GYFTY 72 Cable

1-288 cores  GYFTY non-metallic Optical Fiber Cable

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And, when testing cables designed for low speed LED transmitter type systems, using a LED source similar to the one used in the system and short launch cables may provide as accurate a measurement as is possible under more controlled circumstances, since the LED approximates the system source. For newer sytems using VCSEL sources, one should use at least a LED with a mandrel wrap (see below) or a commercially-available mode modifier.

The Effect on Measurements

If you measure the attenuation of a long fiber in EMD (or any fiber with EMD simulated launch conditions) and compare it to a normal fiber with "overfill launch conditions " (that is the source fills all the modes equally), you will find the difference is about 1 dB/km, and this figure is the "transient loss". Thus, the EMD fiber measurement gives an attenuation that is 1 dB per Km less than the overfill conditions.

Fiber manufacturers use the EMD type of measurement for fiber because it is more reproducible and is representative of the losses to be expected in long lengths of fiber. But with connectors, the EMD measurement can give overly optimistic results, since it effectively represents a situation where one launches from a smaller diameter fiber of lower NA than the receive fiber, an ideal situation for low connector loss.

The difference in connector loss caused by modal launch conditions can be dramatic. Using the same pair of biconic or SMA connectors, it is possible to measure 0.6 to 0.9 dB with a fully filled launch and 0.3 to 0.4 dB with a EMD simulated launch. Which is a valid number to use for this connector pair's loss ?

That depends on the application. If you are connecting two fibers near a LED source, the higher value may be more representative, since the launch cable is so short. But if you are connecting to a cable one km away, the lower value may be more valid.