Simultaneous Strain and Temperature Measurement Using Long-Period Fiber Grating Sensors

Long-period fiber grating (LPFG) sensors have been demonstrated to exhibit negative and positive strain sensitivity coefficients with cladding modes of LP05 and LP07, respectively. Their temperature sensitivity remains positive for both cladding modes. Based on the unique properties of CO2 laser-induced gratings, two designs of LPFG sensors have been presented in this paper for simultaneous strain and temperature measurement. One double-LPFG sensor integrates two separate groups of gratings in series, corresponding to two cladding mode measurements such as LP05 and LP07. It can measure strain and temperature simultaneously with a measurement error of less than 2%. However, the spacer between two tandem gratings can only survive in low temperature environments. The other single-LPFG sensor uses the measurements with two cladding modes on one group of gratings. Under various disturbances, different intrinsic cladding modes of the single LPFG, such as LP05 and LP07, are tracked simultaneously for strain and temperature discrimination. The single-LPFG sensor can measure strains in high temperature up to 700 degrees C.