Abstract The simultaneous advances in artificial neural networks and photonic integration technologies have spurred extensive research in optical computing and optical neural networks (ONNs). The potential to simultaneously exploit multiple physical dimensions of time, wavelength and space give ONNs the ability to achieve computing operations with high parallelism and large-data throughput. Different photonic multiplexing techniques based on these multiple degrees of freedom have enabled ONNs with large-scale interconnectivity and linear computing functions. Here, we review the recent advances of ONNs based on different approaches to photonic multiplexing, and present our outlook on key technologies needed to further advance these photonic multiplexing/hybrid-multiplexing techniques of ONNs.

Photonic multiplexing techniques for neuromorphic computing

Metasurfaces are subwavelength structured thin films consisting of arrays of units that allow the controls of polarization, phase and amplitude of light over a subwavelength thickness. The recent developments in topological photonics have greatly broadened the horizon in designing the metasurfaces for novel functional applications. In this review, we summarize recent progress in the research field of topological metasurfaces, firstly from the perspectives of passive and active in the classical regime, and then in the quantum regime. More specifically, we begin by examining the passive topological phenomena in two-dimensional photonic systems, including both time-reversal broken systems and time-reversal preserved systems. Subsequently, we move to discuss the cutting-edge studies of the active topological metasurfaces, including nonlinear topological metasurfaces and reconfigurable topological metasurfaces. After overviewing the topological metasurfaces in the classical regime, we show how the topological metasurfaces could provide a new platform for quantum information and quantum many-body physics. Finally, we conclude and describe some challenges and future directions of this fast-evolving field. 

/pdf/topological-metasurface-from-passive-toward-active-and-29raljds.pdf

Topological metasurface: from passive toward active and beyond

Abstract The multi-dimensional multiplexing technology is very promising for further increasing the link capacity of optical interconnects. A 96-channel silicon-based on-chip reconfigurable optical add-drop multiplexer (ROADM) is proposed and demonstrated for the first time to satisfy the demands in hybrid mode/polarization/wavelengthdivision-multiplexing systems. The present ROADM consists of a six-channel mode/polarization de-multiplexer, a 6 × 16 array of microring-resonator (MRR)-based wavelength-selective switches, and a six-channel mode/polarization multiplexer. With such a ROADM, one can add/drop optical signals to/from any channels of the multimode bus waveguide arbitrarily. For the designed and fabricated ROADM chip, there are more than 1000 elements integrated monolithically, including 96 MRRs, 576 waveguide crossings, 192 grating couplers, 96 micro-heaters, 112 pads, six polarization-splitter-rotators (PSRs), four asymmetric adiabatic couplers and four asymmetric directional couplers. For any channel added/dropped with the fabricated ROADM, the on-chip excess loss is about 5–20 dB, the inter-mode crosstalk is <−12 dB, and the inter-wavelength crosstalk is <−24 dB. The system experiments are demonstrated by using 10-GBaud quadrature phase shift keying (QPSK) signals, showing that the observed optical signal noise ratio (OSNR) power penalties induced by the ROADM are less than 2 dB at a BER of 3.8 × 10−3.

/pdf/96-channel-on-chip-reconfigurable-optical-add-drop-2rq6pwag.pdf

96-Channel on-chip reconfigurable optical add-drop multiplexer for multidimensional multiplexing systems

Electro-optic modulators are key components in data communication, microwave photonics, and quantum photonics. Modulation bandwidth, energy efficiency, and device dimension are crucial metrics of modulators. Here, we provide an important direction for the miniaturization of electro-optic modulators by reporting on ultracompact topological modulators. A topological interface state in a one-dimensional lattice is implemented on a thin-film lithium-niobate integrated platform. Due to the strong optical confinement of the interface state and the peaking enhancement of the electro-optic response, a topological cavity with a size of 1.6 × 140 μm2 enables a large modulation bandwidth of 104 GHz. The first topological modulator exhibits the most compact device size compared to reported LN modulators with bandwidths above 28 GHz, to the best of our knowledge. 100 Gb/s non-return-to-zero and 100 Gb/s four-level pulse amplitude modulation signals are generated. The switching energy is 5.4 fJ/bit, owing to the small electro-optic mode volume and low capacitance. The topological modulator accelerates the response time of topological photonic devices from the microsecond order to the picosecond order and provides an essential foundation for the implementation of large-scale lithium-niobate photonic integrated circuits. 

High-speed electro-optic modulation in topological interface states of a one-dimensional lattice.

To keep up with the growing bandwidth demands, photonic integrated circuits (PICs) have been widely employed in various application scenarios where high capacity and high-bandwidth-density interconnects are required. However, it is challenging to scale the PICs toward future petabit per second capacity requirements. We study the scalability bottlenecks of PICs in terms of guiding materials, dense integration approaches, wide-band optical sources, and high-efficiency tunable and modulation devices. We also look for possible solutions to address these challenges. In the end, we provide a perspective on future PIC development. Moore’s law for integrated photonics may last for a much shorter time than that in the microelectronics industry, requiring significant innovations and technological breakthroughs in PIC research. 

