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• Our paper realized a high resolution vibration fiber sensor based on the mechanism of fiber macrobending loss. • In the confirmation experiments, the measured resolutions are as high as 0.04 Hz. • In addition, on the basis of the experimental data this suggested fiber senor is enable to provide the multi-frequency vibration scenario. A new optical fiber sensor for vibration measurement has been proposed and demonstrated. This paper realizes vibration sensing based on the macrobending loss in a standard single-mode fiber loop. The experiments shows the suggested sensor enable us to measure multi-frequency vibrations with the resolution as high as 0.04 Hz. The crosstalk between temperature and vibration can be ignored because the macrobending loss is insensitive to the ambient temperature. [ABSTRACT FROM AUTHOR]

• An optical fiber strain sensor based on metal welding has been presented. • The proposed fiber sensor has the strain sensitivity of 9.11 pm/με. • The fiber sensor has the advantages of easy fabrication and low cost. • The relationship between capillary length and strain sensitivity is investigated. This paper presents a strain sensor based on Fabry-Perot (F-P) interference. The F-P strain sensor is composed of a single-mode fiber (SMF) and a silica capillary, which are encapsulated by metal welding. The relationship between the cavity length and sensitivity is analyzed theoretically, then the F-P strain sensor is calibrated and tested. The experiment results show that the strain sensitivity of the fabricated sensor reaches 4.4 pm/με when the length of the silica capillary is 12 mm and the F-P cavity length is 436 μm. In addition, the temperature sensitivity of the F-P sensor is 11.7 pm/°C. With the increase of the capillary, the strain sensitivity increases. When the capillary length is 20 mm and the cavity length is 341 μm, the sensitivity of the sensor reaches 9.11 pm/με. The sensor proposed fabricated by metal welding, has the advantages of high sensitivity, simple fabrication, simple structure, and low cost. [ABSTRACT FROM AUTHOR]

• A compact concentration sensor based on the modified thin-diameter polarization-maintaining fiber is demonstrated. • The relationship between the measurement sensitivity and the length and corroded time of the TDPMF is discussed. • The SC concentration sensor exhibits a high linear sensitivity response up to 4.65 nm/%, and the temperature sensitivity is 0.08179 nm/°C. • The MMF-MTDPMF-MMF structure sensor has excellent repeatability and stability in SC concentration sensing. Sodium citrate, as strong alkali and weak acid salt, play an essential role in the food and pharmaceutical industries. However, lack of detection method selectivity, limited biocompatibility and complexity of the manufacturing process are challenges that hinder the practical application of the sensors. In this paper, a improved-sensitivity sodium citrate concentration sensor with simple and compact interrogation setup is proposed. The sensor is fabricated by fusion splicing the sandwich structure of modified thin-diameter polarization-maintaining fiber, two pieces of multimode fiber, and single-mode fiber. In theoretical modeling, the properties of the structure are investigated. In the experiment, the evanescent field generated by the modified thin-diameter polarization-maintaining fiber plays a vital role in the detection of different sodium citrate concentrations. The sodium citrate concentration sensor exhibits a high linear sensitivity response up to 4.65 nm/%. The sensor provided refractive index sensitivity of 3173.2 nm/RIU. In addition, the response, repeatability, and stability are also evaluated by the experiments. Our results show that the sodium citrate concentration sensor holds great promise for serving as a soft and biocompatible probe for monitoring sodium citrate in situ. [ABSTRACT FROM AUTHOR]

• Distributed temperature fiber optic sensor based on OFDR using Al-coated sensing fiber is demonstrated. • Enhanced temperature sensitivity and resolution has been achieved. • Minimum measurable temperature recorded was 1 °C with a spatial resolution of 5 cm. In this paper, we present a distributed-optical fiber temperature sensor with enhanced sensitivity with Al coating using the Rayleigh backscattering (RBS) spectrum shift measured via optical frequency-domain reflectometry (OFDR). The Al-coated sensing fiber having a larger thermal expansion coefficient than that of silica produces a strain-coupled shift in the RBS under an increase in temperature. This effect leads to an enhanced temperature sensitivity of the distributed measurement scheme. In order to analyze the principle of temperature sensitivity enhancement, a simple analysis was performed by considering the fiber directional strain and force equilibrium between the optical fiber and Al-coated layer. Our results revealed that the temperature sensitivity achieved with the Al-coated fiber in OFDR was ∼56% higher relative to that of a single-mode fiber. [ABSTRACT FROM AUTHOR]

