An OTDR sends short light pulses into a fibre and measures what comes back and when, producing a trace of loss against distance. It is a fibre-testing instrument: it tells you about the fibre, not about the temperature along it.
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Call +86 130 4890 6506 to discuss selection and site conditions Email usOptical time domain reflectometry works on the same time-of-flight principle as distributed temperature sensing. A pulse of light is launched into the fibre. A very small fraction of it scatters back — mostly by Rayleigh scattering, at the same wavelength — and the instrument records how much returns as a function of time.
Because light travels in the fibre at a known speed, time maps directly onto distance. The result is a trace: returned power against distance along the fibre.
Reading that trace gives the practical information an OTDR is bought for:
This is why an OTDR is a commissioning and fault-finding instrument. Before a fibre is trusted for anything else, someone wants to know it is continuous, that the splices are sound and that the loss is what it should be.
DTS uses the same time-of-flight idea but a different scattering effect and a different question. A DTS instrument reports temperature against distance; an OTDR reports loss against distance. Both can be built into the same family of equipment, and the principle is shared, which is exactly why they get confused.
Rayleigh scattering is the effect an OTDR relies on. It is elastic — the light returns at its original wavelength — and it carries no meaningful temperature information.
Raman scattering is the effect DTS relies on, and it is inelastic. It returns at a shifted wavelength and does depend on temperature.
Spatial resolution means different things in the two instruments. An OTDR's resolution concerns how sharply it can separate two nearby loss events; a DTS system's resolution concerns the length of fibre averaged into one temperature value. The numbers are not comparable.
"An OTDR measures temperature." It does not, and no setting changes that. It reports loss and reflections. If you need temperature, you need a different measurement.
"A clean OTDR trace means the temperature monitoring will work well." A clean trace is useful groundwork, not a guarantee. A trace tests the fibre — continuity, loss and reflections. Whether temperature monitoring works well also depends on the fibre's construction, on its connectors and thermal contact, and on the route having been laid so that the fibre runs where the assets are.
"If the trace shows a break, the whole route is unmonitored." Not usually. A break means no measurement beyond that point; the fibre before it can still be measured. Knowing where the break is, to the metre, is precisely what makes this useful during fault-finding.
"OTDR resolution and DTS resolution are the same number." They are different measurements with different resolutions. Comparing them directly is a category error.
"The trace is only for the installer." Keeping traces as a baseline is what makes later comparison meaningful. Without a commissioning trace, a change found months later cannot be attributed to anything.
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An OTDR is the right instrument when the question is about the fibre: is it continuous, where does loss sit, where is the fault, did the installation meet its numbers. It is standard practice before commissioning a fibre-based measurement of any kind, and during fault-finding afterwards.
It is the wrong instrument when the question is about the temperature of what the fibre runs alongside. For that, the trace is context, not the measurement.
Optical time domain reflectometry. The name describes the method: send light, time what comes back, and use the timing to work out where along the fibre each feature sits.
No. It measures loss and reflections along a fibre. Temperature measurement on a fibre uses a different scattering effect and a different instrument.
Locating splices, connectors and breaks; verifying that loss meets specification; and establishing a baseline. It is a commissioning and troubleshooting record, and it stays useful only if it is kept.
Because they share the same underlying trick — timing returned light to find distance. The difference is what is being measured: loss in one case, temperature in the other.
It is good practice to establish that the fibre is sound and to record a baseline before relying on it. It does not by itself establish that a temperature measurement will perform well, because that also depends on the fibre construction and the installation.
Yes — the fibre up to the break. Sections beyond it cannot be measured until the break is repaired. Knowing the distance of the break is the main thing an OTDR contributes in that situation.
If you are commissioning a fibre route and want to know what to record before you rely on it, tell us the route and what it is for. We will say which baseline measurements are worth keeping — and which ones people usually regret not taking.
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