Inverse-designed ultra-compact star-crossings based on PhC-like subwavelength structures for optical intercross connect

Inverse-designed ultra-compact multi-channel and multi-mode waveguide crossings

In this work, we use the inverse design method to design three-channel and four-channel dual-mode waveguide crossings with the design regions of 4.32 µm-wide regular hexagon and 6.68 µm-wide regular octagon, respectively. Based on the highly-symmetric structures, the fundamental transverse electric (TE0) and TE1 modes propagate through the waveguide crossings efficiently. Moreover, the devices are practically fabricated and experimentally characterized. The measured insertion losses and crosstalks of the three-channel and dual-mode waveguide crossing for both the TE0 and TE1 modes are less than 1.8 dB and lower than -18.4 dB from 1540 nm to 1560 nm, respectively. The measured insertion losses of the four-channel and dual-mode waveguide crossing for the TE0 and TE1 modes are less than 1.8 dB and 2.5 dB from 1540 nm to 1560 nm, respectively, and the measured crosstalks are lower than -17.0 dB. In principle, our proposed scheme can be extended to waveguide crossing with more channels and modes.

Inverse-designed polarization multiplexing non-uniformly distributed gratings for one-dimensional beam steering

Non-uniformly distributed gratings on the silicon-on-insulator platform for one-dimensional beam steering are designed by direct binary search inverse-design method. The gratings exhibit good emission directionality and far-field characteristics. Within a relatively small wavelength tuning range of 1517-1577 nm, the longitudinal scanning angle for TE and TM light is 23.65° and 10.81°, respectively, both of which are much larger than their uniform counterparts. By polarization multiplexing and etching depth optimization, a remarkable longitudinal scanning angle of 32.10° and high beam steering efficiency of 0.55°/nm are obtained. This work may pave the way for the development of miniaturized optical phased arrays with excellent beam steering performance.

Self-adjusting inverse design method for nanophotonic devices

Nanophotonic devices, which consist of multiple cell structures of the same size, are easy to manufacture. To avoid the optical proximity effect in the ultraviolet lithography process, the cell structures must be maintained at a distance from one another. In the inverse design process, the distance is maintained by limiting the optimized range of the location. However, this implementation can weaken the performance of the devices designed during transmission. To solve this problem, a self-adjusting inverse design method based on the adjoint variable method is developed. By introducing artificial potential field method, the location of one cell structure is modified only when the distances between this cell structure and other cell structures are smaller than a threshold. In this case, the range of the location can be expanded, and thus the performance of the designed devices can be improved. A wavelength demultiplexer with a channel spacing of 1.6 nm is designed to verify the performance of the proposed method. The experiment reveals that the transmission of the designed devices can be improved by 20%, and the self-adjusting inverse design process is 100 times faster than the inverse-design process based on the genetic algorithm.

Series of ultra-low loss and ultra-compact multichannel silicon waveguide crossing

Ultra-compact waveguide crossing (UC-WC) is a basic component in optoelectronic fusion chip solutions, as its footprint is smaller in the orders of magnitude than that of traditional photonic integrated circuits (PICs). However, a large loss of UC-WC (decibel level) becomes a barrier to scaling and practicality. Here, we propose a series of ultra-low loss UC-WC silicon devices using an advanced hybrid design that combines the adjoint method with the direct binary search (DBS) algorithm. Simulation results show that our 2 × 2 UC-WC has an insertion loss as low as 0.04 dB at 1550 nm, which is about ten times lower than the previous UC-WC results. In the valuable C-band (1530-1565 nm), the insertion loss of UC-WC is lower than -0.05 dB, and the channel crosstalk is lower than -34 dB. Furthermore, for the 3 × 3 UC-WC device, the highest insertion loss in the entire C-band is approximately -0.07 dB, and the highest channel crosstalk is lower than -33 dB. Additionally, the 4 × 4 and more complex 8 × 8 UC-WC devices were also analyzed. The highest insertion loss for 4 × 4 and 8 × 8 UC-WC in the C-band is only -0.19 dB and -0.20 dB, respectively, and the highest channel crosstalk is approximately -22dB and -28 dB, respectively. These results confirm that the designed devices possess two attractive features simultaneously: ultra-compactness and ultra-low insertion loss, which may be of great value in future large-scale optoelectronic fusion chips.

