Development of Sandglass Shape FBG Sensor to Reduce Cross Sensitivity Effect

Fiber Bragg grating (FBG) sensors have received considerable attention in applications of temperature, axial strain, and transverse pressure measurement. However, the cross-sensitivity of strain and temperature measurement is the key problem of FBG sensors. In this paper, a sandglass shape FBG is proposed to divide cross-sensitivity of temperature and transverse pressure. The principle and structure of sandglass FBG are introduced, and the experiment results show cross-sensitivity avoidable in the transverse pressure sensor. 1. Introduction Fiber Bragg grating (FBG) has unique advantages compared to metal sensors such as immunity to electromagnetic interference, compact size, potential low cost, and the possibility of distributed measurement over a long distance, which have been reported as sensors for many measurements including temperature, strain, vacuum, and refractive index. FBG sensors have received great attention for applications in modern stress measurement and optical distribution sensing networks [1]. However, the cross-sensitivity effects of FBGs, like force, displacement, or pressure effects tangled with temperature effect, make it difficult to separately determine the temperature and other parameters by measurement of the wavelength shift of single-FBG sensor, which reduce the utilization value in the field [2, 3]. In this paper, an approach to resolve the cross-sensitivity of FBGs is proposed and demonstrated, which is using the sandglass coat shape of FBG with and polyimide polymers to resolve the cross-sensitivity problem during the demodulation. As an application of this approach, simultaneous measurement of axial strain, and temperature with sandglass FBG coatings is reported. 2. Cross-Sensitivity Effect of FBGs As shown in Figure 1, an FBG of 1？cm in the grating length was inscribed on a standard telecommunication single-mode optical fiber (corning SMF-28). Figure 1: Refractive index distributions of FBG and transmission and reflection characters. The Bragg resonance wavelength of a FBG, , which is the center wavelength of light back reflected from the grating, depends on the effective index of refraction of the core and the periodic spacing of the grating ( ) through the relation . Parameters such as and are affected by changes in strain and temperature. The shift in the Bragg resonance wavelength due to the changes in the temperature and strain can be described by The first term in (1) represents the temperature effect on the FBG and the second term describes the strain effect. The fractional wavelength shift corresponding to a