Tapered optical fiber-based interferometric sensors have potential applications in some fields for the measurements of solution concentration, refractive index, biomolecules, etc. By coating specific functional film onto the surface of a tapered optical fiber, it is possible to further extend the detection range and enhance the measurement sensitivity. In this paper, a tapered optical fiber sensor based on Mach-Zehnder modal interference is studied. An atactic polystyrene thin film was used as a functional layer to coat onto the surface of the tapered region. Detection of aqueous glucose of different concentrations was carried out using the tapered fiber sensor with or without the coating. The experimental results show that the measurement sensitivity can be significantly improved by coating the atactic polystyrene thin film onto the tapered fiber with suitable waist parameter.
Drawing-tower fiber Bragg grating (FBG) array can be used in health monitoring of bridges, pipelines and some largescale projects to realize temperature, stress and vibration quasi-distributed sensing. The grating array sensing system can detect the changes of the tested physical parameters by measuring the central wavelength of the grating. However, for the sensing system based on identical grating array consisting of uniform FBGs, it cannot measure accurately the shift of the reflective wavelength of a grating in the array due to the sidelobe superposition of the gratings. Compared with uniform Bragg grating, apodized FBG has smaller reflection peak sidelobes. It means that the reflection power of the apodized FBG can be concentrated to the central wavelength region, and the reflection peak signal will be not interfered with those of other gratings. In this paper, an apodized FBG array was in-line fabricated in a single mode fiber by on a drawing tower and used for dense quasi-distributed temperature sensing. The obtained apodized FBGs have a full width at half maximum (FWHM) of 0.09 nm and their sidelobes were decreased by -21.9 dB. A 1000-meter-long array consisting of 10000 apodized gratings was successfully fabricated and used for quasi-distributed temperature sensing. These gratings have good consistency, low transmission loss and good mechanical properties. Experimental results show that the array can accurately measure quasi-distributed temperature fields and the temperature sensitivity is 10.15 pm/°C. It is expected that the apodized grating array can find more applications in many fields.
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.
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.
Optical fiber hydrogen sensing system based on weak fiber Bragg grating (WFBG) array deposited with palladium (Pd) film is proposed and experimentally demonstrated. For multi-point measurement, three hydrogen WFBG sensors array are weld in a single optical fiber. A time-division multiplexing (TDM) interrogation system is employed to demodulate the sensing array. Sensing experiments to different hydrogen concentrations ranging from 0 to 3.6% are conducted, and the results show good agreement with standard FBG technology. Due to its strong multiplexing capability of weak FBG, the system is possible to integrate thousands of WFBG hydrogen sensors in a single optical fiber.
Ytterbium-doped silica optical fibers were fabricated using a modified chemical vapor deposition process with chelate precursor. The typical core diameters of the preforms made by the chelate doping method are 5.79mm, and the useful length of the preforms is about 900mm, twice the length of the preforms by the solution doping method. The Ytterbium-doped fiber with octagonal inner cladding shape drawn in a double clad configuration has a background loss of 15dB/km at 1200nm. The Ytterbium ion concentration in the fiber core is up to 0.709wt% and a slope efficiency of 81.9% can be generated when pumped with 915nm laser.
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.
Polyelectrolyte/colloidal TiO2 composite films have higher refractive index than glass, while polyelectrolyte/colloidal SiO2 composite films have lower refractive index. By using a layer-by-layer assembling technique, polyelectrolyte/TiO2 nanoparticle-λ/4-layers and polyelectrolyte/SiO2 nanoparticle-λ/4-layers were alternatively deposited onto optical fibers and microscope glass slides. In this way, λ/4-high-reflection optical coatings were fabricated.
Colloidal TiO2 was prepared by hydrolyzing tetra-n-butyl titauate. Composite multilayer films of Poly(sodium 4-styrensulfonate), PSS, and colloidal TiO2 particles were electrostatically self-assembled onto optic fibers and microscope glass slides. As the PSS/TiO2 film was deposited onto the end face of a glass fiber, the reflected optic intensity periodically oscillated as the bilayer number of the film increased. After a 24-bilayer film was coated onto the both sides of a glass slide, the transmittance at 850 nm decreased more than 20%, which means that the film could serve the function as a reflection-enhancing coating. Data of X-ray diffraction and TEM electron diffraction analysis show that the main crystalline phase of the colloidal TiO2 particles is brookite and that the PSS/TiO2 films are polycrystalline films. Scratching experiments indicate that the composite films are of relatively high hardness.
Colloidal CdS stabilized with (NaPO3)6 was prepared from Cd(NO3)2 and Na2S. The precursor of poly(p-phenylene vinylene), PPV, was synthesized through Wessling's route. PPV precursor/CdS multilayer films were fabricated via the layer-by-layer electrostatic self-assembly technique. After thermal treating the PPV precursor/CdS films, PPV/CdS nanocomposite films were formed. The colloidal CdS and the composite films were characterized using a UV-vis spectrophotometer and a fluorescence spectrophotometer. A photoluminescence peak of the PPV/CdS multilayer film appears at around 380 nm, suggesting that the photoluminescence of the composite films should attribute to the CdS nanoparticles. The electroluminescence spectra show that a strong luminescence band appears in range 350~650 nm, centering around 530 nm, suggesting that the electroluminescence of the composite films should mainly attribute to PPV. For the ITO/(PPV/CdS)20/Al device, the threshold voltage was about 5 V, and the luminescence intensities increased as the biased voltages increased. In the PPV/CdS nanocomposite film-based light emitting device, CdS plays a role mainly in electron-transporting and PPV works mainly in electroluminescence. However, at higher voltages, CdS nanoparticles may also emit light.
Polyelectrolyte multilayer thin films were coated onto flat glass substrates and optic fibers by using the ionic self-assembly technology. The deposition of the multilayers was monitored by using UV-vis spectroscopy, and the assembled thin films were characterized by small-angle X-ray reflectivity. Optic fiber polymer Fabry-Perot cavities were created by assembling polyelectrolytes onto the end faces of glass fibers, and the Fabry-Perot interference phenomena has been observed. It can be expected that novel fiber optical devices, such as Fabry-Perot filters, reflectors and Fabry-Perot cavity-based fiber optic sensors, can be developed via this approach.
Thin film metal/polymer composite electrodes with electrical conductivities on the order of those of bulk metals have been formed on electroactive polymer actuator elements using a novel self-assembly technique. The electrodes exhibit good flexibility and mechanical performance.
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