Inverse design and demonstration of an ultracompact broadband dual-mode 3 dB power splitter

Deep photonic network platform enabling arbitrary and broadband optical functionality

Expanding applications in optical communications, computing, and sensing continue to drive the need for high-performance integrated photonic components. Designing these on-chip systems with arbitrary functionality requires beyond what is possible with physical intuition, for which machine learning-based methods have recently become popular. However, computational demands for physically accurate device simulations present critical challenges, significantly limiting scalability and design flexibility of these methods. Here, we present a highly-scalable, physics-informed design platform for on-chip optical systems with arbitrary functionality, based on deep photonic networks of custom-designed Mach-Zehnder interferometers. Leveraging this platform, we demonstrate ultra-broadband power splitters and a spectral duplexer, each designed within two minutes. The devices exhibit state-of-the-art experimental performance with insertion losses below 0.66 dB, and 1-dB bandwidths exceeding 120 nm. This platform provides a tractable path towards systematic, large-scale photonic system design, enabling custom power, phase, and dispersion profiles for high-throughput communications, quantum information processing, and medical/biological sensing applications.

Compact dual-mode waveguide crossing based on adjoint shape optimization

We design, fabricate, and characterize a compact dual-mode waveguide crossing on a silicon-on-insulator platform. The dual-mode waveguide crossing with high performance is designed by utilizing the adjoint shape optimization. This adjoint-method-based optimization algorithm is computationally efficient and yields the optimal solution in fewer iterations compared with other iterative schemes. Our proposed dual-mode waveguide crossing exhibits low insertion loss and low crosstalk. Experimental results show that the insertion losses at the wavelength of 1550 nm are 0.83 dB and 0.50 dB for TE₀ and TE₁ modes, respectively. The crosstalk is less than -20 dB for the two modes over a wavelength range of 80 nm. The footprint of the whole structure is only 5 × 5 μm².

Bayesian design of mosaic-based mode multiplexers for various wavelength bands

Ultracompact mode multiplexers based on mosaic structure for various wavelength bands designed by Bayesian technique are investigated. C-, O-, and C + O band, TE0-TE1 2-mode multiplexers can be designed with the same footprint, by only changing the mosaic-pattern, showing the great flexibility of mosaic-based devices. Bayesian direct binary search method is used for the design, and it is demonstrated that the Bayesian technique is superior to conventional design method in terms of the best-structure search for the same number of iterations. The designed devices are fabricated for Si-waveguide platform, and the proof-of-concept results are obtained. These results indicate that the mosaic-based devices are promising candidates for future compact optical transceivers.

A Multi-Channel Frequency Router Based on an Optimization Algorithm and Dispersion Engineering

Integrated frequency routers, which can guide light with different frequencies to different output ports, are an important kind of nanophotonic device. However, frequency routers with both a compact size and multiple channels are difficult to realize, which limits the application of these frequency routers in nanophotonics. Here, a kind of bandgap optimization algorithm, which consists of the finite element method and topology optimization, is proposed to design a multi-channel frequency router. Channels supporting photonic edge states with different frequencies are built through the synthetic dimension of translational deformation. Due to the help of the developed optimization algorithms, the number of channels and output ports can be increased up to nine while maintaining ultracompact device size. The device operates within a working band of 0.585-0.665 c/a, corresponding to 1.504-1.709 μm when the lattice constant is set as 1 μm, covering the telecom wavelength of 1.55 μm. The average crosstalk is about -11.49 dB. The average extinction ratio is around 16.18 dB. Because the bus of the device can be regarded as a part of a topological rainbow, the results show that the structure is robust to fabrication errors. This method is general, which can be used for different materials and different frequency ranges. The all-dielectric planar configuration of our router is compact, robust, and easy to integrate, providing a new method for on-chip multi-channel broadband information processing.

Mode-independent thermo-optic switch based on the total-internal-reflection effect

A broadband mode-independent thermo-optic (TO) switch using the total-internal-reflection (TIR) effect is proposed and experimentally demonstrated on a polymer waveguide platform. By optimizing geometric parameters of the TIR switch, a mode-independent TO switching function with a large bandwidth and extinction ratio can be realized for E₁₁, E₁₂, and E₂₁ modes. The measurement results show an extinction ratio larger than 18.1 dB with a driving power of 160 mW for each mode over the wavelength range of 1500-1620 nm. The designed structure can also be cascaded to form a 1 × N switch network for mode-division multiplexing (MDM) systems, which greatly improves the network flexibility.

