An optical time-domain reflectometer (OTDR) draws a picture of a fiber: a line that slopes down with distance, broken by steps and spikes where something happens to the light. This guide shows how the common events look on an OTDR trace, so you can recognize them before you open a real file. It supports acceptance testing in SPL-11 Testing and Acceptance, the files handed over in SPL-12 Documentation and Handoff and fault locating in SPL-13 Emergency Restoration.
How a Trace Is Drawn
The OTDR sends short pulses of light into the fiber and listens for light coming back. The glass scatters a little light back along its whole length, called backscatter. Connectors, breaks and other sharp changes bounce back more.
The OTDR times each return and turns time into distance using the fiber’s index of refraction. That is why a wrong index setting puts every event at the wrong distance.
Every figure on this page uses the same layout as the whole-link trace at the top of the page:
- Distance runs left to right, starting at the OTDR.
- Relative Level runs bottom to top: how much light comes back from each point.
- A slope is loss spread along the fiber. A step down is loss at one point. A spike is a reflection.
The figures have no scale, ticks or values on purpose. Read the shape first. The numbers on a real trace only mean something once you know what kind of event you are looking at.
Launch Cable and Dead Zone
The OTDR’s own connector reflects strongly, and the receiver needs time to recover after a strong reflection. For a short length after it, the OTDR cannot separate events or measure loss. That length is the dead zone. The event dead zone is where two events cannot be told apart. The attenuation dead zone is longer: it lasts until the trace settles back onto the backscatter line and loss can be measured again. Longer pulse settings usually make the dead zones longer.
A launch cable is a length of fiber between the OTDR and the link. It moves the link’s first connector past the dead zone, so that connector shows up as a measurable event. A receive cable at the far end does the same for the last connector, so it is not hidden in the end-of-fiber spike.
Illustrative, not to scale
Text Description
The trace starts with a tall spike, the reflection at the OTDR port. A shaded band marks the dead zone after it, where the trace is still recovering and no event can be read. The launch cable runs past the band as a smooth downward slope. At its far end, the connector into the link shows its own spike and step, clear of the dead zone, so it can be measured.
Backscatter Slope
Between events, the trace is a straight line sloping down from left to right. The slope is the fiber’s attenuation: the light lost per unit of distance. A steeper slope means more loss along the fiber. At the usual pair of test wavelengths, the shorter one (1310 nm) usually shows a steeper slope than the longer one (1550 nm).
A smooth, even slope is what good fiber looks like. A section that slopes more steeply than the rest of the same cable is worth a closer look.
Illustrative, not to scale
Text Description
The trace is a straight line that falls steadily from left to right with no steps or spikes. A second section of fiber drawn with a steeper slope loses more light over the same distance.
Non-Reflective Splice
A good fusion splice bounces almost no light back. On the trace it shows as a small step down with no spike: a non-reflective event. The height of the step is the splice loss seen from that direction. Because one direction can mislead (see Gainers and Bidirectional Averaging below), the true splice loss is the average of both directions.
Illustrative, not to scale
Text Description
The backscatter line slopes down, drops in a small, sharp step at the splice, then carries on at the same slope. There is no spike at the splice.
Connector
Where two connector end faces meet, the joint reflects some light, and any gap or dirt reflects more, so it is a reflective event. It shows a spike followed by a step down. An angled (APC) connector reflects very little, so on many traces it shows only a step, like a splice. Check the route records for where connectors are. An angled connector mated to a flat-polished one reflects strongly. The spike is the reflection, read as reflectance. The step is the connector’s loss. A dirty or damaged connector usually shows a larger step, a taller spike or both. Inspect and clean before you blame the fiber (SPL-11).
Illustrative, not to scale
Text Description
The backscatter line slopes down to the connector, rises in a narrow spike, falls back, and continues lower than before. The spike is the reflection and the drop across the event is the connector’s loss.
Gainers and Bidirectional Averaging
Two fibers spliced together do not always scatter light back the same way. When the fiber after a splice scatters more light back than the fiber before it, the trace can step up at the splice instead of down. That apparent gain is a gainer. The same splice shot from the other end shows a larger step down. Neither reading alone is the true loss.
The fix is the bidirectional average: shoot every fiber from both ends at the same wavelength, then average the two readings for each event. Use the OTDR Splice Average tool to average both directions and flag events over the owner’s threshold.
Illustrative, not to scale
From the OTDR Splice Average tool:
A negative reading in one direction is a "gainer": the fibers on each side of the splice backscatter light differently, so one direction shows a false gain. The two-direction average cancels it.
Text Description
Two panels show the same splice. In the panel shot from end A, the trace steps up at the splice, a false gain. In the panel shot from end B, the same splice sits at its distance from end B, farther along the drawing, and the trace steps down by more than the first panel stepped up. Averaging the two steps gives a small loss, which is the splice’s true loss.
