Self-imaging in single mode-multimode-single mode optical fiber sensors

Multimode interference in optical waveguides has interesting self-imaging properties, which have extensively been investigated and utilized in many integrated optical devices. Although these investigations started with most interest in step-index integrated waveguides, they have later included graded-index waveguides, where the dependence of the interference images on the refractive index grading of the waveguides was observed and utilized in the design and optimization of devices. Later on, multimode interference has also been explored in optical fibers in order to realize fiber devices, including sensors. A basic structure of these devices has been 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 fiber excitation onto the output single mode fiber core. This paper reports on the investigation of the self-imaging properties of these optical fiber structures and their possible use as sensors. Self-imaging in symmetrically excited multimode optical fibers is analytically studied, revealing the effect of refractive index grading on the characteristics of SMS fiber devices. The theoretical results are verified by numerical simulations using the beam propagation method. The experimental investigation of an SMS structure proposed as a bending sensor is then described and a discussion of the results obtained and possible application of the device is presented.