Novel Photonic Crystal Fiber Pressure Sensors

Photonic crystal fibers (PCFs) are novel type of optical fibers that attract tremendous research interests in recent years. The structural flexibility as well as the new light guiding mechanism of PCFs distinguishes them from conventional fibers in many aspects, and various PCFs have been developed targeting for different applications particularly in fiber optic sensing. On the other hand, fiber-optic pressure sensors have many important applications such as oil and gas industry, tube engine pressure measurement, civil engineering health monitoring, etc., due to their distinct advantages over the electrical counterparts, such as EMI, compact size, corrosion resistance, remote sensing capability, and multiplexing capability. In this thesis, we exploit the potential of PCFs for pressure sensing and achieved a variety of novel PCF-based pressure sensors. The main achievements of our study are summarized as follow:1. We experimentally studied the pressure response of FBGs in grapefruit microstructured fibers (GMFs) and theoretically analyzed the sensing principle with a full-vector finite element method (FEM). We first observed the pressure sensitivity of FBG in GMF is three times improved than that for standard single-mode fiber (SMF) in the expriment. Three factors that contribute to pressure sensitivity are considered in our simulation, and we identify the axial pressure is the dominating term among the three. Our theory shows that the large air holes in the cladding of GMF reasons for the sensitivity enhancement. Our calculation results also shows that the pressure sensitivity of large-hole GMF is three times higher than that of SMF, which agree well with the experimental results.2. We proposed and experimentally demonstrated a novel fiber optic sensor configuration employing FBGs in GMF and standard SMF for simultaneous measurement of temperature and hydrostatic pressure. The temperature responses of these two gratings are almost the same whereas the pressure sensitivity of Bragg grating in GMF is much larger (about three times obtained by experiments) than that for SMF, which allows discrimination of these two parameters. In addition, the temperature dependence of pressure response of the sensor was theoretically investigated, and we found that it has little influence on simultaneous measurement of the two parameters.3. We experimentally demonstrate a polarization-maintaining photonic crystal fiber (PM-PCF) based Sagnac interferometer for downhole high pressure sensing application. The PM-PCF serves as a direct pressure sensing probe. The pressure sensitivities of the proposed sensor are4.21nm/MPa and3.24nm/MPa at the wavelength of～1320nm and～1550nm ranges, respectively. High pressure measurement up to20MPa has been done in our experiment. It shows both good linearity in response to applied pressure and good repeatability within the whole measurement range. The proposed pressure sensor is with low temperature cross sensitivity. High temperature sustainability test up to293℃has been performed. These above-mentioned characteristics make it a potential ideal candidate for pressure sensing in harsh environments, such as downhole application.4. We proposed and experimentally demonstrated a highly sensitive salinity sensor using a polyimide-coated Hi-Bi photonic crystal fiber Sagnac interferometer based on the coating swelling induced radial pressure. This is the first time to exploit fiber coating induced pressure effect for salinity sensing. The achieved salinity sensitivity is0.742nm/(mol/L), which is45times more sensitive than that of a polyimide-coated fiber Bragg grating. A bare fiber Bragg grating is incorporated into the fiber loop for temperature compensation.5. We proposed a side-hole PM-PCF with ultrahigh polarimetric sensitivity to hydrostatic pressure. Modal birefringence B as large as2.34×10-3and polarimetric pressure sensitivity dBldp as high as-2.30×10-5MPa-1were achieved at1.55μm for the proposed fiber. The pressure sensitivity is more than nine times higher than the typical PM-1550-01fiber by NKT Photonics. Thanks to its pure-silica material, the temperature sensitivity of the proposed fiber is calculated to be as low as-1.2×10-8K-1. These outstanding merits make it an excellent candidate for future applications of hydrostatic pressure measurement.6. A novel fiber-optic Fabry-Perot interferometer was constructed by splicing a short length of PCF to a standard SMF. The PCF functions as a Fabry-Perot cavity and serves as a direct sensing probe without any additional grating components. Its capability for measuring high pressure and high temperature was demonstrated. Its pressure and temperature responses in the range of0～40MPa and25～700℃were experimentally studied, which are in good agreement with theoretical analysis.