Macrobends at Two Wavelengths
A macrobend is a bend tight enough to leak light, often in a tray, a closure or a slack coil. Bends leak more light at longer wavelengths. So the same event shows a larger step at the longer test wavelength (for example 1550 nm) than at the shorter one (for example 1310 nm). A splice usually shows about the same step at both.
When an event is noticeably larger at the longer wavelength, find its distance on the trace and check the tray or coil at that point. A new event that was not there before the closure was sealed is usually a bend added during closing or coiling (SPL-11).
Illustrative, not to scale
- Shorter Wavelength
- Longer Wavelength
Text Description
Two traces are drawn on the same axes: the shorter wavelength as a solid line and the longer wavelength as a dashed line. At the same point on both, the trace steps down. The step on the dashed longer-wavelength trace is clearly larger than the step on the solid shorter-wavelength trace.
Ghosts
A ghost is a false reflective event. Light from a strong reflection bounces back and forth between reflective points before it returns, so the OTDR draws a spike where nothing is. Ghosts tend to appear at a repeated spacing of a strong reflection, often out past the end of the fiber or in the middle of a clean section.
How to tell a ghost from a real event:
- A ghost has a spike but no step: the trace continues at the same level after it.
- It lines up with a repeated spacing of a strong reflection earlier on the trace.
- It does not match anything on the route records, and it is not seen at the matching point when the fiber is shot from the other end.
Cleaning or improving the strong reflection that causes it usually weakens the ghost.
Illustrative, not to scale
Text Description
The trace starts with the reflection at the OTDR port. A strong reflective spike follows early on the trace. Farther along, as far past it as it is from the port, a smaller spike appears. The trace has the same level before and after the smaller spike, so there is no loss there. That smaller spike is a ghost.
Break or End of Fiber
At the end of the fiber, or at a break, the backscatter stops and the trace drops to the noise floor: a ragged low line where no event can be read. A clean, square end reflects strongly and shows a tall spike before the drop. A crushed or angled break may show only a small spike, or none, before the trace falls into the noise.
The end-of-fiber event on a link that should run farther is how a break is found. The OTDR measures fiber length, not cable length. Measure its distance on the trace with the same index of refraction setting as the baseline trace. Convert that fiber length to sheath length with the cable manufacturer’s helix factor, allow for slack loops and coils, and then compare it with the route records and footage marks to find the break in the field, as described in SPL-13.
Illustrative, not to scale
Text Description
The backscatter line slopes down to the end of the fiber, rises in a spike, then falls away to a ragged, noisy line near the bottom of the figure. Nothing past that point can be read.
Splitter Step
A splitter divides the light among several fibers, so it shows a much larger step down than any splice. Shot from the feeder side, the trace past the splitter is the combined return of all its legs, so events on a single leg are hard to separate from that side. The owner’s process sets how passive optical networks are tested; SPL-11 covers power meter checks at the terminal.
Illustrative, not to scale
Text Description
The backscatter line slopes down, drops in a small step at a splice, then drops in a large step at the splitter, much larger than the splice step. After the step, the trace continues at a much lower level.
Safety
Where Acceptance Values Come From
Reading the shape tells you what an event is. Whether it passes is a separate question. Acceptance values for splice loss, connector loss and reflectance, and the wavelengths and settings to test with, come from the owner: per owner spec. SPL-11 lists what to test, how to average and what to save. When the owner cites a standard, read the values in that standard and the owner’s documents, not from a picture.
Standards and References
ANSI Z136.2for safe use of optical fiber communication systems.- Owner network operations procedure for testing fibers that may carry live traffic.
Telcordia SR-4731for the standard OTDR data file format only. It does not set acceptance values.- Owner acceptance criteria: wavelengths, test settings, splice loss, connector loss, reflectance and file naming.
- OTDR manufacturer instructions for settings, launch and receive cables and event analysis.
On the Map
- OTDR with Launch Cable: the instrument that draws the trace; launch and receive cables reveal the end connectors.
- Splice Tray: where splices sit and where a tight fiber route can show up as a macrobend.
- Closure Interior: trays, baskets and seals; closing it can add the bends a trace catches.
- Aerial Splice Closure: an aerial splice point that appears on the trace as a set of splice events.
- Underground Splice Closure: an underground splice point, found on the trace by distance.
- Aerial Slack Loop: stored cable that adds distance on the trace and can hide a bend.
- Handhole Slack Coil: a coil where a tight wrap shows as a macrobend.
- Splitter Module: the large step on a passive optical network trace.
- Fiber Distribution Hub: the cabinet where many traces start or end.
Related Steps and Tools
- SPL-10 Closure Sealing, Grounding and Mounting: closing the closures before acceptance testing.
- SPL-11 Testing and Acceptance: the bidirectional OTDR and power meter tests.
- SPL-12 Documentation and Handoff: handing over the trace files by fiber ID.
- SPL-13 Emergency Restoration: using the trace to find a break.
- OTDR Splice Average: bidirectional average splice loss with gainer flags.
- Optical Loss Budget: the expected end-to-end loss to compare with power meter results.