Using in-fiber Bragg-grating sensors for measuring axial strain and temperature simultaneously on surfaces of structures

A simple method to simultaneously measure axial strain and temperature using a surface-mounted Bragg-grating sensor is presented. This method uses a single, uniform-pitch Bragg grating with preloading that is only partly glued on the structure. To prevent the cross talk between two Bragg wavelengths reflecting from the glued and free sections, a preloading is applied to the fiber before it is glued. After releasing, two Bragg wavelengths are separately reflected from an in-fiber Bragg-grating sensor. The Bragg wavelength reflected from the sensor section not glued to the structure is used to measure temperature variations; that reflected from the glued sensor section is affected by both strain and temperature variations. Nevertheless, under conditions of no cross-talk sensitivity between strain and temperature, the variation in the spectral separation between two Bragg wavelengths is directly caused by the thermomechanical strain in the structures. After the temperature variation is obtained, the mechanical strain can be calculated if the thermal expansion coefficient of the structure is known. This method, therefore, supplies a simple, simultaneous measurement of axial strain and temperature. Based on the same concept, two alternative setups to measure axial strain and temperature simultaneously using an in-fiber Bragg-grating sensor are also introduced. This approach to the application of smart structures appears to be limited to surface-mounted structures. Nevertheless. it might be redesigned for embedded structures by inserting the Bragg grating fiber into a glass tube and partly fusing the fiber with the tube. (C) 1998 Society of Photo-Optical Instrumentation Engineers.