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Optoelectronics comes of age, part 2: optical connectivity is sufficiently advanced, has been reduced to practice and is available for many near term applications.

Over the last 20 years there have been many process and material technologies used to develop planar polymer waveguides containing films capable of meeting the current and evolving industry requirements. Sorting through the various techniques used to make polymer waveguides, these processes fall into two basic classes, both using photolithographic processes (sometimes with LDI) to define the waveguide. The distinctly different processes for the two classes are:

Ridge technology. Initially, a polymer ridge or trench is constructed through molding, embossing, or etching that has a higher refractive index than the base polymer. See FIGURE 1 for a schematic outlining the generic process steps for ridge and trench polymer waveguide formation. The lower refractive index polymer surrounding the waveguide region creates the specific guiding properties. Different polymer materials have been evaluated depending on whether a ridge or trench is used to form these waveguides.

[FIGURE 1 OMITTED]

Diffusion Technology. This method includes the formation of a high refractive index waveguide by monomer diffusion into the light-exposed guide forming region with no mechanical or chemical etching contact. See FIGURE 2 for a schematic showing the diffusion technique process. An essential process feature here is the photomask-defined light exposure of a mobile monomer waveguide forming region in a polymer matrix that converts the monomer to a polymer. The process of continued monomer diffusion into the surrounding guide imaged region increases the density. The addition of other laminated monomer/polymer diffusing layers with the typical three-plus layer configuration is completely photopolymerized after diffusion is complete. The essential steps include a light induced imaging reaction, a total polymerization light fixing for the entire film, and final cure, all using pre-coated dry materials without waveguide side wall contact. Light and molecular diffusion determine the guide walls.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Several industrial groups and laboratories (some identified in FIGURE 3) exploring the ridge formation style technique also participated in an iNEMI study on optical backplanes. Over 15 groups participated to develop a polymer waveguide technology attribute table. Figure 3 shows the header for the resulting attribute table, listing the two polymer technique class types, with representatives from each subgroup. Both ridge and diffusion technologies have the capability to create self supporting waveguide films that can be micro-machined to provide fully connectable configurations ready for installation. The process techniques can also create waveguide films directly on the final application substrate. A number of unique performance and waveguide configurations can be obtained by the use of the diffusion or ridge guide forming processes.

Practical link configurations for optoelectronic substrate interconnectivity involves connectivity options for parallel and functional links. These critical system building blocks used for practical applications are described below, identifying and elaborating on key system attributes. Self-supporting waveguide films are used as the starting point. The basic features demonstrated confirm that practical connectivity is achievable.

Optical Interconnects and Connectivity Issues

In the following examples the coupling capabilities are demonstrated by the prototype products shown. Critical connectivity issues for practical applications of each are noted.

Type One uses substrate edge ferrules with similarly precise aligned waveguides and butt coupling interfaces. Butt coupling (surfaces cut perpendicular to waveguide or fiber axis) can be coupled to fiber arrays or to waveguide arrays held in similar ferrules. FIGURE 4 shows board edge MT ferrules connecting and aligning 12 waveguides to 12 optical fibers.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Critical connectivity issues include the need to prevent loss producing micro-bends at the connector interface where the film leaves the board edge into the modified MT ferrule. It is necessary to optimize performance to match fiber and waveguide allowed propagating angles (referred to as numerical aperture, or NA, defined as the sine of the half angle of the radiated Gaussian light pattern at the 5% intensity points) and waveguide dimensions, by matching fiber graded index profiles and waveguide step profiles. The connection needs to be matched with center-to-center spacing (pitch) to assure that maximum offsets are not exceeded. This is accomplished by using micro ferrules (machined slots for pin alignment required) designed for small array coupling footprints. In addition, it is necessary to couple to waveguide arrays on a high-density board with the several hundred interconnections using stacked waveguide films, and with a pitch less than the 250 micron limitation associated with standard optical fibers.

Type Two uses I/O mirrors cut at 45 degrees to deflect waveguided light perpendicularly from waveguide films to or from guides, sources or detectors components. An example of a waveguide deflecting metallized mirror is shown is FIGURE 5. Typically, unguided light path distances here are approximately 50 microns. The mirror surfaces are usually metallized to ensure that high angle multimode propagated light is reflected and not passed through a total internal reflection (TIR) mirror (without metallization). Propagating angles with reasonable mode fill are likely to exceed the total internal reflection critical angle allowed. Applications include substrate-attached guides for flip-chips or access to overlying components and top-access configurations used with flexible jumpers and functional devices not attached to the substrate. These mirrors can be cut at 45 degrees by microtome (thin blade used in tissue cutting) or excimer laser on the film edge. For mirrors in the center of waveguide film sheets, excimer laser micromachining will be used to create an "in-situ" mirror when required.

[FIGURE 7 OMITTED]

Precision alignment of the waveguide film arrays for reflected light coupling on substrates is critical, and is achieved first by precision cutting to locate the waveguides in the film strip. FIGURE 6 shows a 12-guide array precisely aligned to flip chip locating solder balls. Since alignment for each guide is critical (within +/- 3 microns for the example shown) runout or accumulated position error must also be within this range.

