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March 2015 • The PCB Magazine 85 polymer binders along with initiators and oth- er constituents are pre-coated on a temporary polyester substrate carrier with a polyester pro- tective sheet. The film is exposed, causing pho- topolymerization in the exposed areas that will become the wave paths. After exposure, mono- mers diffuse from the monomer-rich adjacent areas into the monomer-depleted exposed ar- eas, creating features with a higher refractive index than their surroundings. The exposed film is then laminated between two unexposed films from which more monomers diffuse into the exposed sections. Subsequent flood pho- toexposure and curing form the mechanically and thermally stable structure of the wave- guide and its cladding, without destroying the refractive index difference between guide and cladding. Because of the high optical attenuation of polymer-based waveguides, there has been a continued interest in lower-loss glass-based waveguides. The work described in Reference 2 aims at the formation of electrical-optical cir- cuit boards, using photolithographic and ion exchange processes to form graded-index mul- timode waveguides in thin glass, which is em- bedded in the circuit board. The thin glass was a commercially available product from Schott. The glass is first cleaned, then double-sided coated with an aluminum diffusion mask. Next comes the double sided coating of photoresist, mask alignment, UV exposure and develop- ment. The diffusion mask is then etched and waveguides are then grown in the glass by ion exchange, followed by removal of the diffu- sion mask. The work was done by a consortium, including research institutes (e.g., Fraunhofer IZM), universities, a circuit board manufacturer (Wuerth Elektronik), and suppliers to the indus- try such as Siemens AG. The following communications document continued interest and work on optical back- planes: • In a news release from HP Lab in EE Times (February 6, 2012), HP describes a 30 GByte/s optical backplane it created as a tech demo for its ProCurve 9200 switch. The backplane was built from a hollow metal waveguide bundling 12 10 Gbyte/s optical channels. • Posted in EE Times (October 5, 2012), Al- tera explains the move from copper to optical backplanes. The 56Gb/s backplane technology makes this transition necessary. A standard for this new platform is expected to be developed with two to three years. • In their presentation "Optical Back- planes—Fantasy or Reality?" Beth Murphy and Scott Schaeffer of Tyco/Electronics/AMP give a detailed overview of optical backplane applica- tions such as inter-rack, intra-rack, and inter- device backplanes. They see some extension of copper backplane technology on the basis of lower loss laminates, but see plated through- holes as a major performance issue. • iNEMI offered a "Roadmap for Optical Backplanes—A Copper vs. Opto Business Analy- sis" presented by Jack Fisher at the LEOS HSD Workshop, May 14–17, 2006, Santa Fe, New Mexico. So, it appears that after a hiatus in the mid 2000s there is again a renewed interest in optical backplanes, a development worth fol- lowing. PCB References 1. "Low-Cost Optical Interconnect Expect- ed for GHz Boards," by Masahiro Katoh and Chikashi Horikiri, February 2002 Issue, Nikkei Electronics Asia. 2. "Thin Glass Based Electro-Optical Circuit Boards (EOCB) Using Ion-Exchange Technol- ogy for Graded-Index Multimode Waveguides," Henning Schroeder et al., Proceedings, pg. 268, 2008 Electronic Component and Technology Conference. Karl Dietz is president of Karl Dietz Consulting LLC. he offers consulting services and tutorials in the field of circuit board & substrate fabrication technol- ogy. To view past columns or to reach Dietz, click here. Dietz may also be reached by phone at (001) 919-870-6230. karl's teCh talk OPTICAL INTERCONNECTS continues