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As a special kind of Bragg grating, phase-shifted fiber Bragg grating (PS-FBG) has attracted extensive attention because of its extremely narrow transmission window and excellent sensing performance. The main purpose of this manuscript is to discuss the PS-FBG with special sensing characteristics and explore the influence of different inscription technologies on the sensing characteristics of PS-FBG by comparing the existing inscription methods. The sensing characteristics, advantages and disadvantages of PS-FBG with different structures are analyzed. [ABSTRACT FROM AUTHOR]

• Copper-coated fiber Bragg grating was developed. • Multilayer FBG with nickel, gold, copper showed high strength and sensitivity. • Multilayer FBG can be applied in extremely high-temperature environments. In this study, a copper coating was deposited on the surface of a fiber Bragg grating (FBG) via electroplating. We compared the temperature responses of the uncoated and copper-coated FBGs from 20 °C to 1000 °C. Under high-temperature conditions, copper diffused into the FBG to form copper crystals, and the copper coating gradually underwent high-temperature oxidation. This reduced the sensitivity and spectral consistency of the copper-coated FBG. A multilayer metal-coated FBG was prepared with an inner layer of nickel, outer layer of gold, and copper layer in the middle. The multilayer FBG exhibited a temperature sensitivity of 23.1–23.53 pm/°C. Furthermore, it showed the coating-enhancement effect and good linearity after multiple thermal cycles. The multilayer metal-coated FBG not only shows high-temperature sensitization but also solves the problem of bare FBGs being extremely fragile after experiencing high temperatures. This study provides a feasible solution for the application of FBGs in extremely high-temperature environments. [ABSTRACT FROM AUTHOR]

Temperature monitoring is an important task in temperature engineering to ensure the safety of equipment, where high-sensitivity and high-reliability sensors are required to provide real-time and accurate temperature information. Direct-write FBG fiber optic sensors have good temperature sensitivity and good temperature resistance, but bare FBGs are fragile. Four kinds of metal coatings were prepared on the surface of the optical fiber by chemical deposition and electroplating, and the grating was directly written by femtosecond laser. The thermal characteristics of the sensors were investigated. The metal coating sensor has higher sensitivity than that of the bare FBG sensor, because it can improve the temperature sensitivity of FBG sensor is more than twice times. The reflectance spectrum of the sensor does not change significantly from room temperature to 800 °C. An analytical model of temperature sensitivity of metal coated optical fiber is proposed and four kinds of electroplated metal FBG sensors are tested respectively. The experimental results are in good agreement with the analytical model. The results show that metal coated FBG sensors have great potential for temperature measurement or temperature compensation in temperature engineering by rationally selecting metal coating with different material properties. [ABSTRACT FROM AUTHOR]

• High temperature characteristics of overexposed and unexposed FBGs are compared. • Overexposed FBGs shown better high temperature stability than the unexposed FBGs. • Overexposed FBGs can withstand a high temperature of up to 1000 °C. • The wavelength hysteresis and spectral repeatability is improved by air cooling treatment. The high temperature properties of fiber Bragg gratings (FBGs) fabricated by overexposure using femtosecond laser phase mask technology are investigated. It is found that the overexposed FBG (FBG-II) has exhibited a temperature resistance of up to1000 °C and a reflection intensity of over 12 dB after air cooling treatment at 1000 °C for 5 h, as well as showing better high temperature stability than the unexposed gratings (FBG-I). Moreover, the temperature stability of FBG-II can be enhanced obviously through the rapid air cooling treatment after high temperature annealing, as well as the higher the air cooling rate, the more significant the effect. In addition, the wavelength hysteresis and spectral repeatability of the FBG-IIs can be significantly improved through the method. This research essentially provides a new approach to enhance the thermal properties of FBG and shows great potential in a harsh environment. [ABSTRACT FROM AUTHOR]

