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82 The PCB Magazine • March 2015 by karl Dietz KarL DIETZ COnSuLTInG LLC karl's tech talk Optical Interconnects ColuMn When we hear of optical interconnects or optical signal transmission, we think of the long-distance signal transmission through glass fibers, the conversion of ocean floor cables to glass fibers, and the establishment of glass fi- ber transmission lines between cities in the late 1990s, followed by intra-city glass fiber lines and glass fiber networks that connect buildings [1] . Then, chip-to-chip optical interconnects were explored, and when data rate transmis- sions in backplanes pushed beyond 10Gbits/sec, the limitations of conversional signal transmis- sion through copper over 500 mm line length, and beyond, became apparent, especially with conventional, affordable dielectric material platforms. Signal attenuation and signal shape distortion became unavoidable, even with dif- ferential signaling and back-drilling of metal- ized through-holes. Optical backplanes hold the promise of avoiding these problems. There are practically no transmission losses and no electromagnetic interferences, and the capacity to transmit an enormous amount of data is un- surpassed. Optical transmission can be through optical fibers, waveguides, or through air, or a combination thereof. However, the cost of the optoelectronic components and the cost of pre- cision mounting has been a major hurdle to moving to optical backplanes. Lucent looked at the development of opti- cal backplanes in the early 2000s, but financial problems interfered. Of particular interest were

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