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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]

A highly sensitive strain sensor based on a helical-core long-period fiber grating (HC-LPFG) is proposed and experimentally investigated. The helical core is fabricated in the common single-mode fiber by using a high-frequency CO 2 laser and hydrogen-oxygen flame. This helical shape core of the structure experienced the highly centralized refractive index modulation, which enhances the strain sensitivity and shortens the length of the sensing area to 2 mm. Experimental results indicate that the maximum strain sensitivity of the HC-LPFG reaches -97 pm/µɛ within the measuring range of 0-400 µɛ.

In this paper, a novel refractometer based on Mach-Zehnder Interferometer (MZI) is proposed and experimentally investigated. The MZI is composed of 2 micro bending cores (MBCs), one of which excites the cladding modes and the other couples the modes back. This structure is formed by high-frequency CO 2 laser polishing and oxyhydrogen flame heating. With the unique deformation method, the interaction between the fiber core and the external status gets enhanced, moreover, higher modes in the cladding are excited, which leads to a high refractive index (RI) sensitivity. Due to the high temperature of the oxyhydrogen flame, the core of CO 2 polished fiber is modulated, furthermore, the cladding shape of MBC tends to be circular. Hence, relatively small modulating regions of 500 μm can form for interference. In the experiment, 2 transmission dips are chosen for RI measuring, which possesses the wavelength of 1530.4 nm and 1600.8 nm, respectively. The RI sensitivities of the 2 transmission dips are -271.7 nm/RIU and -333.8 nm/RIU with the RI range of 1.33-1.42. The temperature characteristic is also experimentally analyzed and the temperature sensitivities of which are 0.121 nm/℃ and 0.171 nm/℃ in the range of 34℃-154℃. By solving the matrix equation, the proposed sensor can be applied for simultaneous measurement of RI and temperature.