Study of the Local Oxidation of Silicon Waveguide for Realizing the Intra-Guided Mode Conversions

Abstract

Abstract The impressive achievements in research and developments of silicon-on-insulator (SOI) based silicon photonic integrated (PIC) devices have been made, but the coupling loss between the fibre and SOI waveguide is still a fatal problem. As well known, the butt coupling between a fibre and a low index-contrast silica-on-silicon (SOS) waveguide only causes a much lower optical loss, however, such a process between a fibre and a high index-contrast SOI waveguide creates a much higher optical loss. In this work, a 2-step intra-guide mode conversion of fibre-SOS-SOI structure is modelled and studied, in which a mitigating structure of SOS waveguide is created via a local oxidation of silicon (LOCOS) technique. The LOCOS method is an in-situ chemical process during the waveguides are fabricated without causing any extra fabrication, but it can control the physical parameters of the SOS waveguide through a chemical process. In such a 2-step intra-guide mode conversion, the total optical efficiency of the fibre-SOS-SOI structure is modelled, and the optical efficiencies of both the fibre-SOS mode conversion and the SOS-SOI mode conversion are systematically studied, and the broad dependences of guided mode profile on the chemical parameters of the LOCOS process are discussed. As a result, a total optical loss of <0.5dB for a butt coupling process between fibre and single-mode SOI-waveguide is achieved. So far, no similar practical butt-coupling method has been reported yet.