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The characteristics of directional couplers and power splitters based on three-guide optical couplers in slot waveguide structures are analyzed in detail by a three-dimensional full-vectorial beam propagation method. The numerical results show the achievement of a compact three-guide directional coupler operating as polarization independent with a length of 58.0 [micro]m and having almost evenly spaced propagation constants of the three lowest order modes for quasi-TE and quasi-TM modes. Thus, a high coupling efficiency from one outside waveguide to the other outside waveguide is demonstrated and is over 99.5% for both polarization states. For a three-guide power splitter, multiple sets of waveguide parameters for achieving polarization-independent operation are presented. Tolerances to operating wavelength and structural parameters are also analyzed, and the evolution of the injected field along the propagation distance through the proposed devices is demonstrated. OCIS codes: 130.3120, 230.7380, 130.2790, 250.5300.

A compact and broadband silicon-based polarization beam splitter (PBS) is proposed and investigated in detail, where the two arms of the directional coupler (DC) are, respectively, embedded with subwavelength gratings (SWGs) and vertical slots so that field distributions for the TE mode are significantly changed, effectively weakening coupling strength, whereas those for the TM mode are almost unaffected, nearly analogous to the DC with strip waveguides. By carefully optimizing structural parameters, efficient coupling will emerge between the two waveguides for the TM mode, while TE mode will be confined in the SWG-assisted strip waveguide. Consequently, the two modes can be effectively separated, and thus the realization of a PBS is accomplished. Results show that a compact PBS with a coupling length of 6.45 µm is achieved, together with the extinction ratio (ER) of 27.54/31.88 dB, the insertion loss of 0.12/0.14 dB, and the reflection loss of -43.67/-30.50 d B , respectively, for TE/TM mode at the wavelength of 1.55 µm. The bandwidth, for both modes, is up to 230/100 nm when ER is larger than 15/20 dB. In addition, fabrication tolerances to the critical structural parameters and field evolution through the proposed device are analyzed.