There are several conductivity issues with this type of waveguide. Alignment of the offsets with included runout over the entire waveguide array must match the part requirements before assembly. Runout/offset for vertical and horizontal waveguide positions relative to solder balls must be within tolerance for flip-chip component attachment. Care must be taken at assembly to prevent damage that can compromise waveguide and mirror performance. The waveguide film must fit with a uniform and minimum air gap under the flip-chip components. Alignment and waveguide performance must be maintained when subjected to IR solder reflow used for flip-chip attachment. Temperature stability of waveguide films the film's optical properties must withstand prolonged exposure to temperatures up to 125[degrees]C.

[FIGURE 8 OMITTED]

In Type Three, the I/O mirror coupling to and from an array of lenses demonstrates the application for a flexible self-supporting waveguide link that bridges fiber ribbon arrays through a mirror with a lens through-board interconnection. An example is shown in FIGURE 7, with a side view schematic showing the installation configuration. With lenses, unguided distances can be increased to greater than 1 mm. Lenses facilitate imaging of component I/O with the waveguides through the mirror. This lens waveguide array requires several precise alignment shims to span this distance, in order to couple precisely with the waveguides. Applications can include traversing between top and bottom substrate surfaces or through boards connecting sources and detectors to the waveguide arrays.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

The critical connectivity issue with Type Three is that alignment for waveguides, lens elements and components must be within a few microns to ensure low loss connectivity. Optical fiber to waveguide array interface is achieved with an MT-like ferrule or micro ferrule for the example shown, using a flexible link not attached to the substrate that bridges to the outside of the package. Waveguide assembly, including fiber interface, mirrors, and the lens unit has a system loss of less than 1 dB (20%). The entire system loss is within operational specifications, including coupling from the four VCSELs through the system and back to four detectors. The final Stratos Optical Technologies TxRx unit (without cover) is shown in FIGURE 8. Alignment specs and tolerances for edge connections and mirrors are identical for this interconnect, including transmitter (Tx) and receiver (Rx) optimum-sized waveguides for high performance coupling to graded index fibers. Shown in FIGURE 9, the 60-micron guides are used for the receiver and the 30-micron guides for the transmitter.

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

Type Four is based on board-to-board coupling for perpendicular connected substrates (as used for mother-to-daughter boards) which is achieved with ferrules in latchable housings, using butt coupling multilayer waveguide arrays.

The critical connectivity issues include the alignment and precision issues outlined in Type One (for MT ferule style connectivity). Particular emphasis should be placed on polymer waveguide-to-waveguide interconnection through the daughter board edge to backplane/mother board guide as depicted in Figure 9. To achieve daughter board-to-daughter board connectivity along the backplane, precisely arranged stacked guides and very high-density arrays are likely to be required. Direct zero air gap contact is desirable for the ferrule-to-ferrule interface. Spring-loaded compression combined with flex latchability is required, similar to the latchable MPO or MPX style connector housings that are commercially available.

A conceptual schematic for this housing is shown in FIGURE 10. Balancing daughter board insertion forces with latching of electronic and optical connections requiring a zero air gap are a challenge, and important step in achieving low loss connections with multiple insertion capability. Capitalizing on the bend flex capability inherent with some waveguide films enables the required flex as the daughter board is inserted. In addition, the bending capability has opened opportunities for through-hinge connectivity for cell phones and lap top computers. Alternative techniques using a lens-to-lens coupling at the daughter board-to-backplane optical interface allows an air gap for more forgiving positioning.

In Type Five, waveguide film edge ferrule couplings are used with self-supporting links and/or stand alone functional devices. Arrays of single or multi-layer waveguides are precisely centered or positioned within the ferrule. FIGURE 11 shows several MT ferrule connected flex jumpers and board edge substrate links that are fully inspected and ready for installation.

The critical connectivity issues for this type include alignment, precision, and coupling issues as previously described.

A backplane/motherboard array that is 40 cm long interconnects with high-density waveguide arrays that have been created to interconnect two daughter boards. The 47 cm waveguide optical path (7 cm included for the daughter board) used multiple connections while still operating within the loss budget of 15 dB. The flex circuit shown previously has the substrate attached for the demo board in FIGURE 12.

Summary

Optoelectronic substrate connectivity is sufficiently advanced, practical and available for consideration in near term applications. For many in the industry, the evolution of optical interconnectivity at the substrate level is sufficient to begin initial prototyping. Technologies continue to develop and improve, opening up new application opportunities. We believe that the approaches outlined in this article suggest that the creation of self-supporting polymer waveguide structures are both achievable and cost effective for many applications.

ACKNOWLEDGEMENTS

The authors appreciate the useful additions, suggestions and comments from Davis Hartman of General Dynamics, Dale Murray of W.L. Gore, Diana Williams of Rogers Corporation, and Silvio Bertling of Parkelectrochemical that have been incorporated in this article. In addition, the extensive help in editing and organizing from Kevin Hair of Optical InterLinks was most appreciated.

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 InterconnectTechnology Analysis. He can be reached at fish5er@mindspring.com.
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Title Annotation:OPTICAL INTERCONNECT
Author:Booth, Bruce L.; Fisher, Jack
Publication:Printed Circuit Design & Fab
Date:Mar 1, 2008
Words:1908
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