Upgrade multimode lans with offset launching

Sept. 1, 1997

Upgrade multimode lans with offset launching

By YVONNE CARTS-POWELL

If you already have a multimode fiber cabling plant for your local area network (lan), then upgrading to Fibre Channel or another standard for transferring data at more than 1 Gbit/sec can be challenging. Presently, Gigabit Ethernet in these fibers is limited to distances of less than 1 km. Until wavelength-division multiplexing becomes inexpensive and readily available, other methods for increasing the bandwidth of existing lans are needed--preferably methods that are robust and inexpensive to implement.

Researchers in Bristol, UK, have developed a method that increases the bit-rate distance product by as much as four times without requiring new fiber or new laser designs. The researchers, at the Centre of Communications Research at the Univer-sity of Bristol, and Hewlett-Packard Laboratories in Bristol, found that when they intentionally misalign the laser beam and fiber, the beam enters the edge of the fiber`s core and not all modes of the laser output are propagated. Because there are fewer modes, dispersion is limited, allowing an increase in either the travel distance or the data rate of signals.

Limiting modes

The offset-launch method builds on work published in 1993 by Z. Haas and M.A. Santoro on restricted mode launches. They proposed increasing the bit-rate distance product of multimode networks by allowing only a few optical modes to propagate through the fiber. The center-restricted method uses light launched into the center of the fiber to excite the lowest-order mode.

The offset-launch method, in contrast, uses a larger number of modes and appears to be less sensitive to both on-axis alignment and tilt. Lutz Raddatz and others used micropositioning stages to align the singlemode pigtail from a 1300-nm-output diode laser to a multimode fiber with a 50-micron core and inherent bandwidth of 720 MHzkm. This fiber link transmitted data at 2.5 Gbits/sec over 3 km, which included seven connectors and three splices, to simulate real operating conditions. Raddatz reports, "Similar bandwidth improvements were achieved in 62.5- micron-core multimode fiber."

The system appeared to be less sensitive to vibrations in the fiber and offset connectors than are center-restricted launch links. For some types of fiber, Raddatz explains, center-launched links also are more sensitive to incidence angles than are offset launches: as much as 2° of angle in the input can reduce bandwidth gain to less than 1 in center-launching, whereas with offset launching, the bandwidth gain stays at about 2.5 for angles from 0° to 6°.

The system showed good stability in harsh environmental conditions and provided bit-error rates of less than 10-10. Using the same type of fiber and an 850-nm-output vertical-cavity surface-emitting laser, the system could transmit at 1 Gbit/sec over a 2-km link. This, Raddatz says, "represents a threefold increase in the bandwidth previously possible using the fiber."

The researchers expect that launching could be achieved by directing light into the multimode fiber using lenses alone, butting the cleaved singlemode and multimode fibers close together, or directing the light using a lens formed from the end of a singlemode fiber (see figure).

The method offers up to 400% bandwidth enhancement without disturbing the fiber plant. Although there is a power penalty in not transmitting all the laser modes produced, Raddatz says it is typically less than 2 dB. q

Yvonne Carts-Powell writes from Belmont, MA.

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