Compact silicon-based polarization-independent directional coupler using subwavelength gratings.
A compact and high extinction ratio polarization-independent directional coupler (PIDC) using silicon-based subwavelength gratings (SWGs) is proposed and analyzed in detail, where the SWG structures are located between the two input/output strip waveguides together with input/output SWG-based transitions. The introduced SWG-based structures can effectively enhance the coupling strength for the TE mode, while that for the TM mode is hardly affected. According to the coupled-mode theory, by carefully choosing the structural parameters, the coupling length for both polarizations can be identical; thus a PIDC is realized. Results show that a PIDC with a coupling length of 9.2 μm is achieved, and the extinction ratio, insertion loss, and reflection loss are 33.25/31.41 dB, 0.224/0.121 dB, and -23.11/-26.31  dB, respectively, for the TE/TM mode. In addition, fabrication tolerances to the key structural parameters are analyzed in detail, and field evolution through the present device is also presented.