Research on discriminating measurement of high-temperature and strain based on in-line MPCF-FPI combined with a LPFG

An optical fiber high-temperature strain sensor is proposed and fabricated which is formed by combining an in-line photonic crystal fiber Fabry-Perot interferometer(FPI) with an air bubble cavity and a long-period fiber grating(LPFG) written in a standard single mode fiber using high-frequency CO2 laser.The in-line FPI was constructed by directly splicing a mutimode photonic crystal fiber(MPCF) to a conventional single mode fiber with a commercial splicer.The air microbubble inserted between the two fibers has two smooth glass-air interfaces separated by a distance L as two reflective mirrors of the FPI.Due to big air holes in the cladding of MPCF and its large numerical aperture,this device has higher signal-to-noise ratio and fringe contrast.In addition,the bubble usually does not reduce the ultimate strength of a fusion splice,the since surface tension ensures that they have smooth walls which do not serve as stress concentrations or crack nucleation sites,so such a FPI can be used as an ideal strain sensor under high temperatures.The LPFG written by high-frequency CO2 laser can survive high temperatures up to 800 ℃ and has relatively high temperature sensitivity.Therefore,simultaneous measurements of high-temperature and strain can be realized by monitoring the wavelength shifts of the FPI and LPFG.The experimental results show that this method has strain sensitivity of 2.7 pm/μe and temperature sensitivity of 0.07 nm/℃,meeting the requirements for practical applications.