Photonic Interconnects Research Articles

High capacity, low power, short reach integrated silicon photonic interconnects

The architecture and component technology of a low power, high capacity, short reach optical interconnect are detailed. Measurements from high-performance 300 mm silicon photonics components that comprise the system are shown, along with a quantum-dot mode-locked laser 20-channel comb source with free space wall plug efficiencies up to 17%, advanced packaging techniques for 3D silicon photonic-electronic integration, and schematics for integrated electronics that control the photonic integrated circuits. Techniques for operating such a system in the presence of changing ambient temperature are addressed. Experiments on a 1 Tbps design are conducted with an optical link experiment indicating sub-picojoule/bit energy consumption at scale.

Thin film ferroelectric photonic-electronic memory

To reduce system complexity and bridge the interface between electronic and photonic circuits, there is a high demand for a non-volatile memory that can be accessed both electrically and optically. However, practical solutions are still lacking when considering the potential for large-scale complementary metal-oxide semiconductor compatible integration. Here, we present an experimental demonstration of a non-volatile photonic-electronic memory based on a 3-dimensional monolithic integrated ferroelectric-silicon ring resonator. We successfully demonstrate programming and erasing the memory using both electrical and optical methods, assisted by optical-to-electrical-to-optical conversion. The memory cell exhibits a high optical extinction ratio of 6.6 dB at a low working voltage of 5 V and an endurance of 4 × 104 cycles. Furthermore, the multi-level storage capability is analyzed in detail, revealing stable performance with a raw bit-error-rate smaller than 5.9 × 10−2. This ground-breaking work could be a key technology enabler for future hybrid electronic-photonic systems, targeting a wide range of applications such as photonic interconnect, high-speed data communication, and neuromorphic computing.

Ion trap with in-vacuum high numerical aperture imaging for a dual-species modular quantum computer.

Photonic interconnects between quantum systems will play a central role in both scalable quantum computing and quantum networking. Entanglement of remote qubits via photons has been demonstrated in many platforms; however, improving the rate of entanglement generation will be instrumental for integrating photonic links into modular quantum computers. We present an ion trap system that has the highest reported free-space photon collection efficiency for quantum networking. We use a pair of in-vacuum aspheric lenses, each with a numerical aperture of 0.8, to couple 10(1)% of the 493nm photons emitted from a 138Ba+ ion into single-mode fibers. We also demonstrate that proximal effects of the lenses on the ion position and motion can be mitigated.

Pairwise Tomlinson-Harashima precoding for multiple-lane IM-DD transmissions.

As for the photonic interconnection based on the multiple-lane intensity modulation direct detection (IM-DD) transmission, both intra-channel inter-symbol-interference (ISI) originating from bandwidth constraint, and inter-channel performance discrepancy emerging from inter-channel component differences are the major bottleneck for the throughput enhancement. Here, we propose a pairwise Tomlinson-Harshima precoding (P-THP) scheme, in order to simultaneously deal with both intra-channel ISI and inter-channel performance discrepancy. The effective function of the proposed P-THP scheme is experimentally evaluated by transmitting 4-channel 81-GBaud PAM4 signals over 2 km standard single-mode fiber (SSMF). Compared with the conventional scheme with only applying THP on individual wavelength channel, the required optical received power (ROP) under the back-to-back (B2B) transmission can be reduced by 0.75∼1 dB with the help of proposed P-THP in different experimental component configurations, at the 7% hard decision forward error correction (HD-FEC) threshold of BER = 3.8 × 10-3. After the 2 km SSMF transmission, only the use of proposed P-THP can guarantee to reach the designated HD-FEC threshold, leading to a net rate of >600 Gbit/s.

Surface acoustic wave actuated plasmonic signal amplification in a plasmonic waveguide

Enhancement of nanoscale confinement in the subwavelength waveguide is a concern for advancing future photonic interconnects. Rigorous innovation of plasmonic waveguide-based structure is crucial in designing a reliable on-chip optical waveguide beyond the diffraction limit. Despite several structural modifications and architectural improvements, the plasmonic waveguide technology is far from reaching its maximum potential for mass-scale applications due to persistence issues such as insufficient confined energy and short propagation length. This work proposes a new method to amplify the propagating plasmons through an external on-chip surface acoustic signal. The gold–silicon dioxide (Au-SiO2) interface, over Lithium Niobate (LN) substrate, is used to excite propagating surface plasmons. The voltage-varying surface acoustic wave (SAW) can tune the plasmonic confinement to a desired signal energy level, enhancing and modulating the plasmonic intensity. From our experimental results, we can increase the plasmonic intensity gain of 1.08 dB by providing an external excitation in the form of SAW at a peak-to-peak potential swing of 3 V, utilizing a single chip.

