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New Method Improves Optical Fiber Attachment to Substrates
Feb 2001
The Holy Grail for telecommunications companies today is something called the all-optical network. Constructed entirely of fiber optic pathways -- from long distance underground cables, to local area networks, metropolitan networks, and building networks, to patch cords and cable assemblies connecting computers, servers, routers and other components -- the all-optical network is being touted as the solution to the massive bandwidth crunch that has occurred with the explosion of Internet traffic and other data transmission demands.
    One of the major challenges to getting to the all-optical network is the miniaturization and high volume manufacture of fiber-based components. Tiny laser diodes have been developed to facilitate the transmission of optical signals. Other devices called wave division multiplexers aid optical transmissions by splitting light into multiple frequencies, each capable of carrying thousands of voice signals and hundreds of data transmissions in a single fiber optic cable. But all this downsizing and frequency-splitting comes at a price: getting all these fibers to line up so that the beams of light travel without a hitch is exceedingly difficult, and the smaller the fiber, the tougher it is to make the interconnection.
    Traditionally, fibers have been attached to the structure that holds the laser light source to the circuit board, using a metal coating that solders the fiber to the laser structure, but this method often caused the fiber's position to shift, causing misalignment. Another method commonly used involves placing the fiber in a small metal tube to avoid the direct soldering problem, but again, the result often is misalignment. Other methods call for the fiber to be soldered to the substrate with either metal or a glass composition. Metal solders create depressions in the metal pad holding the fiber to the substrate, which can result in misalignment; glass solders are brittle and prone to break under a variety of conditions.
    This week's Technology of the Week introduces a new method for securing optical fibers to substrates, signaling an important breakthrough in the hot new industry segment of board-level optical solutions. Developed by Tektronix Inc., this new method creates a structurally sound connection between an optical fiber and a substrate using a silicon/gold eutectic alloy.
    This new method starts with a conventional external gold coating on the optical fiber. But instead of using a metal solder to attach the fiber to a substrate, a silicon-retaining member with a fiber-retaining groove in it is placed, with the fiber in it, onto a gold-coated metal pad deposited onto an alumina substrate. Heated to approximately 370 degrees C using a resistor attached to the underside of the substrate, the silicon and gold form an alloy that securely adheres the fiber to the metal pad. The advantages of this method include more precise alignment of the fiber with the light source because the physical manipulation of the fiber is kept to a minimum, and stress-related problems caused by solder-related temperature differentials between the fiber and substrate are avoided. The entire package offers improved strength, durability, and structural integrity. Applications for optical interconnects using this technology include high-speed data and telecommunications; portable diagnostics; and digital signal transmission in severe environments, including shipboard communications, transportation, test & measurement, and avionics.

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