Fabrication Of Long-period Fiber Gratting And Study Of Chemical Sensor On It

Long-period fiber grating (LPG) is a new fiber grating, whose period is often from ten micron to one thousand micron. As it has some advantages such as low loss, no back reflecting and being immune to electromagnetic interference, it has a wide range of applications, such as band stop filter, gain flatness, temperature, stress and refractive index sensing and sensor demodulation and so on.If we make the sensor probes by the sensor mechanism, which can be used to detect some certain chemical constituents, it will has important application value.Long-period fiber grating (LPG) is one of transmission gratings whose transmission wavelength depends on the product of grating's effective refractive index and grating's period, and grating's transmission and bandwidth depend on the change in refractive index along grating and the grating's length. Therefore, by designing grating's period and modulating the change in refractive index we can fabricate long period fiber grating with different transmission wavelength and transmission; in addition, long-period fiber grating has particular sensitivity to the surrounding refractive index.so long-period fiber gratings have unique applications in optical communication and sensor field, such as filters and sensors.This article is mainly about chemical sensing characteristics of LPGs.We conclude the mode-coupled theory of LPGs and explain the reason of chemical sensing characteristic of LPG in theory. Besides, we have done lots of experiments to test our theory. In addition, we modified the surface of LPG using Hydrofluoric acid and ionic self-assembled multilayers(ISAM) technique to improve the optical sensitivity of LPG.which were then used to detect the concentration of nitrite. At the end of this paper, we summarized the key technologies required during practical application of LPGs. During this paper, we obtained some results as follow: 1. Firstly, we deduced waveguide modes of single and multimode fibers by use of Maxwell equation. Meanwhile, we deduced briefly field distribution of fiber gratings using couple mode theory. We strictly deduced couple mode theory of FBGs using weak waveguide approximation, and the relation between wavelength and grating period,wavelength and cladding diameter was obtained, which was the useful tool for analyzing optical spectra of LPGs.2. In experiment, high quality long period gratings were fabricated by the method of UV writing using phase mask, before which we processed the fiber on special-purpose to increase the photosensitivity. The UV light was sent out by KrF (248nm) excimer laser. All of these were prepared for the design and fabrication of high quality of long-period fiber gratings.3. We discuss the sensitivity mechanism and response realation. We investigate the chemical sensing property of LPG. According to couple mode theory, we analyse the chemical sensing property of LPG in theory and obtain the relationship between refractive index and the change of wavelength. Meanwhile, in experiment we put LPG in different alcohol solution. When the surrounding refractive index increases, resonance wavelength moves to short wavelength and amplitude decreases. When the surrounding refractive index is bigger than a certain value, resonance wavelength in longer wavelength appears once again and amplitude increases.4. Finally, we point out the practical significance of chemical sensor of LPG. in industry, sometimes people need measure concentration of some dangerous solution. LPGs sensor are very suitable to this measurement. With the development of study of fiber gratings, the LPG has wide application in various fields.