The all-fiber voltage sensor is difficult for application because of its complex structure, difficult signal demodulation, and sensitivity to the environment. In this paper, a wavelength modulated all-fiber voltage sensor with a simple structure is designed and proposed using straight tapered single-mode fiber and stacked piezoelectric crystal as a voltage sensing unit. The simulation analysis of its performance shows that it can realize the voltage measurement of 0 kV ∼ 10.41 kV, and the accuracy can reach 0.2%. Additionally, the tapered fiber has good strain sensitivity and temperature insensitivity, and its sensitivity is up to 0.03206 nm / μ ε with low bending loss. [ABSTRACT FROM AUTHOR]

• All fiber-optic sensor based on a Balloon-like interferometer for pH measurement was demonstrated. • The Mach-Zehnder interferometer was fabricated by bending standard single-mode fiber into a Balloon shape. • In addition to the high sensitivity, the proposed sensor exhibits a fast response time with excellent stability. In this paper, we present the fabrication of a new fiber-optic pH sensor based on the macro-bending loss properties in a droplet-shaped single-mode fiber (SMF) bending structure for the first time. To form the droplet-shaped probe, a segment of straight SMF is properly bent into a droplet shape using a short length of a thin silica capillary tube and without any splicing. By gradually varying the bending diameter of the droplet-shaped structure, interference results between the core and cladding modes, and a Mach–Zehnder interferometer (MZI) is effectively introduced within the bending section. Sodium hydroxide (NaOH) or hydrochloric acid (HCl) were added to the pH indicator utilizing buffers solution to achieve a wide pH range from 1 to 14. Experimental results show that the proposed sensor exhibits maximum sensitivity of 3.264 nm/pH and 5.336 nm/pH for acidic and alkaline solutions, respectively. In addition to the high sensitivity, the proposed sensor structure provides high sensitivity with a high linear response and good repeatability. [ABSTRACT FROM AUTHOR]

Optical fiber based sensors capable of measuring temperature distribution over a given length are particularly attractive in the biomedical field, especially in the case of real-time monitoring of minimally invasive hyperthermal treatments of tumors, such as laser ablation, given their intrinsic multiplexing along a single fiber span and the unique properties of optical fibers in terms of flexibility, size and electromagnetic compatibility. The paper compares two sensing approaches: one is based on an array of Bragg gratings inscribed in the core of a single-mode fiber, the other detects Rayleigh backscattering from an unmodified single-mode fiber using coherent optical frequency domain reflectometry. Following an introduction, which describes the most distinctive features of the two approaches, the paper presents three comparative experiments, each devised to highlight a peculiar aspect of the sensors. In the first experiment the sensors are exposed to various constant temperature distributions to evaluate the uniformity of the readings along the fiber length. In the second experiment, a known temperature profile is generated through an ad hoc setup to evaluate the actual capability of the two approaches to reconstruct the temperature distribution. Finally, in the third experiment the two sensors are employed in a simulated tumor laser ablation procedure to measure the temperature distribution in the irradiated area. Both sensors provide results whose error, mainly due to cross sensitivity between temperature and strain, is acceptable for the considered biomedical application. [ABSTRACT FROM AUTHOR]

