Research On The Methods Of Enhancing The Refractive Index Sensing Sensitivity Of Long-period Fiber Grating

Long-period fiber grating (LPFG) is a kind of optical fiber device which rose and developed fleetly in recent years, besides some important advantages,such as electrically passive operation, big signal bandwidth, high sensitivity and multiplexing capabilities, low weight, small configuration and suitable to be used in the high temperature, corrosive and dangerous environment, which are common characters belong to general fiber gratings, compared with the fiber Bragg grating(FBG), there is no backward reflection in it, it also has higher temperature, stress, refractive index and curvature sensitivity, so it is widely used in sensing fields. The paper takes LPFG as the object, deeply discussed the methods of enhancing the refractive index sensing sensitivity of LPFG, we also use these methods to detect the nitrite in food. The main research works includes:1. Based on the coupled mode theory, we deduced mode field distribution of long period fiber gratings, and obtained the relationships between the resonant wavelength, transmissivity and the period, length of LPFG, it is the basic to analyze the characteristic of LPFG. We obtained the transmission spectrum by solving the coupled equation of fiber core fundamental mode and first order cladding modes which transmit co-rotating. We took three-layer cylindrical waveguide model as the object, and used the theory of weak waveguide approximation, studied the principle of LPFG refractive index sensing, obtained the relationships between the wavelength of resonance peaks and the period, diameter, ambient refractive index of LPFG in refractive index sensing.2. LPFG, which had different period and length, were fabricated on telecommunication fibers of SMF- 28(Corning) using the method of amplitude mask, we studied the mode coupling of it, the results indicated that resonance wavelength red shifts and amplitude increasing happened in the transmission spectrum with increasing of modulating intensity, there were reorganization of the modes, when the modulation reached a certain degree, the resonance peak on the right became smaller and smaller, it disappeared at last, a new resonance peak appeared on the left, the second cladding mode would be then the third one, the first cladding mode would be the second mode, and so forth.3. The refractive index sensing property of LPFG was studied in the experiments. We obtained the relation between the resonance wavelength shift and the ambient refractive index by measuring a series of sucrose solution with different concentrations: when the refractive index was lower(1.0～1.4), LPFG was insensitive to the change of refractive index; when the ambient refractive index approached to the refractive index of the cladding, LPFG had high sensitivity, so there are limitations of naked LPFG, sensitivity enhancement is necessary in order to use LPFG on the detection of solution concentration, especially some trace substance.4. The paper put forward some methods to improve the sensitivity of refractive index sensing by cladding modification of LPFG. We used the methods, such as etching the cladding of LPFG, depositing thin film by electro-static self-assembly(ESA) and depositing organic polymer of higher refractive index than the cladding by ESA after etching the cladding, to change the quality of LPFG in the experiments, the last method was most effective, the results indicated that the sensitivity of LPFG to ambient refractive index could be enhanced obviously. Compared with the naked LPFG, their sensitivity to ambient was improved as much as 8 times, especially, when the LPFG was in the lower refractive index ambient, great shift (about 150nm) of resonance wavelength of the attenuations bands was happened while the change of ambient refractive index was about 0.1.5. LPFG was used in the detection of the nitrite in foods because of the high sensitivity of LPFG to the lower solution concentrations when it was deposited a thin film. The results indicated that there was no resonance wavelength shifts in the different nitrite solution when the LPFG was naked, but resonance wavelength shift was about 17.5nm when the concentration of the nitrite solution increased from 0.001mol/L to 0.2mol/L after the PDDA/PSS organic polymer was deposited on the LPFG, the lowest measurable nitrite concentration was 0.001mol/L, this value was better than the national standard of nitrite, which was 0.004mol/L.