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• We propose a highly-sensitive phase-interrogated RI sensor based on twin-core fiber with inherent noise suppression. • It is the first time that optical fiber sensing structure is combined with digital holography technology, which works as the phase interrogation method, for measuring environmental parameters. • The background signal is reduced to 1/25 of the original noise level by using phase subtraction method within a single measurement. • Experimental results show that a short response time of about 13 s and a high sensitivity of about 835 rad/refractive index unit (RIU) are obtained. • The achieved high sensitivity is about 47 times higher than the sensitivity of a refractive index plasmonic sensor realized by enabling its quadrupole resonance in phase interrogation (17.46 rad/RIU). In this work, we propose a highly-sensitive phase-interrogated RI sensor based on twin-core fiber with inherent noise suppression. This is the first time that optical fiber sensing structure is combined with digital holography technology, which works as the phase interrogation method, for measuring environmental parameters. The proposed system consists of a Mach–Zehnder interferometer formed by interference between the reference Gauss beam and the sensing dual-beam from the twin-core fiber (TCF). In the sensing optical path, a non-core fiber (NCF) and TCF offset splicing structure works as the differential fluid RI sensing element, which provides the capability of suppressing background signal by calculating the phase difference between the two cores of the TCF. This simple and practical phase subtraction method realizes direct background subtraction within a single measurement and reduces the background signal to 1/25 of the original noise level. The fluid is continuously introduced into a fluid tube in which the sensing fiber is installed to realize in-line measurement of fluid RI. Experimental results show that a short response time of about 13 s and a high sensitivity of about 835 rad/refractive index unit (RIU) are obtained. The high sensitivity benefits from direct interaction between light path and fluid, which is about 47 times higher than the sensitivity of a refractive index plasmonic sensor realized by enabling its quadrupole resonance in phase interrogation (17.46 rad/RIU). This scheme will find many applications in little sample consumption and high sensitivity required analytical measurement owing to its merits of high sensitivity, little sample consumption, fast response, and inherent background noise suppression. [ABSTRACT FROM AUTHOR]

A novel and in-line fiber low-frequency vibration sensor based on Butterfly-Shape Mach-Zehnder Interferometer (BSMZI) is proposed and demonstrated experimentally. The sensing element is a tapered hollow-core fiber (HCF) sandwiched between two single-mode fibers (SMFs). The frequency measured by the Butterfly-Shape vibration sensor can well match with the applied frequency, and the presence of the higher harmonics can verify that the sensor does not distort the measured vibration signal. The maximum relative error of vibration frequency is 0.27%. The intensity of vibration signal (of the fiber sensor) is linearly proportional to the driving power of vibration platform. The vibration measurement sensitivity is 13.575 dB/w in the range of 0.1 w to 0.5 w under a constant vibration frequency of 50 Hz, and the linear fitting coefficient is 0.9556. The proposed sensor has a negligible temperature sensitivity in the range of 30.3 °C–69.5 °C. Because of its compact structure, easy preparation, low production cost, high frequency measurement accuracy of low-frequency vibration, and good linearity of vibration sensing, the proposed BSMZI sensor has potential application prospect in low-frequency vibration sensing field. [ABSTRACT FROM AUTHOR]

Mach–Zehnder interferometers have attracted attention due to their flexible structures and extensive applications. In this study, a simple and highly sensitive curvature sensor based on an asymmetrical Mach–Zehnder interferometer comprising an up-taper and a peanut-like section was theoretically and experimentally characterized. A simulation was conducted to investigate the influence of structural parameters on the interference spectrum. Experimental measurements show that a high curvature sensitivity of −35.3 nm/m−1from 1.35 to 2.20 m−1was obtained and high mechanical strength was demonstrated. The temperature–curvature cross-sensitivity of 4.44 × 10−4 m−1/°C was lower than the curvature resolution. The high curvature sensitivity and low temperature cross-sensitivity make this sensor a candidate for practical applications. [ABSTRACT FROM PUBLISHER]

