Printed Circuit Design & Fab - February 2008 - (Page 36)

OPTICAL INTERCONNECT for the development and implementation of polymer optical interconnections for optoelectronic substrate connectivity. the motherboard and the daughter board ferrules is being explored to allow for air gap and alignment tolerance instead of direct zero air gap contact coupling. Multiple stacked-waveguide film layers at the ferrule interface allow for array interconnections from daughter board to daughter board. These interconnection options include a several critical optical interfaces used for: a) waveguides to/from optical fibers or b) waveguides to/from waveguides or c) waveguides from VCSEL’s, edge-emitting diode lasers, and to photo detector connections. These three basic optical interfaces are listed in 1 through 3 below and show in FIGURE 2 and 3. The fourth interface below is a fundamental optical guiding interface important for waveguides to/ from graded index fibers. 1. Out-of-plane I/O (input/output) mirrors that deflect signals perpendicular to the waveguide film surface usually formed at the edge of a waveguide array film. Deflecting mirrors, optimized for each waveguide, are constructed in the film sheet, not at an edge. 2. Butt coupling, where the interface is typically cut perpendicular to both the waveguide film and the waveguides in the film plane. 3. Waveguide tolen array coupling including either 1 or 2 from the two previous examples above but with sufficient lens coupling distance of approximately 100 microns. 4. Optimum coupling between standard graded-index optical fibers requires compatibility or the near match of the propagating and transmitting angles (referred to as NA or numerical aperture match). Also, the lowest-loss coupling to and from these fibers requires reduced guide size for waveguide to fiber in order to couple into the higher fiber core index region at the core center for transmitter Tx interfaces, and for receiver Rx interfaces, guide sizes equal to or larger than the fiber core size to efficiently collect all fiber-to-waveguide coupled light.. Obviously bi-directional coupling requires a compromise on guide dimensions to best optimize the systems performance. To meet specific application requirements, optical functionality can be imaged or incorporated within the parallel link waveguide films with appropriate coupling interfaces and configurations described above. Useful functionality may include crossovers, splitters, couplers, and combiners. Practical Application Configurations Waveguide interconnections can be designed and installed to be compatible with board interconnectivity for backplane/motherboard routing and coupling to either daughter boards or single boards. Interconnectivity to the sources and detectors associated with electronic chips is facilitated by versatile optical configurations. A flexible yet strong encapsulating film (as shown in FIGURE 1) provides a thermomechanically-stable self-supporting structure. Versatile designs and flexible films permitting a variety of parallel optical link interconnect options are shown in FIGURE 2 and 3 and described below: 1. Substrate-surface-attached waveguides for board edge-to-chip interfaces or chip-to-chip coupling. Coupling to or from the chip is typically accomplished through flip-chip solder bump procedures using out-of-plane I/O mirrors. Solder bumps/balls are used for chip alignment and attaching components to the substrate. 2. Combinations that include on-substrate surfaces as in 1) above and a design that goes off-surface using flexible jumper style links to connect to the chip top surface. This configuration allows diverse connections such as board edge-to-chip, chip-to-chip or chip-to-off-board fiber-array ferrules. 3. Jumper style interconnectivity on both ends facilitating off-board or device package I/O leading to chip top access, and chip-to-chip options. 4. Hybrid options including either on- or off-substrate link to I/O mirror and lens arrays for interconnectivity to the underside of a substrate or to an optical layer in between substrates. 5. On-substrate or below-substrate surface waveguide arrays bending 90 degrees perpendicularly off board to connector ferrule and daughter board with edge connection ferrules. Using guide bending flexibility within motherboard ferrules enables daughter-board to motherboard connection compatibility with insertion requirements for connectors also on the daughter board. Lens coupling between 36 General Description for Optical Waveguides Operationally, waveguides are “wire like” regions of higher refractive index within a surrounding matrix of a lower refractive index. Light propagating within the waveguide remains trapped inside as long as the glancing angle at the edge of the waveguide is not so large that light can escape. This phenomenon can be compared to the refraction of light as it passes through water. Glass fibers are used for optical transmission applications in both telecommunication and data communications have a higher refractive index core that traps propagating signals. If the size of the higher index waveguide core is about 6 microns and embedded in 125 microns of lower index glass, light in or near the core can only travel in one path, with slight angular deviations. Think of it as a single lane tunnel through a mountain, where a driver has no option except to travel on the single path. An optical waveguide designed in this way is called a single-mode (SM) waveguide. Single mode waveguides are used for high data rate telecom signals that travel over long distances. All signals exit at nearly the same time and they are not blurred in the process. Larger core waveguides allow many signals to follow multiple paths. Again, using the analogy of a multi-lane highway tunnel, the driver can move back and forth taking either a straight or curving path through the tunnel. Thus, some “travelers” will get through the tunnel sooner than others. In optics, this type of waveguide is referred to as a multiple mode (MM) with specific angular values based on constructive and destructive interference of the nearly monochromatic laser light. A view of the output end of a 50 micron square multimode waveguide propagating many optical signals is shown in FIGURE 4. The granular light appearance is evidence of the many modes being propagated. Transmission times for these modes do not sufficiently blur the signal’s arrival, so MM waveguides are used for shorter (less than 100 meter) runs, even with very high frequency signals. Other defining factors in optics require FEBRUARY 2008 PRINTED CIRCUIT DESIGN & FAB

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Printed Circuit Design & Fab - February 2008

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