Transmitter dispersion eye closure quaternary assessment based on linearCNN with 1 × 1 convolutional kernel.

Transmitter dispersion eye closure quaternary (TDECQ) is a vital metric to characterize the quality of four-level pulse amplitude modulation (PAM-4) optical signals. However, the traditional TDECQ assessment scheme is complex and time consuming, with heavy iterative operations. Therefore, accelerating the TDECQ assessment has great significance for photonic data-center interconnection (DCI) applications. Here, we propose and experimentally demonstrate a TDECQ assessment based on linear-convolutional neural network (L-CNN) with the 1 × 1 convolutional kernel to reduce the implementation complexity. Our experimental results verify that the lightweight L-CNN can realize the accurate TDECQ assessment, without the involvement of nonlinear activation functions (NAFs). The mean absolute error (MAE) of 26.5625 and 53.125 GBaud PAM-4 signals are 0.16 dB and 0.18 dB, respectively, over a TDECQ range from 1.5 to 4.0 dB. Meanwhile, in comparison with existing CNN-based schemes, the L-CNN based TDECQ assessment scheme only needs 2048 multiplications, which have been reduced by five orders of magnitude.

Valley-conserved topological integrated antenna for 100-Gbps THz 6G wireless.

The topological phase revolutionized wave transport, enabling integrated photonic interconnects with sharp light bending on a chip. However, the persistent challenge of momentum mismatch during intermedium topological mode transitions due to material impedance inconsistency remains. We present a 100-Gbps topological wireless communication link using integrated photonic devices that conserve valley momentum. The valley-conserved silicon topological waveguide antenna achieves a 12.2-dBi gain, constant group delay across a 30-GHz bandwidth and enables active beam steering within a 36° angular range. The complementary metal oxide semiconductor-compatible valley-conserved devices represent a major milestone in hybrid electronic-photonic-based topological wireless communications, enabling terabit-per-second backhaul communication, high throughput, and intermedium transport of information carriers, vital for the future of communication from the sixth to X generation.

Weighted Decision Enabled Phase Retrieval Receiver With Adaptive Intensity Transformation

Phase-retrieval (PR) receivers can reconstruct complex-valued signals from two de-correlated intensity measurements by the use of iterative PR algorithm, without the assistance of any optical carriers. However, both the computational complexity with hundreds of iterations and the limited PR accuracy prevent it from being applied to high-speed photonic interconnections. In current submission, we propose a weighted decision enhanced PR receiver with adaptive intensity transformation (WD_AIT_PR). The weighted decision based on a compressed sigmoid nonlinear function is helpful to avoid the hard-decision (HD) errors, leading to the performance enhancement of the PR receiver, in terms of both convergence speed and PR accuracy. When the 56 GBaud 16QAM signal after transmission over 80 km standard single mode fiber (SSMF) is reconstructed, the WD_AIT_PR receiver is numerically verified to possess the faster convergency speed with only 70 iterations and the best steady BER performance by reducing the required optical signal-to-noise ratio (OSNR) by 0.8 dB, 1.8 dB and 1.8 dB in order to reach 7% HD forward error correction (HD-FEC), in comparison with the HD_AIT_PR receiver by replacing WD with HD, the PR receiver with the phase reset (PR_PR) and the PR receiver with adaptive intensity transformation (AIT_PR), respectively. Meanwhile, the WD_AIT_PR receiver is robust to transmission impairments, including transmitter-side I/Q imbalance, receiver-side skew, the noise imbalance between two intensity measurements, laser phase noise, and laser wavelength drift. Finally, the enhanced performance of the WD_AIT_PR receiver is experimentally verified through recovering 25 GBaud 16QAM signals under the condition of 40 km SSMF transmission.

