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Optic fiber sensors for refractive index (RI) measurement have brought about widespread attention in an extensive range owing to their unique properties. In this paper, we theoretically analyzed and experimentally verified a brand-new type of long period fiber grating with two resonant peaks. This structure is obtained by side-polishing method and melting tapered method. Firstly, a single-mode fiber (SMF) is produced into staggered V-shaped grooves on symmetrical sides by high-frequency CO2 laser. It is then heated and stretched by optic fiber melting tapered system for bending the core towards two-sides grooves. The sinusoidal core is closer to external environment and exhibits a stronger refractive index modulation, which prompts this structure more sensitive to RI. The structure possesses two resonant peaks of 1382.6 nm (dip-1) and 1473.4 nm (dip-2), which can be used to measure temperature and RI simultaneously. The experimental results present that the maximum RI sensitivity of the proposed sensor exhibits 620 nm/RIU (RI unit) in dip-2. Furthermore, compact sensing area, low cost and strong mechanical property of the proposed sensor make itself more suitable for high-accuracy RI measurement. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate an all-fiber strain sensor based on dual side V-grooved long-period fiber grating (DSV-LPFG). This fiber structure is achieved by polishing a series of asymmetrical V-shaped grooves on a single mode fiber (SMF), which is fabricated by a high-frequency CO2 laser. The asymmetrically periodic structure enhances the sensitivity of strain and shortens the total length of fiber grating. Experimental results show that the strain sensitivity of the structure reaches 44.6 pm/ $\mu \varepsilon $ in the dynamic range of 0- $1000~\mu \varepsilon $. Meanwhile, the strain sensitivity of the sensor with the different periodic numbers and the offset distance are investigated. Temperature sensitivity of 53 pm/°C is obtained, which exhibits a small temperature cross-sensitivity of $1.18~\mu \varepsilon /^{\circ }\text{C}$. Furthermore, the proposed strain sensor exhibits the advantages of simple structure, convenient fabrication and low cost. [ABSTRACT FROM AUTHOR]

We propose and experimentally investigate a spiral-polished single-multi-single-mode (SP-SMS) fiber sensor for the simultaneous measurement of strain and temperature. This special spiral shape on a multimode fiber is fabricated by a high-frequency CO2 laser, which excites higher order cladding modes in the interference process and leads to high strain sensitivity. Experimental results show that the strain sensitivity for the proposed structure in the transmission spectrum of two marked interference dips is − 17 pm/με and − 1 pm/με in the range of 0–600 με. Moreover, temperature sensitivities of 10 pm/℃ and 13.3 pm/℃ are obtained in the range of 30–150 ℃. Furthermore, the proposed sensor is inexpensive, is easy to fabricate and has a compact structure. [ABSTRACT FROM AUTHOR]