Plasmonic waveguides, as a competitive candidate, have been widely studied in rapid developing photonic integrated circuits (PICs) and optical interconnection fields. However, crosstalk between plasmonic waveguides is a critical issue that has to be considered in practice. Actually, crosstalk dominates the ultimate integration density of the planar photonic circuits. This paper reviews the recent research work on evaluation methods and crosstalk suppression approaches of plasmonic waveguides. Three crosstalk evaluation methods based on comparison of specific parameters of waveguides have been summarized. Furthermore, four specific approaches to reduce crosstalk have been illustrated as two categories according to their impacts on waveguide performances and the whole circuit. One means of crosstalk suppression is changing the placement of waveguides, which could maintain the transmission characteristics of the original waveguide. The other means is inserting medium, which has the advantage of occupying smaller space compared to the first method. Consequently, to suppress crosstalk between plasmonic waveguides, one should choose suitable approach.
In optical interconnection field, inconsistent coupling behaviors of TE and TM modes may cause abnormal operation. Therefore, it is very significant to implement a polarization-independent directional coupler. In this paper, a hybrid-plasmonic-waveguide-based polarization-independent directional coupler has been proposed. Since the energy of the hybrid plasmonic waveguides for TE and TM modes is distributed in different layers, we manage to achieve polarization independence by adjusting the material properties and dimensions of the corresponding layer. We optimize the parameters of our proposed directional coupler such as radius of silicon waveguide layer, height of silica waveguide layer, and the distance between the two waveguides, etc. Then performance of the directional coupler has been evaluated. It is worth mentioning that the length of the coupling section is only 4.25 μm. Meanwhile, the polarization-dependent loss is only 0.393 dB, and the maximum coupling efficiency of TE and TM modes can reach 86.4% and 94.6%, respectively. Besides, the coupling efficiency of TM mode remains above 90% over the entire C-band, while the coupling efficiency also keeps at least 80% for TE mode. Finally, the manufacture process for the proposed directional coupler has been discussed. In brief, the improved polarization-independent directional coupler features small size, low energy loss, good polarization-independent characteristics, and wavelength insensitivity simultaneously. Compared to the other counterparts ever proposed, our proposed coupler can provide a perfect trade-off between device size and loss, which shows important potential applications in the fields of PIC and optical interconnection.
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