OTDR fibre optic testing: what it is and how to read the report

What is OTDR measurement

OTDR stands for Optical Time Domain Reflectometer: an instrument that sends short light pulses into the fibre and analyses how much light is reflected back from individual points along the route and with what delay. From this, the route length can be calculated, the location and magnitude of attenuation at each splice, connector or bend can be precisely determined, and a fibre break can potentially be located, all without needing to expose the route in any way.

The principle is similar to an echo: the sooner the pulse reflects back, the closer the location; and the stronger the reflection or signal drop, the greater the attenuation at that point. The device processes this data into a graph that tells an experienced technician more about the route's condition than any visual inspection could reveal.

When to order an OTDR measurement

• after completion of the new route or FTTH connectionsto ensure that the work carried out meets the specifications
• upon handover of the work to the investor or internet provider, where the protocol serves as proof of quality
• In case of suspected fault, where measurement helps locate a break or increased attenuation without unnecessary digging.
• during regular inspection of critical backbone routes

Measurement is also worthwhile as a preventive measure, regularly tested routes allow comparison of the current status with the initial protocol and reveal gradually increasing attenuation before it manifests as an outage. This is particularly valued by network operators whose service quality for end customers depends directly on the reliability of backbone routes.

How measurements are carried out in practice

High-quality measurements are performed bidirectionally, i.e., from both ends of the route. This is important because unidirectional measurement can distort attenuation values at splices with different core diameters, in one direction the splice appears better, in the other worse, and only the average of both measurements gives a real figure. Measurements are standardly taken at two wavelengths, usually 1310 and 1550 nanometres, as the route's behaviour may vary slightly at these wavelengths. The result is an attenuation profile graph along the entire route and a clear table of individual events.

The measurement itself takes minutes per route; for longer routes with multiple fibres, the time is multiplied by the number of measured fibres and directions. The output is an electronic protocol that can be attached to the handover documentation or used as a basis for resolving complaints or faults.

How to read the protocol: a practical guide for customers

The OTDR measurement protocol may appear technical at first glance, but the key data are understandable even without in-depth knowledge of the field:

• Total route attenuation (in dB) - the lower the value, the better. As a guideline, it should correspond to the sum of the cable attenuation and the attenuation of all splices and connectors over the given length.
• Events on the curve - Each "step" or deviation in the graph represents a splice, connector or cable bend. The size of the jump corresponds to the attenuation at that point.
• Reflectance (return loss) - A high value at the connector often indicates that it is either poorly cleaned or not fully seated.
• Route length - The OTDR calculates it based on the speed of light in the fibre, so the stated value may differ slightly from the physical cable length due to coiling and reserves in the cassettes.
• End of route - if the curve drops before the route should physically end, it usually indicates a break or a severely damaged point.

If you are unsure what the individual figures in a specific protocol mean, have them explained by the technician who carried out the measurement. Comparing results with the expected condition of the route is often more important than absolute values, and this is best assessed by the person who performed the measurement.

Connection with fibre splicing

A good fibre splice It is the foundation of low attenuation along the route. OTDR measurement is a tool that objectively verifies and documents the quality of both splices and the entire route, therefore, both activities usually go hand in hand during implementation as well as repair.

Likewise, OTDR measurements are also useful outside new construction, for example in repairs to severed cables, where it is often the only way to locate a fault without unnecessarily exposing the entire route metre by metre. For older routes lacking documentation, it can sometimes be the only reliable source of information on the cable's actual path and the number of splices it contains.