[Display omitted] • The eccentric multi-mode FBG can measure the vector bending and temperature simultaneously. • Rectangular structure was adopted to resolve bending direction. • The intensity difference of two opposite FBGs realized the sensitivity multiplication. • The curvature sensitivity reaches 4.42 dB/m-1 and the temperature sensitivity is greater than 13 pm/°C. • The fluctuation of intensity with temperature is weakened by multiple temperature cycles. Vector bending sensing is a key parameter in posture detection and structural health monitoring. Eccentric fiber Bragg gratings (FBGs), which can measure vector bending and temperature simultaneously, were fabricated by femtosecond laser direct writing in multi-mode fiber. The intensity of the FBG is only sensitive to curvature when the temperature is below 260 ℃. Rectangular structure was adopted to resolve bending direction. The intensity difference of the symmetrically inscribed FBG realizes the multiplication of the bending sensitivity, and the bending sensitivity is 4.42 dB/m−1. In addition, the difference between the two symmetrical peaks eliminates the power fluctuation noise caused by the fluctuation of the light source and the contamination of the fiber end. The wavelength of the FBG is only sensitive to temperature when the curvature is in the range of −2.353 m−1–2.353 m−1, and the temperature sensitivity is more than 13 pm/℃. [ABSTRACT FROM AUTHOR]

• The resonant wavelength of FBG was fine-tuned by stress annealing. • The fine-tuning of resonance wavelength depends on the applied stress. • FBG fine-tuned by stress annealing is suitable for high-temperature conditions. • Elastic-plastic transition of silica fiber is analyzed by nonlinear shift of resonant wavelength. The resonant wavelength is an important parameter of fiber Bragg grating (FBG). When FBGs are fabricated by femtosecond (fs) laser direct writing, it can be fine-tuned through the process of stress annealing. By changing applied stress, it can be permanently fine-tuned in the range of 20.76 nm relative to the original one, and the tunable sensitivity is 1.48 nm/MPa. Besides, the elastic limit strength of silica can be measured at high temperature by analyzing nonlinear shift of FBG resonance wavelength during the stress annealing process. The temperature sensing properties of FBG are measured from 20 °C to 1150 °C. The sensitivity and high-temperature stability of FBG are improved after stress annealing, which means that the fine-tuned FBG is still suitable for high-temperature sensing, high-power fiber lasers and other high-temperature conditions. [ABSTRACT FROM AUTHOR]

• A stress assisted FBG fabricated by femtosecond laser was proposed. • The FBGs shown better temperature stability and smaller wavelength hysteresis. • The FBGs with larger stress have exhibited better temperature characteristics. • The fabrication method was simple and controllable. A strain assisted fiber Bragg grating (S-FBG) has been fabricated by using femtosecond laser mask phase method. The spectral characteristics of the FBG with different pre-stress has been investigated. It was found that the wavelengths of the S-FBGs shift linearity towards shorter wavelength direction as the applied strain increases and show a sensitivity of 0.1596 pm/με and 0.8796 pm/με before and after the stress released, respectively. Moreover, the S-FBGs show better temperature characteristics than the normal FBG, including smaller decay rate of the reflection intensity, smaller wavelength hysteresis effect and better high temperature stability. Meanwhile, the temperature test has demonstrated that the S-FBG with larger pre-stress shows better temperature characteristics. In addition, the S-FBG has advantages of compact size, easy fabrication and high reflection, which is of high interest in developing high temperature sensing platforms. [ABSTRACT FROM AUTHOR]

• A novel PS-FBG was inscribed by overlapped a normal FBG and a TFBG. • The intensity and wavelength of PS-FBGs can be controlled by tilt angles. • The method has merits of simple fabrication and good repeatability. • The PS-FBG shows a high strain and temperature sensitivity. A phase-shifted fiber Bragg grating (PS-FBG) fabricated by femtosecond laser phase mask technique is demonstrated. The PS-FBG fabricated by overlapped two types of fiber Bragg gratings (FBGs), which is a normal FBG and a titled fiber Bragg grating (TFBG). They are inscribed one over the other on the same section in a single-mode fiber. We researched the effect of different tilt angles to the Bragg wavelength of TFBG. The intensity and wavelength of PS-FBGs can be changed by controlling tilt angles. The method has advantages of simple and efficient fabrication as well as high-quality spectrum. Meanwhile it has a good repeatability. Additionally, the strain and temperature sensitivities of the 0° + 3° PS-FBG are experimentally measured, which are 0.867 pm/με and 14.91 pm/℃, respectively. [ABSTRACT FROM AUTHOR]