Chuang Wu

High-pressure and high-temperature characteristics of a Fabry–Perot interferometer based on photonic crystal fiber

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 low-cost, and compact in size, which makes it very attractive for high-pressure and high-temperature sensing applications.

High pressure sensor based on photonic crystal fiber for downhole application

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.

Characterization of Pressure Response of Bragg Gratings in Grapefruit Microstructured Fibers

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.

Intermodal coupling of supermodes in a twin-core photonic crystal fiber and its application as a pressure sensor

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.

Salinity sensor based on polyimide-coated photonic crystal fiber

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-line microfluidic refractometer based on C-shaped fiber assisted photonic crystal fiber Sagnac interferometer.

We propose and demonstrate a highly sensitive in-line photonic crystal fiber (PCF) microfluidic refractometer. Ultrathin C-shaped fibers are spliced in-between the PCF and standard single-mode fibers. The C-shaped fibers provide openings for liquid to flow in and out of the PCF. Based on a Sagnac interferometer, the refractive index (RI) response of the device is investigated theoretically and experimentally. A high sensitivity of 6621 nm/RIU for liquid RI from 1.330 to 1.333 is achieved in the experiment, which agrees well with the theoretical analysis.

Ultrahigh birefringence index-guiding photonic crystal fiber and its application for pressure and temperature discrimination

In this Letter, we reported on an ultrahigh birefringence photonic crystal fiber (PCF) with a germanium-doped elliptical core, which is fabricated in our lab using the stack-and-draw method. An ultrahigh birefringence of 1.1×10(-2) is obtained experimentally, which is close to the theoretical value of 1.4×10(-2) at the wavelength of 1550 nm. To our knowledge, this is the highest birefringence reported to date for fabricated index-guiding PCF. Fiber Bragg gratings (FBG) were written in the fiber to confirm its ultrahigh birefringence, and we demonstrated the capability to simultaneously measure the FBGs pressure and temperature experimentally. Because of the large separation of the two FBG peaks (>12 nm), such fiber is a promising candidate for a single polarization device.

In-line open-cavity Fabry–Pérot interferometer formed by C-shaped fiber fortemperature-insensitive refractive index 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. Whats 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.

Side-Hole Photonic Crystal Fiber With Ultrahigh Polarimetric Pressure Sensitivity

We propose a side-hole polarization-maintaining photonic crystal fiber (PM-PCF) with ultrahigh polarimetric sensitivity to hydrostatic pressure. The proposed fiber has a very simple structure with a core surrounded by a double row of large air holes and a central row of small air holes. A pair of ultralarge side holes was symmetrically introduced into the silica cladding of the fiber to enhance the polarimetric response to hydrostatic pressure. Modal birefringence B as large as 2.34 × 10-3 and polarimetric pressure sensitivity dB/dp as high as -2.30 × 10-5 MPa-1 were achieved at 1.55 μm for the proposed fiber. Combining the advantages of both side-hole fibers and PM-PCFs, it is believed to be an excellent candidate for future applications of hydrostatic pressure measurement.

Polarimetric heterodyning fiber laser sensor for directional acoustic signal measurement

A DBR fiber grating laser acoustic sensor based on polarization beat signal modulation analysis has been demonstrated for directional acoustic signal measurement. The acoustic sensor was fabricated in birefringent erbium-doped fiber, and the influences of external-acoustic pressure on fiber grating laser sensor were analyzed, considering the effect of relative orientation of the acoustic wave on the degrees of birefringence modulation. In experiment, the birefringence in sensing fiber was modulated by ultrasonic pressure. Agreement between theoretical and experimental results was obtained for ultrasound wave propagating from different directions (0-360 degrees in 15 degrees intervals) corresponding to a nonlinearly change in beat frequency modulation rates. The results demonstrate that the DBR fiber grating laser acoustic sensor has an orientation recognizable ability, offering a potential for acoustic vector signal detection.