Optical time-domain reflectometry (OTDR)
A measurement technique that maps optical loss along a fiber by sending pulses and recording the time-resolved Rayleigh backscatter. The standard field instrument for fiber installation, splice characterization, and fault location.
OTDR sends a short optical pulse into a fiber under test and records the optical power backscattered or reflected back to the launching end as a function of time. Two distinct mechanisms produce returning light:
| Mechanism | Signal characteristic |
|---|---|
| Rayleigh scattering | Continuous (every point of fiber scatters); produces the smooth exponentially decaying baseline of an OTDR trace |
| Fresnel reflection | Discrete (at connectors, splices, fiber ends, breaks); produces sharp peaks |
The position of any feature corresponds to time-of-flight: , where is the group velocity and the factor of 2 accounts for the round trip. At (typical silica fiber), 1 μs of OTDR time corresponds to 102 m of fiber distance.
OTDR trace interpretation:
| Feature | Trace appearance | Cause |
|---|---|---|
| Continuous slope | Exponential decay (in log power vs distance, a straight line) | Fiber Rayleigh attenuation |
| Step down | Sudden drop without reflection peak | Lossy splice or bend |
| Step down + peak | Drop with reflection peak | Connector (lossy + reflective) |
| Step up | Sudden increase | Splice between dissimilar fibers (backscatter coefficient mismatch) |
| Sharp peak then noise | Fresnel reflection at large step, then noise | Fiber end, break, or open connector |
| Saturation | Receiver overload | Strong reflection (e.g., return from end facet of un-terminated fiber) |
OTDR measurement parameters:
| Parameter | Typical range |
|---|---|
| Pulse width | 5 ns – 20 μs |
| Spatial resolution | 0.5 m – 2 km (set by pulse width: ) |
| Dynamic range | 25 – 50 dB |
| Operating wavelength | 1310, 1550, 1625 nm (matching telecom bands) |
| Dead zone | 0.5 – 30 m (region near launch where strong launch reflections mask features) |
Why OTDR is essential.
- Installation verification — confirms each splice meets specification ( 0.1 dB loss)
- Fault location — pinpoints fiber breaks to within meters
- Loss budget verification — confirms link total loss matches design
- Long-term monitoring — detects gradual degradation (bends, water ingress)
- Splice acceptance testing — bidirectional OTDR measurement averages splice loss measurements from both directions
Limitations:
- Dead zone — close-in features near the launch end are masked by the launch reflection (use a launch fiber, 1+ km long, to separate measurement features from the dead zone)
- Pulse width tradeoff — shorter pulses give better spatial resolution but worse SNR (less energy per pulse); longer pulses give better SNR but worse resolution
- Asymmetry effects — splice loss measured by OTDR can appear different from each direction due to backscatter coefficient differences; true loss is the average of bidirectional measurements
Brillouin and Raman OTDR are extensions that exploit inelastic scattering (Brillouin scattering, Raman scattering) instead of Rayleigh backscatter. These enable distributed sensing of strain (BOTDR) and temperature (RDTS) along the same fiber that is used for communication.