A fiber-optic Fabry–Perot interferometer was constructed by splicing a short length of photonic crystal fiber to a standard single-mode fiber. The photonic crystal fiber functions as a Fabry–Perot cavity and serves as a direct sensing probe without any additional components. Its pressure and temperature responses in the range of 0–$40\mathrm{MPa}$ and $25°\mathrm{C}$–$700°\mathrm{C}$ were experimentally studied. The proposed sensor is easy to fabricate, potentially low- cost, and compact in size, which makes it very attractive for high-pressure and high-temperature sensing applications.

We demonstrate a polarization-maintaining (PM) photonic crystal fiber (PCF) based Sagnac interfero meter for downhole high pressure sensing application. The PM PCF serves as a direct pressure sensing probe. The sensor is transducer free and thus fundamentally enhances its long-term sensing stability. In addition, the PM PCF can be coiled into a small diameter to fulfill the compact size requirement of downhole application. A theoretical study of its loss and birefringence changes with different coiling diameters has been carried out. This bend-insensitive property of the fiber provides ease for sensor design and benefits practical application. The pressure sensitivities of the proposed sensor are 4.21 and $3.24\mathrm{nm}/\mathrm{MPa}$ at $\sim 1320$ and $\sim 1550\mathrm{nm}$, respectively. High pressure measurement up to $20\mathrm{MPa}$ was achieved with our experiment. It shows both good linearity in response to applied pressure and good repeatability within the entire measurement range. The proposed pressure sensor exhibits low temperature cross sensitivity and high temperature sustainability. It functions well without any measurable degradation effects on sensitivity or linearity at a temperature as high as $293°\mathrm{C}$. These characteristics make it a potentially ideal candidate for downhole pressure sensing.

We report an open-cavity optical fiber Fabry-Pérot interferometer (FPI) capable of measuring refractive index with very low temperature cross-sensitivity. The FPI was constructed by splicing a thin piece of C-shaped fiber between two standard single-mode fibers. The refractive index (RI) response of the FPI was characterized using water-ethanol mixtures with RI in the range of 1.33 to 1.36. The RI sensitivity was measured to be 1368 nm/RIU at the wavelength of 1600 nm with good linearity. Thanks to its all-glass structure, the FPI exhibits very low temperature cross-sensitivity of 3.04 × 10⁻⁷ RIU/°C. The effects of cavity length on the performance of the sensor were also studied. A shorter cavity gives rise to broader measurement range while offering larger detection limit, and vice versa. What’s more, the effect of material dispersion of analyte on the sensitivity of open-cavity FPIs was identified for the first time. The sensor is compact in size and easy to fabricate. It is potentially useful for label-free optical sensing of chemical and biological samples.

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 is 0.742 nm/(mol/L), which is 45 times 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.

In this paper, we experimentally demonstrated the fabrication and hydrostatic pressure characteristics of a twin-core photonic crystal fiber (TC-PCF). Mode couplings in the TC-PCF for x- and y-polarizations were analyzed simultaneously using group effective index of guiding modes. The output spectrum of the TC-PCF was modulated due to the combined couplings of the two polarizations. To the best of our knowledge, it is the first time to measure hydrostatic pressure through the dual-polarization mode coupling in a TC-PCF. The measured sensitivity of the pressure sensor was −21pm/MPa. The length of the TC-PCF used for pressure measurement was 20cm, which is much shorter than pressure sensor based on PM-PCF, and does not require any external polarizing components, meaning that it is a good candidate for compact pressure sensor.

In this paper, the pressure response of fiber Bragg gratings in grapefruit microstructured fibers was experimentally investigated and theoretically analyzed with full-vector finite element method. The theoretical calculation agrees well with the experimental results. The pressure changes the refractive index and induces axial strain of the fiber. The radial pressure induces positive strain, but axial pressure induces negative strain. The negative strain dominates. The existence of large air holes make grapefruit microstructured fibers experience larger negative strain than standard single-mode fiber, therefore fiber Bragg gratings in grapefruit microstructured fibers have larger pressure sensitivity than that in SMF.