As bandwidth requirements and integration of photonic components in computing systems increase, the optical micro-ring resonator are becoming an important building block for dense, high-bandwidth interconnects. Ring resonators are small in size and can operate at data rates up to 60Gb/s NRZ,1 making them well-suited for integrating many rings operating at different wavelengths into a single device. These devices are a promising solution for complex interconnected systems, such as chip-to-chip interconnects.2 However, using these rings can be challenging as they are sensitive to fabrication and temperature variations and need constant tuning to lock them to their assigned optical wavelengths.3 This tuning is commonly done by inserting embedded heaters in or above the ring. Existing techniques that tune the rings by the optical power coupled into the rings require extensive characterization and spectrum analysis, or out-of-band signalling, to account for rings drifting across optical channels and fabrication variations. In this work, we use four cascaded micro-rings implemented in a silicon photonic device operating in the C-band. Each ring taps a single wavelength from a bus waveguide. The remaining light in the bus waveguide is then fed into a photodiode, which is used to monitor the tuning of all rings. We show an algorithm that, based only on information about the design of the chip, tunes the rings to the exact desired optical channel. With this algorithm the use of more complex techniques to directly measure a ring's coupled wavelength can be avoided, reducing system complexity.

Automated tuning and channel selection for cascaded micro-ring resonators

1School of Microelectronics, Xidian University, Xi’an 710071, China 2Institute of Microelectronics, Tsinghua University, Beijing 100084, China 3Department of Micro/nanoelectronics, Peking University, Beijing 100871, China 4Frontier Institute of Chip and System, Fudan University, Shanghai 200438, China 5Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117546, Singapore 6Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

Preface to the Special Issue on Beyond Moore: Three-Dimensional (3D) Heterogeneous Integration

Abstract Integrated photonics is widely regarded as an important post-Moore’s law research direction. However, it suffers from intrinsic limitations, such as lack of control and satisfactory photonic memory, that cannot be solved in the optical domain and must be combined with electronics for practical use. Inevitably, electronics and photonics will converge. The photonic fabrication and integration technology is gradually maturing and electronics-photonics convergence (EPC) is experiencing a transition from device integration to circuit design. We derive a conceptual framework consisting of regulator, oscillator, and memory for scalable integrated circuits based on the fundamental concepts of purposeful behavior in cybernetics, entropy in information theory, and symmetry breaking in physics. Leveraging this framework and emulating the successes experienced by electronic integrated circuits, we identify the key building blocks for the integrated circuits for EPC and review the recent advances. Graphical Abstract 

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Circuit-level convergence of electronics and photonics: basic concepts and recent advances

Abstract Due to the rise of 5G, IoT, AI, and high-performance computing applications, datacenter traffic has grown at a compound annual growth rate of nearly 30%. Furthermore, nearly three-fourths of the datacenter traffic resides within datacenters. The conventional pluggable optics increases at a much slower rate than that of datacenter traffic. The gap between application requirements and the capability of conventional pluggable optics keeps increasing, a trend that is unsustainable. Co-packaged optics (CPO) is a disruptive approach to increasing the interconnecting bandwidth density and energy efficiency by dramatically shortening the electrical link length through advanced packaging and co-optimization of electronics and photonics. CPO is widely regarded as a promising solution for future datacenter interconnections, and silicon platform is the most promising platform for large-scale integration. Leading international companies (e.g., Intel, Broadcom and IBM) have heavily investigated in CPO technology, an inter-disciplinary research field that involves photonic devices, integrated circuits design, packaging, photonic device modeling, electronic-photonic co-simulation, applications, and standardization. This review aims to provide the readers a comprehensive overview of the state-of-the-art progress of CPO in silicon platform, identify the key challenges, and point out the potential solutions, hoping to encourage collaboration between different research fields to accelerate the development of CPO technology. Graphical Abstract 

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Co-packaged optics (CPO): status, challenges, and solutions

The dynamic buckling of circular rings is a pervasive instability problem with a major impact in various fields, such as structural, nuclear and offshore engineering, robotics, electromechanics, and biomechanics. This phenomenon may be simply seen as the complex motion that occurs deviating from the original circular shape under, for instance, any kind of time-dependent forcing load. Despite the fact that this topic has progressively gained importance since the mid-20th century, it seems that the same points have not been made completely clear. In fact, even some subtleties in the derivation of classical static buckling load may still give rise to misinterpretations and lead to misleading results. A fortiori, research concerning the nonlinear dynamics of rings still suffers the inherent difficulties associated with different possible analytical formulations of post-buckling dynamics. Advancement in this respect would be relevant, both from a theoretical and a practical point of view, since the applications are endless, with countless possibilities, especially in the biomedical and biotechnological fields: buckling-driven transformations of thin-film materials for applications in electronic microsystems, self-excited oscillations in collapsible tubes and pliable fluid-carrying shells, vocal-fold oscillations during phonation and snoring, pulse wave propagation in arteries, closure and reopening of pulmonary airways, stability of cardiac and venous valves during vascular surgery, stability of annuloplasty devices, flow-induced deformation and ultimate rupture of a cerebral aneurysm, and much more. The present article, in the framework of a critical review of the classic formulation of elastic ring buckling, proposes a straightforward approach for the nonlinear dynamics of an elastic ring that leads to a Mathieu–Duffing equation. In such a manner, some possible evolutions of the system under pulsing loads are analyzed and discussed, showing the inherent complexity of its dynamic behavior.

