Fiber circuit shrinks board size

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John Wallace

As the number of channels in DWDM systems grows, so does the tangle of fiber on the systems` circuit boards. Aiming to untie the knots, researchers at NTT Photonics Laboratories (Ibaraki, Japan) and NTT Network Innovation Laboratories (Kanagawa, Japan) have developed a flexible fiberoptic circuit that can connect hundreds of fibers and devices together.

The circuit is made by pressing the fibers onto an adhesive sheet in very specific patterns, a task done by a wiring machine. Multiple fibers can be run alongside one another; curved and crossed patterns can be created with no mix-up in fiber order. The fibers are positioned to an accuracy of about 10 µm. Once all the fibers are in place, a second adhesive sheet is sandwiched on top.

The fibers are connected to outside fibers and devices that are terminated with physical-contact (PC) connectors. Fibers at the periphery of the circuit itself extend out from the circuit by several millimeters and are terminated merely by polishing the fiber ends; these ends are slid into close-fitting holes in the PC connectors until they make contact with the fiber ends within the connectors. A small additional displacement of the connectors toward the fiber circuit pushes the circuit fiber end into the shape of a wave, creating a spring force that maintains fiber-to-fiber contact. The same kind of connection can be made to planar waveguide circuits; in this case, the fiber contacts the end of the waveguide itself.

Because the spring force stresses the fiber, not just any fiber material will do. The researchers chose 125-µm-diameter polymer-skin-coated fiber for its high resistance to handling damage. A bent fiber length of 7 mm results in a buckling force of 0.7 N. A bending amplitude of no greater than 0.3 mm helped to keep the insertion loss down below 0.5 dB. Return losses were greater than 50 dB for fiber-to-fiber and 40 dB for fiber-to- planar-waveguide-circuit connection. Because the circuit fibers themselves have no connectors, high-density multifiber connections are possible.

Using a fiber circuit, the researchers built a compact 8 × 16 delivery and coupling switch that makes possible a high-density integrated board for photonic transport systems (see figure). The switch is made up of eight sets of 1 × 16 switches and 16 sets of 8 × 1 couplers. The 128-fiber circuit and fiber PC connectors eliminated the need to fusion-splice 128 fibers between the switches and couplers.

The configuration includes four sets of a dual 1 × 16 thermo-optics switch and two sets of an octad 8 × 3 coupler. There are a total of 304 PC points on the board. Containing electronics as well as optics, the board is half the volume of a conventional fusion-spliced board. Average insertion loss of the 128 routes of the delivery and coupling switch is 15.2 dB, only 1 dB higher than for a fusion-spliced board, even though the researchers added an additional chip-chip connection. For more information, contact Masaru Kobayashi at koba@iba.iecl.ntt.co.jp.

Fiber circuit reduces the volume of an 8 × 16 delivery and coupling switch board by a factor of two.

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