Sensing And Filtering Characteristics Of Novel Long Period Fiber Grating

Long period fiber grating (LPFG) is one of the most important passive devices in optical communications, optical sensing and optical information processing technologies.As sensors, 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, we developed a four-layer theory that is capable of calculating LPFGs with real metallic coatings using matrix method on consideration of simplicity and flexibility of calculation. In the four-layer step-index model, the effective refractive index of cladding mode which may excite a surface plasma wave (SPW) was calculated, and the spectral characteristics of LPFGs with an nm-thick metallic film, which has complex refractive index with very small real part and very high imaginary part, are investigated in detail. The problem of unlimited bound of secondary type of Bessel function in the case of large argument that encountered in the four-layer matrix model was solved. With the model, the transmission resonant spectrum of LPFGs with arbitrary metallic coatings can be theoretically obtained. The sensitivity, i.e., the resonant transmission wavelength shifting with the change of refractive index of the external medium can be quantitatively derived at different thicknesses of the overlay coatings. It is found theoretically that the LPFGs with gold coating are more sensitive than the naked LPFGs .The optimal measurement sensitivity can be obtained when the gold thickness is ~20 nm. If the refractive index of the thin film coating on the long fiber grating is appropriately designed, surface plasmonic resonance occurs which will significantly enhance the detecting sensitivity of the external refractive index.Fiber gratings are important devices in many applications such as communication systems, all-fiber tunable lasers, and fiber sensors. In this thesis, a novel strain based tunable bandpass filter using phase-shift long period fiber grating (PS-LPGs) has been proposed and demonstrated. The PS-LPGs were fabricated with different physical deformations based on line by line technology with the line beam ~200μm wide and ~ 6 mmlong, which can be easily adjusted to irradiate on the fiber. The characteristics and dependence of the wavelength tunability on the physical deformation of the long period grating is examined quantitatively. It is found that the wavelength tunable range can reach ~10nm with a 16.5 mm-long grating of 37 periods. The transmission and the bandwidth of the bandpass filter maintain unchanged during the tuning process, and is also temperature insensitive. This novel strain-based bandpass filter with wide tunability provides a more flexible way than that of temperature-based tuning.