Optical meta-waveguides for integrated photonics and beyond

The growing maturity of nanofabrication has ushered massive sophisticated optical structures available on a photonic chip. The integration of subwavelength-structured metasurfaces and metamaterials on the canonical building block of optical waveguides is gradually reshaping the landscape of photonic integrated circuits, giving rise to numerous meta-waveguides with unprecedented strength in controlling guided electromagnetic waves. Here, we review recent advances in meta-structured waveguides that synergize various functional subwavelength photonic architectures with diverse waveguide platforms, such as dielectric or plasmonic waveguides and optical fibers. Foundational results and representative applications are comprehensively summarized. Brief physical models with explicit design tutorials, either physical intuition-based design methods or computer algorithms-based inverse designs, are cataloged as well. We highlight how meta-optics can infuse new degrees of freedom to waveguide-based devices and systems, by enhancing light-matter interaction strength to drastically boost device performance, or offering a versatile designer media for manipulating light in nanoscale to enable novel functionalities. We further discuss current challenges and outline emerging opportunities of this vibrant field for various applications in photonic integrated circuits, biomedical sensing, artificial intelligence and beyond.

Ultra-Compact Power Splitters with Low Loss in Arbitrary Direction Based on Inverse Design Method

The power splitter is a device that splits the energy from an input signal into multiple outputs with equal or uneven energy. Recently, the use of algorithms to intelligently design silicon-based photonic devices has attracted widespread attention. Thus, many optimization algorithms, which are called inverse design algorithms, have been proposed. In this paper, we use the Direct Binary Search (DBS) algorithm designed with three 1 × 3 power splitters with arbitrary directions theoretically. They have any direction and can be connected to other devices in any direction, which greatly reduces the space occupied by the optical integrated circuit. Through the simulation that comes about, we are able to get the insertion loss (IL) of the device we designed to be less than 5.55 dB, 5.49 dB, and 5.32 dB, separately. Then, the wavelength is 1530–1560 nm, so it can be used in the optical communication system. To discuss the impact of the footprint on device performance, we also designed another device with the same function as the second one from the above three devices. Its IL is less than 5.40 dB. Although it occupies a larger area, it has an advantage in IL. Through the design results, three 1 × 3 power splitters can be freely combined to realize any direction, multi-channel, ultra-compact power splitters, and can be better connected with different devices to achieve different functions. At the same time, we also show an example of a combination. The IL of each port of the combined 1 × 6 power splitter is less than 8.82 dB.

Ultra-compact X-shaped waveguide crossings with flexible angles based on inverse design

When photonics integrated circuits (PICs) become more massive in scale, the area of chip can't be taken full advantage of with 2×2 waveguide crossings with a 90° intersection angle. Crossings with small angles would be a better idea to further improve the area utilization, but few works have researched 2×2 crossings with different angles. In this paper, in order to have an ultra-compact footprint and a flexible intersection angle while keeping a high performance, we report a series of compact X-shaped waveguide crossings in silicon-on-insulator (SOI) waveguides for fundamental transverse electric (TE₀) mode, designed by using finite-difference frequency-domain (FDFD) numerical analysis method and a global optimization method. Thanks to inverse design, a compact footprint as small as 4.5 µm² and various angles between two input/output waveguides of 30°, 45°, 60°, 80° and 90° are achieved. Simulation results show that all crossings have good performance of insertion losses (ILs) within 0.1∼0.3 dB and crosstalks (CTs) within -20∼-50 dB in the wavelength range of 1525∼1582 nm. Moreover, the designed crossings were fabricated on a commercially available 220-nm SOI platform. The measured results show that the ILs of all crossings are around 0.2∼0.4 dB and the CTs are around -20 dB∼-32 dB; especially for the 30° intersection angle, the crossing has IL around 0.2 dB and CT around -31 dB in C band. Besides, we theoretically propose an approach of a primary structure processing technique to enhance the device performance with a more compact footprint. This technique is to remove the redundant structures in conjunction with the electric field distribution during the optimization procedure of inverse design. For the new 90° crossing structure produced by it, simulation results show that ILs of 0.29 ± 0.03 dB and CTs of -37 ± 2.5 dB in the wavelength range of 1500∼1600 nm are achieved and the footprint is shrunk by 25.5%.

