Printed Circuit Design & Fab - March 2009 - (Page 34) optoeLectronIcs OPtICAL WAvegUIDe PCB Interconnects Optical technology continues to displace copper for for highspeed data transmission, but wide spread adaptation is still five years away. by JACK FIShER Increased computing power is the primary driver in a move towards the optical interconnect.1 This drives miniaturization, high density circuits and high date rates resulting in a bottleneck at the electrical connections. Electrical (copper) interconnections, under these conditions, compromise performance and require increasing costs to shield and to condition signals to overcome electrical interconnection issues. Optical technology generally continues to displace copper for data transmission over shorter distances. The higher the data rate required, the shorter the distance over which optical methods become superior. Telecommunications, from long haul down to the metro links, are a few kilometers in length and are dominated by single mode optical technologies. between optical and copper data transmission in more detail by including specific distance-bandwidth combinations and an indication with $$ signs of the cost to fill the application utilizing copper methods. The general consensus is that a copper pair is a viable media for transmitting up to 18 Gb/s over a maximum distance of one meter using conditioning/detection circuits. Whether or not copper can be used at greater bandwidth/distance combinations, marked as “Frontier” in this table, is an open issue. Converting to optical methods at any of the frontiers can be an economic or technical issue, or both. At the longer distances, the issue is usually economic with optical innovation continually overcoming the economic barrier, resulting in adoption. taBLe 1. Speed / Distance Status. 5cm – 1m 1 Gb/s 3 Gb/s 5 Gb/s 10 Gb/s 20 Gb/s 40 Gb/s Copper Copper $ Copper $$ Copper $$$ Copper $$$$ Frontier 1 – 2m Copper $ Copper $$ Copper $$$ Copper $$$$ Frontier Frontier At the shorter distances, technology is not nearly as developed, and the economic issues are not yet clear, implying that it will be longer until those frontiers are overcome. PCB Applications As stated earlier, it is believed that copper can be used to transmit data at rates up to 18 Gb/s for distances up to one meter if signal-conditioning circuit- The Transition from Copper to an Optical Connection fIGure 1 is a graphic of the current fIGure 1. Current generic data transmission environment. generic data transmission environment with increasing distance horizontal and increasing bandwidth vertical.2 It illustrates the general state of optical methods replacing copper with optics and beginning to challenge copper in backplane applications. taBLe 1 illustrates the battlefront 34 2 - 4m Copper $$ Copper $$$ Copper $$$$ Frontier Frontier Optics 4 – 10m Copper $$$ Copper $$$$ Frontier Frontier Optics Optics 10 – 30m Copper $$$$ Optics Optics Optics Optics PRINTED CIRCUIT DESIGN & FAB MARCH 2009
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