1550 nm Video Overlay Fails After Tight Drop Bend
A technician is commissioning a GPON FTTH drop in a 12-unit MDU. The feeder is 9 km of G.652.D, followed by a 1:32 splitter and a 45 m drop cable. At the ONT, the 1490 nm downstream reads -24.8 dBm, but the 1550 nm RF video overlay reads -37.9 dBm and will not lock; the design threshold is -30 dBm. The 1310 nm upstream arrives at the OLT at -28.9 dBm, just inside the -28 dBm receiver threshold. The drop is a 3.0 mm G.652.D cable stapled around a corner with an inside radius of about 8 mm; the installer used two plastic staples and left no slack. The 1490 nm signal passes because its margin is larger. A power meter at the splitter output shows normal levels. The OTDR trace from the CO is dominated by the splitter and shows no clear drop event.
1. What is the most likely cause of the video overlay failure, and how would you confirm it without replacing the drop?
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The most likely cause is macrobending at the 8 mm-radius corner. Bend loss is wavelength-dependent: it is greatest at 1550 nm, lower at 1490 nm, and usually much lower at 1310 nm. That matches the pattern: the 1550 nm video channel is far below threshold, the 1490 nm downstream still passes, and the 1310 nm upstream is only marginal.
Confirm it by temporarily relieving the bend and re-measuring all three wavelengths at the ONT with a calibrated power meter. If 1550 nm improves by several dB while 1310 nm changes little, the bend is confirmed. An OTDR from the ONT side, with a short launch cable, can also show a localized loss at the bend, but the splitter prevents a useful CO-side OTDR shot through the PON. Do not use a visual fault locator as the primary test; a tight bend may not scatter enough visible light to be seen.
2. What corrective action and acceptance criteria should the technician apply?
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Reroute the drop so the installed bend radius is at least 10 times the cable outside diameter; for a 3.0 mm cable that is about 30 mm installed and about 60 mm during pulling. Replace staples with cable ties or hook-and-loop fasteners that do not pinch or crush the jacket, and leave a service loop. If the route cannot provide that radius, use a bend-insensitive G.657.A2 drop cable rather than forcing G.652.D around the corner.
After correction, verify 1550 nm at the ONT is at or above -30 dBm with margin, and recheck 1490 nm and 1310 nm. Also inspect and clean the connectors before declaring the link good. Acceptance should include the design threshold plus margin, not merely a signal that happens to lock. If the 1550 nm level remains low after relieving the bend, then test for a damaged connector or a crushed cable section.