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From static buckling to nonlinear dynamics of circular rings

As Moore’s law approaching its end, electronics is hitting its power, bandwidth, and capacity limits. Photonics is able to overcome the performance limits of electronics but lacks practical photonic register and flexible control. Combining electronics and photonics provides the best of both worlds and is widely regarded as an important post-Moore’s direction. For stability and dynamic operations considerations, feedback tuning of photonic devices is required. For silicon photonics, the thermo-optic effect is the most frequently used tuning mechanism due to the advantages of high efficiency and low loss. However, it brings new design requirements, creating new design challenges. Emerging applications, such as optical phased array, optical switches, and optical neural networks, employ a large number of photonic devices, making PCB tuning solutions no longer suitable. Electronic-photonic-converged solutions with compact footprints will play an important role in system scalability. In this paper, we present a unified model for thermo-optic feedback tuning that can be specialized to different applications, review its recent advances, and discuss its future trends.

Towards electronic-photonic-converged thermo-optic feedback tuning

Microring resonator (MRR) is a versatile photonic device that finds a wide range of applications in communication, computing, and sensing. However, it is susceptible to fabrication, thermal, and laser wavelength variations. This paper presents a PWM wavelength locker for a MRR. The wavelength locker and the MRR have been fabricated in a monolithic photonic BiCMOS platform. The effectiveness of this design is verified by extensive computer simulations. To the best of our knowledge, this is the first closed-loop PWM wavelength locker for MRRs reported in the literature.

First Demonstration of Closed-Loop PWM Wavelength Locking of a Microring Resonator in a Monolithic Photonic-BiCMOS Platform

Micro-ring resonator (MRR) is a key photonic device that has a wide range of applications but suffers from wavelength uncertainties. For almost all practical applications, a wavelength controller is required for each MRR. The wavelength controller is usually much larger than the MRR. With more complicated control algorithms, the controller size becomes even larger. Equipping each MRR with a wavelength controller will not be scalable. We propose a pipelined time-division-multiplexing (PTDM) control scheme that achieves high scalability while maintaining good loop bandwidth by exploiting the speed mismatch between the heater and the controller. To verify this proposed scheme, a hybrid integrated controller supporting four MRRs is designed. Measurement results show that it achieves a sine tracking speed of about 15 nm/s while achieving a locking accuracy of 7 pm and a tuning range of 9 nm.

Resolving the scalability challenge of wavelength locking for multiple micro-rings via pipelined time-division-multiplexing control.

This paper proposes an electronic-photonic converged adaptive-tuning-step (ATS) pipelined time-division-multiplexing (PTDM) scheme to achieve fast wavelength locking of multiple micro-ring resonators (MRRs) using a shared controller without deteriorating the locking accuracy. The ATS technique breaks the trade-off between wavelength tracking speed and locking accuracy. The PTDM control enables controller sharing while maintaining a high loop bandwidth by exploiting the speed mismatch between the heater and the controller. Custom-designed integrated circuits provide interfacing and clocking functions. A hybrid-integrated four-channel prototype of the proposed scheme has demonstrated a record-breaking tracking speed of 180 nm/s with a locking accuracy of about 7 pm and a tuning range of about 9 nm. This scheme could be extended to applications with dozens or even hundreds of MRRs.

An Electronic-Photonic Converged Adaptive-Tuning-Step Pipelined Time-Division-Multiplexing Control Scheme for Fast and Scalable Wavelength Locking of Micro-Rings

This paper presents a dual-band matching network based on composite right/left-handed transmission line (CRLH-TL) unit cell to optimize the impedance matching of dual-band Doherty power amplifier (DPA). The number of CRLH unit cells in the matching network can be flexibly determined by theoretical calculation to achieve impedance matching at any two frequencies. Moreover, A dualband DPA is designed and fabricated using GaN HEMT CGH40010H to verify the performance of the proposed network. The designed DPA can deliver a saturation output power over 43.3 dBm and the drain efficiency (DE) over 55.2% in the frequencies of 2.6 GHz and 3.5 GHz. Meantime, the drain efficiency at the 5dB output power back-off (OPBO) is over 44% at 2.6 GHz and 3.5 GHz. key words: Doherty power amplifier, dual-band, composite right/lefthanded transmission, GaN Classification: Microwave and millimeter wave devices, circuits, and modules

Zhicheng Wang

Papers

Towards electronic-photonic-converged thermo-optic feedback tuning

First Demonstration of Closed-Loop PWM Wavelength Locking of a Microring Resonator in a Monolithic Photonic-BiCMOS Platform

Resolving the scalability challenge of wavelength locking for multiple micro-rings via pipelined time-division-multiplexing control.

An Electronic-Photonic Converged Adaptive-Tuning-Step Pipelined Time-Division-Multiplexing Control Scheme for Fast and Scalable Wavelength Locking of Micro-Rings

A concurrent 2.6/3.5GHz dual-band Doherty power amplifier with matching network based on composite right/left-handed unit cell