A Temperature Self-Compensated LPFG Sensor for Large Strain Measurements at High Temperature

In this paper, a CO₂ laser-induced long-period fiber-grating (LPFG) optic sensor was packaged with a hybrid mechanism of elastic attachment and gauge length change for large strain measurements in a high-temperature environment. An emphasis was placed on the use of two cladding modes (LP₀₆ and LP₀₇) of a single LPFG sensor for simultaneous strain and temperature evaluations so that exact temperature was used to compensate strain measurements. Both strain and temperature sensitivities of the LPFG sensor, as well as the strain transfer ratio due to a combined effect of elastic attachment and gauge length change, were analytically derived and validated with tension tests at elevated temperatures. The strain sensitivity of the LPFG sensor switched sign from negative for LP₀₆ or lower modes to positive for LP₀₇ or higher modes, whereas its temperature sensitivity remained positive. The sign switch for the strain sensitivity resulted from two competing changes of grating period and effective refractive index as the gratings are subjected to an axial strain. The LPFG sensor was demonstrated to be operational up to 700°C for a strain measurement of up to 1.5%.