Highly sensitive LPG temperature sensor employing polyamic acid-coating

A highly sensitive long period fiber grating(LPG) temperature sensor was designed and demonstrated. Cladding of the LPG was etched and encapsulated in metal to maintain constant strain, then was sealed in a test tube containing polyamic acid(PAA), which had a relatively large thermo-optic coefficient as a temperature-sensitive material. The resonance wavelength shifted with temperature for different PAA refractive index values and different LPG cladding diameters were measured and discussed. The results show that the sensitivity of the LPG temperature sensor becomes higher when the refractive index of PAA becomes larger for a certain LPG of determined diameter. Additionally the sensitivity of the LPG temperature sensor becomes higher when the cladding diameter of the LPG decreases for a determined refractive index of PAA. A sensitivity of 1.26 nm/℃ was achieved for this improved LPG temperature sensor, ten times greater than that of the sensor made of common LPG and 100 times greater than that made of common fiber Bragg grating (FBG). This new sensor exhibits good linearity of 99.80% at temperatures of 2-35℃, which improves its potential application for biomedical measurement and ocean detection, where the temperature range of interest is close to room temperature.