Scalability of Large-Scale Photonic Integrated Circuits

Valley‐Hall photonic topological insulators (VPTIs) provide an intriguing approach to suppress backscattering, enhance the robustness of transport, and shrink the footprints of the topological devices. However, previous works focused on the transmission characteristics of the symmetric topological waveguides with few reports on functional devices involving edge mode coupling, which hinders the implementation of various functional devices such as optical couplers/splitters, switches, filters, and etc. In this paper, a new method to manipulate the mode field distributions in topological waveguides is demonstrated by introducing asymmetric edge states, thus the mode coupling with an arbitrary coupling ratio can be achieved between topological waveguides. As an example, a topological power splitter with a 33:67 splitting ratio is demonstrated experimentally. In addition, to verify the robust transmission property of the topological photonic devices, a high‐speed data transmission experiment for a topological power splitter is performed and a high data rate of 128 Gb s−1 is achieved. These results promise a new method to manipulate the mode field distribution in topological waveguides and devices that may find wide applications in diverse fields such as optical communications, nanophotonics, and quantum information processing.

Asymmetric Topological Valley Edge States on Silicon‐On‐Insulator Platform

On-chip spatial mode operation, represented as mode-division multiplexing (MDM), can support high-capacity data communications and promise superior performance in various systems and numerous applications from optical sensing to nonlinear and quantum optics. However, the scalability of state-of-the-art mode manipulation techniques is significantly hindered not only by the particular mode-order-oriented design strategy but also by the inherent limitations of possibly achievable mode orders. Recently, metamaterials capable of providing subwavelength-scale control of optical wavefronts have emerged as an attractive alternative to manipulate guided modes with compact footprints and broadband functionalities. Herein, we propose a universal yet efficient design framework based on the topological metamaterial building block (BB), enabling the excitation of arbitrary high-order spatial modes in silicon waveguides. By simply programming the layout of multiple fully etched dielectric metamaterial perturbations with predefined mathematical formulas, arbitrary high-order mode conversion and mode exchange can be simultaneously realized with uniform and competitive performance. The extraordinary scalability of the metamaterial BB frame is experimentally benchmarked by a record high-order mode operator up to the twentieth. As a proof of conceptual application, an 8-mode MDM data transmission of 28-GBaud 16-QAM optical signals is also verified with an aggregate data rate of 813 Gb/s (7% FEC). This user-friendly metamaterial BB concept marks a quintessential breakthrough for comprehensive manipulation of spatial light on-chip by breaking the long-standing shackles on the scalability, which may open up fascinating opportunities for complex photonic functionalities previously inaccessible. 

/pdf/metamaterial-enabled-arbitrary-on-chip-spatial-mode-1g0bqawm.pdf

Metamaterial-enabled arbitrary on-chip spatial mode manipulation

We propose an on-chip mode division (de)multiplexer based on asymmetric directional couplers (ADCs) for multi-band operation. In mode-coupling process, the minimum coupling length is wavelength-dependent. The longer the wavelength is, the shorter the minimum coupling length is. A light of longer wavelength can be coupled back and forth multiple times with a total coupling length which equals to the minimum coupling length of a shorter wavelength light, thus realizing multi-band transmission at different wavelengths. As a proof-of-concept experiment, a four-mode (de)multiplexer for joint operation in the C- and O-Bands is designed and experimentally demonstrated. For the four modes (TE0, TE1, TE2 and TE3), the measured insertion losses (ILs) and crosstalk (CT) of the (de)multiplexer are < 4.7 dB and < -10.1 dB respectively from 1290 nm to 1360 nm, and they are < 3.5 dB and < -11.8 dB respectively from 1510 nm to 1580 nm.

On-chip mode division (de)multiplexer for multi-band operation.

Coupled‐waveguide devices are of central importance in on‐chip photonic circuits. However, their performance usually suffers from high wavelength and structure sensitivity, which makes it challenging to achieve broadband and fabrication tolerant optical functions. Topological pumping of edge states has emerged as a promising route for robust light transport in coupled waveguides. Here, the Thouless pumping process is explored in finite Rice‐Mele modeled silicon waveguide arrays and the robust power coupling and mode‐order conversion are experimentally demonstrated in this system. Thanks to the topological protection of light transport, the directional coupler and mode‐order converter based on the waveguide arrays show an ultrabroad bandwidth of 120 nm and are quite tolerant to significant structural deviations (−50–150 nm). As compared to the robust optical coupling in other topological waveguide arrays reported in recent works, the approach is more advantageous in terms of robustness against the variations in both waveguide width and gap distance, which is greatly favored in large‐scale photonic integration. This work could serve as a design principle for a new class of broadband and fabrication tolerant coupled‐waveguide devices, which can find applications in many fields including optical simulations of condensed matter physics, on‐chip optical communications, and quantum information processing.

Broadband and Fabrication Tolerant Power Coupling and Mode‐Order Conversion Using Thouless Pumping Mechanism

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