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           Fluorosilicic Acid
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           Magnesium fluorosilicate
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SMS (Single mode-Multimode-Single mode) optical fiber compared with fiber Bragg grating structure, the structure is very simple, easy to implement, low cost, so it has been widely used. In this paper, multi-mode fiber optical fiber structure as part of the SMS research object, the use of hydrofluoric acid etching of the cladding corrosion law, corrosion was found 10 minutes, multimode cladding portion is substantially complete corrosion. And through experiments on flat multimode optical fiber structure different lengths SMS refractive index of sodium chloride solution, SMS fiber structure multimode fiber bending length and angle of different transmission spectra in the SMS fiber structure and multi-mode portion of the curved the refractive index of the liquid sodium were measured. The results showed that the longer the length of the straight multimode fiber, the higher the sensitivity, the multimode length of 35mm, the sensitivity 0.04154nm / RIU; when the bending angle is not the same, the shorter the length of the bending, the bending angle of the center wavelength the more sensitive; and when the SMS fiber structure of the same length of multimode fiber is bent, the measured center wavelength than the sensitivity of the refractive index of liquid SMS straight fiber structure sensitivity is higher when 0.0453nm.

Based on LPFG mode restructuring mechanism for thin film caused by the change, studied the effects of fiber radius variation cladding on film sensor characteristics LPFG results showed that, at the same film parameters to reduce the cladding radius can effectively improve sensitivity of the sensor, and the sensor is increased in response to changes in the parameters of the dynamic range of the film, but the sensitizing effect of reducing the radius of the cladding of the sensor decreases with increasing film thickness by hydrofluoric acid etching to reduce the radius of the cladding, electrostatic self-assembly method cladding surface plating PAH / PAA film coating process spectral data output fiber grating confirmed theoretical analysis results show that: a radius of 39 μm, a thickness of 424 nm LPFG of thin-film sensor sensitivity in the detection of the pH value of 3.93 nm / pHU, sensitivity than the standard packet layer of 1-fold increase.