Low-Complexity Conditional Generative Adversarial Network (c-GAN) Based Nonlinear Equalizer for Coherent Data-Center Interconnections

Nonlinear impairments emerging from both fiber optical links and optoelectrical devices are the major bottleneck for enhancing the throughput of photonic data-center interconnection (DCI). Here, we propose a data-driven nonlinear equalizer based on a conditional generative adversarial network (c-GAN) for high-speed and large-capacity photonic DCI. Its performance is experimentally evaluated by transmitting C-band 40-channel 60-GBaud dual polarization-16 quadrature amplitude modulation (DP-16QAM) dense wavelength division multiplexing (DWDM) signals over 20-km standard single-mode fiber (SSMF). Our experimental results verify that, the proposed c-GAN equalizer outperforms both traditional nonlinear equalizers and other data-driven counterparts, in terms of both the bit-error ratio (BER) and computational complexity. Only the proposed c-GAN equalizer can guarantee the BER performance of all 40 channels to reach the threshold of 20% soft decision forward error correction (SD-FEC) at BER=0.027, leading to a net transmission capacity of 16 Tb/s. Meanwhile, the computational complexity of the proposed c-GAN equalizer can be significantly reduced by 31.8%, 80.7%, and 98.8%, respectively, in comparison with Volterra filter equalizer (VFE), multilayer perceptron (MLP), and long short-term memory neural network (LSTM-NN) schemes.

New trends in photonic switching and optical networking architectures for data centers and computing systems [Invited

The rapid increases in data traffic coupled with user preferences are driving the data center and computing system service providers to offer energy-efficient, intelligent, flexible, cost-effective, high-capacity, and low-latency data services without added complexity to the users. Disaggregated heterogeneous reconfigurable computing systems realized by photonic switching and interconnects can enhance throughput and energy efficiency for artificial intelligence/machine learning (AI/ML) workloads, especially when aided by the AI/ML-enhanced control plane. Photonic switching and new optical networking architectures are expected to solve many of these challenging problems. This paper discusses new trends in photonic switching and optical network architectures for future data centers and computing systems summarized as follows: (1) flat reconfigurable disaggregated computing enabled by high-radix photonic switching and interconnects in data centers; (2) chiplet-based computing architectures empowered by embedded photonics toward heterogeneous reconfigurable computing; (3) nanosecond-scale photonic switching in data centers and computing systems; (4) AI/ML in self-driving, application-aware, and situation-aware data centers; (5) the emergence of flexible networking for cloud computing, edge computing, and split computing, as well as flexible networking for 5G/6G RF-optical networks; and (6) the deployment of embedded co-designed silicon photonics being considered for future data centers.

Brain-inspired nanophotonic spike computing: challenges and prospects

Nanophotonic spiking neural networks (SNNs) based on neuron-like excitable subwavelength (submicrometre) devices are of key importance for realizing brain-inspired, power-efficient artificial intelligence (AI) systems with high degree of parallelism and energy efficiency. Despite significant advances in neuromorphic photonics, compact and efficient nanophotonic elements for spiking signal emission and detection, as required for spike-based computation, remain largely unexplored. In this invited perspective, we outline the main challenges, early achievements, and opportunities toward a key-enabling photonic neuro-architecture using III–V/Si integrated spiking nodes based on nanoscale resonant tunnelling diodes (nanoRTDs) with folded negative differential resistance. We utilize nanoRTDs as nonlinear artificial neurons capable of spiking at high-speeds. We discuss the prospects for monolithic integration of nanoRTDs with nanoscale light-emitting diodes and nanolaser diodes, and nanophotodetectors to realize neuron emitter and receiver spiking nodes, respectively. Such layout would have a small footprint, fast operation, and low power consumption, all key requirements for efficient nano-optoelectronic spiking operation. We discuss how silicon photonics interconnects, integrated photorefractive interconnects, and 3D waveguide polymeric interconnections can be used for interconnecting the emitter-receiver spiking photonic neural nodes. Finally, using numerical simulations of artificial neuron models, we present spike-based spatio-temporal learning methods for applications in relevant AI-based functional tasks, such as image pattern recognition, edge detection, and SNNs for inference and learning. Future developments in neuromorphic spiking photonic nanocircuits, as outlined here, will significantly boost the processing and transmission capabilities of next-generation nanophotonic spike-based neuromorphic architectures for energy-efficient AI applications. This perspective paper is a result of the European Union funded research project ChipAI in the frame of the Horizon 2020 Future and Emerging Technologies Open programme.

