An ultracompact and low-loss TM-pass polarizer on silicon is proposed and demonstrated experimentally with a subwavelength-grating (SWG) waveguide. The SWG waveguide is designed to support Bloch mode for TM polarization so that the incident TM-polarized light goes through the SWG waveguide with very low excess loss. On the other hand, for TE polarization, the SWG waveguide works as a Bragg reflector, and consequently the incident TE-polarized light is reflected. For a fabricated ∼9  μm long polarizer (with the period number N=20), the measured extinction ratio is ∼27  dB and the excess loss is ∼0.5  dB at the central wavelength 1550 nm. The bandwidth to achieve an extinction ratio of 20 dB is about 60 nm (from 1520 to 1580 nm). When increasing the period number to N=40, the measured extinction ratio is up to 40 dB (which is not as high as the expected theoretical value 65 dB due to the limit of the measurement system).

A low-loss and broadband silicon thermo-optic switch is proposed and demonstrated experimentally by using a Mach-Zehnder Interferometer with 2×2 3 dB power splitters based on bent directional couplers (DCs). The bent DCs are introduced here to replace the traditional 2×2 3 dB power splitters based on multimode interferometers or straight DCs, so that one achieves a coupling ratio of ∼50%∶ 50%, as well as low excess loss over a broadband. The demonstrated Mach-Zehnder switch (MZS) has a ∼140  nm bandwidth for an excess loss of <1  dB and an extinction ratio of >20  dB. The present MZS also shows excellent reproducibility and good fabrication tolerance, which makes it promising for realizing N×N optical switches.

An improved 8-channel silicon mode demultiplexer is realized with TE-type and TM-type grating polarizers at the output ends, and these gratings serve as fiber-chip couplers simultaneously. The present 8-channel silicon mode demultiplexer includes a three-waveguide PBS (for separating the TE0 and TM0 modes) and six cascaded ADCs (for demultiplexing the high-order modes of both polarizations). The grating polarizers with high extinction ratios are used to filter out the polarization crosstalk in the 8-channel hybrid multiplexer efficiently and the measured crosstalk for all the mode-channels of the improved 8-channel mode multiplexer is reduced greatly to ~-20dB in a ~100nm bandwidth.

A compact 64-channel hybrid demultiplexer based on silicon-on-insulator nanowires is proposed and demonstrated experimentally to enable wavelength-division-multiplexing and mode-division-multiplexing simultaneously in order to realize an ultra-large capacity on-chip optical-interconnect link. The present hybrid demultiplexer consists of a 4-channel mode multiplexer constructed with cascaded asymmetrical directionalcouplers and two bi-directional 17 × 17 arrayed-waveguide gratings (AWGs) with 16 channels. Here each bi-directional AWG is equivalent as two identical 1 × 16 AWGs. The measured excess loss and the crosstalk for the monolithically integrated 64-channel hybrid demultiplexer are about -5 dB and -14 dB, respectively. Better performance can be achieved by minimizing the imperfections (particularly in AWGs) during the fabrication processes.

A monolithically integrated 64-channel hybrid demultiplexer on silicon is demonstrated experimentally to enable wavelength-division-multiplexing and mode-division-multiplexing simultaneously for realizing an ultra-large capacity optical-interconnect link. The present hybrid demultiplexer consists of a four-channel mode multiplexer realized with three cascaded asymmetrical directional-couplers and four identical arrayed-waveguide gratings (AWGs) with 16 channels. For the fabricated hybrid multiplexer, the excess loss and the crosstalk are about -7 and -10  dB, respectively. Better performances can be achieved by minimizing the imperfections (particularly in AWGs) in the fabrication processes. The present hybrid demultiplexer is scalable to have more channels by utilizing more wavelengths, modes, and polarizations.