Compact polarization rotator for silicon-based cross-slot waveguides using subwavelength gratings

A compact and broadband polarization rotator (PR) for silicon-based cross-slot waveguides using subwavelength gratings (SWGs) is proposed and analyzed. To significantly break the symmetry of the waveguide structure, the diagonal regular Si wires of the cross-slot waveguides are replaced with the full etching SWGs. Moreover, the special properties of the SWGs-whose effective index is adjustable-can effectively enhance the modal birefringence between the two lowest-order hybrid modes, resulting in a more compact device. By utilizing interference effect of the hybrid modes, both transverse electric to transverse magnetic (TE-to-TM) and TM-to-TE conversion can be efficiently realized. Numerical results show that a PR of 12.6 μm in length at a wavelength of 1.55 μm is achieved, where the polarization conversion efficiency (PCE) and insertion loss (IL) are, respectively, 97.2% and 0.71 dB, and the reflection loss is below -20.5  dB for both cases. Moreover, a wide bandwidth of ∼260  nm for both polarizations is obtained for keeping the PCE over 90% and IL below 1 dB. In addition, fabrication tolerances to the structural parameters are analyzed in detail, and field evolution along the propagation distance is also presented.