Multimode optical fiber splice loss: Relating system and laboratory measurements

We examine the splice loss occurring along a multimode fiber regenerator span and compare the results to a "standard" laboratory test condition. Large variations in the splice loss sensitivity to transverse offset of a "reference" splice were seen 1) at the ends of 5 different 8.9-km fiber concatenations and 2) at various locations along one 8.9-km path. These data indicate that significant variations in the modal power distribution exist even at the receiver end of a regenerator span. This is explained by the long coupling lengths (long compared to splice spacing) of low loss fibers and the random amount of mode coupling introduced by the concatenation splices. Splice loss measurements made with an "overfilled" launch condition and a 3-turn 20-mm mandrel wrap mode filter (commonly used in fiber loss measurements) accurately approximated the average splice loss sensitivity along the span for offsets less than 4 μm. i.e., losses less than 0.2 dB. For offsets greater than 4 μm, the mandrel wrap mode filter configuration yields a pessimistic estimate, but is within 30 percent of the average "system" loss for most cases of interest. This demonstrates the usefulness of the mode filter in establishing splice measurement conditions that yield results representative of those seen in a regenerator span.