All-fiber Mach-Zehnder interferometers (MZIs) working as curvature sensors have been attracting increasing attention. However, previously reported MZI-based curvature sensors with offset or abrupt-taper structures suffer from weak mechanical strength and low curvature sensitivity. In this letter, a robust and highly-sensitive MZI-based curvature sensor formed by cascading two up-tapers in a single mode fiber (SMF) is proposed and experimentally demonstrated. By optimizing structural parameters of the MZI both in simulation and in experiment, a high quality interference spectrum with a maximum extinction ratio (ER) of around 20 dB is obtained, which is crucial in curvature sensing with spectrum intensity variation and interference dip identifiability being considered. Experimental results show that a high curvature sensitivity of −35.41 nm/m −1 is obtained in the range of 0.6846 m −1 – 1.0606 m −1 , which is 2.4 times higher than that of the interferometer with single-mode-multimode-single-mode (SMS) structure. The two up-tapers, featured by great mechanical strength, are fabricated by only fusion pushing together a whole SMF without cleaving procedure, contributing to excellent repeatability and consistency in the sensor fabrication process. Such an all-fiber sensor exhibits advantages of simple configuration, ease of fabrication, excellent mechanical strength, low cost, and high curvature sensitivity. [ABSTRACT FROM AUTHOR]

A four-loop shaped structure of fiber Bragg grating (FBG) acoustic emission (AE) sensor based on additive manufacturing (AM) technology is proposed in the letter. The finite element analysis (FEA) method was used to model and analyze the sensor structure. We aimed at improving the sensitivity, the static load analysis, and the dynamic response analysis of the normal FBG acoustic emission sensor and the FBG AE sensor with improved structure parameters. We constructed the FBG AE sensor experimental system based on a narrowband laser demodulation method and test on real acoustic emission signals. The results demonstrated that the response sensitivity of the FBG acoustic emission sensor was 1.47 times higher than the sensitivity of the normal FBG sensor. The sensitivity coefficient of PLA-AE-FBG2 sensor was 3.057, and that of PLA-AE-FBG1 was 2.0702. Through structural design and parameter optimization, the sensitivity and stability of the FBG AE sensor are improved. The four-loop shaped sensor is more suitable for the health monitoring in fields such as aero-engine blade, micro-crack of structure, and crack growth in bonded joints. While ensuring the sensing characteristics, sensitivity, and stability of the four-loop shaped sensor have been enhanced. It is possible to apply the FBG AE sensor in some complex engineering environments. [ABSTRACT FROM AUTHOR]

• Electric field intensity of multilayer of GNPs coupled with gold film was studied. • The RI sensitivity of the double-layer gold nanospheres sensor was 15747 nm/RIU. • The RI sensitivity of the double-layer gold nanorods sensor was 25642 nm/RIU. • The LOD of double-layer gold nanospheres biosensor is 10 ng/mL. • The LOD of double-layer gold nanorods biosensor is 4.6 ng/mL. The main challenge for all biosensors is how to measure the biological samples at very low concentrations. In this paper, a surface plasmon resonance (SPR) fiber biosensor using multi-layer gold nanoparticles (Au NPs)/Au film coupling to enhance sensitivity was proposed. The coupling effect between such longitudinally aligned multilayer nanoparticles increases the intensity of the local electromagnetic field and the depth of propagation to the outside. We used the finite element method to analyze the local enhancement effect of multi-layer Au nanospheres and Au nanorods, and the fabricated double-layer Au NPs (Au nanospheres and Au nanorods)/Au film coupling enhancement sensors. These two sensors' refractive index sensitivities are 15,747 nm/RIU and 25,642 nm/RIU, respectively. By changing biomolecules immobilized on the sensor's surface, the highly sensitive double-layer Au nanoparticle/Au film fiber sensor enables high-precision measurement of different biomarkers at low concentration. For detection of human IgG, the detection limit of gold nanosphere sensor is 10 ng/mL and gold nanorod sensor is 4.6 ng/mL. [ABSTRACT FROM AUTHOR]

