Ultra-compact and high-performance polarization beam splitter assisted by slotted waveguide subwavelength gratings

We propose an ultra-short polarization beam splitter (PBS) consisting of two slot waveguides assisted by slotted waveguide subwavelength gratings (SWSWGs), located between the two slotted waveguides. By controlling the optical momentum of evanescent waves with the anisotropic characteristics of the SWSWGs, we considerably suppress and enhance the couplings of transverse-electric (TE) and transverse-magnetic (TM) modes, respectively, concurrently improving performances and reducing length of the proposed PBS, compared with conventional slotted waveguide couplers (CSWCs). Exceptionally, a transition point is found to show almost zero crosstalk between waveguides for the TE mode, i.e., infinite coupling length. Differing from conventional single-material SWGs, the SWSWGs not only simplify the fabrication process but improve polarization extinction ratio (PER). Numerical results demonstrate the improvement in PER_TM (PER_TE) from approximately 13 (23) dB for the CSWCs to 26 (24) dB for the present structure, with a > 70% reduction in device length, operating at the wavelength of λ = 1,550 nm. Our design achieves performance of PER_TM > 25 dB and PER_TE > 20 dB, and insertion loss (IL) < 0.05 dB for TE and < 0.3 dB for TM modes within a bandwidth width (BW) of ~ 50 nm from λ = 1,530 to 1,580 nm. Additionally, geometry deviation is also investigated to assess experimental tolerance. The present idea provides an approach for improving PER, device length, and operating BW of PBSs composed of various waveguide couplers.