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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]

• A highly sensitive strain optical fiber sensor is achieved by means of side-polishing and high-temperature oxyhydrogen flame heating. • The highest strain sensitivity of the M-LPFG reaches to −19.05 pm/ με. • A compact length is obtained, which is 5 mm. • Low cost, simple structure and easy fabrication. Optical sensors for strain measurement have been significantly concerned due to their unique characteristics of quick response, anti-electromagnetic interference and compact size. In this article, a brand-new type of "M"-shaped core long-period fiber grating (M-LPFG) based on core-modulated method is proposed and experimentally testified. By means of side-polishing and high-temperature oxyhydrogen flame heating, the cladding of fiber tends to be circular and the core will bend slightly towards the polished area. The micro-bending core is suffered strong refractive index modulation and this phenomenon also excites the evanescent field adequately. By analyzing the simulated light transmission and the transmission spectrum, considerable degree of micro-bending core is achieved. In the experiment, the proposed sensor with 10 periods (5 mm) displays a high strain sensitivity of −19.05 pm/ με with range of 0–500 με, and the temperature sensitivity of which is obtained as 51.16 pm/°C. Due to relatively small influence on cladding and self-circularity of structure, the M-LPFG possesses a high mechanical strength. With the above advantages of high sensitivity, compact size, low-cost and strong mechanical strength, the method of fabricating M-LPFG has great potential in strain measurement. [ABSTRACT FROM AUTHOR]

• A new type of waved core fiber structure is proposed and experimentally demonstrated. • The highest sensitivity of the sensor reaches 125.5 pm/με within the measuring range of 0–200 με. • Simultaneous measurement for strain and temperature can be achieved. • Compact in length and high mechanical strength. Strain and temperature, as two important parameters to evaluate structural health, is increasingly paid numerous attentions for years. In this article, we developed and experimentally tested a highly sensitive strain sensor based on SMS structure with waved core multi-mode fiber (MMF), which has ability to monitor temperature simultaneously. By introducing a section of bent-core fiber into the portion of MMF, the higher-order cladding mode is excited to enhance the sensitivity. Furthermore, the portion of bent-core displays significant influence on effective refractive index (ERI) of the MMF section as strain applied, which brings about higher strain sensitivity. The experimental results display that the highest sensitivity of the sensor reaches 125.5 pm/με within 0–200 με and the measuring range achieves 0–1200 με. By demodulating two dips, simultaneous measurement for strain and temperature is achieved by solving the equation matrix. This simple method displays minor influence on mechanical property and enhances the strain sensitivity considerably. It is strongly believed the proposed sensor brings about a competitive choice in engineering fields. [ABSTRACT FROM AUTHOR]

In this paper, we developed and experimentally evaluated a new type of long-period fiber grating. This structure is fabricated by CO 2 laser polishing and subsequent heating with an oxyhydrogen flame. Firstly, a single-mode fiber is produced using a high-frequency CO 2 laser to form periodic trapezoidal structures. It is then heated with an oxyhydrogen-flame-based fiber optic tapering system to bend the core slightly toward the polished area. The micro-bent cores exhibit strong refractive index modulation, which make the core-modulated LPFG more sensitive to tensile strain. The experimental results show that the proposed sensor exhibits a strain sensitivity as high as 30.43 pm/ μ ε over the 0–100 μ ε range and a temperature sensitivity of 9.29 pm/ ∘ C. Based on the results of a general theoretical analysis and simulations, we attribute the threshold point of its strain sensitivity to the degree of micro-bending of the core. Furthermore, the structure is low cost and compact and exhibits high mechanical strength and thus can be used in a variety of applications in several fields. • A new method for fabricating LPFGs is proposed and experimentally demonstrated. • The microbent core exhibits strong refractive index modulation, which makes the core-modulated LPFG more sensitive to tensile strain. • The strain sensitivity of the fabricated CM-LPFG is as high as 30.43 pm/ μ ε for the 0–100 μ ε. • Low cost, compact size, high mechanical strength. [ABSTRACT FROM AUTHOR]