Fiber Loop Converters

Fiber Loop Converters are sometimes required to operate over multimode fiber because an existing span of installed multimode fiber may already be in place between two points requiring FLC transmission. The FLCs were designed for use over singlemode fiber, and if there is a choice, singlemode fiber should be installed. However, FLC operation over multimode fiber is possible and, in fact, is a functioning application with several custom-ers. LAN applications over short distances have been successful. The following aspects of this application require attention.

Launch Conditions
A 10 meter (33') singlemode jumper with a physical finish on the connector endfaces should be installed in the FLC transmitter. This singlemode jumper can then be con-nected to the multimode fiber in the span. To prevent higher order modes from being launched directly into the multimode fiber, a minimum of 10 meters (33') of singlemode fiber is required out of the transmitter. The multimode fiber can terminate directly into the FLC's receiver at the other end of the span.

Connections in the Span
Ideally, the only connection in the span will be the junction from singlemode to multimode. If fiber joining is required anywhere else in the span, fusion splicing is recommended. Because the connectors in a multimode span are not required to be in physical contact at the fiber endfaces, Fresnel reflections and alignment mismatches can degrade perfor-mance to an unacceptable level. If connectors are in the span, they should be of singlemode quality with physically contacting, radius-polished fiber endfaces. Multimode SMA and biconic fiber connectors are unacceptable. D4, SC, FC and ST ® connectors can be used, if the endface polish and the concentricity tolerances are singlemode quality. The number of connectors should be kept to the absolute minimum.

Optical Budget, Bandwidth and Fiber Properties
Multimode fiber has higher loss per distance properties than singlemode fiber. Tradi-tional calculations of optical budget for the span will not take into account the pulse spreading and interference resulting from sending a singlemode signal down a multi-mode fiber. The optical budget for the DS3 FLC is 13 dB at 1300 nm wavelength. Standard Corning 62.5 mm fiber has attenuation of 0.7 dB/km. This works out to a theoretical capability of 18.5 km (11.5 miles) based on optical budget alone. When bandwidth is considered, the theoretical distance limit decreases. Bandwidth is not a limiting factor for T1/E1 FLCs and QFLCs. Bandwidth may become the limiting factor when the bit rate increases to DS3 rates or above.

The bandwidth of the fiber, which is measured in the units of MHz-kilometers, should be known. Standard Corning graded-index 62.5 micron core multimode fiber typically exhibits a bandwidth of 400 MHz/km. This means that an FLC operating at 51.84 Mbps (which translates into an analog rate of 25.92 MHz) is theoretically capable of transmit-ting over a distance of 15.43 km (9.56 miles). However, more conservative distances are recommended. See Table 1 (which lists the minimum of 200 MHz/km) and Table 2 for the theoretical limits of the FLCs. The bandwidth cells of the fibers listed in the tables are minimums. Fiber of greater bandwidth capacity is available from the manufactur-ers. QLX modules operate at the same rates as the QFLC.

DS3 FLCs have been successfully implemented over a distance of 10.0 km (6.2 miles on multimode fiber).

Both attenuation and bandwidth of the multimode fiber should be examined and the limits calculated before installing FLCs and QLXs for use over multimode fiber. Charac-terizing the fiber plant or records indicating the exact fiber type will help to determine how well FLCs and QLXs can work over multimode fiber.

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