Irradiation effect on strain sensitivity coefficient of strain sensing fiber Bragg gratings

The effect of irradiation on the strain sensitivity coefficient of strain sensing fiber Bragg gratings(FBGs) has been investigated through experiments. FBGs were fabricated in single mode fibers with 3 mol% Ge-concentration in the core and with a H2-loading treatment. In experiments, the FBGs were subjected to γ-radiation exposures using a Co60 source at a dose-rate of 25 Gy/min up to a total dose of 10.5 kGy. The GeO defect in fiber absorbs photons to form a GeE’ defect; the interaction with H2 is a probable reason for the γ-radiation sensitivity of gratings written in hydrogen loaded fibres. The effect mechanism of radiation on the strain sensitivity coefficient is similar to that of radiation on the temperature sensitivity coefficient. Radiation affects the effective index neff, which results in the change of the thermo-optic coefficient and the strain-optic coefficient. Irradiation can change the strain sensitivity coefficient of FBGs by 1.48%–2.71%, as well as changing the Bragg wavelength shift(BWS) by 22 pm–25 pm under a total dose of 10.5 kGy. Our research demonstrates that the effect of irradiation on the strain sensitivity coefficient of FBG is small and that strain sensing FBGs can work well in the radiation environment.     

Customized MCVD fabrication of different application-type silica optical fibers and their FBG inscription characteristics

We report here the experimental realization of Fiber Bragg Gratings (FBG) of desired specifications in different application-types of in-house fabricated silica optical fibers, namely, standard telecom fiber of type SM-28, Er³⁺-doped SiO₂–GeO₂ fiber and photosensitive high GeO₂ fiber. Fibers for this purpose were fabricated by customizing the process variables of the Modified Chemical Vapor Deposition (MCVD) technique used for our laboratory fabrication of optical fibers. To check the performance of the drawn fibers in terms of FBG characteristics, gratings were written by exposing a short length of fibers to interference fringes derived from a frequency-doubled Ar-ion laser using a modified two-beam interferometer system in single phase-mask configuration. Results of inscribing selected wavelength FBGs demonstrate the efficacy of our approach in achieving, reproducibly, high-quality FBGs in all application-fibers.     

Combined time-wavelength interrogation of fiber-Bragg gratings based on an optical time-domain reflectometry

A reflectometric method for the combined time-wavelength multiplexing of the fiber-Bragg-grating (FBG) signals is proposed. The method is based on the spectral filtering of the probe pulses generated by a fiber-optic reflectometer using a bandpass filter consisting of a fiber circulator and FBG. The interrogated Bragg gratings are recorded on a fiber line in groups with identical resonance wavelengths inside groups and different wavelengths from different groups. The separation of the signals of FBGs that have different resonance wavelengths is due to the tuning of the filter passband, and the separation of the signals of FBGs with identical wavelengths involves the time separation of the responses of the Bragg gratings to the probe pulse. The threshold sensitivity of the method in the measurement of the relative elongation of FBG is 0.5 × 10^?4. The considerable practical prospects of the method are related to its simplicity, reliability, and the application of the conventional reflectometric equipment.     

Sensing properties of fiber Bragg gratings in small-core Ge-doped photonic crystal fibers

We report about fiber Bragg gratings (FBGs) inscribed in two different types of small-core Ge-doped photonic crystal fibers with a UV laser. Sensing properties of the FBGs were systematically investigated by means of demonstrating the responses of Bragg wavelengths to temperature, strain, bending, and transverse-loading. The Bragg wavelength of the FBGs shifts toward longer wavelengths with increasing temperature, tensile strain, and transverse-loading. Moreover, the bending and transverse-loading properties of the FBGs are sensitive to the fiber orientations. The reasonable analyses for these sensing properties also are presented.     

