Fiber-optic modal coupler using periodic microbending.

Periodic coupling between the various modes in an optical fiber is of current interest in interferometric and sensor applications. Coupling between the two cross-polarized modes of a single-mode birefringent fiber has been studied,1 as has coupling between the LP0 1 and LP 11 modes in a circularly symmetric fiber. 2 The latter case was achieved by periodically squeezing the fiber. It has been demonstrated in theory, however, that coupling between the LP01 and LP11 modes can be achieved much more efficiently by introducing microbends into the fiber instead of squeezing it.3 This Letter describes such an efficient LP01 to LP11 modal coupler that was built and tested. Microbends were introduced into the fiber, spaced by a beat length between the two modes. The optical-frequency dependence of the modal coupler is investigated. The polarization characteristics and the LP mode approximation both inside and outside the coupling region are discussed. Finally the insertion loss of the modal coupler was measured. The length of fiber over which the relative phase delay between the LP01 and LP11 modes is 2r (LB) is