Inverse-designed ultra-compact high efficiency and low crosstalk optical interconnect based on waveguide crossing and wavelength demultiplexer

In this paper, we use the inverse design method to design an optical interconnection system composed of wavelength demultiplexer and the same direction waveguide crossing on silicon-on-insulator (SOI) platform. A 2.4 μm × 3.6 μm wavelength demultiplexer with an input wavelength of 1.3-1.6 μm is designed. When the target wavelength of the device is 1.4 μm, the insertion loss of the output port is - 0.93 dB, and there is - 18.4 dB crosstalk, in TE0 mode. The insertion loss of the target wavelength of 1.6 μm in TE0 mode is - 0.88 dB, and the crosstalk is - 19.1 dB. Then, we designed a same direction waveguide crossing, the footprint is only 2.4 μm × 3.6 μm, the insertion loss of the wavelength 1.4 μm and 1.6 μm in TE0 mode is - 0.99 dB and - 1 dB, and the crosstalk is - 12.14 dB and - 14.34 dB, respectively. Finally, an optical interconnect structure composed of two devices is used, which can become the most basic component of the optical interconnect network. In TE0 mode, the insertion loss of the output wavelength of 1.4 μm at the output port is - 1.3 dB, and the crosstalk is - 29.36 dB. The insertion loss of the output wavelength of 1.6 μm is - 1.39 dB, and the crosstalk is - 38.99 dB.

Broadband and compact silicon mode converter designed using a wavefront matching method

A broadband and compact TE0-TE1 mode converter for a mode division multiplexing system designed using a wavefront matching method is realized. We present the first experimental demonstration of a silicon waveguide device designed by a wavefront matching method. In order to achieve broadband operation of the silicon mode converter, seven wavelengths are considered in its optimization process. The designed silicon mode converter is fabricated via a standard complementary metal-oxide-semiconductor technology, which enables low-cost mass production. Measurements performed using the fabricated mode converter correlate strongly with the calculated results.

Silicon Integrated Nanophotonic Devices for On-Chip Multi-Mode Interconnects

Mode-division multiplexing (MDM) technology has drawn tremendous attention for its ability to expand the link capacity within a single-wavelength carrier, paving the way for large-scale on-chip data communications. In the MDM system, the signals are carried by a series of higher-order modes in a multi-mode bus waveguide. Hence, it is essential to develop on-chip mode-handling devices. Silicon-on-insulator (SOI) has been considered as a promising platform to realize MDM since it provides an ultra-high-index contrast and mature fabrication processes. In this paper, we review the recent progresses on silicon integrated nanophotonic devices for MDM applications. We firstly discuss the working principles and device configurations of mode (de)multiplexers. In the second section, we summarize the multi-mode routing devices, including multi-mode bends, multi-mode crossings and multi-mode splitters. The inverse-designed multi-mode devices are then discussed in the third section. We also provide a discussion about the emerging reconfigurable MDM devices in the fourth section. Finally, we offer our outlook of the development prospects for on-chip multi-mode photonics.

Inverse-designed arbitrary-input and ultra-compact 1 × N power splitters based on high symmetric structure

Based on high symmetric structure, we propose the arbitrary-input and ultra-compact 1 × 2 and 1 × 3 power splitters by utilizing inverse design method. These devices can realize the functionality of power splitting, when the optical field is launched from arbitrary port. The shapes of their structures are 3.8 μm-wide regular hexagon and 4.0 μm-wide regular octagon, respectively. By utilizing 3D fine difference time domain solutions, the simulated results indicate that the excess loss of the 1 × 2 power splitter is less than 1.5 dB from 1,500 to 1,600 nm, and the excess loss and crosstalk of the 1 × 3 power splitter are less than 1.9 dB and lower than − 15.5 dB over 100 nm bandwidth at the centered wavelength of 1,550 nm respectively. In addition, the tolerances to fabrication errors are also investigated.

Scaling and cascading compact metamaterial photonic waveguide filter blocks

In this work, we reported the design and fabrication of a compact and scalable metamaterial longpass filter with an ultrasmall footprint of 5.1µm×5.1µm. In the stopband, light transmission can be blocked and reflected with ∼25dB attenuation. In the passband, light can pass through with a low insertion loss around -0.28dB. The transition band can be redshifted or blueshifted by scaling up or down the filter block; i.e., scaling down 1% can produce a transition band blueshift of 11.4 nm. The power roll-off can be enhanced by cascading multiple filter blocks, i.e., 1.34 dB/nm by cascading three filters. These results demonstrate the great potential of the metamaterial-based waveguide devices for scalable photonic filtering applications.

