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A simple interrogation technique for refractive index measurement using multimode interference structure

A simple interrogation technique for refractive index measurement is proposed, using a multimode interference-based fiber tip structure. The fiber probe is a section of a multimode fiber, spliced to a single-mode fiber and interrogated in reflection. The interrogation technique uses two fiber Bragg gratings as discrete optical sources; by means of relative intensity variation of the reflected signals, those sources will provide a measurement of refractive index changes, while taking advantage of the MMI-based fiber tip. The read-out system uses a WDM and two photodetectors to separate both signals. A sensitivity of -5.87/RIU, in the refractive index range 1.30- 1.38, was achieved with the proposed configuration.