A compact silicon-based polarization demultiplexer (P-DEMUX) is proposed by using a microring resonator, where the metal-loaded strip waveguides are used as the bus channels. The modal characteristics of transverse electric (TE) modes for the hybrid plasmonic waveguides are analogous to those for the strip dielectric ones, while those of transverse magnetic (TM) modes show significant differences, leading to strong polarization-dependence. Consequently, the input TE mode can efficiently output from the drop port at the resonant wavelength, while the input TM mode directly outputs from the through port with nearly negligible coupling. The present P-DEMUX can be easily employed to construct an on-chip wavelength/polarization division multiplexing optical transmission system, further increasing the optical interconnect capacity. Results show that a P-DEMUX with a radius of 2.003 μm for the microring is achieved at the wavelength of 1550 nm, where the extinction ratio and insertion loss are, respectively, ∼22.36 (20.09) dB and ∼0.39 (0.84) dB for the TE (TM) mode. In addition, fabrication tolerances to the key structural parameters are investigated and field evolution through the P-DEMUX is also presented.
A compact polarization beam splitter (PBS) for silicon-based slot waveguides is proposed based on a multimode interference coupler, where an asymmetrical multimode waveguide (AMW), cut by a right angle at one corner, is employed to efficiently separate the TE and TM modes. With the unique modal properties of the slot waveguides and corresponding optimized designs, the input TE mode can almost pass through the AMW and then enter into the bar port, while a mirror image is formed at the cross port for the input TM mode due to the self-imaging effect. Meanwhile, tapered waveguide structures and S-bend are incorporated into the designed PBS for further enhancing the performance. According to the numerical results, a PBS with an AMW of 2.3 μm in length is achieved, where the extinction ratios are 16.6 and 20.9 dB, respectively, for TE and TM modes, and the insertion losses are 1.37 and 0.81 dB, respectively, at the wavelength of 1.55 μm. For keeping extinction ratios over 15 and 20 dB for TE and TM modes, the bandwidths are around 59 and 73 nm, respectively, both covering the entire C-band. In addition, field evolution along the propagation distance through the PBS is also demonstrated.
A design scheme of compact polarization rotator (PR) for silicon-based slot waveguides is proposed, where the slot including the upper claddings is filled with liquid crystals (LCs). With this design, the transverse field components of eigenmodes supported by the slot waveguides have almost identical amplitudes due to the anisotropic features of the LCs, leading to a high modal hybridness, which plays a pivotal role in designing an efficient PR. As a consequence, the input TE (TM) mode can be converted efficiently to the TM (TE) mode at the output port within a short length. The numerical results show that a PR of 11.3m in length at the operating wavelength of 1.55m is achieved with the extinction ratio and insertion loss of 12.6 (11.5) dB and 0.22 (0.30) dB, for TE-to-TM (TM-to-TE) conversion, respectively. Moreover, field evolution along the propagation distance through the PR is also demonstrated.
A compact crossing for silicon-based slot and strip waveguides is proposed by utilizing a strip-multimode waveguide (SMW) crossing at the center and two logarithmically tapered slot-to-strip mode converters at the ports with slot waveguides. For input/output ports with slot waveguides, the guided modes are efficiently transformed through the mode converter and then enter into the SMW (without mode converters for those ports with strip waveguides), where the fields converge at the center of the intersection due to the self-imaging effect. Hence, the size of the input beam is much smaller than the width of the SMW at the crossing center, leading to significant reductions of the crosstalk and radiation loss. The numerical results show that a hybrid waveguide crossing operating at the wavelength of 1.55 μm with the insertion loss, crosstalk, and reflection of 0.14/0.164 , −35.88/−38.79, and −35.35/−40.5 dB for input ports with slot/strip waveguides, respectively, is achieved. Moreover, the fabrication tolerances to the structural parameters are investigated by using a finite-difference time-domain method and evolution of the injected field along the propagation distance through the crossing structure is also demonstrated.
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