Ultra-compact and efficient 1 × 2 mode converters based on rotatable direct-binary-search algorithm

In this paper, we innovatively demonstrate a rotatable direct-binary-search algorithm. Based on this unique inverse design method, the coupling region of nanophotonic device can be realized with multi-shape and multi-rotation pixels. In addition, the novel 1× 2 mode converters with multipurpose design goals on a 220 nm-thick top silicon-on-insulator platform are proposed by utilizing this enhanced algorithm, which can simultaneously achieve power splitting and mode conversion. By 3D fine difference time domain solutions, the 1 × 2 mode converter that converts TE₀ mode into TE₁, with a footprint of 2.7 µm × 2.4 µm, exhibits the excess loss of 0.1 - 0.2 dB (TE₁ mode), crosstalk of lower than -20.6 dB (TE₀ mode) and reflection loss of lower than -19.5 dB (TE₀ mode) from 1500 nm to 1600 nm. The 1 × 2 mode converter that transforms TE₀ into TE₂ occupies the footprint of 3.6 µm × 3 µm. The excess loss is 0.3 - 0.4 dB (TE₂ mode) in the wavelength range of 1500 - 1600 nm. The crosstalks are lower than -17.5 dB (TE₁ mode) and -25.1 dB (TE₀ mode), and the reflection loss is lower than -18.3 dB (TE₀ mode). Besides, the fabrication tolerances caused by both expansion or contraction of etched pattern contour and round corner effect are also investigated.

Multimode waveguide crossing with ultralow loss and low imbalance

The mode-division-multiplexing (MDM) technology has become an alternative solution to further increase the link capacity in optical communication systems. Ultralow loss waveguide crossings for multimode waveguides are requisite in on-chip MDM systems. We propose and demonstrate an ultralow loss silicon multimode waveguide crossing using a combination of fully etched and shallowly etched waveguides in the multimode-interference coupler region to reduce the imbalance for two transverse electric polarized (TE) modes. By engineering the geometries and the proportion of the two waveguides, the self-imaging positions for different modes can coincide exactly. Simulated results show that the insertion losses are 0.043 and 0.084 dB for the fundamental TE (TE₀) mode and the first-order TE (TE₁) mode at 1550 nm, while the experimental values are 0.1 and 0.12 dB, respectively. The measured crosstalk is less than -30 dB for both modes within a 75 nm wavelength span.

Low-loss, low-crosstalk waveguide crossing for scalable integrated silicon photonics applications

A waveguide crossing based on multi-mode interference is designed and experimentally characterized on the silicon platform. The insertion loss of the device is measured as 43 ± 4 mdB per crossing, with a crosstalk of < -50 dB between 1550 and 1560 nm, in good agreement with predictions from 3D finite-difference time-domain simulations. Furthermore, the device backscatter was investigated using white light reflectometry and no significant backscatter was observed from 160 waveguide crossings in the time domain. In the frequency domain, the backscatter of the waveguide crossing device was measured experimentally for the first time, achieving a backscatter of -55 dB. The crossing has a footprint of 14.3 x 14.3 µm² and can be fabricated in a single step.

Ultra-compact reflective mode converter based on a silicon subwavelength structure

Mode converters play an essential role in mode-division multiplexing systems. A reflective mode converter (RMC), which is utilized to accomplish the mode conversion in the contra-propagation process, can fold the optical path and realize the mode exchange in an optical network. In this paper, we propose and experimentally demonstrate an RMC based on a silicon subwavelength structure. It can convert the input fundamental mode (${\text{TE}_0}$TE₀) into the first-order mode (${\text{TE}_1}$TE₁) in a ${2.0}\;\unicode{x00B5} \text{m} \times {2.0}\;\unicode{x00B5} \text{m}$2.0µm×2.0µm footprint. The simulated insertion loss and cross talk are lower than 0.6 dB and $ - {20.3}\;\text{dB}$-20.3dB in 1525-1565 nm. Experimental results verify the functionality of the device. The measured insertion loss and cross talk are lower than 2.2 dB and $ - {16.2}\;\text{dB}$-16.2dB. To further prove the generality of the methodology, we design another two RMCs realizing the mode conversion functions of ${\text{TE}_0}$TE₀ to ${\text{TE}_2}$TE₂ and ${\text{TE}_0}$TE₀ to ${\text{TE}_3}$TE₃ modes. The simulated insertion losses are lower than 1.1 dB and 1.8 dB.

State-of-the-Art and Perspectives on Silicon Waveguide Crossings: A Review

In the past few decades, silicon photonics has witnessed a ramp-up of investment in both research and industry. As a basic building block, silicon waveguide crossing is inevitable for dense silicon photonic integrated circuits and efficient crossing designs will greatly improve the performance of photonic devices with multiple crossings. In this paper, we focus on the state-of-the-art and perspectives on silicon waveguide crossings. It reviews several classical structures in silicon waveguide crossing design, such as shaped taper, multimode interference, subwavelength grating, holey subwavelength grating and vertical directional coupler by forward or inverse design method. In addition, we introduce some emerging research directions in crossing design including polarization-division-multiplexing and mode-division-multiplexing technologies.

