STUDIES ON LONG-PERIOD GRATINGS IN OPTICAL WAVEGUIDES

Abstract

In this thesis, an ultra high wavelength band rejection filter based on Long Period Waveguide Gratings is proposed. A long-period grating is capable of coupling light between the guided mode and the co-propagating cladding modes at specific wavelengths and hence results in a series of sharp rejection bands in the transmission spectrum. Long-period grating in fibers, i.e., long-period fiber gratings (LPFGs) have been extensively studied and found numerous applications, such as filters, gain flatteners for erbium-doped fiber amplifiers, and sensors, etc. However, the geometry and material constraints of a fiber impose significant limitations on the functions that an LPFG can achieve, especially on the realization of active devices. Moreover, optical fiber is not suitable for making compact low-cost devices and does not satisfy the demand for mass production and integration. Therefore, to remove the constraints of the fiber and develop integrated-optical filters, long-period waveguide gratings (LPWGs), i.e., long-period gratings formed in planar waveguides have been proposed recently. Although the light-coupling mechanisms in an LPWG and an LPFG are basically the same, LPWGs exhibit much richer optical characteristics because of the additional degrees of freedom available in the design and fabrication of optical waveguides. The V design, of LPWGs in various waveguide geometries constitutes the major part of this thesis. To start with, by using the coupled-mode theory, general formulae for the calculation of the characteristics of an LPWG, including the resonance wavelength, the coupling coefficient, and the transmission spectrum are derived. The general theory is next applied to the analysis of LPWGs in slab waveguides. Extensive simulation results are presented to highlight the features of these LPWGs. It is found that LPWGs in different waveguide structures exhibit different characteristics. For instance, the cladding thickness is an effective parameter for the control of the transmission properties of LPWGs in slab waveguides. The possibility of controlling the transmission spectrum of an LPWG device by controlling the profile of the waveguide cladding is investigated. We show that a low-index trench in an otherwise uniform cladding can result in interesting transmission properties. We obtained an ultra wide-band rejection in the transmission using such profile. The study should be useful in designing integrated-optics wavelength filters.