Long Period Waveguide Grating Refractive Index Sensor Characteristics

Long-period fiber grating (LPFG) is one of most important optical fiber components in a variety of lightwave applications. However, optical fibers are mostly round in shape and made of silica, the geometry and material constraints of fiber impose certain limitations on the functions that LPFG can achieve. To diminish such constraints, long-period grating in thin-film optical waveguide are proposed recently.As we know, in the past decades, considerable research has been devoting to waveguide gratings, and as a result, large amount of optical devices based on waveguide grating has been proposed, improved and widely used in optical communications and sensors. In this thesis, the transmission characteristics and the potential application as refractive index sensors of LPWG are investigated.First, by using coupler-mode theory, the transmission characteristics of corrugated long-period gratings formed in slab waveguide are systematically analyzed. The dependence of field distribution, resonance wavelengths and mode coupling coefficient on structural parameters such as dielectric refractive index and thickness are simulated, and lots of interesting results are shown, which will be a basis for further theoretical investigations and practical applications on LPWGs.Secondly, as we know, at specific wavelengths, there are distinct rejection bands in the transmission spectra of LPWG resulting from the coupling of co-propagating core mode and cladding modes, and these wavelengths are influenced by ambient refractive index due to the fact that the effective refractive index of the cladding modes are dependent on the refractive index of ambient refractive index. Therefore, LPWG can be used as ambient refractive index sensors. By choosing different structure parameters, the responding of resonance wavelengths on the changing of ambient refractive index are investigated. Moreover, we also found the LPWG' s sensitivity to ambient refractive index can be greatly improved by the deposition of an overlay with higher refractive index than that of the above...