大模场啁啾光纤光栅的光谱特性

 将一种矩阵算法拓展应用于数值求解多模耦合模微分方程, 并使用该矩阵算法对大模场啁啾光纤光栅的光谱特性进行了理论研究。结果表明, 大模场多模光纤光栅因模式的自耦合和互耦合而使反射谱存在多个反射峰, 这与单模光纤光栅的反射谱不同。由于光栅周期存在啁啾, 大模场光纤光栅的反射峰分裂, 且峰值反射率减小。使用高斯切趾函数可使反射峰分裂在一定程度上得到改善。     

Application of optical time-domain reflectometry for the interrogation of fiber Bragg sensors

A reflectometric approach is proposed for the polling and multiplexing of sensitive elements on fiber Bragg gratings (FBGs). The method is based on the power measurement of the radiation reflected by the FBG using a conventional fiber-optic time-domain reflectometer. The multiplexing of sensors is based on the time separation of signals. Requirements on the Bragg diffraction gratings that provide the linear dependence of the received signal on the FBG mechanical stress and temperature are determined. In the measurements of the FBG relative elongation and temperature, the threshold sensitivities are 0.8 × 10^?4 (80 μstrain) and 5°C, respectively. Due to its simplicity and efficiency, the reflectometric method of FBG polling and multiplexing can be used to solve various measurement problems, in particular, the safety monitoring of the stressed-strained elements in building structures.     

荧光光纤光栅传感特性的实验研究

在载氢掺铒光纤上写入Bragg光纤光栅，得到新型光子学器件-荧光光纤光栅.分别对其Bragg波长（λB）及荧光寿命（τ）的温度（T）及应变（ε）响应特性进行了实验研究，并且给出了λB和τ分别关于（T，ε）的拟合方程.实验结果表明：荧光光纤光栅的λB对T和ε的响应具备一般Bragg光纤光栅的优良特性，测得温度灵敏度为11.1pm /℃，应变灵敏度为1.19pm/με；而且τ对T和ε的响应也具有良好的线性关系，温度灵敏度为0.59 μs/℃，应变灵敏度为6.16 ns/με.实验结论为解决温度应力交叉敏感、实现温度应力的同时监测提供一条新颖的途径.     

Suspended-core fiber Bragg grating sensor for directional-dependent transverse stress monitoring

This Letter presents simulation and experimental results of orientation-dependent transverse stress fiber sensors using fiber Bragg gratings (FBGs) inscribed in four-hole suspended-core fibers. Resonant peak shifts and splitting of FBGs were studied as functions of the applied transverse load and fiber orientation. Both simulation and experimental results revealed that the response of FBGs in suspended-core fibers is sensitive to both the orientation and magnitude of an applied transverse stress.     

用紫外光直接在光纤内写入布拉格反射光栅的实验研究

用紫外光直接在光纤内写入布拉格反射光栅的实验研究杜卫冲谭华耀＊刘颂豪华南师范大学，激光生命科学研究所，广州５１０６３１＊香港理工大学，电机系自１９８９年Ｍｅｌｔａｚ［１］首次提出采用两束波长位于掺Ｇｅ－石英光纤吸收带内的相干紫外激光相干涉，从侧面照射...     

High sensitive and large dynamic range quasi-distributed sensing system based on slow-light π-phase-shifted fiber Bragg gratings

In this paper, we theoretically analyze the slow-light π-phase-shifted fiber Bragg grating (π-FBG) and its applications for single and multipoint/quasi-distributed sensing. Coupled-mode theory (CMT) and transfer matrix method (TMM) are used to establish the numerical modeling of slow-light π-FBG. The impact of slow-light FBG parameters, such as grating length (L), index change (Δn), and loss coefficient (α) on the spectral properties of π-FBG along with strain and thermal sensitivities are presented. Simulation results show that for the optimum grating parameters L = 50 mm, Δn = 1.5×10⁻⁴, and α = 0.10 m^-1, the proposed slow-light π-FBG is characterized with a peak transmissivity of 0.424, the maximum delay of 31.95 ns, strain sensitivity of 8.380 με^-1, and temperature sensitivity of 91.064 °C^-1. The strain and temperature sensitivity of proposed slow-light π-FBG is the highest as compared to the slow-light sensitivity of apodized FBGs reported in the literature. The proposed grating have the overall full-width at half maximum (FWHM) of 0.2245 nm, and the FWHM of the Bragg wavelength peak transmissivity is of 0.0798 pm. The optimized slow-light π-FBG is used for quasi-distributed sensing applications. For the five-stage strain quasi-distributed sensing network, a high strain dynamic range of value 1469 με is obtained for sensors wavelength spacing as small as 2 nm. In the case of temperature of quasi-distributed sensing network, the obtained dynamic range is of 133 °C. For measurement system with a sufficiently wide spectral range, the π-FBGs wavelength grid can be broadened which results in substantial increase of dynamic range of the system.     

