Chemo-sensor Based On Evanescent Wave Of Long Period Fiber Grating

We mainly make some investigations to long-period fiber grating(LPFG) by using its sensitivity of refractive index. First of all, the influences of methylene and hydroxyl group on response of the polyol alcohols to LPFG were explored.the sensing sensitivity was utilized to analyse the response of alcohols to LPFG, the results showed hydroxyl group has a greater contribution than methylene in the shift of central wavelength of LPFG spectral attenuation band. Secondly, through preparing different concentrations of sucrose and fructose solutions and selecting high refractive index of benzaldehyde, ethyl benzoate, triethanolamine, the law of the sensitivity of LPFG to refractive index were studied. The experiments displayed that when the refractive index of the surrounding media is less than that of LPFG cladding, with increasing refractive index, the central wavelengths of LPFG spectral attenuation band gradually shifted toward short-wave. The more is the refractive index of environmental media close to that of grating cladding, the greater the range of movement the central wavelengths is. When the refractive index of the surrounding media is equivalent to that of cladding, there is only one attenuation band in the LPFG transmission spectrum. When refractive index of the environmental media is higher than that of the grating cladding, the central wavelengths of attenuation band gradually shifted toward long-wave, the depths of attenuation band become smaller and the response sensitivity of LPFG deseases. To further improve the sensitivity of LPFG, polyelectrolyte PDDA/PSS films were assembled on the grating surface, with increase of the assembly layers, the center wavelengths of attenuation band gradually move toward short-wave, food additive sodium nitrite was detected by using swelling of polyelectrolyte PDDA/PSS films, the experiments showed LPFG assembled thin films had a better response to sodium nitrite concentrations over the range of 0.001-0.2mol/L, the central wavelengths of attenuation bands moved to the long-wave as the concentrations increased, and the responses has a better reversibility.In order to explore the response law of LPFG assemblyed films to refractive index of ambient media, polyelectrolyte PDDA/PSS and PAH/PSS films were assembled onto the surface of LPFG cladding. Methanol, ethanol, isopropanol, air and water were used surrounding media of LPFG cladding. The experiments indicated LPFGs assembled PAH/PSS films have an optimal sensitivity following isopropanol, ethanol, methanol, water, air in order, respectively. LPFG assembled PDDA/PSS films made an appearance of the best sensitivity region following to isopropanol, ethanol, air, methanol, water, repectively, that is, there are the best sensitive regions to different refractive index of materials. Based on above appearances, we infer that the sensitivity law of LPFG assembled to different refractive index, when bilayers number is lesser, the films thickness is smaller, the average refractive index of films and media material of the higher refractive index is close to that of LPFG cladding. Under this circumstance, LPFG is sensitive to higher refractive index of material. With the rise of the assembly bilayers number, polyelectrolyte film thickness further increases, making average refractive index of films and a relatively low refractive index material approach that of LPFG cladding. In the case, LPFG is more sensitive to the lower refractive index material. LPFG assembled films incarnated selectivity of different refractive index by phenomena of the best sensitive performance in different regions to different refractive index. Therefore, by controlling assembly layers number to make LPFG have the best sensitivity to different refractive index, selectivity of LPFG to different refractive index of environmental media can be realized. This provides a good way for resolving poor specificity of LPFG, and is promising to play a larger role in the chemical and biological testing in the next step. Due to its high immunity to electromagnetic interference, low background reflection, minor insertion loss, light weight, small size, corrosion-resistant, high sensitivity, to ambient media characteristics of easily implanting and connecting with other the device, LPFG will show more and more promising prospects for applications in a wide range of nondestructive and fast on-line real-time detection of chemical and biological detection and industry measurement in the future.