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Addressing Embedded Optical Waveguides in PCBs

Addressing Embedded Optical Waveguides in PCBs
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by Joshua Kihong Kim, Principal Engineer, Hirose Electric

Today’s high-speed electronic interconnection is mainly based on copper traces of PCB for transmission line structures (i.e., strip line and microstrip). The IEEE 802.3 standard defines three classes of interconnection systems depending on the reach length and structure for electronic signal: chip-to-chip, chip-to-module, and backplane. PCB technology has delivered reliable and rigid circuit assembly for almost six decades with a more than fair degree of system performance. However, electronic systems keep evolving in terms of data processing speed performance expectations. This situation becomes clearer in high-speed equipment such as data center servers and switches. Unfortunately, this trend is now approaching the limit of physics called Shannon-Hartley’s maximum channel bandwidth. Scientists and engineers have been looking into this problem to create the breakthrough solution for the industry for many decades.

One attempt, called ‘PCB with embedded optical waveguide’ or ‘Optical Printed Circuit Board’ (OPCB), has been studied for decades but with no clear success. The OPCB technique continues to utilize “copper trace” for conventional interconnection needs in the same PCB, and it starts to use “embedded optical waveguides,” particularly for the high-speed interconnection portion. Although this has been in the research field for many years, it is only very recently (September 2021) that OPCB has drawn significant attention as a potential industry solution. The main reason for this recent change in trend is that this embedded optical waveguide PCB solution, called MWIS (Multimode Waveguide Interconnect System), solves not only the physical bandwidth limitation of copper, but it also solves the imminent power consumption issues on facility operation given the fixed power.

Note that today’s electrical SERDES not only increases its pJ/bit due to PCB Df (Dissipation Factor) loss due to increasing fundamental frequency (Nyquist), but it also increases DSP power consumption due to the increased number of impaired bits from channel impedance discontinuities (e.g., connectors etc.)

Hirose has recently published the background of these issues and solutions. Hirose proposed this to the standard body, COBO, to form a working group called MWIS and is now working together with other companies (more than 30) to build a related standard and prototype.

MWIS is composed of three key channel elements: Media Adaptor(MA), Photonic Integrated Circuit Connector (PICON) and Multimode Waveguide embedded in PCB (MMWG).

–              MA is a very simple electrical-to-optical converter, linear or non-linear. Inside the system (or box), MA always exists in a pair, one at each end of MMWG.

–              PICON provides optical transition between active optical component to MMWG. There are many types of active optical components, including Silicon Photonics (One type of Photonic Integrated Circuit) and standalone laser diode/photo diode. PICON is generally part of MA.

–              MMWG can have many different cross-sectional shapes and sizes. The physical size is the only reason to use multimode rather than single mode waveguide. For design exercise by general electronic engineers, the standard will specify electrical equivalent S-parameters for overall channel characterization.

Deploying equipment with on-board optics results in a shorter deployment cycle and a more reliable installation. The COBO MWIS working group aims to lay the foundation for optical interoperability and integration.

Learn more: https://www.onboardoptics.org/

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