H. Y. Fu

Pressure sensor realized with polarization-maintaining photonic crystal fiber-based Sagnac interferometer

A novel intrinsic fiber optic pressure sensor realized with a polarization-maintaining photonic crystal fiber (PM-PCF) based Sagnac interferometer is proposed and demonstrated experimentally. A large wavelength-pressure coefficient of 3.42 nm/MPa was measured using a 58.4 cm long PM-PCF as the sensing element. Owing to the inherently low bending loss and thermal dependence of the PM-PCF, the proposed pressure sensor is very compact and exhibits low temperature sensitivity.

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.

Temperature-Insensitive Fiber Bragg Grating Based Tilt Sensor With Large Dynamic Range

We present a novel and simple temperature-insensitive fiber Bragg grating (FBG) based smart tilt sensor. The sensor design is free from any inherent mechanical joints/frictions with a capability to measure magnitude as well as the direction of the inclination. The first design is with a sensitivity of 0.00282 nm/° in a very large dynamic range of ± 35°. The sensor response is optimized against design parameters and observed to be completely reversible. Since there is a strain discrepancy on individual FBGs of the first design, a modified design is then proposed for optimization. An excellent sensitivity of 0.0395 nm/° , resolution of 0.013° and accuracy of 0.051° have been achieved. These features along with the freedom to tune the dynamic range, accuracy of measurement and the sensitivity of the proposed sensor to a desired operation range make the proposed sensor of extreme importance for practical engineering applications.

Multiplexing of polarization-maintaining photonic crystal fiber based Sagnac interferometric sensors

Three multiplexing schemes are presented for polarization-maintaining photonic crystal fiber based Sagnac interferometric sensors. The first technique is wavelength division multiplexing using coarse wavelength division multiplexers (CWDMs) to distinguish signals from each multiplexed sensor in different wavelength channels. The other two schemes are to multiplex sensors in series along a single fiber link and in parallel by using fiber-optic couplers. While for the CWDM scheme, the multiplexed sensing signal can be obtained by direct measurement; for the other two multiplexing techniques, the sensing signal is more complex and cannot be easily demultiplexed. Thus, some signal processing methods are required. In this regard, two mathematical transformations, namely the discrete wavelet transform and Fourier transform, have been independently and successfully implemented into these two schemes. The operating principles, experimental setup, and overall performance are discussed.

Intensity-modulated fiber Bragg grating sensor system based on radio-frequency signal measurement

An intensity-modulated, fiber Bragg grating (FBG) sensor system based on radio-frequency (RF) signal measurement is presented. The RF signal is generated at a photodetector by two modulated optical signals reflected from the sensing FBG and a reference FBG. Wavelength shift of the sensing FBG changes intensity of the RF signal through changing the delay between the two optical signals, with temperature effect being compensated automatically by the reference FBG. It also exhibits important features including potentially high-speed measurement, low cost, and adjustable sensitivity. In the experiment, strain measurement with a maximum sensitivity of -0.34 microV/micro epsilon has been achieved.

Polarization-maintaining photonic-crystal-fiber-based all-optical polarimetric torsion sensor

An application of a polarization-maintaining photonic crystal fiber (PM-PCF) for torsion sensing is proposed and experimentally demonstrated. The response of the sensor is theoretically validated using the Jones matrix. High normalized sensitivity of ∼0.014/° was measured within a linear twist angle range from 30° to 70°. The sensor response was observed to be highly repeatable over a 90° twist in both the clockwise and counterclockwise directions. The proposed sensor exhibits reduced temperature sensitivity due to the low thermal coefficient of PM-PCF, making it an ideal candidate for torsion sensing.

A Novel Fiber Bragg Grating Sensor Configuration for Long-Distance Quasi-Distributed Measurement

A novel fiber Bragg grating (FBG) sensor configuration using three broadband light sources of different wavelength bands for the interrogation of FBG sensors distributed over 75 km of fiber is proposed and demonstrated. Rayleigh backscattering was reduced and a 60-dB effective dynamic range was demonstrated.

Highly birefringent four-hole fiber for pressure sensing

A highly birefringent four-hole flber (FHF) with a pair of large air holes and a pair of small air holes are proposed for air/hydrostatic pressure sensing. The birefringence of the FHF can be up to 0.01 due to the rectangle-like flber core surrounded by four air holes. Therefore, a FHF with a length of only several centimeters is required for high-sensitivity pressure sensing based on a Sagnac interferometer. Optical properties of the FHF such as efiective index and birefringence are investigated. Pressure sensor based on the FHF depends on the pressure-induced refractive index change or pressure- induced birefringence. The stress distribution of the FHF subjected to an air/hydrostatic pressure is represented. Simulations show that the principal stress component parallel to the slow axis of the of the FHF under the air/hydrostatic pressure is greatly enhanced due to the existence of two large air holes, which consequently results in a high sensitivity of the FHF-based pressure sensor. Relationships between the pressure-induced bireflrngence and the radius of the large air hole, the external diameter of the FHF, or the ellipticity of the elliptical FHF are investigated. The polarimetric pressure sensitivity of the FHF can be up to 607rad/MPa/m.

Single-PPLN-assisted wavelength-/time-selective switching/dropping/swapping for 100-GHz-spaced WDM signals

We propose an approach to implementing wavelength- and time-selective optical switching, dropping and swapping based on the sum-frequency generation (SFG) or cascaded sum- and difference-frequency generation (cSFG/DFG) in a periodically poled lithium niobate (PPLN) waveguide. Analytical solutions are derived, showing the parametric depletion effect for optical switching and the narrow-band operation due to quasi-phase matching (QPM) condition of PPLN. Using parametric depletion effect of SFG process, we demonstrate wavelength- and time-selective optical switching for ITU-grid compatible 40-Gbit/s wavelength-division multiplexed (WDM) signals with a channel spacing of 100 GHz. Less than 1-dB power penalty at a bit-error rate (BER) of 10(-9) is measured for the wavelength- and time-selective switching channel. Negligible impacts are observed on other channels of WDM signals. Using combined effects of parametric depletion and wavelength conversion of cSFG/DFG processes, we demonstrate wavelength- and time-selective optical dropping for ITU-grid compatible 100-GHz-spaced 40-Gbit/s WDM signals. Moreover, we demonstrate optical swapping between two 100-GHz-spaced 40-Gbit/s signals. The obtained theoretical and experimental results confirm single-PPLN-assisted wavelength- and time-selective optical switching, dropping and swapping for 100-GHz-spaced WDM signals, which might potentially be extended to WDM signals with narrower channel spacing.