Abstract
In strain measurement applications, the matched fiber Bragg gratings (FBG) method is generally used to reduce temperature dependence effects. The FBG parameters have to be designed to meet the requirements by the particular application. The bandwidth and slope of the FBG has to be balanced well, according to the measurement range, accuracy and sensitivity. A sensitivity enhanced strain demodulation method without sacrificing the measurement range for FBG sensing systems is proposed and demonstrated utilizing a pair of reference FBGs. One of the reference FBGs and the sensing FBG have almost the same Bragg wavelength, while the other reference FBGs has a Bragg wavelength offset relative to the sensing FBG. Reflected optical signals from the sensing FBG pass through two reference FBGs, and subtract from each other after the detection. Doubled strain measurement sensitivity is obtained by static rail load experiments compared to the general matched grating approach, and further verified in dynamic load experiments. Experimental results indicate that such a method could be used for real-time rail strain monitoring applications.
Highlights
Optical fiber sensors (OFSs) have advantages of small dimensions, high sensitivity, and immunity against electromagnetic interference (EMI) [1,2,3]
A variety of OFS have been developed for strain and temperature measurements, including the distributed techniques based on the Brillouin, Raman or Rayleigh scattering methods and the discrete fiber sensors based on fiber Bragg gratings (FBG) [1]
The sensing FBG3 is bonded to the bottom of the rail, and two reference FBGs are placed near the rail without external stress
Summary
Optical fiber sensors (OFSs) have advantages of small dimensions, high sensitivity, and immunity against electromagnetic interference (EMI) [1,2,3]. The simplest detection scheme for the FBG sensor is the reflection matched grating method, which uses the reflection and transmission power spectra of two FBGs [1,15] This method has the advantages of fast measurement and low cost, but it is mainly used for the applications where precision is not of concern, such as axes counting in railways [1] or the tunnel transport system. Reflected optical signals from the sensing FBG pass through the two reference FBGs individually and are detected through simple power measurements Using such scheme, small strain values or variations can be detected by subtracting the two signals from the reference gratings. The DP-FBG demodulation hardware and software are further verified utilizing static and dynamic rail load experiments
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