Ultracompact programmable inverse-designed nanophotonic devices based on digital subwavelength structures

Inverse design is a powerful approach to achieve ultracompact nanophotonic devices. Here, we propose an ultracompact programmable near-infrared nanophotonic device platform to dynamically implement inverse-designed near-infrared devices with different functions by programming the state of the phase-change material filled in each pixel. By tuning PCM block by block, the subwavelength condition for inverse-designed ultracompact devices is satisfied with large tuning pixel size. Based on the inverse-design device platform with a footprint of 6.4µm×8µm, we design and theoretically demonstrate four power splitters with different split ratios and one mode multiplexer working in the near-infrared band. The average excess losses for the power splitters with ratios of 0:1,1:1, 2:1, and 3:1 are less than 0.82, 0.65, 0.82, and 1.03 dB over a wavelength span of 100 nm, respectively. Meanwhile, the insertion losses of the mode multiplexer are 1.4 and 2.5 dB for T E 0 and T E 1 mode, respectively, and the average crosstalk is less than -20 and -19d B, respectively. The five different devices could be configured online in a nonvolatile way by heating phase change materials with an off-chip laser, which may significantly enhance the flexibility of on-chip optical interconnections.

93-THz ultra-broadband and ultra-low loss Y-junction photonic power splitter with phased inverse design

Optical power splitters with ultra-broadband and ultra-low insertion loss are desired in the field of photonic integration. Combining two inverse design algorithms for staged optimization, we present the design of a Y-junction photonic power splitter with 700 nm wavelength bandwidth (from 1200 nm to 1900 nm) within a 0.2 dB insertion loss, corresponding to a 93 THz frequency bandwidth. The average insertion loss is approximately -0.057 dB in the valuable C-band. Moreover, we comprehensively compared the insertion loss performance of different types and sizes of curved waveguides, and also give the cases of 1:4 and 1:6 cascaded power splitters. These scalable Y-junction splitters provide new alternatives for high-performance photonic integration.

Compact, broadband, and low-loss power splitters using MZI based on Bézier bends

We experimentally demonstrate wavelength-independent couplers (WICs) based on an asymmetric Mach-Zehnder interferometer (MZI) on a monolithic silicon-photonics platform in a commercial, 300-mm, CMOS foundry. We compare the performance of splitters based on MZIs consisting of circular and 3^rd order (cubic) Bézier bends. A semi-analytical model is constructed in order to accurately calculate each device's response based on their specific geometry. The model is successfully tested via 3D-FDTD simulations and experimental characterization. The obtained experimental results demonstrate uniform performance across different wafer sites for various target splitting ratios. We also confirm the superior performance of the Bézier bend-based structure, compared to the circular bend-based structure both in terms of insertion loss (0.14 dB), and performance consistency throughout different wafer dies. The maximum deviation of the optimal device's splitting ratio is 0.6%, over a wavelength span of 100 nm. Moreover, the devices have a compact footprint of 36.3 × 3.8 μ m ².

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.

Compact, ultrabroadband and temperature-insensitive arbitrary ratio power splitter based on adiabatic rib waveguides

We propose a compact, ultrabroadband and temperature-insensitive adiabatic directional coupler based on rib silicon waveguide-enabling arbitrary splitting ratios. Simulation results show that the device can achieve arbitrary splitting ratios from 1400 to 1600 nm, equal to 50%:50%, 60%:40%, 70%:30%, 80%:20%, and 90%:10% for the fundamental transverse electric mode. The designed device has an excess loss of less than 0.19 dB on the operational waveband. Furthermore, the proposed device shows a great robustness to fabrication imperfection, with a waveguide width deviation of 50 nm and ambient temperature change from 0°C to 200°C. These properties make the design a potential candidate for ultrahigh-density photonic integration chips.

Broadband multimode 3 dB optical power splitter using tapered couplers

A design of a 1 × 2 multimode 3 dB optical power splitter using tapered couplers is proposed and investigated in this paper. As an example, a 1 × 2 splitter processing five-lowest order transverse-electric-polarized modes is designed and optimized by utilizing finite difference time domain method and particle swarm optimization algorithm. To verify the feasibility of this novel design, the optimized device is fabricated on a silicon-on-insulator platform. The coupling lengths of tapered couplers are respectively 6.5 µm, 6.0 µm, 3.5 µm, 5.0 µm, 5.0 µm, 7.5 µm, 6.0 µm, 5.0 µm, and 8.0 µm. Measurement results reveal that, for the fabricated splitter, the power uniformity varies from 0.041 to 0.88 dB, the crosstalk ranges from -23.96 to -14.12 dB, and the insertion loss changes from 0.089 to 1.50 dB within a bandwidth from 1520 to 1600 nm.

