Abstract
A sensitivity-enhanced fiber-optic refractive index (RI) sensor based on a tapered single-mode thin-core diameter fiber is proposed and experimentally demonstrated. The sensor head is formed by splicing a section of tapered thin-core diameter fiber (TCF) between two sections of single-mode fibers (SMFs). The cladding modes are excited at the first SMF-TCF interface, and then interfere with the core mode at the second interface, thus forming an inter-modal interferometer (IMI). An abrupt taper (tens of micrometers long) made by the electric-arc-heating method is utilized, and plays an important role in improving sensing sensitivity. The whole manufacture process only involves fiber splicing and tapering, and all the fabrication process can be achieved by a commercial fiber fusion splicer. Using glycerol and water mixture solution as an example, the experimental results show that the refractive index sensitivity is measured to be 0.591 nm for 1% change of surrounding RI. The proposed sensor structure features simple structure, low cost, easy fabrication, and high sensitivity.
Highlights
Fiber-optic refractive index (RI) sensors are of considerable interest due to their small size, high sensitivity, immunity to electromagnetic interferences, and flexibility of directly embedding into the system structure
The cladding mode of long-period gratings (LPGs) is sensitive to the RI of the surrounding medium, allowing it to be employed as a RI sensor
We present an improved in-line inter-modal interferometer (IMI) for highly sensitive RI sensing, which is formed by splicing a tapered thin-core diameter fiber (TCF) between two sections of single-mode fibers (SMFs)
Summary
Fiber-optic refractive index (RI) sensors are of considerable interest due to their small size, high sensitivity, immunity to electromagnetic interferences, and flexibility of directly embedding into the system structure. FBGs always use removal of the fiber cladding to increase the sensing sensitivity. Core mismatch [10,11], fiber tapers [12], and photonic crystal fibers [13,14] have been used to form interferometers in RI sensing applications. IMIs have several favorable features, including simple fabrication, low cost, and high sensing sensitivity. We present an improved in-line IMI for highly sensitive RI sensing, which is formed by splicing a tapered thin-core diameter fiber (TCF) between two sections of single-mode fibers (SMFs). For the IMI-based RI sensor, the higher order cladding modes generally exhibit higher sensitivities. The simple sensing structure offers the advantages of low cost, simple fabrication, and high sensitivity, and may find potential applications in chemical or biological sensors
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