Reflection-type optical fiber refractive-index sensor using a multimode interference structure with high sensitivity

A high-sensitivity reflection-type refractive-index (RI) sensor using a multimode interference structure has been fabricated by forming a gold mirror at the end of a sensor fiber. The sensor part is composed of single-mode fibers (SMFs) attached to a multimode fiber (MMF) with an end-face mirror. This structure has the same function as an MMF sandwiched by SMFs, which is called a multimode interference (MMI) structure. The fabricated sensor shows a sensitivity as high as 6.8×10%6 (at 1 pm wavelength resolution), which is estimated from the relationship between the change in RI and the shift in wavelength in a wide RI range, and 1.0×10%5 (at 0.01 dB power resolution), which is estimated from the relationship between the change in RI and the variation of interference power at a fixed wavelength, despite the sensor having only half the length of a conventional two-port multimode interference RI sensor.