Bending characteristics of single mode-Multimode-Single mode optical fiber structures

Multimode interference has been used in many integrated and fiber-optic devices, including sensors. A basic structure proposed for sensing using this phenomenon is the Single mode-Multimode-Single mode (SMS) fiber section concatenation, where multimode interference in the multimode section leads to the formation of a self-image of the single mode excitation onto the output single mode fiber core. The perturbation of modal interference in the multimode section by a changing measurand provides the basis for sensing. This paper reports on an investigation of the bending characteristics of these structures for their possible use as bending sensors. Numerical simulations using the beam propagation method are used to explore the sensitivity of SMS structures of parabolic and step-index multimode fibers to bending. The results of experimental examination of a realized SMS fiber structure are related to its bending characteristics obtained by simulations. It is concluded that SMS fiber structures of step-index multimode sections are expected to be more suitable than graded-index fiber structures for use as bending sensors in the range of bending radii of practical interest.