A dual-polarization 10-channel mode (de)multiplexer is proposed and realized with cascaded dual-core adiabatic tapers on a silicon-on-insulator (SOI) platform. The mode demultiplexer has a 2.3 μm-wide multimode bus waveguide, which supports six mode-channels of TE polarization and four mode-channels of TM polarization. These ten mode-channels are (de)multiplexed with five cascaded dual-core adiabatic tapers based on SOI nanowires. The widths for these dual-cores are chosen optimally according to the dispersion curves of the dual-core SOI nanowire, so that the desired highest-order modes of TE-and TM-polarizations are extracted simultaneously. These two extracted mode-channels are coupled very efficiently to the fundamental modes of TE-and TM-polarizations (TE 0 and TM 0 ) in the narrow waveguide, respectively, which are then separated by using a polarization beam splitter based on bent directional couplers. A chip consisting of a pair of 10-channel mode (de)multiplexers is fabricated and then tested with data transmission of 30Gbps/channel. The measurement results show that all TM-and TE mode-channels have low crosstalks (-15ß-25 dB) and low excess losses (0.2ß1.8 dB) over a broad wavelength band of ß90 nm, which makes it WDM (wavelength-division-multiplexing)-compatible and thus suitable for high capacity on-chip optical interconnects.

A three-mode (de)multiplexer based on two cascaded asymmetric Y junctions is proposed and experimentally demonstrated on a silicon-on-insulator platform for mode-division multiplexing applications. Within a bandwidth from 1537 to 1566 nm, the best demultiplexing crosstalk of the fabricated device, composed of a three-mode multiplexer, a multimode straight waveguide, and a three-mode demultiplexer, is up to -31.5  dB, while in the worst case it is -9.7  dB. The measured maximum insertion loss is about 5.7 dB at a wavelength of 1550 nm. The mode crosstalk and insertion loss can be further improved by high-quality fabrication processes.

A silicon-based on-chip reconfigurable optical add-drop multiplexer (ROADM) is presented for hybrid wavelength-division-multiplexing-mode-division-multiplexing systems. The present ROADM consists of a four-channel mode demultiplexer, four wavelength-selective thermo-optic switches based on microring resonators, and a four-channel mode multiplexer. With the present ROADM, one can add/drop one of wavelength channels of any mode to/from the multimode bus waveguide successfully with an excess loss of 2-5 dB and an extinction ratio of ∼20  dB over a wavelength range of 1525-1555 nm.

A lateral-apodized add-drop filter is demonstrated in a multimode asymmetric waveguide Bragg grating. This design utilizes two individual superposed gratings with the same sidewall corrugation depth. The strong side lobes of the grating filter are efficiently suppressed by mapping the target apodization profile into lateral shifts between the periods of the two gratings. Compared with other apodized technology, this device is easier to be realized. Experimental results show that the side-lobes suppression ratio can reach 18.5 dB, and a bandwidth of 9.5 nm is achieved by a large corrugation width of 150 nm. The insertion loss at the drop port is only 0.8 dB, and the extinction ratio is up to 24 dB at the through port.

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