Anisotropic Thermo‐Optic Mach–Zehnder Interferometer on LNOI for Polarization Handling and Multiplexing

AbstractThin‐film lithium niobate (LN) on insulator (LNOI) is emerging as a promising integrated photonic platform. Thermo‐optic (TO) Mach–Zehnder interferometers (MZIs) on LNOI are demonstrated to be a viable and often preferred alternative to their electro‐optic counterparts for low‐speed and static applications, where stability and repeatability are crucial. Harnessing the unique and strong anisotropic TO effect of LN, a novel and versatile anisotropic TO MZI on x‐cut LNOI for polarization handling and multiplexing is proposed and experimentally implemented. Each MZI arm consists of a two‐section anisotropic TO phase shifter along the y‐ and z‐directions of the LN crystal, leading to anisotropic temperature dependence of the effective refractive indices for the transverse‐electric (TE) and transverse‐magnetic (TM) polarizations. A polarization‐insensitive switch and a polarization beam splitter are implemented with the MZI device, which features low excess loss of ≈0.2–1.8 dB and a high extinction ratio of ≈20–45 dB in the telecom C‐band (1530–1565 nm). More intriguingly, arbitrary splitting ratios and even arbitrary combinations of unitary transmission matrices can be further implemented for both polarizations simultaneously. The versatile configurations of this anisotropic TO MZI have important implications for large‐scale photonic computing and interconnect.

Design and fabrication of conversion fiber with double-core structure for photonic integrated circuits high-density interconnect

To relax photonic integrated circuits (PIC) and its optical interconnect densification limitations from the standard fiber, we proposed a special fiber with a double core structure, named conversion fiber, which can convert both its spot size and its outer diameter, simultaneously. The simulation in this study clarified that the proposed conversion fiber had more capability of spot size and the outer diameter conversion with a large conversion ratio, as compared with a conventional fiber with a single core structure, and had large tolerance for the refractive index fluctuation. After that, we fabricated a conversion fiber based on the design parameters. The insertion loss of fabricated conversion fiber, including the coupling loss and the excess loss due to the mode conversion was 0.2 dB, which well agrees with the simulation results. From these results, it is expected that the proposed fiber can achieve higher density optical interconnect for PIC.

High-order mode waveguide amplifier with high mode extinction ratio written in an Er3+-doped phosphate glass.

Inscription of fiber-compatible active waveguides in high-gain glass, followed by direct interconnection with few-mode fibers, is one of the most promising solutions for all-optical mode-division multiplexing. In this work, based on the femtosecond laser writing technique, we propose a general fabrication scheme for inscribing high-order mode waveguides in glass, by carefully tailoring the cross-section of the waveguides to match the mode intensity distribution via an improved multi-scan approach. Specifically, we design and fabricate two kinds of waveguides, namely, LP01-mode waveguide and LP11-mode waveguide in a highly Er3+-doped phosphate glass, enabling the insertion loss of the waveguides to be as low as 1.88 dB, and the mode extraction factor of the LP11-mode waveguide up to ∼24 dB. Importantly, we have successfully achieved optical amplification of the waveguides, with an on-off gain as high as 3.52 dB. This novel high-order mode waveguide amplifier has broad application prospects in monolithic on-chip integrated photonic light sources and optical interconnection with few-mode fiber and/or silicon-based waveguide.

Frenkel-Poole Mechanism Unveils Black Diamond as Quasi-Epsilon-Near-Zero Surface.

A recent innovation in diamond technology has been the development of the "black diamond" (BD), a material with very high optical absorption generated by processing the diamond surface with a femtosecond laser. In this work, we investigate the optical behavior of the BD samples to prove a near to zero dielectric permittivity in the high electric field condition, where the Frenkel-Poole (FP) effect takes place. Zero-epsilon materials (ENZ), which represent a singularity in optical materials, are expected to lead to remarkable developments in the fields of integrated photonic devices and optical interconnections. Such a result opens the route to the development of BD-based, novel, functional photonic devices.

