Sensing Characteristics Of Refractive Index Using Long Period Fiber Gratings With Metallic Coatings

Fiber grating, including both fiber Bragg grating(FBG) and long period fiber grating(LPFG) is one of the most important passive devices in optical communications, optical sensing and optical information processing technologies. Fiber gratings, especially LPFGs, are very sensitive to the variation of refractive index of external media. Viewing the increasing importance of the refractive index sensing in biochemistry, environment monitoring, medical diagnostics, and oil industry, the fiber grating based sensing technology has been attracting more and more attentions.In this thesis, the fundamental sensing characteristics of LPFGs based devices are presented both theoretically and experimentally. The main content includes: progress of research and development of fiber grating based sensing technology is reviewed firstly; the spectral characteristics of LPFGs with different grating parameters are analyzed using coupled mode theory; the theoretical basis of LPFGs based sensing technology is presented, in which the sensing sensitivities of LPFG on temperature and stress are analyzed. A novel single-ended reflection type of LPFG with metallic coatings is proposed, which breaks the limitation of conventional transmission type of LPFG based sensors. An experimental setup that is used for writing LPFG using CO2 laser is established, in which various LPFGs based refractive index sensors are fabricated, and the sensing characteristics of various sensors are investigated experimentally. It is found that different measurement sensitivities can be obtained using different metallic coatings and different thicknesses of same metallic material. The higher sensitivity can be obtained when the refractive index of the external media is closer to that of the cladding of the fiber. The proposed single-ended reflection type of LPFGs with metallic coatings are found to be more sensitive than the conventional transmission type of LPFGs.