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.
A polarization splitter (PS) based on a directional coupler (DC) composed of vertical multiple-slotted waveguide (MSW)
and silicon nanowire is proposed and designed by using a modified three-dimensional full-vectorial beam propagation
method (BPM). By utilizing the unique modal properties of the slot waveguide, the coupling of the DC in quasi-TE
modes can be almost neglected, and only that in quasi-TM is considered, leading to a compact PS. Moreover, the MSW
has the advantages of both strong field confinement and high birefringence, which improves the performance of the
present device. The numerical results show that a PS with a length of 20.5μμm in the coupling region is achieved whose
bandwidth is up to 22nm (ranged from 1.540 to 1.562μm) with the crosstalk lower than -20dB. In addition, the
fabrication tolerances to the structural parameters are investigated in detail. And the evolution of the injected field along
the propagation distance through the PS is also demonstrated.
A three-guide directional coupler composed of two slot waveguides and one silicon wire with a function of transverse electric (TE)-pass/transverse magnetic (TM)-splitting is proposed and designed using a modified 3-D full-vectorial beam propagation method. In our design, quasi-TE modes fed in the middle silicon wire pass through without coupling, whereas quasi-TM modes fed in the same channel are evenly split into the outer slot waveguides. With properly chosen parameters, a TE-pass/TM-splitting directional coupler with a length of 22.6 μm, including the straight coupling region and the S-bends, is achieved with the crosstalk of −31 and −35 dB for quasi-TE and quasi-TM modes, respectively. In addition, the tolerances to the structural parameters and operating wavelength are also investigated in detail.
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