High-fidelity mode scaling via topological-optimized on-chip metalens for compact photonic interconnection

High-fidelity mode scaling via topological-optimized on-chip metalens for compact photonic interconnection

Petabit-Scale Silicon Photonic Interconnects With Integrated Kerr Frequency Combs

Silicon photonics holds significant promise in revolutionizing optical interconnects in data centers and high performance computers to enable scaling into the Pb/s package escape bandwidth regime while consuming orders of magnitude less energy per bit than current solutions. In this work, we review recent progress in silicon photonic interconnects leveraging chip-scale Kerr frequency comb sources and provide a comprehensive overview of massively scalable silicon photonic systems capable of capitalizing on the large number of wavelengths provided by such combs. We first consider the high-level architectural constraints and then proceed to detail the corresponding fundamental device designs supported by both simulated and experimental results. Furthermore, the majority of experimentally measured devices were fabricated in a commercial 300 mm foundry, showing a clear path to volume manufacturing. Finally, we present various system-level experiments which illustrate successful proof-of-principle operation, including flip-chip integration with a co-designed CMOS application-specific integrated circuit (ASIC) to realize a complete Kerr comb-driven electronic-photonic engine. These results provide a viable and appealing path towards future co-packaged silicon photonic interconnects with aggregate per-fiber bandwidth above 1 Tb/s, energy consumption below 1 pJ/bit, and areal bandwidth density greater than 5 Tb/s/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

RF Waveguide Pattern Engineering to Mitigate Bonding Surface Nonuniformities in CMOS Compatible Fabrication Processes

Heterogeneously integrating silicon photonic devices and circuits with other optical materials (III/V semiconductors, LiNbO3, and so on) is a promising approach toward bolstering the capabilities of silicon photonics and improving the manufacturability of other photonic platforms. An approach to heterogeneous integration is to directly bond an optical material to the surface oxide of a silicon photonic circuit or device, which requires locally and globally smooth bonding surfaces to facilitate sufficient bonding quality. However, embedded silicon photonic metal structures, such as RF waveguides for high-speed photonic devices and electrical interconnects, can impact the surface topography during planarization, resulting in nonideal bonding surfaces. In this article, we discuss and demonstrate a method of patterning the ground planes of RF waveguides to achieve a more uniform distribution of embedded metal density, in turn, providing more constant planarization rates. This provides a more uniform bonding surface which is a requirement for the high-volume manufacturing of these technologies.

XPoM-based 8-port photonic interconnection with low polarization sensitivity utilizing S–E–S hybridization

In this paper, we propose a hybrid amplifier-based 8-port Cross-Polarization Modulation (XPoM) interconnection with low polarization sensitivity unlike conventional Semiconductor Optical Amplifier (SOA)-based switching. The three-stage configuration utilizes Erbium-Doped Fiber Amplifier (EDFA) as an intermediate stage to reduce the effects of inherent polarization modulation creating large Transverse Electric – Transverse Magnetic (TE–TM) relative phase shifts. The appreciable error rate of [Formula: see text] and Extinction Ratio (ER) of 14.166 dB have been obtained with 100 GHz of port-spacing at different input powers. The low Polarization-Dependent Loss (PDL) of 1.0045 dB, small variations of azimuth and ellipticity with device angle in S–E–S configuration in contrast to that of SOA, adds to the superiority of the proposed model.

Toward compact high-efficiency grating couplers for visible wavelength photonics.

Although grating couplers have become the de-facto standard for optical access to integrated silicon photonics platforms, their performance at visible wavelengths, in moderate index contrast platforms such as silicon nitride, leaves significant room for improvement. In particular, the index contrast governs the diffraction efficiency per grating tooth and the resulting overall coupler length. In this work, we develop two approaches to address this problem: a dielectric grating that sums multiple optical modes to increase the overall output intensity; and an embedded metal grating that enhances the attainable refractive index contrast, and therefore reduces the on-chip footprint. We present experimental results that can be developed to realize compact efficient visible wavelength photonic interconnects, with a view toward cryogenic deployment for quantum photonics, where space is constrained and efficiency is critical.