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 MPa and 25°C-700°C were experimentally studied. The proposed sensor is easy to fabricate, potentially lowcost, and compact in size, which makes it very attractive for high-pressure and high-temperature sensing applications. . These advantages make them very suitable for deployment in space-limited harsh environments such as turbine engines and power plants and in the oil and gas industry, where pressure and temperature are the two most common measurands [3][4][5][6][7]. Many methods have been developed to fabricate fiberoptic FPIs. These include using conventional hollow-core fiber [3][4][5], chemical etching [7,8], and laser micromachining [9,10]. Recently, FPIs made from hollow-core photonic bandgap fiber [11] and solid-core photonic crystal fiber (PCF) [12] by simply cleaving and splicing have been demonstrated as strain and temperature sensors. The former needs a coating film onto the fiber endface, thereby increasing the complexity of the fabrication process. The latter as a temperature sensor is not stable, because the air holes are exposed to external environment [13].In this Letter, we report a fiber-optic FPI constructed by splicing a solid-core PCF to a standard single-mode fiber (SMF), and the other end of the PCF was collapsed with arc discharge using a fusion splicer. The entire fabrication process was performed using a commercially available fusion splicer. The principle of the FPI sensor is also presented here. We experimentally investigated the high-pressure and high-temperature characteristics of the proposed FPI sensor. Measurements of pressure up to 40 MPa and temperature up to 700°C were carried out. The pressure and temperature coefficients of the FPI sensor were measured to be −5:8 pm=MPa and ∼13 pm=°C, respectively. The experimental results agree well with the theoretical analysis. Figure 1(a) illustrates the experimental setup of our measurement. A broadband source (BBS) centered at 1530 nm is used to illuminate the FPI through an optical circulator. The reflection spectrum of the FPI is observed by an optical spectrum analyzer (OSA). Figures 1(b) and 1(c) show the schematic diagram and photograph of the fabricated FPI. A short length of a solid-core PCF (PM-1550-01 by Blaze Photonics) was first spliced to a standard SMF using the technique reported in [14]. Because of the small difference in refractive index between the SMF and the PCF, a mirror with relatively low reflectivity was formed at the fiber interface. The PCF was then cleaved to a length of several millimeters. A second lowreflection mirror at the end of the solid core was formed due to Frésnel reflection. As a result, a relatively weak FPI was constructed with an intensity contrast of ∼6 dB as sh...

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 -21 pm/MPa. The length of the TC-PCF used for pressure measurement was 20 cm, 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.

We demonstrate a polarization-maintaining (PM) photonic crystal fiber (PCF) based Sagnac interferometer 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 nm= MPa at ∼1320 and ∼1550 nm, respectively. High pressure measurement up to 20 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°C. These characteristics make it a potentially ideal candidate for downhole pressure sensing.

Abstract: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. ©2011 Optical Society of America