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JANUARY 2021 I PCB007 MAGAZINE 95 As mentioned, the "integrated waveguide" is a major fabrication challenge, but equally as challenging is the "alignment" of the VCSEL lasers and the optical sensors to the embedded PCB waveguide. This was addressed by UCL and Xyratex in the UK (Figure 19). Embedded optical waveguides in printed circuits have progressed aggressively in the last few years due to cooperation between university and industrial partners. There are now several polymer materials available, sev- eral fabricated by industrial firms capable of building these boards in production. The 10 Gb/s threshold per channel has been achieved and now the 16 Gb/s and 25 Gb/s are in prog- ress. Soon, the 40 Gb/ s channel will also be achieved. A second, but similar approach was used by IBM for their 12x10 Gb/s optical link dem- onstrator (Figure 20). A high-speed flex-cable containing the silicon optical components (VC- SEL or photodiodes-TIA) are passively aligned with MT fiber ferrules. Conclusion Telecommunication rates continue to im- prove; long-line optical communications is the standard. The challenge now is to bring fast- er optical communications rates into the Metro area and to the individual or commercial user. The performance limitations of electrical in- terconnections and wires (traces) due to their physical field limitations can be overcome with optical interconnections. The basic technolo- gies are optical waveguide materials, wave- guide fabrication, low-cost optical components (transmitters and receivers) and optical assem- bly. These technologies can be made compat- Figure 20: Alignment of VCSEL and the mirror in an embedded waveguide will require a new level of SMT precision. Various approaches are being researched to find the most cost effective.