Temperature insensitive fiber optic pressure sensor with a pi-phase FBG on microstructured fiber

Fiber optic sensing technology has become a competitive device for strain measurement in many applications such as structural health monitoring and machine condition monitoring. Such success is achieved due to its advantages such as lightweight, electrically non-conductive, electromagnetic field and harsh environment immune, relatively high sensitive to strain change and the compatibility with wavelength division multiplexing method to measure many points from just one fiber cable. However, the use of fiber optic sensors in pressure measurement in gas and fluid media encounters some challenges such as temperature cross-talk to the pressure measurement, low sensitivity, and slow response in gas medium. In this work, I demonstrate both analytically and by experiments that an fiber Bragg grating (FBG) pressure sensor, inscribed in a microstructured fiber with two side holes in its cladding, can be used to measure pressure and temperature simultaneously to remove the temperature effect on measurements. The sensor has a 𝝅-phase shifted FBG which intrinsically has a much narrower linewidth than that of the conventional FBGs and can significantly improve the sensitivity of the pressure measurement. The microstructured fiber has two different refractive indices along their two principal axes caused by its birefringence. Two FBG peaks in the measured spectra related to two principal axes change with different rates when the pressure and/or temperature is applied which makes it possible to measure the change of pressure and temperature. My results also show that the sensor responds to the pressure change instantaneously if the separation of two FBG polarization peaks is used as the measurand, which makes it suitable for dynamic pressure measurements. In addition, the sensor uses the FBG only with no transducer required and its size is extremely small, only one to two centimeter long and with a diameter of about 0.3 millimetre.