Considerations for the Design of Asymmetrical Mach–Zehnder Interferometers Used as Polarization Beam Splitters on a Submicrometer Silicon-On-Insulator Platform

Asymmetrical Mach-Zehnder Interferometer (MZI) structures on a submicrometer silicon-on-insulator platform are designed and analyzed when used as a polarization beam splitter (PBS). The analysis shows that it is allowed to use 2×2 power splitter/combiner couplers with a balanced ratio (50:50) for only one polarization by selecting the output ports for TE and TM polarizations appropriately. This makes the design much easier and more flexible. Examples for the design of MZI-based PBSs using directional couplers as well as multimode interference couplers are given. Finally, compensating the positive or negative wavelength shifts due to fabrication errors is proposed by using a thermal-optical effect on the two MZI arms separately. The numerical calculation shows that a small temperature increase (e.g., 30°C) is enough to compensate the wavelength shift due to an arm-waveguide width deviation of 50 nm. The parameters of MZI arm waveguides are chosen appropriately in order to make the MZI wavelength shifts (due to the fabrication errors) almost polarization independent, so that the thermal compensation approach could push the central wavelengths for both polarizations back to the designed value.