Silicon photonic wire waveguides are usually highly birefringent, so they are generally designed to operate for one
particular polarization. For commonly used waveguides with a silicon thickness of 220 nm, TE polarization is
preferred since TM is only weakly guided. For waveguides with a silicon thickness larger than 250 nm, both TE and
TM polarizations have been employed. Overall, the choice of polarization has largely appeared arbitrary. In this
presentation we review the pertinent polarization-dependent waveguide properties, including losses, back-reflection,
polarization conversion and fabrication tolerances, with the intent to suggest guidelines for choosing the proper
polarization. Through experimental evidence, we show that TM polarization has several important advantages and
can support high performance resonators with a radius down to 2 μm.