Knowledge base · Measurement
What is an OTDR and how does it work
An OTDR, an optical time domain reflectometer, is connected to one end of a fibre. It sends short pulses of light into the fibre and listens to what comes back. The time the light takes to return gives the distance; how much of it returns gives the loss. The result is a trace: a plot of signal level along the whole link, which shows where every splice and every connector sits and where the fibre ends.
No file of your own? The app has an Open an example button that opens a real trace, and everything on this page can be seen on it.
By Vera (new to the field) ·
What the instrument sees in the light that comes back
The trace is built from two kinds of light that come back: scattered and reflected.
Backscatter
Glass scatters a small share of the light at every point along it, and some of that scattered light travels back to the instrument. This faint, even return runs the whole length of the fibre and draws a sloping line on the plot. Its slope is the fibre's attenuation per kilometre, and a step in it means loss at one place.
Reflection
Wherever the light meets a boundary between glass and air, part of it is reflected back. Those places are drawn as a sharp peak: a connector, a mechanical splice, the end of the fibre, a break. A fusion splice hardly reflects at all, so it shows only as a step down.
Events
The instrument finds the steps and peaks itself and gathers them into an event table: where each event is, how much was lost there and how much was reflected. What each type means is covered in the guide on event types.
How time becomes distance
The instrument measures how long the light took to reach a point and come back, then halves the path. It takes the speed of light in glass from the refractive index set in its settings. A wrong index therefore shifts every distance at once, while the curve itself stays the same.
The setting that matters most is the pulse width. A long pulse carries more light and reaches further; a short one tells apart events that stand close together. The price of a long pulse shows after every strong reflection: that is where the dead zone begins, a stretch where the instrument is blinded and cannot separate events.
How far the instrument can see is its dynamic range: the difference between the level at the start of the link and the noise level the curve sinks into. A longer pulse and averaging, which means a longer acquisition, both increase it. And so that the first connector of the link is not lost in the instrument's own dead zone, a launch cable goes between the instrument and the link.
OTDR vs OLTS
An optical loss test set, or OLTS, works differently. A light source stands at one end of the link and a power meter at the other, and the set compares how much light went in with how much came out. The result is one figure per fibre and wavelength: the total loss of the link. It is measured directly, with light that has passed through the whole link, so an OLTS gives the more accurate total. What it cannot tell you is where that loss was picked up.
An OTDR needs only one end of the link and answers the question "where and what": how far away a splice is, how much a connector lost, where the break is. Its total loss is worked out from backscatter and can differ from the OLTS figure. Fibres with different backscatter meeting at a splice read too high from one end and too low from the other, and the true value is the average of two measurements from opposite ends. That is why acceptance testing often asks for both: the OLTS for total loss, the OTDR for a map of the link.
What opens here are OTDR files: the trace, the events and the acquisition parameters from an instrument of any make. We do not open OLTS results. There is no curve in them, only a table of losses per fibre, and our reader has no use for one.
Next
A trace is stored in the instrument's file, most often a .sor. You do not need the instrument maker's software to look at it: the file opens in the browser.