Printed Circuit Design & Fab - February 2008 - (Page 33)

cal substrate based applications are also essential for a broad range of diverse optical interconnectivity solutions. This article reviews the generic capabilities for a practical optoelectronic substrate interconnection system, the substrate associated polymer waveguide capabilities and requirements, and evolving practical embodiments under development today. Interconnect Options for Parallel Links – 1 1. Board Edge – on substrate 2. On and off substrate Historical Comments Test installation on manufactured low loss optical glass fibers for telecom applications first began in the late 1970’s. By 1985, a multimode optical fiber link used by Bell Atlantic (now Verizon) between Washington and New York was proudly demonstrated to one of the authors. Shortly thereafter, single mode glass optical fiber was manufactured for installation, and as quality improved over the next decade, glass optical fiber became practical for central office and undersea installations. The rest is history, as optical fiber now brings phone, TV, and internet to the home as well as spanning oceans for telecommunications. During the mid 1980s as optical fibers moved onto PCBs and into computers, it was recognized that short interconnecting links and optical function components, such as splitters, combiners, etc., would be needed at low cost.. Many fiber technology approaches were developed and commercialized to provide the required functionality. In addition, laboratories worldwide invested considerable effort in the development of planar sheets of polymer waveguides. 3. Off substrate FIGURE 2. Interconnect options for Parallel Link – 1. Optical Signal Distribution Requirements A broad range of optical capabilities must be utilized to facilitate a complete optical interconnection system. Optical fibers and polymer waveguides are providing practical hybrid solutions that are beginning to meet industry requirements. Both point-to-point optical links and functionality for signal distribution options combined with appropriate connectivity solutions are being explored, prototyped, and implemented. Subsets of these capabilities are being utilized for stand-alone off-substrate functional components. Optical InterLinks polymer waveguide technology exemplifies how the critical attributes can and are currently being met for specific application developments and will be used as examples of evolving practical systems. Capabilities for a complete optical interconnection system involve the following categories. 1. Optical input and output connections at the edges of the motherboard or backplane. For some time optical fibers at the board edge have been used to connect directly to photo detectors and laser sources. Single fiber, multiple fibers in ribbons, or cables are used for these board inputs. Solving problems caused by copper transmission lines on the board, such as high signal input/output density, EMI, high energy consumption and associated heat dissipation, plus physical constraints such as the forces required to couple high-pin-count/high-density connectors, are increasingly driving the use of optical transmission into the motherboard and on through to the daughter boards. 2. Backplane/motherboard (BP/MB) optical signal distribution. Board edge connector coupled optical signals are distributed across the backplane to the daughter board assuming there are no active components on the BP/MB. Depending on the board size and the length of the optical runs required, either fibers/fiber ribbons or polymer waveguides can be used. Since optical fibers have lower optical loss, they are preferred FEBRUARY 2008 for long runs typically covering from 30 to 100 cm. Fibers will likely be in the form of bundles or ribbons, and are frequently encapsulated into commercially-available polyimide sheets pre-arranged in a flexible film package. Fiber shuffles for signal distribution can be mounted on the BP/MB to uniformly route optical I/O between all daughter boards as required. Parallel polymer waveguide links can be used to distribute signals from short millimeter lengths (up to 50 cm runs) depending on optical loss at the operating wavelengths and the allowable system loss budget. In some board systems this is in the 10 to 15 dB (~5% signal remaining) range. It can also be advantageous to choose polymer waveguides that have unique Build 3D full-wave models for interconnects and via-holes Introducing Simbeor™ 2007 • 3D full-wave analysis with all dispersion and loss effects • Broadband dielectric models • Metal surface roughness • Metal plating, skin, and proximity effects • Geometry synthesis wizard for impedance-controlled via-holes • S-parameters for via-holes • RLGC(f) parameters for transmission lines • Fits into any design flow Download a free trial at www.simberian.com © 2008 Simberian Inc. PRINTED CIRCUIT DESIGN & FAB 33 http://www.simberian.com http://www.simberian.com

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Printed Circuit Design & Fab - February 2008

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