To further increase the capacity of the optical transmission system, the hybrid mode- and polarisation-division multiplexing (MDM-PDM) technology has been proved to be an efficient approach by multiplexing dual polarisations for each orthogonal eigen mode. A hybrid (de)multiplexer [(De)MUX] is one of the most important fabrics for the hybrid MDM-PDM networks. A compact silicon 10-mode hybrid (De)MUX is proposed based on three cascaded asymmetric directional couplers (ADCs) based sections, three adiabatic tapers, and a polarisation beam splitter (PBS). The phase-matching conditions can be achieved by varying the widths of the bus waveguides for the TM modes and then by varying the widths of the access waveguides for the TE modes. The simulated results show that a compact total coupling length for TM1 ~ TM3 and TE1 ~ TE5 modes can be achieved to be 55.4 μm. In addition, the total loss of the proposed hybrid (De)MUX can be reduced benefitting from the fewer tapers compared with the conventional cascaded ADCs. The PBS is also optimised with a compact length of 7.0 μm and high extinction ratios of 32.9 dB and 15.4 dB for the TM0 and TE0 modes, respectively.

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A silicon mode-selective switch (MSS) is proposed by using a horizontal metal-oxide-semiconductor (MOS) capacitor incorporated with the epsilon-near-zero (ENZ) indium-tin-oxide (ITO). The carrier concentration of the double accumulation-layers in ITO can be adjusted via the applied gate-voltage to achieve the desired switching state. The MOS-type mode of the central MOS-capacitor based triple-waveguide coupler is introduced and optimised by using the full-vectorial finite element method to switch the “OFF” and “ON” states. The thickness of the accumulation layer and the optimal design are studied by using the 3D full-vectorial eigenmode expansion method. The optimised quasi-TE0 and quasi-TE1 modes based MSSes are with the extinction ratios of 28.52 dB (19.05 dB), 37.29 dB (17.8 dB), and 37.29 dB (23.7 dB), at “OFF” (“ON”) states for the accumulation-layer thicknesses of 1.5, 5.0, and 10.0 nm, respectively. The operation speed can achieve to be 6.3 GHz, 6.2 GHz, and 6.2 GHz for these three accumulation-layer thicknesses, respectively. The performance of the proposed MSS with a 2.5 V gate-voltage is also studied for preventing the oxide breakdown. The proposed MSS can be applied in the mode-division-multiplexing networks for signal switching and exchanging.

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