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.

Inverse design of a dual-mode 3-dB optical power splitter with a 445 nm bandwidth

Optical power splitters are fundamental blocks for photonic integrated circuits. Conventional 3-dB power splitters are either constrained to single-mode regime or to the limited optical bandwidth. In this paper, an alternative design approach is proposed via combined method of topology optimizations on both analog and digital meta-structure. Based on this approach, a dual-mode power splitter is designed on silicon-on-insulator with an ultra-broad bandwidth from 1588 nm - 2033nm and an ultra-compact footprint of only 5.4 µm × 2.88 µm. The minimum feature size is 120 nm which can be compatible with silicon photonic foundry process. The simulated excess loss and crosstalk over this wavelength range for the two lowest TE modes are lower than 0.83 dB and -22 dB, respectively. To the best of our knowledge, this is a record large optical bandwidth for an integrated dual-mode 3-dB power splitter on silicon.

Efficient mode exchanger-based silicon photonic switch enabled by inverse design

A novel and energy efficient mode insensitive switch building block is proposed and experimentally demonstrated on a silicon-on-insulator platform. Based on a Mach-Zehnder interferometer, the switch uses a relatively compact mode insensitive phase shifter which includes a mode exchanger. The novel structure realizes the exact same phase shift for all modes by exchanging the modes midway within the phase shifter. The design approach leads to reduced power consumption otherwise not possible. Switching the first two quasi transverse electric (TE) modes simultaneously consumes 25.6 mW of power, an approximately 30% reduction from previous reported demonstrations. The measured insertion loss is 3.1 dB on average with a worst-case crosstalk of -14.9 dB over a 40 nm optical bandwidth from 1530 nm to 1570 nm. The design methodology enables scalability up to four optical modes.

Super strong wide TM Mie bandgaps tolerating disorders

This study demonstrates the appearance of super intense and wide Mie bandgaps in metamaterials composed of tellurium, germanium, and silicon rods in air that tolerate some disordering of rod position and rod radius under transverse magnetic (TM) polarized light waves. 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This strong tolerance of disordering of TM modes in tellurium, germanium, and silicon metamaterials opens a new way to design small, high-efficient, and feasible fabrication optical devices for optical integrated circuits.

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.

Gradient-probability-driven discrete search algorithm for on-chip photonics inverse design

The inverse-designed photonic device, with the characteristics of high performance and ultra-high compactness, is suitable for on-chip photonics applications. The gradient-based algorithms have high convergence efficiency. However, they depend on the continuous independent variable, so they cannot be directly applied to the pixel-based discrete search methods. In this paper, we propose a gradient-probability-driven discrete search (GPDS) algorithm for photonics inverse design. The algorithm establishes a connection between the gradient and the discrete value set by introducing the method of probability sampling. As an intrinsic discrete search algorithm in which the values of pixels are selected from a finite number of the discrete set, no additional discretization process is needed. Compared with the traditional brute-force search (BFS) method and traditional gradient method, the probability sampling process of our proposed GPDS algorithm can improve device performance efficiently and provide better stability to the initial states. We illustrate several component designs which are commonly used in the silicon photonics platform, and the results show that the algorithm can achieve high-performance structures within fewer iterations and has the ability of multi-objective optimization. With good flexibility and manufacturing-friendly geometry control, the algorithms are potential to be a powerful tool in solving multi-objective problems.

Ultra-compact reconfigurable device for mode conversion and dual-mode DPSK demodulation via inverse design

The mode multiplexing/de-multiplexing devices are key components for mode-division multiplexing (MDM) technology. Here, we propose an ultra-compact and reconfigurable mode-conversion device via inverse design, which can selectively implement multichannel mode conversion controlled by input phase shifts (Δφ). The device can transform input TE₀ (TE₁) mode to TE₄ (TE₃) mode at Δφ=0, or from TE₀ (TE₁) to TE₁ (TE₂) at Δφ=π spanning the wavelength range of 1525-1565 nm. We further demonstrate an integrated monolithic module based on the mode conversions to directly demodulate the dual-mode difference phase shift keying (DPSK) signal which significantly reduces the device size and benefits for future dense integrations in MDM systems.

