Optical properties of a five-layer slab with long-period gratings.

摘要

In recent years optical devices based on fiber gratings have been widely used in telecommunication systems as well as in various precision measurements as sensing elements. They prove to be effective, robust and low cost solutions. The subject of this thesis is the design and analysis of one of such device---a planar waveguide structure with a long period grating that is sensitive to a small refractive index variation and therefore suitable for all optical switching system.;The two-channel waveguide is investigated theoretically with perturbation theory. We have found an approximate closed form expression for modal indices in that structure. This analytical solution is compared with numerical simulations from a commercial simulation tool, and we find a reasonably good agreement. Then we introduce a long period grating in our mathematical model and evaluate its spectral transmission characteristics. Under certain geometrical and material parameters, the transmission characteristics exhibit dual peak resonance, i.e. a phase matching condition is satisfied at two distinct wavelengths. By optimizing the above parameters, the dual peak resonance can be made very sensitive to small internal refractive index changes. Our numerical simulations show that the transmission spectrum of the five-layer slab with a long period grating exhibits a wavelength shift for the loss peak Δλ = 8 nm for a refractive index change of Δn = 10−6 . Such a high sensitivity makes it attractive for use in all optical switching systems. We discuss possible applications and the effect of parameters fluctuations and physical constrains.