Shiquan Tao

Optical fiber humidity sensor based on evanescent-wave scattering

The phenomenon of evanescent-wave scattering (EWS) is used to design an optical-fiber humidity sensor. Porous solgel silica (PSGS) coated on the surface of a silica optical-fiber core scatters evanescent waves that penetrate the coating layer. Water molecules in the gas phase surrounding the optical fiber can be absorbed into the inner surface of the pores of the porous silica. The absorbed water molecules form a thin layer of liquid water on the inner surface of the porous silica and enhance the EWS. The amount of water absorbed into the PSGS coating is in dynamic equilibrium with the water-vapor pressure in the gas phase. Therefore the humidity in the air can be quantitatively determined with fiber-optic EWS caused by the PSGS coating. The humidity sensor reported here is fast in response, reversible, and has a wide dynamic range. The possible interference caused by EWS to an optical-fiber gas sensor with a reagent-doped PSGS coating as a transducer is also discussed.

High dynamic range fiber optic relative humidity sensor

We present a fiber optic relative humidity sensor. The sensor was fabricated by coating a thin polyvinyl alcohol/CoCl 2 film on a 200-μm core plastic-cladding fiber; the film was deposited after removing the cladding of the fiber. It was found that bending the fiber leads to drastic changes in the response behavior. The sensor was compared against a commercially available relative humidity sensor and was found to be sensitive to relative humidity ranging from 3 to 90%. The sensor response is usually very fast, but varies at different humidity levels and has a good dynamical range. The sensor response is repeatable and fully reversible, and should be useful for monitoring relative humidity in harsh industrial environments.

Optical-fiber sensor using tailored porous sol-gel fiber core

A new concept in optical-fiber chemical sensors, the active fiber core optical sensor (AFCOS), is presented. In this sensor, the fiber core acts as a transducer. The sensitivity of an AFCOS sensor is compared with that of an active coating [evanescent wave (EW)] based optical-fiber sensor. Requirements for a fiber core to act as a chemical sensor are discussed. Novel techniques for making a porous sol-gel silica fiber, doping chemical reagents into the fiber, and constructing a chemical sensor using the porous fiber as a transducer have been developed. The microstructure of the fabricated sol-gel silica fiber and the effect of the fibers microstructure on the capability of the porous sol-gel silica fiber for guiding light are discussed. A humidity sensor employing a CoCl/sub 2/-doped porous sol-gel fiber as a transducer has been constructed as an example. The test results for the humidity sensor verified a theoretical analysis indicating that an optical-fiber chemical sensor using an active fiber core as a transducer has a much higher sensitivity than that of an EW-based sensor.

Optical fiber sensor using tailored porous sol-gel fiber core

Novel techniques for fabricating a porous sol-gel fiber, doping chemical reagent into the fiber, and constructing an optical fiber chemical sensor using the porous fiber as a transducer, have been invented. A moisture sensor employing a CoCl/sub 2/ doped porous sol-gel fiber as a transducer has been constructed as an example. The test result of the moisture sensor verified a conclusion from theoretical analysis. That is an optical fiber chemical sensor using an active fiber core as a transducer should have a much higher sensitivity than that of active coating based sensor.

Porous solgel fiber as a transducer for highly sensitive chemical sensing.

A novel solgel process for making porous silica fiber and doping the fiber core with sensing material is described. A CoCl(2) -doped solgel fiber was fabricated and was used to construct an active-core optical fiber moisture sensor. Test results show that the sensitivity of the active-core optical fiber sensor is much higher than that of an evanescent-wave-based optical fiber sensor.

Determination of elemental mercury by cavity ringdown spectrometry

Cold vapor cavity ringdown spectroscopy has been successfully applied to the detection of elemental mercury. Using an absorption cell 0.18 m in length, detection limits of 0.027 and 0.12 ng were obtained using peak area and peak height measurements, respectively. For the peak area measurement, this corresponds to a gas phase concentration of less than 25 ng m−3. For comparison, using a similar absorption cell, standard AAS yielded a Hg detection limit (peak height) of 9 ng, (gas phase concentration of ≡ 8.3 μg m−3).

Mercury atomic absorption by mercury atoms in water observed with a liquid core waveguide as a long path absorption cell

Atomic absorption by mercury atoms in water was observed by using a liquid core waveguide (LCW) as a long pass absorption cell. Mercury atoms in an aqueous solution were generated through mixing a mercury standard solution and a NaBH4 solution. Atomic absorption spectrum by the generated mercury atoms in the solution was observed. Also atomic absorption by mercury atoms dissolved in water from the gas phase was detected. The observed mercury atomic absorption spectrum has a broadband structure with peak absorption at 255 nm and a bandwidth of 20 nm. The peak wavelength of mercury atomic absorption in water is very close to that of mercury atomic absorption in the gas phase. The bandwidth of mercury atomic absorption in water is much broader than that of mercury atomic absorption in the gas phase, but narrower than that of the absorption spectrum of simple molecules in an aqueous solution. The possible application of this discovery is discussed. Although the absorption cross-section of mercury atomic absorption in water is much smaller than that of mercury atomic absorption in the gas phase, the application of a LCW as a long path absorption cell for detecting mercury atomic absorption in an aqueous solution significantly improved the sensitivity of mercury detection.

A highly sensitive hexachromium monitor using water core optical fiber with UV LED

A simply structured, cheap hexachromium monitor was developed. The monitor is based on UV/VIS absorption technique. A 2-m long water core optical fiber was employed as a long path length sample cell and a UV light emitting diode (LED) was used as a light source. The emission profile of the UV LED fits very well with the absorption spectrum of chromate ions in water. Therefore, the light-dispersing element, which is usually used in an optical spectrometer, is not necessary in this monitor design. The water core fiber as a long path length makes the monitor highly sensitive for hexachromium detection. This monitor is specific for hexachromium detection without interference from tri-valence chromium ions. A detection limit of 0.1 ng Cr(VI) ml(-1) was obtained with this simple monitor.

Sol-Gel Synthesis of Palladium-Doped Silica Nanocomposite Fiber Using Triton X-100 Micelle Template and the Application for Hydrogen Gas Sensing

Palladium-doped silica nanocomposites were synthesized via a sol-gel technique combined with a template of Triton X-100 micelle. The freshly prepared sol sample of Pd-doped silica nanocomposites was investigated by TEM. Determined from the TEM image, the sizes of the Pd nanoparticles are narrowly distributed, which are around 30 nm in diameter. The prepared sol solution of the sample was injected into a Tygon Microbore Autoanalysis tubing. After 14 days gelatinization, a transparent porous optical fiber was obtained. The response of the fiber to hydrogen gas was tested by using a fiber-optic spectrometric method. The palladium-doped silica nanocomposite fiber is sensitive upon exposure to hydrogen gas and the response is reversible. This palladium-doped silica nanocomposite fiber can be applied as a new kind of hydrogen gas sensor

On-line mercury speciation in exhaust gas by using solid-phase chemical reduction

Speciation of mercury species in exhaust gas from combustion sources is important for both the design of equipment for mercury pollution control and incinerator operation control. A simple, portable atomic absorption spectrometer is described that can monitor, in real time, the mercury species present in stack gas. A SnCl2-loaded reduction column was used to convert molecular mercury to mercury atoms, which were then detected by atomic absorption spectrometry. The adoption of solid-phase reduction of the molecular species simplified the instrument design and made analysis easier. Results presented in this paper demonstrate the potential of the proposed method for field application.