Mid-infrared refractive index sensor with high sensitivity and wide detection range based on multimode interference in tellurite optical fibers

Abstract

In this paper, a section of tellurite no-core optical fiber (NCF) is compactly sandwiched between two segments of tellurite single-mode optical fibers (SMF) to form multimode interference (MMI) sensor for refractive index (RI) measurement. Theoretical investigation reveals that the sensitivity of this MMI-based RI sensor can be enhanced by reducing the diameter of the NCF. As the NCF diameter is lowered to 20 µm, the sensor presents good RI sensing performance in the mid-infrared range of 2–5 µm and the sensitivity rises with the increase of the wavelength. At 4–5 µm and within the detectable RI range of 1.36–1.63, the maximum sensitivity is 3069.1 nm/RIU. The proposed sensor combines the material superiority of tellurite glass and the structural advantage of the SMF-NCF-SMF sensing unit, exhibiting high sensitivity and wide detection range in terms of RI sensing. It provides a new method for detecting the concentration of toxic organic compounds with high RI.