Polarization-selective microring resonators.

Polarization-selective microring resonators (MRRs) are designed and demonstrated to work with resonances for only one of TE and TM polarizations for the first time, which can be used for realizing the crossing-free, compact and scalable multi-wavelength coherent receiver with dual polarizations. The present polarization-selective MRRs are realized by manipulating the polarization-dependence of the bending loss of the micro-resonators and the coupling ratio of the bent directional couplers introduced here. The demonstrated TM-type MRR and the TE-type MRR work well as a wavelength-selective optical filter for TM polarization and TE polarization, respectively, as designed. On the other hand, the resonance of the undesired orthogonal polarization mode is depressed significantly with very high extinction ratio. For the realized TM-type MRR, the peak-transmission at the drop port is depressed to be lower than -50dB and the loss of the transmission at the through port is very low (<0.5dB) when TE polarization mode is input. For the realized TE-type MRR, the peak-transmission at the drop port is depressed to be ~-40dB and the loss of the transmission at the through port is very low (<0.5dB) when TM-polarization mode is input. The present polarization-selective TM- and TE-type MRRs are useful for the applications with multiple wavelengths and dual polarizations.

Low-loss and broadband 2 × 2 silicon thermo-optic Mach-Zehnder switch with bent directional couplers.

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.

Compact monolithically-integrated hybrid (de)multiplexer based on silicon-on-insulator nanowires for PDM-WDM systems.

A compact silicon hybrid (de)multiplexer is designed and demonstrated by integrating a single bi-directional AWG with a polarization diversity circuit, which consists of an ultra-short polarization-beam splitter (PBS) based on a bent coupler and a polarization rotator (PR) based on a silicon-on-insulator nanowire with a cut corner. The present hybrid (de)multiplexer can operate for both TE- and TM- polarizations and thus is available for PDM-WDM systems. An 18-channel hybrid (de)multiplexer is realized with 9 wavelengths as an example. The wavelength-channel spacing is 400GHz (i.e., Δλ(ch) = 3.2nm) and the footprint of the device is about 530µm × 210µm. The channel crosstalk is about -13dB and the total excess loss is about 7dB. The excess loss increases by about 1~2dB due to the cascaded polarization diversity circuit in comparison with a single bi-directional AWG.

Experimental demonstration of simultaneous mode and polarization-division multiplexing based on silicon densely packed waveguide array.

A silicon mode and polarization-division multiplexing scheme based on a densely packed waveguide array structured as a bus waveguide is introduced. A short adiabatic taper is adopted for (de)multiplexing. Such a structure shows theoretical insertion losses that are <0.05 dB and crosstalk that is <-20 dB over a wide wavelength band for all five supported modes. The structures for (de)multiplexing are fabricated and characterized experimentally. A device, which consists of a multiplexer, a 50-µm-long straight-bus waveguide, and a demultiplexer, exhibits insertion losses that are <0.6 dB and crosstalk that is <-15 dB over an 80 nm wavelength band. The demonstrated (de)multiplexer has a total length of 60 µm, and the bus waveguide has an effective width of 1.58 µm.

Improved 8-channel silicon mode demultiplexer with grating polarizers.

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.

Silicon hybrid demultiplexer with 64 channels for wavelength/mode-division multiplexed on-chip optical interconnects.

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.

High-order microring resonators with bent couplers for a box-like filter response.

High-order microring resonator (MRR) filters with bent directional couplers are proposed and demonstrated to achieve a box-like filter response. When using bent couplers, the coupling ratio can be adjusted easily by choosing the length of the coupling region, and the excess loss is almost zero while the perimeter of the microring length is unchanged. For the present fabricated five-microring filters with bent directional couplers, the excess loss is less than 1.0 dB, the out-of-band extinction ratio is ∼36 dB, and the response has rising and falling edges as sharp as 48 dB/nm. The thermal tunability of the high-order MRR filter with a Ti-microheater is also demonstrated and the thermally tuning efficiency is about 0.10 nm/mW.

Low-loss ultracompact transverse-magnetic-pass polarizer with a silicon subwavelength grating waveguide.

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).

EPMF: Efficient Perception-Aware Multi-Sensor Fusion for 3D Semantic Segmentation.

We study multi-sensor fusion for 3D semantic segmentation that is important to scene understanding for many applications, such as autonomous driving and robotics. For example, for autonomous cars equipped with RGB cameras and LiDAR, it is crucial to fuse complementary information from different sensors for robust and accurate segmentation. Existing fusion-based methods, however, may not achieve promising performance due to the vast difference between the two modalities. In this work, we investigate a collaborative fusion scheme called perception-aware multi-sensor fusion (PMF) to effectively exploit perceptual information from two modalities, namely, appearance information from RGB images and spatio-depth information from point clouds. To this end, we first project point clouds to the camera coordinate using perspective projection. In this way, we can process both inputs from LiDAR and cameras in 2D space while preventing the information loss of RGB images. Then, we propose a two-stream network that consists of a LiDAR stream and a camera stream to extract features from the two modalities, separately. The extracted features are fused by effective residual-based fusion modules. Moreover, we introduce additional perception-aware losses to measure the perceptual difference between the two modalities. Last, we propose an improved version of PMF, i.e., EPMF, which is more efficient and effective by optimizing data pre-processing and network architecture under perspective projection. Specifically, we propose cross-modal alignment and cropping to obtain tight inputs and reduce unnecessary computational costs. We then explore more efficient contextual modules under perspective projection and fuse the LiDAR features into the camera stream to boost the performance of the two-stream network. Extensive experiments on benchmark data sets show the superiority of our method. For example, on nuScenes test set, our EPMF outperforms the state-of-the-art method, i.e., RangeFormer, by 0.9% in mIoU. Compared to PMF, EPMF also achieves 2.06× acceleration with 2.0% improvement in mIoU. Our source code is available at https://github.com/ICEORY/PMF.