Pre-irradiation-induced graft reaction of maleic anhydride onto polypropylene

The radiation induced graft polymerization is a well-known method to obtain new materials. Until recently, only conventional radiation sources, such as Co-60 and electron beams, were used. Moreover, part of the damage induced in polymers by heavy ions can produce active sites (peroxides and hydroperoxides) that are useful to initiate grafting reactions. Maleic anhydride (MAH) was grafted onto polypropylene (PP) wax with a number-average molecular weight (Mn) of 8000 by gamma pre-irradiation technique. Effects of total dose, monomer concentration, reaction time, and temperature on percentage of grafting are studied in detail. It is shown that the optimum conditions for grafting are temperature of 70 ^°C and total dose of 14.4 kGy. PP-g-MAH is characterized by infrared spectrum. Differential scanning calorimetry shows that the compatibility of PP-g-MAH is better than that of PP.     

High-temperature-resistant chemical composition Bragg gratings in Er3+-doped optical fiber

Chemical composition gratings (CCGs), unlike standard fiber Bragg gratings (FBGs), do not suffer a significant decrease in reflectance or an irreversible wavelength shift when they are exposed to elevated temperatures. To date, the growth of CCGs has been related to the fluorine content of the fibers in which they are written. It is shown that FBGs with high thermal stability, resembling CCGs, can be fabricated in Er3+-doped optical fibers that do not contain any fluorine.     

Spectral-mismatch-induced resolution limit of interferometric fiber Fabry-Perot sensor system

The mismatch of fiber Bragg gratings (FBGs) in spectral profiles can lead to a severe degraded resolution of the constructed fiber Fabry-Perot (FFP) sensor system through its effect on the fringe visibility. The variation of visibility induced by spectral mismatch and the corresponding phase resolution limit are analyzed theoretically and experimentally. Theoretical analyses are based on the approximation of Gaussian profiles to the reflection spectra of weak FBGs, especially with consideration of side lobes. The investigation provides an insight into the evolution of the fringe visibility caused by spectral mismatch, and shows good agreement with experimental results. An optimum phase resolution of about 55 μrad/Hz^1/2 above 100 Hz is achieved for a nearly 4 m-long FFP sensor by matching spectral profiles of the gratings and balancing path length differences of the tandem interferometers.     

Influence of γ-ray radiation grafting on interfacial properties of aramid fibers and epoxy resin composites

This research used Co⁶⁰ γ-ray radiation to modify Armos fibers in 1,2-epoxy-3-chloropropane. After the treatment, the interlaminar shear strength (ILSS) values of aramid/epoxy composites were improved by about 20%. Surface elements of Armos fibers were determined by XPS analysis, which indicated that the oxygen/carbon ratio was increased. The surface of the fibers treated was rougher than that of the untreated fibers when examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Fourier transform infrared (FT-IR) spectra confirmed that the epoxy group was grafted onto the fibers. The wettability of the fibers' surface was also enhanced by the treatment. Nanoindentation technique analysis showed that the nanohardnesses of the various phases (the fiber, the interface and the matrix) in the composite, whose fibers were treated, were correspondingly higher than those in the composite, whose fibers were untreated. The results indicate that γ-ray irradiation grafting technique, which is a suitable batch process for industrialization, can modify the physicochemical properties of Armos fibers and improve the interfacial adhesion of its composite.     

FBG温度传感器探头的封装设计方案

提出了一种基于FBG(布拉格光纤光栅)温度传感器封装的新设计方案。一个密闭的容器被活塞分成两部分,一部分充满水银,另一部分抽成真空,两个完全一样的布拉格光纤光栅分别安装在活塞两边。当温度发生变化时,通过水银的热胀冷缩推动活塞移动,把温度的变化通过水银热胀冷缩转换成对光纤光栅的拉力和压力。而光纤光栅和金属外壳本身的热胀冷缩对两段布拉格光栅的影响是相同的,基本上可以相互抵消,因此,消除了温度和应力交叉敏感的问题。另外,理论分析表明该布拉格光纤光栅传感头的温度灵敏度可以达到2.83×10-4/℃。