Mode conversion in tapered submicron silicon ridge optical waveguides

The mode conversion in tapered submicron silicon ridge optical waveguides is investigated theoretically and experimentally. Two types of optical waveguide tapers are considered in this paper. One is a regular lateral taper for which the waveguide width varies while the etching depth is kept the same. The other is a so-called "bi-level" taper, which includes two layers of lateral tapers. Mode conversion between the TM fundamental mode and higher-order TE modes is observed in tapered submicron silicon-on-insulator ridge optical waveguides due to the mode hybridization resulting from the asymmetry of the cross section. Such a mode conversion could have a very high efficiency (close to 100%) when the taper is designed appropriately. This enables some applications e.g. polarizer, polarization splitting/rotation, etc. It is also shown that this kind of mode conversion could be depressed by carefully choosing the taper parameters (like the taper width, the etching depth, etc), which is important for the applications when low-loss propagation for the TM fundamental mode is needed.