MDU video overlay failure after a tight bend and a UPC/APC mix
A 36-unit MDU is served from a basement FDT with four 1:8 splitters. Drops are preconnectorized SC/APC, 45 m average. The installer used SC/UPC pigtails on the splitter outputs because APC patch cords were backordered, and at unit 24 the drop was secured with a staple that pulls the fiber into a 7.5 mm radius behind the faceplate. Unit 24 receives −21.4 dBm at 1490 nm, which is within specification, but the 1550 nm RF video overlay fails: optical power is −31.8 dBm and MER is 28 dB where 33 dB is required. An OTDR at 1550 nm from the FDT shows a 3.6 dB event 2 m from the ONT with −52 dB reflectance; at 1490 nm the same location shows 0.4 dB. At 1310 nm the splitter outputs show about −45 dB reflectance. The property manager wants unit 24 fixed today and asks whether the UPC/APC mix must be replaced building-wide.
1. Identify the cause of the 1550 nm failure and the immediate corrective action. Why is 1490 nm unaffected?
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The key evidence is that the same physical location shows 3.6 dB at 1550 nm but only 0.4 dB at 1490 nm. Connector loss and splice loss are only weakly wavelength-dependent; a 9× difference at one point is the signature of a macrobend. A 7.5 mm radius is below the installation bend limit for typical drop cable, and 1550 nm is much more sensitive to bend loss than 1490 nm.
The staple is the likely culprit. Open the faceplate, remove the staple, replace it with a proper cable clip or standoff, and restore at least 25–30 mm of bend radius with slack. Inspect the fiber for a permanent kink; if the bend has damaged the coating or the fiber, replace the drop rather than re-stapling it.
After correcting the bend, re-measure at 1550 nm. The 3.6 dB event should disappear and the RF video power and MER should recover. If MER remains below 33 dB after power is restored, then investigate reflections and the UPC/APC mix as a secondary cause.
2. Assess the UPC/APC mixing at the splitter outputs. Must all four splitters be reterminated today?
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Mating a UPC connector to an APC connector creates a small air gap and an index mismatch. The result is typically 0.3–1.0 dB of added loss and reflectance in the −45 to −50 dB range, which is consistent with the 1310 nm reading. The 1490 nm level of −21.4 dBm shows that the digital path still passes with margin.
For GPON at 1490 nm, −45 dB reflectance is generally tolerable. For an RF video overlay, reflections can degrade CSO/CTB and MER through multipath interference, so the mix is a real defect and should be corrected. However, it is not the cause of a 10.4 dB gap between 1490 and 1550 nm; a bend is. Replacing all four splitter output jumpers today would interrupt service and would not fix unit 24.
Decision: correct the bend immediately. Schedule replacement of the UPC pigtails with SC/APC jumpers during the next maintenance window, and inspect every unit for additional UPC/APC mating points. Record the interim condition and retest video performance on all 36 units after the jumper replacement.
3. Define the acceptance test for all 36 drops after repair.
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Test each drop at both 1490 nm and 1550 nm with an OLTS or calibrated power meter, and record the loss against the design value. At 1550 nm, require ORL of at least 32 dB. Verify MER of at least 33 dB at every video outlet, not only at unit 24. Run a 1550 nm OTDR from the FDT toward each ONT with a launch fiber; reject any localized event above 0.1 dB other than a normal connector event, and reject any bend-induced loss that appears at 1550 but not at 1490.
Perform a visual inspection behind every faceplate for bend radius and staple damage. Record the splitter port, drop length, connector type, test wavelength, and instrument calibration date for each unit. Only then should the building be accepted.