Haihu Yu

Distributed OTDR-interferometric sensing network with identical ultra-weak fiber Bragg gratings

We demonstrate a distributed sensing network with 500 identical ultra-weak fiber Bragg gratings (uwFBGs) in an equal separation of 2m using balanced Michelson interferometer of the phase sensitive optical time domain reflectometry (φ-OTDR) for acoustic measurement. Phase, amplitude, frequency response and location information can be directly obtained at the same time by using the passive 3 × 3 coupler demodulation. Lab experiments on detecting sound waves in water tank are carried out. The results show that this system can well demodulate distributed acoustic signal with the pressure detection limit of 0.122Pa and achieve an acoustic phase sensitivity of around -158dB (re rad/μPa) with a relatively flat frequency response between 450Hz to 600Hz.

Pure Directional Bending Measurement With a Fiber Bragg Grating at the Connection Joint of Eccentric-Core and Single-Mode Fibers

A novel fiber Bragg grating (FBG) structure based on an eccentric core fiber (ECF) and a single-mode fiber (SMF) was proposed and experimentally demonstrated for distinguishing the bending effect from the axial strain effect and measuring the pure directional bending. The structure is fabricated by writing a FBG on the fusion splice junction between an ECF and a SMF. The FBG formed on the ECF-SMF junction has two different resonant peaks because of the different refractive indices of the two fibers. The peak relating to the ECF is sensitive to both directional bending and axial strain, while the peak corresponding to the SMF is only sensitive to strain. By using the unique FBG structure, one can directly know whether a bending is accompanied by an axial strain, and can obtain both the direction and curvature of the bending. Experimental results show that the bending sensitivities of the FBG in the ECF part are 49.3 and -50.3 pm/m-1 at directions of 0° and 180°, respectively, and the strain sensitivities of the FBG in the ECF and SMF are 0.74 and 0.70 pm/με, respectively.

Reflectivity measurement of weak fiber Bragg grating (FBG)

One method based on the end-face reflection as a reference was used to measure weak grating reflectivity of 0.01%–1%. For measuring ultra-weak grating reflectivity, the grating group with an identical wavelength was used. By using anhydrous alcohol and deionized water as the reflecting medium, the singlepulse FBG reflectivity of a highly doped-Ge photosensitive fiber was calculated to be about 0.747% and 0.739%, respectively; and the single-pulse FBG reflectivity of ordinary SMF fiber was about 0.016% and 0.015%, respectively. The consistent results by the two different reflecting media showed that the end-face reflection method on the grating group was feasible.

Novel polyimide coated fiber Bragg grating sensing network for relative humidity measurements.

A novel relative humidity (RH) sensing network based on ultra-weak fiber Bragg gratings (FBGs) is proposed and demonstrated. Experiment is demonstrated on a 5 serial ultra-weak FBGs sensing network chopped from a fiber array with 1124 FBGs. Experimental results show that the corresponding RH sensitivity varies from 1.134 to 1.832 pm/%RH when ambient environmental RH changes from 23.8%RH to 83.4%RH. The low-reflectance FBGs and time-division multiplexing (TDM) technology makes it possible to multiplex thousands of RH sensors in single optical fiber.

Analysis of the Spectrum Distortions of Weak Fiber Bragg Gratings Fabricated In-Line on a Draw Tower by the Phase Mask Technique

Identical weak optical fiber Bragg grating (FBG) arrays can be fabricated in-line on a draw tower using the phase mask technique. On the FBG arrays, some grating spectrum distortions, for example, asymmetric side lobes or deformed side lobes, low extinction ratio of the main reflection peak and comb filter-type peaks, can be observed. We collected different distorted reflection spectra of the in-line-fabricated weak FBGs and compared them with the simulated spectra of weak FBGs. By comparing the experimental and theoretical weak FBG spectra, it is found that the different kinds of spectrum distortions are caused by phase shifting, asymmetric apodization and nonlinear chirp. This analysis is helpful to improve the FBG array fabricating process by phase mask in-line on a draw tower. And we got good FBG arrays which have good central wavelength consistency, good reflectivity consistency and minor spectral distortion.

All Fiber Grating (AFG): a new platform for fiber optic sensing technologies

A versatile all fiber grating sensor network based on ultra-weak fiber Bragg gratings (FBGs) was firstly proposed and demonstrated. On-line writing identically weak fiber Bragg grating array by the phase mask technique was developed. The sensing network is interrogated with time- and wavelength-division multiplexing method. The proposed ultra-weak FBG system was very promising for the large-scale sensing network.

Conductive atomic force microscopy study of silica nanotrench structure

Conductive atomic force microscope had been applied to study the electrical transportation mechanism together with topographic information on a periodical silica nanotrench structure. The bottom of the trench is covered by a 4nm silica thin film, while the ridges between the trenches are made up of 90nm thick silica film. On the same bias, the current transport through the bottom is larger than the current transport through the ridges. In the bottom, rectifying characteristics are observed; the conduction mechanism can be ascribed as a Schottky emission. Strong inhomogeneous electrical properties are also observed. A high resolution with a lateral resolution below 6nm is demonstrated in the conductive atomic force microscope study.

Fabry-Perot Interferometric High-Temperature Sensing Up to 1200 °C Based on a Silica Glass Photonic Crystal Fiber

A Fabry-Perot interferometric sensor for temperature measurement was fabricated based on a silica glass solid-core photonic crystal fiber with a central air-bore. By splicing a stub of photonic crystal fiber to a standard single-mode fiber, an intrinsic Fabry-Perot cavity was formed inside the photonic crystal fiber. Sensing experiment results show that the sensor can work stably for a consecutive 24 h under temperatures up to 1100 °C, and the short-term operation temperature can reach as high as 1200 °C (<30 min). In the measurement range of 300–1200 °C, the temperature sensitivity of the peak wavelength shift can reach as high as 15.61 pm/°C, with a linearity of 99.76%. The presented interferometric sensor is compact in size and possesses advantages such as an extended working range and high sensitivity, showing promising application prospects.

Distributed acoustic sensing system based on continuous wide-band ultra-weak fiber Bragg grating array

A distributed acoustic sensing system (DAS) with low-coherence ASE and Michelson interferometer based on continuous width-band ultra-weak fiber Bragg grating (UW-FBG) array is proposed and experimentally demonstrated. The experimental result shows that the proposed system has better performance in detecting acoustic waves than the conventional hydrophone.

Theoretical calculations of crosstalk and time delay in identical FBG array in PM fiber

The spectral shadowing, multiple reflections and polarization crosstalk in an identical FBG array in PM fiber were theoretically studied. For a PM fiber FBG array (PMF-FBG array) with FBG peak reflectivity of -30 dB, grating spacing of 2 m and total fiber length of 2000 m, the reflection power of the PMF-FBG changed from -30.0 dB to -36.9 dB at the end of the array because of the spectral shadowing and the multiple reflections. Equations for the signal time delay of the PMF-FBG array were given. Higher birefringence in the PM fiber will result in a longer time delay.