Study On Chemical Sensing Based On Long Period Fiber Gratings

Optical fiber gratings are widely used in the optical communicationand sensor field because of easy fabrication, low cost, littleinsert loss, small volume. Optical fiber gratings are fabricatedas dispersion compensators, band-rejection filters and sensors oftemperature and pressure and so on. A long period fiber grating (LPG)can couple the forward propagating core mode to one or a few of theforward propagating cladding modes, and has no backward reflection.It is a typical transmission device. Because LPG resonance peaksare changed with different environment index, the long period fibergrating can be used as the chemical sensor to measure solutionconcentration or index.This article is mainly about chemical sensing characteristics oflong period fiber gratings. We have explained the reason of chemicalsensing characteristic of LPG in the theory, and we have done lotsof experiments to test our theory. At the same time, we haveintroduced some advanced research in the LPG's chemical sensingcharacteristic in the world, and brought forward the realisticvalue of our research.Firstly, We deduced waveguide modes of single and multimode fibersby use of Maxwell equation. We deduced briefly field distributionof Bragg fiber gratings using couple mode theory. We strictlydeduced couple mode theory of LPGs using weak waveguideapproximation, and the relation of wavelength and grating periodwas obtained, which is the useful tool for analyzing optical spectraof LPGs. Combing the relation mentioned above and weak waveguideapproximation, we obtained theoretical formula and calculatedresults affecting optical spectra features of LPGs, andexperimental results approaching theory were obtained.Additionally, we analyzed respond of resonance peaks and strengthof LPGs with refractive-index change of surrounding's media.Moreover we show that LPGs can be used as chemical sensor.Secondly, based on all the theories described above, we design andfabricate some amplitude masks with different length and period.Through a series of experimental fabrications, we summarizedfactors of affecting spectra of LPGs including the length of LPGs,grating period, order of couple modes, refractive-index changes offiber and the quitting of Hydrogen. Then we focused on theexperiments of chemical sensing characteristic of long period fibergratings. According to the experiment results, we drew pictures ofwavelength and index. At the same time we put the same long periodfiber grating in different solution, and observed the shift ofresonance peaks. When LPG was put in water, mode LP06 shifted 3.2um,mode LP05 shifted 1.6um. Through polynomial fit, we can get theconcentration of solution according to the shift of resonancepeaks.