Using phase mask technology, the method of online preparation of wire-drawing tower grating (DTG) under single pulse laser was studied. the self-prepared DTG separately tested its breaking strength, the consistency of hundreds of identical gratings (center wavelength and reflectivity), and the temperature response of thousands of gratings in a single fiber. The results show that under the optimal process, the mechanical strength of DTG is not only consistent with the original fiber, but also relatively 6% higher than that of FBG; the isotropic grating at different positions, the center wavelength is stable within 80pm, and the reflectance difference is less than 3dB, and the temperature response consistency is very high. Therefore, the proposed single-pulse laser based on the phase mask to prepare the drawing tower grating can fully meet the requirements of the internet of things for large-capacity, fast response, long-distance, densely distributed fiber sensor network technology.
An in-fiber Mach-Zehnder interferometer (MZI) based on dual side-hole fiber (DSHF) was demonstrated for highly sensitive measurement of curvature. The MZI-based bending sensor is fabricated by fusion splicing a piece of DSHF in between two standard single mode fibers (SMF) with cladding alignment. Due to the existence of the two air holes and the asymmetrical cross-section of the DSHF, the DSHF-based MZI is a core-cladding interferometer which is sensitive to directional bending. The bending characteristics are investigated experimentally within the curvature range of 0-8.172m-1. The bending sensitivities of the sensor are respectively 1.464 nm/m-1 and -1.394 nm/m-1 at their two opposite bending directions.
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.
We demonstrate a high spatial resolution multiplexing scheme for fiber Bragg grating (FBG) sensors based on single-arm frequency-shifted interferometry (SA-FSI). The SA-FSI system uses an incoherent broadband source, a slow detector, and an electro-optic modulator (EOM). By sweeping the frequency of EOM and taking the fast Fourier transform (FFT) of the interference signal, we resolved the locations of FBGs distributed both in parallel and in series along fiber links despite their reflection spectral overlap. Eighteen weak FBGs (~5% reflectivity) separated by ~0.1 m were clearly resolved experimentally, sweeping EOM modulation frequency in the range of 2-11 GHz.
We present and demonstrate a novel fiber Michelson interferometer based on cascaded twin core fiber (TCF) and side-hole fiber (SHF) for curvature measurement. The interferometer is fabricated by fusing the TCF and SHF. The interferometer can measure curvature and temperature by monitoring the wavelength change of interferometer spectrum. The experiment investigated sensor’s spectral responses to the curvature at different bending directions. Experimental results show that the bending sensitivities of the sensor are -6.968 nm/m-1 and 6.978 nm/m-1 at the bending direction of 0° and 180°, respectively. The temperature sensitivity of the interferometer is 10.37pm/°C.
A novel optic fiber hydrogen sensor is proposed in this paper. Two Bragg gratings with different reflectivity were written in single mode fiber with phase mask method by 248 nm excimer laser. The end-face of singe mode fiber was deposited with WO3-Pd-Pt multilayer films as sensing element. The peak intensity of low reflectivity FBG is employed for hydrogen characterization, while that of high reflectivity FBG is used as reference. The experimental results show the hydrogen sensor still has good repeatability when the optic intensity in the fiber is only 1/3 of its initial value. The hydrogen sensor has great potential in measurement of hydrogen concentration.
Large scale distributed sensor networks based on fiber Bragg gratings (FBGs) can find a wide range of uses. However, when identical FBGs are connected in series, the crosstalk will occur because of multiple reflections among the FBGs. In this article, theoretical simulations were done on the crosstalk in an array with FBG peak reflectivity of -35dB. An array consisting of identical weak FBGs with peak reflectivities ranging from -33dB to -37dB and a spatial resolution of 2.5m was prepared in-line on a fiber draw tower. The testing results of the FBG array are in close agreement with the theoretical simulations and ghost gratings can be observed on the reflection spectrum of the FBG array.
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 reflectance spectra of the in-line fabricated FBGs and compared them with the simulated spectra of FBGs. By comparing the experimental and theoretical FBG spectra, it is found that the different kinds of spectral distortions are caused by phase shifting, asymmetric apodization and nonlinear chirp.
An air-silica microstructure optical fiber based on anti-resonant reflecting optical waveguide (ARROW) principle was used to develop spectral absorption gas sensor. The ARROW fiber has an air core and an air cladding layer. An ARROW fiber with length of 725 mm was used to construct a sensing system to detect acetylene gas. The gas was injected into the fiber from one end of the fiber. The transmission spectra were collected using an optical spectrum analyzer. The results indicate that the system can detect the gas of different concentrations and has good system linearity. The response time of the system is about 200 s.
Fiber Bragg grating (FBG) is a passive optical fiber component with the refractive index modulated along the fiber length and has been widely applied in fiber sensing systems. High-temperature stable fiber gratings are promising for uses at high temperature and receive extensive attentions. In this paper, FBGs were inscribed in hydrogen loaded standard single mode fibers with 248 nm excimer laser, and regenerated gratings were obtained through heat treatment. The center wavelength of the regenerated FBG has a good linearity with temperature, and the reflectivity of the regenerated FBG can almost keep unchanged at 800 °C.
An interrogation system based on two semiconductor optical amplifiers for weak fiber Bragg gratings(FBGs) was proposed in this study. The first Semiconductor Optical Amplifier (SOA) was used to modulate the light and the second SOA separated the signal through delayed switching. The proposed system has lower insertion loss and higher spatial resolution, and can interrogate time-division and wavelength-division multiplexed FBG array. Up to 50 FBGs, with a reflectivity of 0.2% and a spatial resolution of 5m along the optical fiber, were distinguished to demonstrate the interrogation system.
KEYWORDS: Signal to noise ratio, Roads, Scattering, Fiber optics sensors, Signal processing, Analytical research, Spatial resolution, Acoustics, Signal detection, Temperature metrology
A differential double-pulse pair Brillouin optical time-domain analysis (DDP-BOTDA) combined with the double-pulsed technique and the differential pulse-width pair technique is proposed and simulated to detect small temperature and strain changes. Using a symmetrical double-pulse pair, the system can detect small Brillouin shift with high spatial resolution and large dynamic range. Sub-meter spatial resolution is decided by the difference between the pulse-width and the peak and valley of the spectrum which is derived from differential pump depletion signals, and detectable Brillouin shift is less than 1 MHz.
An interrogation system for the time-division multiplexed sensor array based on 1310-nm band ultra-weak fiber Bragg gratings (FBGs) was proposed in this study. Power spectral density was introduced to budget the optical power and evaluate the interrogation capability of the proposed system. The proposed system consists of one tunable laser and two semiconductor optical amplifiers, where the former scans the wavelength, and the latter modulates the light and separates the reflected pulses from different FBGs through delayed switching. Up to 1000 serial FBGs, with a peak reflectivity of −40 to −45 dB and a spatial resolution of 5 m along an optical fiber, were interrogated to demonstrate the feasibility of the interrogation system.
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