Design, Fabrication and Measurement of SOI-based photonic wire devices, including Bragg-grating structures

Abstract

Summary form only given: We present photonic wire device structures based on silicon on insulator (SOI) - in particular tapered Bragg gratings. Different types of taper specification have been introduced to enable efficient coupling of light into the grating region. The structure has been simulated using the 2D FDTD method in the Fullwave software from Rsoftreg- and shows substantial improvement in reflectivity within the stop-band. Reductions in the spectral side-lobe levels have been obtained. In recent years, photonic wires have become one of the major fields of research due to their compactness and capability of reduced losses. Their high index contrast makes them suitable for design requirements such as sharp bends, T-junctions and Mach-Zehnder structures. The waveguide Bragg gratings have been widely studied and understood. They provide high optical confinement due to the high index contrast between the core and its surrounding cladding, which leads to a small volume and a compact structure. Photonic wires have great potential for applications such as distributed Bragg reflectors. wavelength division multiplexing and non-linear optics. Bragg gratings and waveguide micro-cavities have been realised at 1.55 mum with clear controllability of the stop band. 2D and 3D FDTD simulation methods have been studied and analysed recently for these structures, with relatively good agreement. Simulation of the first order tapered Bragg gratings has been carried out the using the Fullwave 2-D FDTD simulation tool. A rectangular recess structure was introduced into the sidewalls of the waveguide with 4-period tapered structures of various types at the input and the output of the grating. The modelled tapered Bragg grating waveguide has a 32 period grating (Lambda=360 nm), recess=250 nm and used a 500 nm width photonic wire with a 260 nm thick silicon (Si) core. The grating period is derived from the famous Bragg equation, Lambda=mlambda0/2neff where Lambda is a grating period, neff=effective index, lambda0=free space wavelength and m=1 for first order grating. This study shows that the tapering of the amplitude has increased the reflectivity to almost 95% for both linear and parabolic tapers. But the oscillations in the side lobe structure in the longer wavelength region are large, due to the length of the grating region. Non-symmetrical behaviour between the side- lobes is observed, but the situation is further improved by means of tapering both the amplitude and the phase of the gratings. A reflectivity of >96% is observed, together with reduction of the side lobe oscillations in the longer wavelength region by almost 50%. Photonic wire tapered Bragg grating based on silicon on insulator have been successfully simulated- and show an improvement of reflectivity by almost 10%, as compared with the classical waveguide Bragg Grating. 2D FDTD simulations are preferable because of the reduced time consumption and memory requirements, when compared with 3D FDTD simulation. They show good a predictable trend for the stop band. We now plan to investigate different types of taper structure (i.e. linear, parabolic and cosine) using 3D FDTD simulation. Comparisons will be made to determine the accuracy of the 2D and 3D FDTD simulations and how these compare with the results of measurements