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

A novel switching-linear hybrid dynamic regulator architecture with dual-supply low dropout regulators (LDOs) is presented in this paper. This architecture leverages the intrinsic dual supplies to extend the operating range of the LDOs. Furthermore, it increases the thermo-optic tuning efficiency by reducing the LDO dropout voltage through dynamic supply modulation. This architecture is suitable for large-scale thermo-optic tuning in silicon photonics. The efficiency improvement is particularly effective when tracking signals of several adjacent channels are close to each other, e.g., wavelength tuning of a microring array. The principle of this architecture is general and can be implemented using different switching converters and LDOs. A specific design with extensive post-layout simulation results is used to verify the effectiveness of our architecture. Implemented in a 130 nm CMOS process, this design can simultaneously regulate eight output channels with an output swing of 0.8 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text V_{\text {pp}}$ </tex-math></inline-formula> . Its peak efficiency when driving 100 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega $ </tex-math></inline-formula> loads is 92% at 1 V output, and the dynamic efficiency is around 86% when tracking 50 kHz sinusoidal signals. To the best of our knowledge, this is the first time that dynamic supply modulation has been applied to multi-channel thermo-optic tuning of silicon photonic devices. 

https://figshare.com/articles/preprint/An_Eight-Channel_Switching-Linear_Hybrid_Dynamic_Regulator_with_Dual-Supply_LDOs_for_Thermo-Optic_Tuning/19165589/1/files/34052459.pdf

An Eight-Channel Switching-Linear Hybrid Dynamic Regulator With Dual-Supply LDOs for Thermo-Optic Tuning

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.

Yuhang 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