Compact broadband polarizer based on shallowly-etched silicon-on-insulator ridge optical waveguides

A new way to make broadband polarizers on silicon-on-insulator (SOI) waveguides is proposed, analyzed and characterized. The characteristics of the eigenmodes in a shallowly-etched SOI ridge optical waveguide are analyzed by using a full-vectorial finite-different method (FV-FDM) mode solver. The theoretical calculation shows that the loss of TE fundamental mode could be made very low while at the same time the TM fundamental mode has very large leakage loss, which is strongly dependent on the trench width. The leakage loss of the TM fundamental mode changes quasi-periodically as the trench width w(tr) varies. The formula of the period ∆w(tr) is given. By utilizing the huge polarization dependent loss of this kind of waveguide, a compact and simple optical polarizer based on a straight waveguide was demonstrated. The polarizer is fabricated on a 700 nm-thick SOI wafer and then characterized by using a free-space optical system. The measured extinction ratio is as high as 25 dB over a 100 nm wavelength range for a 1 mm-long polarizer.