On-chip silicon photonic controllable 2 × 2 four-mode waveguide switch

Multimode optical switch is a key component of mode division multiplexing in modern high-speed optical signal processing. In this paper, we introduce for the first time a novel 2 × 2 multimode switch design and demonstrate in the proof-of-concept. The device composes of four Y-multijunctions and 2 × 2 multimode interference coupler using silicon-on-insulator material with four controllable phase shifters. The shifters operate using thermo-optic effects utilizing Ti heaters enabling simultaneous switching of the optical signal between the output ports on four quasi-transverse electric modes with the electric power consumption is in order of 22.5 mW and the switching time is 5.4 µs. The multimode switch exhibits a low insertion loss and a low crosstalk below - 3 dB and - 19 dB, respectively, in 50 nm bandwidth in the third telecom window from 1525 to 1575 nm. With a compact footprint of 10 µm × 960 µm, this device exhibits a relatively large width tolerance of ± 20 nm and a height tolerance of ± 10 nm. Furthermore, the conceptual principle of the proposed multimode switch can be reconfigurable and scalable in multifunctional on-chip mode-division multiplexing optical interconnects.

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.

Broadband dual-mode 2 × 2 3 dB multimode interference couplers with a shallowly etched multimode section

A broadband dual-mode 2×23dB power splitter on silicon is proposed by utilizing multimode interference (MMI) couplers with a shallowly etched multimode region. The present MMI coupler is designed optimally to be with reduced phase errors for all the excited higher-order modes in the MMI region by engineering the refractive-index profile with the assistance of a shallowly etched multimode region. Simulation results show the device works well for the TE₀ and TE₁ input modes over the C-band. For the TE₀ input mode, the excess losses (ELs) and the imbalances (IBs) are, respectively, less than 0.4 and 0.12 dB in the wavelength range of 1.5-1.6 µm, while the ELs and the IBs for the TE₁ input mode are, respectively, less than 0.5 and 0.2 dB in the wavelength range of 1.53-1.58 µm. Besides, the intermode cross-talk is less than -22dB for TE₀ and TE₁ modes in the wavelength range of 1.5-1.6 µm. The proposed dual-mode MMI coupler can be extended for more than two input modes.

Robust inverse-design of scattering spectrum in core-shell structure using modified denoising autoencoder neural network

Neural network-based inverse design of nanophotonic device network is computationally and time efficient, but in general suffers the problems of robustness and stability against variation of the input target electromagnetic response. The inverse design network is required to be robust against the input electromagnetic response and to be capable of approximating the given electromagnetic response, even under the circumstances that the exact target response may not exist. We introduce a modified denoising autoencoder network to ensure the robustness of neural network-based inverse design, which consists of (1) a pre-trained network as a substitute of numerical simulation and (2) an inverse design network. We further purposely train the network with certain random disturbances added to the training dataset generated by the pre-trained network. Consequently, our modified denoising autoencoder network is more robust and more accurate than the conventional fully connected neural network. The strength and flexibility of our proposed network is illustrated via three concrete examples of achieving the desired scattering spectra of layered spherical scatterers.

Voxelized topology optimization for fabrication-compatible inverse design of 3D photonic devices

Topology optimization for photonic device design, has been mostly used to optimize binary structures based on refractive index as the free parameter for each design cell. Typically, a constraint on the optimization variable to be z-invariant and a smoothing operation on small features are applied to make the structure fabricable by conventional lithography. To enable topology optimization to design fabricable 3D structures using emerging methods like grayscale lithography and focused ion beam milling, we propose here a framework that uses the refractive index step position as the free parameter for each 3D voxel. This choice of framework enables us to reuse the same mesh in each iteration and thereby reduce the time for optimization. We apply the framework to the fabricable design of both free-space and integrated photonic devices, at different wavelengths, demonstrating high-efficiency ultra-compact designs with wide wavelength tunability.

Arbitrarily routed mode-division multiplexed photonic circuits for dense integration

On-chip integrated mode-division multiplexing (MDM) is an emerging technique for large-capacity data communications. In the past few years, while several configurations have been developed to realize on-chip MDM circuits, their practical applications are significantly hindered by the large footprint and inter-mode cross talk. Most importantly, the high-speed MDM signal transmission in an arbitrarily routed circuit is still absent. Herein, we demonstrate the MDM circuits based on digitized meta-structures which have extremely compact footprints. 112 Gbit/s signals encoded on each mode are arbitrarily routed through the circuits consisting of many sharp bends and compact crossings with a bit error rate under forward error correction limit. This will significantly improve the integration density and benefit various on-chip multimode optical systems.

On-chip mode-division multiplexing has many challenges including crosstalk, losses, and footprint.  Here the authors use a nanohole metastructure to create multiplexed bends and crossings for photonic data communications circuit routing with high density that combats these challenges.