The dependence of absorbance on sample concentration and sample length in evanescent wave spectroscopy, carried out by means of weakly guiding step index optical fiber sensors, is investigated theoretically. The absorbance is found to depend on the product of the sample bulk attenuation coefficient (α), the length of the fiber in contact with the sample (L), and the inverse of the normalized frequency (V) of the sensing section of the waveguide. When the mode volume of the fiber sensor is filled, the absorbance is predicted to vary nonlinearly with concentration (or sample length) at low values of αL/V and to increase as the cube root of αL/V at high values. However, if the mode volume of the sensing section of the fiber is only partially filled, a linear dependence of absorbance on αL/V is indicated. The results of the model are compared with experimental data already published by various groups.
An investigation of the suitability of monomode fluoride glass fibers for the sensing of fluid concentration by evanescent wave absorption in the mid-infrared spectral region is reported. The test system consisted of a HeNe laser ((lambda) equals 3.39 micrometers ), a polished semi- coupler, and a cooled PbSe detector. Evanescent wave absorption in isopropanol was measured for a range of cladding overlayer thicknesses. Results are in broad agreement with the predictions of a theoretical model based on a planar waveguide structure under similar conditions. Calculations based on this model indicate that more elaborate sensor designs are required in order to achieve sufficient sensitivity for the sensing of gaseous species.
The use of low V-number infrared transmitting fibers in the detection of gases, via the attenuation of the evanescent wave power in the porous polymer claddings, is discussed. The attenuation process is modelled, MDC values are predicted for some gases, and various intensity referencing techniques are suggested.
Many fiber optic sensors for chemical species rely on a change in the fluorescence characteristics of a dye in response to its chemical environment. Such sensors often require the complex immobilization of the fluorescent species on or near the fiber surface. A novel process, whereby a highly transparent porous glass can act as a support for sensor dyes, is presented. The sol-gel process for low-temperature fabrication of porous glass has been used as a means of entrapping chemically-sensitive dyes for fiber optic sensor applications. As a proof of principle, fluorescein dye was used to investigate the performance of a pH sensor. Because of its potential for distributed sensing and planar integrated optics sensors, evanescent-wave rather than direct excitation of fluorescence was used. A short length of unclad fiber was coated with a thin layer of porous cladding within which the pH-sensitive dye is entrapped. The sensor displayed maximum sensitivity in the range of pH 3.5 to pH 6.5 with a response time of less than 5 seconds. The suitability of the technique for distributed and integrated-optic sensing of gaseous and liquid species is discussed.
Evanescent wave spectroscopy at 3.3gm on the surface of the core of an IR
transmitting fluoride fibre is used to detect the concentration of flammable
gases such as propane or methane in the environment of the fibre. Three probe
designs are discussed. In one the gas diffuses through the teflon cladding of
a multimode fibre causing an attenuation of the evanescent field and thereby a
reduction in transmittance of the fibre. In the second a short section of
cladding is removed from multimode fibre to access the evanescent field
region. In the third singlemode fibre is used with its cladding reduced
locally by polishing. Referencing is carried out at non absorbing wavelengths
in the 3.2 to 3.6pm region.
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