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JANUARY 2025 I DESIGN007 MAGAZINE 37 ogy detects, generates, transports, and pro- cesses light. PICs are the driving technology behind next-generation, miniaturized, and high-performance devices. By leveraging advanced technologies such as distributed Bragg reflectors (mirrors), dielectric wave- guides, vertical couplers, and electro-optic modulators, PICs enable unprecedented integration densities. This allows for a wide range of applications, from communications to sensing and packaging. Onboard optical interconnects can manage extremely high data rates and provide a greater number of data channels compared to electri- cal interconnections. Moreover, as optical sig- nal transmission is impervious to electromag- netic interference (EMI), it is ideal for mixed- signal systems, such as data acquisition and signal processing, where sensor applications need high accuracy of analog electronics. Optical waveguides on PCBs require not only low attenuation but also a reliable man- ufacturing process for the optical layer. In an optical PCB, the fabrication steps and material properties of the waveguides need to be com- patible with the manufacturing and assembly techniques prevalent in the PCB industry. Apart from the optical path in an optical inter- connection system, there must be coupling elements that can transfer optical signals into and out of the waveguides. Additionally, com- mon pick-and-place machines must be capable of suitably and automatically mounting these coupling elements without any active align- ment between the optical waveguide and the coupling element. Using structured polymer foils helps this integration. To construct an EOCB, laminate low-cost glass sheets with a conventional organic base material such as FR-4. You can use two struc- tures together: Glass can be a core layer inside the conventional PCB base material and as the main carrier—a photonic interposer carrying all the PIC and electronic chip components. You can plate copper on the glass and the organic material. e processing is compatible with regular PCB processing but needs high- precision assembly. Figure 2 illustrates the structure of a typi- cal EOCB. e blue layer denotes a glass layer with integrated single-mode optical wave- guides. e electrical processor is located on a glass interposer with a cooling surface on top. A critical point is the connection between the transceiver and the glass layer. While struc- turing through-glass electrical vias (TGVs) is an established process, steering the photonic signals is the challenge. Currently, there are at least two techniques under development: either a periscope placed from the top or a small mirror positioned from the bottom of the EOCB. However, industry standards or multi- source agreements for these techniques still need to be established. Figure 2: Structure of the electro-optical circuit board. (Source: Rajesh Uppal)