Fiber Run Fails OTDR Acceptance After Installation
A contractor installs 1.8 km of single-mode OS2 fiber between two buildings using a mix of indoor riser cable and outdoor loose-tube cable, joined at an entrance facility by a fusion splice. There are LC connectors at both ends and one splice. The link is tested at 1310 nm with an OTDR from the A end. The OTDR shows a splice loss of 0.35 dB, connector loss of 0.6 dB at the far end, and an overall attenuation of 0.42 dB/km. The specification requires splice loss of 0.3 dB or less, connector loss of 0.75 dB or less, and attenuation of 0.4 dB/km or less at 1310 nm. The B-end connector reading is taken from a single-direction trace.
1. Which parameters fail, and how should the single-direction OTDR readings be interpreted?
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Attenuation at 0.42 dB/km exceeds the 0.4 dB/km limit. Splice loss at 0.35 dB exceeds the 0.3 dB limit. Connector loss at 0.6 dB is within the 0.75 dB limit. However, a single-direction OTDR trace has a directional bias: splice and connector losses measured from one side include gain or loss artifacts from mode-field mismatch, backscatter differences between cable types, and the ghost/reflection at the far connector. Bidirectional averaging is standard practice for splice loss acceptance. The 0.35 dB splice reading should be confirmed from the B end and averaged before declaring failure; the averaged value may fall under 0.3 dB. The attenuation figure is a length-averaged slope and is less affected by direction, so the 0.42 dB/km is more likely a real failure.
2. What could explain elevated attenuation on a mixed indoor/outdoor cable run, and how would you isolate it?
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Causes include tight bend radius at the entrance facility, especially where riser cable transitions to loose-tube; a kink at the splice tray; contaminated or poorly polished connectors; and mismatch between cable types at the splice. Isolate by examining the OTDR trace for localized events rather than uniform slope. A step loss at the splice or at a bend point indicates a discrete fault; a uniformly steep slope suggests the wrong fiber type, a damaged cable section, or a launch condition problem. Use a visual fault locator at the entrance facility and inspect connectors with a scope. Re-test with a launch fiber to remove the near-end dead zone.
3. Outline the acceptance and remediation path.
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Re-test bidirectionally and average splice and connector losses. If the averaged splice is within 0.3 dB and connectors within 0.75 dB, only the attenuation fails. Locate the high-loss section from the trace, inspect and re-terminate or re-splice as needed, and check bend radii against the cable's minimum. After remediation, re-run bidirectional OTDR and a power meter/light source insertion-loss test as a cross-check. Document results against the spec and do not accept the link until attenuation is at or below 0.4 dB/km and all discrete events pass.