Study On Chemical Sensing Based On Long Period Fiber Gratings

Optical fiber gratings are widely used in the optical communication and sensor field because of easy fabrication, low cost, little insert loss, small volume. Optical fiber gratings are fabricated as dispersion compensators, band-rejection filters and sensors of temperature and pressure and so on. A long period fiber grating (LPG) can couple the forward propagating core mode to one or a few of the forward propagating cladding modes, and has no backward reflection. It is a typical transmission device. Because LPG resonance peaks are changed with different environment index, the long period fiber grating can be used as the chemical sensor to measure solution concentration or index.This article is mainly about chemical sensing characteristics of long period fiber gratings. We have explained the reason of chemical sensing characteristic of LPG in the theory, and we have done lots of experiments to test our theory. At the same time, we have introduced some advanced research in the LPG's chemical sensing characteristic in the world, and brought forward the realistic value of our research.Firstly, We deduced waveguide modes of single and multimode fibers by use of Maxwell equation. We deduced briefly field distribution of Bragg fiber gratings using couple mode theory. We strictly deduced couple mode theory of LPGs using weak waveguide approximation, and the relation of wavelength and grating period was obtained, which is the useful tool for analyzing optical spectra of LPGs. Combing the relation mentioned above and weak waveguide approximation, we obtained theoretical formula and calculated results affecting optical spectra features of LPGs, and experimental results approaching theory were obtained. Additionally, we analyzed respond of resonance peaks and strength of 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 and fabricate some amplitude masks with different length and period. Through a series of experimental fabrications, we summarized factors of affecting spectra of LPGs including the length of LPGs, grating period, order of couple modes, refractive-index changes of fiber and the quitting of Hydrogen. Then we focused on the experiments of chemical sensing characteristic of long period fiber gratings. According to the experiment results, we drew pictures of wavelength and index. At the same time we put the same long period fiber grating in different solution, and observed the shift of resonance peaks. When LPG was put in water, mode LP06 shifted 3.2um, mode LP05 shifted 1.6um. Through polynomial fit, we can get the concentration of solution according to the shift of resonance peaks.With the C chain increasing,the idex increase from 1.3285 to 1.4185.The wavelength increase 2 nm when one CH2 increased.In industry, sometimes people need measure concentration of some dangerous solution. Long period fiber gratings are very suitable to this measurement. We can master the concentration change on time through observing the shift of resonance peaks.Along with development of study of fiber gratings, the application field of long period fiber gratings will be broader. The properties of long period fiber gratings will be used better.