Printed Circuit Design & Fab - February 2008 - (Page 34)

Interconnect Options for Parallel Links – 2 4. Off / On substrate – lens array to through board and both sides accesse Encapsulated board option 5. 90 degree board interconnection with surface and underside access FIGURE 4. Multimode Waveguide viewed at the output film end. FIGURE 3. Interconnect options for Parallel Links – 2. functionality, like low-loss crossovers for routing, splitters, star couplers for mixing and distributing, and/or combiners, that can be incorporated in planar waveguide films. Both fibers and polymer waveguides films can be attached or bonded to substrates, be embedded within or between boards, or be a flexible off-substrate circuit attached only where needed at the end connections. BP/MB optical signal distribution typically provides routing to right angle oriented daughter boards where active light sources, detectors and logic chips are located. Of course, for a single board system containing active components, fiber arrays connected to fiber or polymer guides at the board edge are used to directly interconnect components. 3. Right angle connectorization at the daughter board (DB) junction with the BP/MB. Right angle boardto-board connectivity is required to couple light between DBs and BP/MB that interconnect 90 degrees to each other. Low cost, high-performance, right angle array connectors are critical for this application, and considerable efforts in the industry have been made to achieve a practical solution compatible with electronic counterparts on the daughter board. To achieve DB-to-DB connectivity along the BP/MB, waveguide arrays need to enter the BP/MB connector housing from both directions. To achieve dual-directionality typically requires multiple waveguide array interconnects be stacked in at least two or more layers, in order to be able to direct signals from daughter card to daughter card along the BP/MB. The use of closely spaced waveguides enables very high-density interconnections within a standard MT (multi-terminal) form factor, or other custom two pin aligned ferrule options. An MT connector is a commonly used fiber optic ribbon connector. With appropriate machining, it is modifiable to provide free standing board edge planar polymer waveguide array coupling alignment. The MT ferrule connector is very popular due to its small form factor and the ability to connect 24 channels with 12 channels on two rows (greater numbers, space permitting) while maintaining conventional optical fiber 250 microns pitch spacing. Machining modifications enable coupling of up to several hundred closely-spaced polymer waveguides for highdensity interconnectivity. Typically, MT ferrules are inserted into flexible aligning and locking housings to provide a latchable connector tightly holding the coupling interfaces together with a zero 34 air gap. These latchable housings are commercially sold under manufacturer’s names, such as MPX or MPO connectors. For right angle board-to-board optical interconnections, when the daughter card is inserted, the locking mechanism assures a zero air gap interconnect between adjoining ferrules, allowing sufficient insertion force flex to be compatible with adjoining electronic pin connectors on the daughter board. This configuration assures good coupling while avoiding a hard stop insertion of the board. Thus the natural bend flex of self-supporting waveguide films is important. On the other hand, unique lens configurations are also being explored to provide practical coupling solutions allowing a more forgiving air gap. Both are useful for high vibration environments. Both electronic and optical connector housings should have nearly identical height and at least compatible form factors at the edge of the DB and on the BP/MB. For these applications, spring-loaded latchable housings with internal custom ferrules are the preferred choice for optical configuration. Once the optical signals are on the DB, polymer waveguides offer unique versatility where array routing and optical functionality is required to distribute signals through the daughter card. 4. Optical signal distribution on the DB or into Tx/ Rx link packages. Optical signals on DBs are distributed directly to components or to small interposer boards for subsequent distribution to and from typically VCSELs (vertical cavity surface emitting lasers) and detector components. Traversing the offset space between the parallel DB and the interposer board requires either flex optical waveguides and edge connectors, or lens and I/O mirror arrays to couple the DB and small board offset air gap. Waveguide array transmitted signals are coupled directly to detectors (PD) or from the VCSELs, with both components close to their amplifiers or driver chips. Increasingly the capability and versatility inherent in polymer waveguides is a prime consideration for DB signal distribution. Some of the enabling capabilities and attributes for polymer waveguides include: ■ Point-to-point dense arrays as flexible links (unattached except at the terminations) or substrate-attached links or combinations. ■ The ability to provide complex routing under, over or around surface mounted components or with waveguides embedded between board substrates to be routed from the underside of the board to the surface component side. ■ Functionality, including splitters, combiners, star couplers FEBRUARY 2008 PRINTED CIRCUIT DESIGN & FAB

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Printed Circuit Design & Fab - February 2008

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