SiNx-Si interlayer coupler using a gradient index metamaterial

In this work, we demonstrate a metamaterial (MM)-based SiN_x-Si interlayer coupler. A gradient index (GRIN) MM was employed to achieve refractive index matching, balancing the large difference in the refractive index between Si (∼3.42) and SiN_x (∼2). In addition, a two-layer SiN_x interlayer coupler was proposed to further minimize the coupling loss and interlayer crosstalk. Finally, we demonstrated a three-dimensional (3D) SiN_x-Si interlayer coupler with the -0.6  dB insertion loss per layer propagation, with the 40 nm 1-dB-bandwidth (1530-1570 nm) and <-45  dB interlayer crosstalk. These promising results demonstrate the great potential of a GRIN MM-based SiN_x-Si interlayer coupler in 3D photonic integration.

Inverse-designed low-loss and wideband polarization-insensitive silicon waveguide crossing

Waveguide crossings are an essential component for constructing complex and functional on-chip photonic networks. Polarization-insensitive waveguide crossings are desired because photonic networks usually involve light with different polarizations. Here, we propose a polarization-insensitive waveguide crossing on a 250-nm silicon-on-insulator platform by using an inverse design method. In simulation, the designed waveguide crossing can maintain insertion loss below 0.18 (0.25) dB in the wavelength range of 1440-1640 nm for the TE₀ (TM₀) mode and achieve minimal insertion loss as small as 0.08 (0.07) dB at the wavelength of 1550 nm. The cross talk maintains below -32  dB and -35  dB for the TE₀ and TM₀ modes, respectively. Experimentally, the fabricated waveguide crossing achieves measured insertion loss less than 0.20 (0.25) dB for the TE₀ (TM₀) mode with minimal insertion loss as small as 0.1 dB. The measured cross talk is below -28  dB and -31  dB for the TE₀ and TM₀ modes, respectively. Therefore, our proposed waveguide crossing can be widely applied in photonic integrated circuits to construct photonic systems with the capabilities of polarization control and mode (de)multiplexing.

Polarization-insensitive silicon waveguide crossing based on multimode interference couplers

A polarization-insensitive waveguide crossing based on multimode interference (MMI) couplers is proposed and demonstrated on a silicon-on-insulator (SOI) platform. By utilizing two orthogonal MMIs, the footprint of the device is about 23  μm×23  μm. The proposed device, easily fabricated with only one fully etched step, is characterized with low insertion losses and low crosstalks for both transverse-electric and transverse-magnetic polarizations from 1520 to 1610 nm bands.

High performance ultra-compact SOI waveguide crossing

Waveguide crossing is an important integrated photonic component that will be routinely used for high-density and large-scale photonic integrated circuits, such as optical switches and routers. Several techniques have been reported in achieving high performance waveguide crossings on a silicon-on-insulator photonic platform, i.e., low-loss and low-crosstalk waveguide crossings based on multimode interference, bi-layer tapering, optical transformation, metamaterials, and subwavelength gratings. Until recently, not much attention has been given to the reduction of the footprint of waveguide crossings. Here we experimentally demonstrate an ultra-compact waveguide crossing on silicon photonic platform with a footprint only ~1 × 1 μm². Our simulations show that it has a low insertion loss (< 0.175 dB) and low crosstalk (< -37dB) across the whole C-band, while the fabricated one has an insertion loss < 0.28 dB and crosstalk around -30 dB for the C-band.

Ultra-compact mode (de) multiplexer based on subwavelength asymmetric Y-junction

We propose and experimentally demonstrate a novel multimode ultra-compact mode (de)multiplexer for highly integrated on-chip mode-division multiplexing systems. This device is composed of a wide divergence angle asymmetric Y-junction based on subwavelength structure and optimized using an inverse design method. The proposed device occupied a footprint of only 2.4 × 3 µm². The measured insertion loss and crosstalk were less than 1dB and -24 dB from 1530 nm to 1590 nm for both TE₀ mode and TE₁ mode, respectively. Likewise, a three mode multiplexer is also designed and fabricated with a compact footprint of 3.6 × 4.8 µm². Furthermore, our scheme could also be expanded to include more modes.