Thus, researchers or engineers could potentially use FBG and LPFG sensors with the same measurement apparatus, such as the ASE light source and these optical spectrum analyzer (OSA), to monitor changes of strain, temperature, and liquid-level over time.2.?Fiber Grating SensorsWe used fiber grating sensors, FBG and LPFG sensors, as FOSs for the NDE of smart infrastructure materials. Figure 1(a) shows a schematic of the experimental setup of a reference dual-wavelength grating FBG sensing system. The FBG sensors were used to examine the temperature and strain responses of infrastructure materials. The fiber sensor was consisted of a bare grating pair (��1 and ��2) and a packaged reference grating (��3). The bare grating pair was constructed by fusion splicing two fiber Bragg gratings in cascade with different Bragg wavelengths.
The three Inhibitors,Modulators,Libraries fiber Bragg gratings at wavelengths of ��1, ��2, ��3 were interrogated using a broadband ASE source and an OSA. A fiber Inhibitors,Modulators,Libraries coupler was used for coupling the reflected light signals of the sensor to the Inhibitors,Modulators,Libraries OSA. The reference grating was used to measure only the temperature effect. The change in the Bragg center wavelengths ����i of the two gratings from the changes in temperature (��Ti) and strain (����i) could be obtained using Equation (1).Figure 1.Schematic of experimental setup of (a) a reference dual-wavelength grating FBG sensing system for temperature and strain measurements; (b) an LPFG sensing system either for temperature or liquid-level measurements.Figure 1(b) displays the schematic of experimental setup of an LPFG sensing system.
The LPFG sensors were used to examine Inhibitors,Modulators,Libraries the temperature and liquid-level responses of infrastructure materials. The LPFG sensors were used as either a temperature sensor (wavelength of ��4) or a liquid-level sensor with five LPFGs in cascade with different wavelengths (wavelengths of ��5, ��6, ��7, ��8, and ��9, about 20-nm wavelength interval).2.1. Fiber Bragg Grating SensorsFor discriminating the strain and temperature effects, the use of reference grating [7], the use of dual wavelength gratings [8], and the use of two sensors associated with different strain and temperature responses were reported [9�C14]. Another approach is to use the dual wavelength technique involving writing two superimposed Bragg gratings [8], in which the responses to temperature (��1T,��2T) and strain (��1��,��2��) at the same location on the structure are different.
The change in the Bragg center wavelengths GSK-3 ����i of the two gratings from the changes in temperature (��Ti) and strain (����i) is given by the following matrix expression:����i=��i?��?i+��iT��Ti i=1,2(1)where ��i�� = ?��/?��i is the strain coefficient of material related to the Poisson ratio, photoelastic constant and effective refractive index, and ��iT = ?��/?Ti is the temperature new product coefficient related to the thermal expansion and thermo-optic coefficients.