Multimode optical fiber sensors based on self-imaging

Multimode Interference (MMI) has been extensively investigated and utilized in integrated optical devices for both the communication and sensor industries. It has later been explored in optical fibers to realize fiber devices, including sensors. This paper reports on the investigation of simple optical fiber sensors based on the MMI phenomenon of self-imaging with possible use as bending and intrusion detectors. The sensors are composed of Single mode ´ Multimode ´ Single mode (SMS) fiber section concatenations. MMI leads to the formation of a self-image of the single mode fiber excitation onto the output single mode fiber through the multimode fiber, where modal interference is sensitive to disturbances applied to that section. This structure is investigated and explored for the realization of new fiber sensors. The transmission spectra of two proposed devices under different conditions are measured and the operation of a sensor developed for intrusion detection is demonstrated using a laser source of 1.55μm wavelength. Self-imaging in symmetrically excited optical fibers is analytically studied, revealing the effect of refractive index grading on the characteristics of SMS fiber devices. Experimental investigations of the proposed sensors are then described and a discussion of the results obtained and possible applications of the devices is presented.