[Paper Review] Review Of Integrated Photonic Elastic WDM Switches For Data Centers

Akhilesh S. P. Khope, Anirban Samanta+3 more|arXiv (Cornell University)|May 23, 2021

Photonic and Optical Devices78 references5 citations

TL;DR

This paper presents a comprehensive review of integrated photonic elastic Wavelength Division Multiplexing (WDM) switches for data centers, focusing on wavelength-selective switching (WSS) technologies, electronic-photonic integration, packaging techniques, and wavelength locking. It highlights the role of silicon photonics and III-V platforms in enabling high-port-count, reconfigurable, low-power optical switches that dynamically adapt bandwidth to bursty workloads, with key demonstrations of 8×8, 32×32, and 8×8×8 switches using MZI, SOA, MEMS, and AWG architectures.

ABSTRACT

In this review paper, we present an elaborate discussion on wavelength selective switches and their demonstrations. We also review packaging and electronic photonic integration of switches; a topic neglected in other review papers. We also cover wavelength locking which is paramount in switching networks with many tunable filters.

Motivation & Objective

• To provide a detailed review of integrated photonic wavelength-selective switches (WSS) tailored for data center applications.
• To address the lack of coverage in existing literature on packaging and electronic-photonic integration of WSS devices.
• To examine wavelength locking and stabilization techniques critical for maintaining performance in temperature-varying data center environments.
• To evaluate the feasibility and performance of elastic WDM switching in enabling dynamic, reconfigurable bandwidth allocation in data center networks.
• To highlight recent advances in high-port-count, chip-integrated WSS architectures using silicon photonics and hybrid integration platforms.

Proposed method

• Systematic review of integrated photonic WSS architectures, including broadband, single-wavelength, and elastic/multiwavelength selective switches.
• Analysis of photonic integrated circuit (PIC) technologies such as Mach-Zehnder interferometers (MZI), ring resonators (MRR), and arrayed waveguide gratings (AWG) for WSS implementation.
• Examination of packaging techniques including flip-chip bonding, ceramic interposers, fiber ribbon alignment, and 3D integration with CMOS drivers.
• Evaluation of electronic-photonic co-integration using fabless foundries, including 50 µm copper pillar technology and through-oxide vias for reduced parasitic capacitance.
• Review of wavelength locking mechanisms such as feedback control using contactless photonic probes, heater actuation, dithering, and in-resonator photodetectors.
• Investigation of control systems using FPGAs and real-time calibration algorithms for dynamic reconfiguration and power equalization in Benes and WSS topologies.

Experimental results

Research questions

• RQ1 How do elastic WDM switches based on integrated photonics improve bandwidth scalability and energy efficiency in data center networks?
• RQ2 What are the key challenges and solutions in packaging and electronic-photonic integration of high-port-count WSS devices?
• RQ3 How do wavelength locking and stabilization techniques maintain performance across temperature variations in data center environments?
• RQ4 What are the performance limits of current integrated photonic WSS in terms of port count, wavelength count, and switching speed?
• RQ5 How do emerging integration technologies like 3D integration and hybrid CMOS-PIC processes enable higher density and lower power operation?

Key findings

• A 32×32 non-blocking, polarization-diversity WSS was demonstrated using a flip-chip bonded chip with a 74-port 127 µm fiber array and 0.18 mm pitch electrodes.
• An 8×8×8 SiP MEMS switch was achieved using 3D integration with 50 µm copper pillar technology, enabling high port count and compact packaging.
• A 4×4 MZI-based switch with integrated CMOS driver was demonstrated using flip-chip bonding, achieving full electronic-photonic integration.
• Wavelength locking using a single monitor signal enabled stable operation over >40°C temperature range with 3×20 Gb/s OOK and 3×75 Gb/s DMT modulation.
• A 32×32 switch with high-Δ PLC connector was demonstrated, showing improved mechanical and thermal stability for large-scale integration.
• FPGA-based tuning algorithms enabled real-time, hitless tuning of MRRs and automatic calibration of 4×4 Benes switches, enhancing reconfigurability and reliability.