Passive optical components for optical interconnection using hybrid optical printed-circuit boards (PCBs) where electrical and optical layers are integrated into one board has been studied. We present detailed fabrication processes and optical characteristics of optical PCBs and connectors for optical coupling between vertical and horizontal directions. Two kinds of optical PCBs, polymer-waveguide-embedded and silica-fiber-embedded PCBs, were prepared. For the polymer-waveguide-embedded PCB, the polymer waveguide was formed lithographically on a FR-4 board and its core has 100 μm width and 60 μm thickness. The waveguide-defined board was covered with another FR4 plate and then laminated at 185°C under the pressure of 35 kg/cm2. After lamination the transmission loss of the waveguide was -0.53 dB/cm. For the fiber-embedded PCB, fibers with 100 μm core diameter were inserted in grooves formed on a FR-4 board and they followed a similar lamination processes. The propagation loss of the fiber-embedded board at 850 nm was negligible in board scale. We also prepared 2 types of connectors for optical coupling between the surface mounted transmitter or receiver modules and the optical PCBs; 45°-ended fiber block and 90°-bent fiber connector. The insertion losses of the 2 kinds of connectors were, respectively, -0.15 dB and -0.25 dB. The best combination between the optical PCBs and connectors in view of optical characteristics and packaging is fiber-embedded board and 90°-bent fiber connector. They show successfully optical link of 2.5 Gbps with a very low coupling losses of -4.4 dB and a low optical crosstalk of -53 dB.
A simple method for fabricating fiber-embedded boards using a grooving technique is described that is quite cost effective and fully compatible with conventional printed circuit board (PCB) processes with no necessity for a specially designed wiring machine. FR-4 plates are grooved using a dicing saw machine and followed by placing optical fibers into the grooves. The fiber-embedded PCBs are laminated by conventional PCB processes at a temperature of 180°C for 1 h under 47 kg/cm2 of pressure. The 50/125-µm glass fibers, and the polyimide-coated glass fibers are laminated successfully. In the fiber-embedded boards with a length of 10 cm, the variation of center positions of the embedded glass fibers is about ±5 µm. The transmitted optical power through the fiber-embedded boards shows a good uniformity of less than ±0.5 dB variation from the average value for the 12 fiber channels. Data transmission through the board at data rates of 2.5 Gbits/s is achieved. After confirming the successful laminations and the data transmission with the small-scale fiber-embedded boards, a large-scale prototype of the fiber-embedded board for a backplane application is successfully fabricated.