Simple method to measure temperature and axial strain simultaneously using one in-fiber Bragg grating sensor

A simple method to simultaneously measure temperature and axial strain for a surface mounted Bragg grating sensor is presented. This method uses a single, uniform pitch Bragg grating that is only partially glued on the structure. Thermomechanical strain fields will produce two different Bragg wavelengths from the one fiber Bragg grating sensor. The Bragg wavelength reflected from the sensor section not glued to the structure is used to measure temperature variations; and the Bragg wavelength reflected from the sensor section glued to the specimen is affected by both of strain and temperature variation. However, under conditions of no cross-talk sensitivity between temperature and strain, the spectral separation between two Bragg wavelengths is directly interpreted as a wavelength shift caused by the thermomechanical strain in structures. After the temperature variation is obtained, the mechanical strain can be calculated if the thermal-expansion coefficient of structures is known. Therefore, this method supplies a simple measurement in temperature and axial strain simultaneously.