎛ diameter. The coupling ratio and mode extinction were 50% and -13dB, respectively. We are going to extend the novel method to realize fused type mode selective couplers in near future.