• Recycling fused optical fiber to develop a miniature self-referred Fabry-Perot microcavity sensor. • The physical theory and Finite Element Method (FEM) of the Fabry-Perot microcavity were considered to fabricate an efficient sensor. • This sensor simultaneously measurements the refractive index and temperature. • This sensor has the advantages of low cost, high repeatability, and small dimension, making it a desirable sensor for chemical and biomedical applications. In this paper, a fiber-optic Fabry-Perot microcavity sensor is presented based on splicing a catastrophic fused fiber (CFF) to a single-mode fiber (SMF) for the simultaneous measurement of the refractive index (RI) and temperature. CFF formed by the catastrophic fuse effect consists of a periodic void in the fiber core that creates a teardrop shape or elliptical air microcavity after splicing. By simulation, the elliptical microcavity sensor is selected to measure external RI changes by Fast Fourier Transform (FFT) analysis. The RI sensitivity was obtained to be −2.5 ± 0.0074/RIU with a resolution of about 4 × 10−5 RIU. Also, the experimental results showed that temperature is obtained independently from the FFT parameter without being affected by the RI. The sensor temperature was determined by tracking the wavelength shift of the interference spectrum. Consequently, the temperature sensitivity of 7 pm/°C in the range of 20 °C to 95 °C was achieved. Therefore, the RI and the temperature were simultaneously measured by analyzing the spatial frequency and wavelength shift. The proposed dual-microcavity FP sensor can compensate for any power fluctuation in the input light source. [ABSTRACT FROM AUTHOR]

This paper proposes a biosensor based on side-hole fibers (SHF) formed by the structure of single-mode fiber (SMF)-coreless fiber (CLF)- SHF-CLF-SMF for the detection of bovine serum albumin (BSA). Among the fibers, the SHF's misalignment splicing makes one hole exposed, which is very sensitive to the change of the external environment. Due to the unique structure of the SHF, two Mach-Zehnder interferences (MZI) are generated in this fiber sensor, which further improves the sensitivity of the sensor. Experiments show that the refractive index (RI) sensitivity of this sensor reaches 13421.638 nm/RIU, and for the BSA concentration detection, the sensitivity reaches 2.413 nm/(mg/ml). The proposed biosensor is simple, easy to manufacture, cost-effective, and can be used for many chemical and biological analyses. [ABSTRACT FROM AUTHOR]

• A second-order PbS fiber Solc-Sagnac interferometer is proposed. • The Birefringence effect in PbS fiber is applied for temperature sensing. • The sensor is verified by theoretical simulation and experiment. • The transmission spectrum is red shifted with increasing temperature. • The temperature sensitivity can be up to 55.1 pm/°C. In this paper, a temperature sensor based on a second-order PbS fiber Solc-Sagnac interferometer is presented. The PbS fiber and the two sections of single-mode fiber (SMF) are spliced together, it is named SMF-PbS-SMF structure. The light passes through the coupler and is divided into two beams of equal power, which are propagated in the Sagnac loop and travel through the two PbS fibers. The interference occurs after re-coupling of the two polarization controllers. The theory of temperature-sensitive birefringence effect in PbS fiber is analyzed and used for temperature sensing. The experimental results show that the transmission spectrum is red-shifted with increasing temperature, and the temperature sensitivity is 55.1 pm/°C in the range of 33.3 °C–78.8 °C. The proposed sensor has a simple structure and can be widely used in industrial production, biomedicine and other fields. [ABSTRACT FROM AUTHOR]

• The development of a simple, compact, inexpensive and accurate distributed optical fiber sensor for short-range applications is proposed and implemented. • A simple, compact, stable and inexpensive solution, namely the Transmission–Reflection Analysis technique, is used. • A novel and inexpensive interrogation system based on a magnesium and erbium co-doped optical fiber is proposed. • Distributed measurements of strain were performed with the proposed interrogation system and an accuracy of up to tens of millimeters was analytically demonstrated. Transmission–Reflection Analysis (TRA) is an inexpensive technique used in distributed monitoring systems for single perturbation tracking. However, TRA is limited to long distance applications (~ km), since it has poor spatial resolution (~ m) when combined to standard silica fibers. This paper proposes the use of a magnesium and erbium co-doped optical fiber in conjunction with the TRA technique. The goal is to improve spatial resolution and accuracy for detection of loss-inducing disturbances along the sensing fiber. Bending losses are induced at several points of the fiber spaced at 50 cm over a 5.4 m fiber length, where each curve is clearly identified with a maximum hysteresis of 1.65%. The localization of a strong disturbance with an estimated accuracy of ± 0.8 mm near the source end and ± 7.2 mm near the fiber end is analytically demonstrated. Regarding weak disturbances, the proposed approach is vulnerable to noisy measurements. However, the response accuracy has been estimated up to ± 36.7 mm. [ABSTRACT FROM AUTHOR]