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Optical Fiber Sensing

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Optical Fiber Sensing
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  6.Discussion The sensor is based on the phenomenon of evanescent wave absorption where a small portion of the optical power in the guided modes, extended to the cladding region, interacts with the coated sensing thin film of PVA and CoCl2. A dry CoCl2 has a strong absorption around 690 nm. Thus, at low humidity, CoCl2 is dry and a strong absorption of light passing through the sensing fiber element was observed. Whereas, at higher humidity, CoCl2 absorbs lesser optical power, leading to an increase in the output power. Throughout the experiment, we tried to vary the humidity quite slowly to carefully monitor the sensor response at different levels of humidity. In order to determine specifically the effect of film configuration and optimize the sensor performance was examined by varying the film thickness. This variation in the sensing film thickness was realized by repeated dip coating on the  bare fiber core. We fabricated sensor with three film thicknesses using 2-dip and 3-dip coatings. After each coating, the probe was left for more than 24 h to dry before the next coating. 4.Observation The following observation can be made from these results. First and most important, the increase in the film thickness as we go from 1-dip to 2-dip, increases the sensitivity. To explain the existence of the optimum film thickness, it should be noted that a thicker film would contain higher CoCl2 concentration. Thus, as the film thickness was increased from 1-dip to 2-dip, the increased CoCl2 concentration in the cladding region resulted in higher interaction with water vapour and maximum strength of intensity modulation, hence increasing the sensitivity. More importantly, the film thickness for 1-dip coated probe was much less than the penetration depth of the evanescent wave ( d   p), whereas that for 2-dip was of the order of it. This lead to the maximum possible interaction of evanescent wave in the case of 2-dip coated probe. A dry CoCl2 has a strong absorption in the wavelength ranging from 550 to 700 nm and has almost no absorption if wet, forming a compound CoCl2·6(H2O). Hence, anhydrous CoCl2 was immobilized in a suitable polymer matrix and dip-coating method was used to realize a thin sensing film. Poly vinyl alcohol (PVA) was used as the binding polymer matrix for anhydrous CoCl2. 2.Introduction Measurement of humidity is required in a numerous range of applications,   including the chemical and food processing industry, air-conditioning,   horticulture, and electronics processing. Thus, the relative humidity measurement has been extensively studied over last decades. A remarkable progress in employing optical fiber technology for the development of relative humidity sensors has been made. However potentially competent conventional like capacitance based, wet-dry bulb and solid-state humidity sensors are also widely deployed and extensively used. But they are complicated to fabricate and have disadvantages like they are cost effective,  sensitive to electrostatic discharge and EM signals, and are limited by reversible shift if operated at high RH. Thus, because of considerable and distinct advantages of optical fiber sensors over the conventional sensors like immunity to electromagnetic noise, small size, remote and distributed sensing etc., so a wide range of optical fiber (OF) humidity sensors have been in use practically.  3.Working Principle: The aim of this project is to realize an optical fiber RH sensor with maximum achievable sensitivity and large operational range. An evanescent wave model is used. The cladding of the plastic-clad silica (PCS) multi-mode fiber was removed from the central portion of the fiber, where a highly humidity sensitive film was then deposited carefully. As the light at a wavelength close to the peak absorption wavelength of the sensing film propagates through the sensing region, the optical power in evanescent tail of the propagating mode is absorbed with the change in the humidity. This results in a modulated output from the fiber, which is used as the criterion for detecting and determining the relative humidity in the surrounding environment. 5.Results: The sensor response is approximately linear in the measured RH range accept at very low humidity values as the sensor responds slowly at lower humidity (till ∼ 30%) and very quickly at higher humidity. Thus the sensitivity increases steeply and linearly with RH in a range from ∼ 30 to 80%. 1.Materials required:    Optical fibre,    Air tight container    Detector    Monochromatic light source    Hygroscope    Poly vinyl alcohol(PVA)    Cobalt chloride    Hair dryer    Steam source    Multimeter
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