• InVO 4 was first applied to the sensing electrode for potentiometric ammonia sensor. • The sensor has considerable sensitivity of -86.7 mV/decade and response value of −143 mV to 320 ppm NH 3 at 600 °C. • The sensor with InVO 4 electrode exhibits good NH 3 sensitivity, selectivity and durability. Orthorhombic InVO 4 powder was synthesized by precipitation method and sintered at 900, 1000 and 1100 °C to prepare the sensing electrodes for potentiometric ammonia sensors. The influence of sintering temperature on electrode morphology and NH 3 sensing properties was investigated at 600 °C operating temperature. With the increase of sintering temperature, the layer thickness decreases and porosity increases for InVO 4 electrode. The resulting reduction of gas-phase consumption of NH 3 and the increment of three-phase boundary (TPB) enhance the NH 3 sensitivity from −79.9 mV/decade of 900 °C to −86.7 mV/decade of 1000 °C. The 1100 °C-sintered sensor has the lowest sensitivity of −25.9 mV/decade due to the TPB reduction caused by over-loosened electrode from more intense sintering. The response and recovery times decrease from 68 s and 151 s for 900 °C-sintered sensor to 19 s and 29 s for 1100 °C-sintered sample. The highest NH 3 response is obtained at 600 °C with the response values of −143 mV to 320 ppm and −13 mV to 10 ppm NH 3. The recovery rate accelerates with increase of operating temperature from 550 to 650 °C, but response rate slows down. The low cross-sensitivity to various gases especially NO x indicates good NH 3 selectivity of the sensor due to the outstanding electrocatalytic activity of InVO 4 for NH 3 oxidation electrode reaction. Considered the good NH 3 sensitivity, selectivity, repeatability, durability and resistance to oxygen/vapor interference, it is believed that InVO 4 is a promising ammonia sensing electrode material. [ABSTRACT FROM AUTHOR]

The NH 3 sensitivity of a potentiometric YSZ-based planar sensor utilizing ZnFe 2 O 4 sensing electrode (SE) was tuned by sputtering Au on the SE. ZnFe 2 O 4 was prepared by sol-gel method and calcined at 900 °C. ZnFe 2 O 4 -SE was prepared by screen printing and sintered at 1100 °C for 3 h. SEM results show that Au is uniformly distributed in the network structure of the ZnFe 2 O 4 -SE. The response characteristics of the sensor to 5–80 ppm NH 3 were evaluated between 650 °C and 750 °C, and it was found that the response rate and response signal increased significantly after gold sputtering on SE. The performance improvement by gold sputtering may originate from enhanced catalytic activity of the anodic electrode reaction, which was observed by the polarization curve test. At 700 °C, the sensor exhibited the largest sensitivity with the slope of −31.3 mV/decade, and its response to 80 ppm NH 3 was −42.3 mV, and it has relatively short response and recovery time of 25 s and 49 s for 80 ppm NH 3 , respectively. It could even displayed a considerable signal for 5 ppm NH 3 at each temperature. The oxygen concentration had non-negligible influence on the NH 3 response signal. It showed tiny cross sensitivities toward NO x and C 3 H 8 , but weak CO cross sensitivity. When the water vapor concentration is within 10%, the water vapor has a small influence on the response signal. The sensor also displayed slight signal drifts in weekly tests in six weeks, which showed that ZnFe 2 O 4 had a good long-term stability. • ZnFe 2 O 4 was used as NH 3 sensing electrode for potentiometric gas sensor for the first time. • The response signal and response/recovery rate were greatly enhanced after the ZnFe 2 O 4 -SE was tuned by sputtering Au. • The sensor displays a considerable signal for 5 ppm NH 3 between 650 °C and 750 °C. [ABSTRACT FROM AUTHOR]