Printed Circuit Design & Fab - February 2008 - (Page 37)

OPTICAL INTERCONNECT that the waveguide refractive index (or, the core to surround index) inside to be greater than the outside, in order to sustain higher propagating angles in the waveguide. If this index is too low the light escapes. Also, if the input angles are too great for a given index the light will also escape, analogous to porous tunnel walls. Summarizing, SM waveguides are typically about 6 microns in width with a refractive index of approximately 0.006 over the surround sustaining propagation angles of within +/-7 degrees and are primarily used in telecom applications. MM waveguide cores range from 10 to 100 microns in width/diameter with a refractive index of approximately 0.02 to 0.035 over the surround and are primarily used for data communication links and circuit board interconnectivity. This high index difference sustains propagation angles of within +/-18 degrees inside the guiding core. For low-coupling loss, the interconnection offset precision for single mode waveguides requires +/-0.5 microns, while multimode guides of 40 microns widths requires within +/-5 microns offset precision. Thus, for substrate based optical interconnections, MM waveguides are preferred, given their higher tolerance for alignment offset and as such are less costly to manufacture. Other important factors useful in optical interconnection includes smooth low optical absorption and low optical scattering waveguide walls, so that a sufficient percentage of the light gets through the link to solidly couple between sources and detectors. erances required for multimode (MM) waveguides will be a challenge for any polymer system under a high temperature and pressure fabrication condition. These issues include (but are not limited to) thermally induced softening, pressure distortions as well as differential thermal expansion during heating and bonding. Even if a polymer waveguide system survived, creating the precise optical interfaces needed to connect components with waveguides trapped between two rigid millimeter thick (or possibly thicker) boards would seem to be an extremely complex difficult undertaking. Some industry or university groups may have made progress in developing this technology, but that is beyond the scope of this article. Assuming that the technology advances to the point that embedded waveguide systems can survive the manufacturing process, maintaining waveguide array alignment and effective interconnection under these adverse pressure and temperature conditions is still and issue. Finally, manufacturing yield must be considered. There is still a question as to whether the potential manufacturing yield loss cost would be acceptable, since a process or material failure, or an alignment problem would likely necessitate scrapping of the entire board, which would be an expensive loss. Alternatively, waveguide film sheets could be constructed and inserted with no prior machining for connecting two completed substrate boards, avoiding the high temperature and pressure construction process. Without pre-machining of the connection and optical interfaces, manufacturing difficulties for creating and metalizing in-situ mirrors at the base of open vias would be as much a challenge as insuring alignment under high temperature/pressure processing explained previously. To emphasize, alignment between surface components, vias, lenses, mirrors and waveguide structures must have less than 5 microns misalignment offset for acceptable interconnect system performance. PCD&F Ed note: In March we will continue with Part 2 of Optoelectronics Comes of Age. It continues with detailed coverage of the applications and implementation of wave guides and the systems issues that are impacting wide spread adaptation of this technology. DR. BRUCE BOOTH is president, founder and CTO of Optical Interlinks. He can be reached at blbooth@opticalinterlinks. com. JACK FISHER is a consultant at Interconnect Technology Analysis. He can be reached at fish5er@mindspring.com. ACKNOWLEDGEMENTS: The authors appreciate the useful suggestions and comments from Silvio Bertling of Parker Nelco, Diana Williams of Rogers Corp., and Dale Murray of W.L.Gore as well as the extensive help in editing and organizing from Kevin Hair of Optical InterLinks. Embedded Waveguides Between Substrates Considerable discussion within the industry relative to creation and application of waveguide optical interconnections has revolved around attempting to embed optical layers within board substrates during the multi-layer board fabrication process. The idea is to align a polymer waveguide film sheet or link between two multilayer substrates (like FR-4 boards) then subject the board to the high temperature and pressure required to bond the substrate layers together in the board construction process. It is the authors’ perspective that this will cause problems in the construction of embedded polymer waveguides. Maintaining the less than 5 micron waveguide alignment offset tolFEBRUARY 2008 PRINTED CIRCUIT DESIGN & FAB 37 http://www.pcb-pool.com http://www.pcb-pool.com

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Printed Circuit Design & Fab - February 2008

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