Formulas
| Quantity | Formula | Example |
|---|---|---|
| Distance from the OTDR round-trip time | d = c · t / (2 · n) | 100 µs at n = 1.468 → 10.21 km |
| One-way latency | t = L · n / c | 100 km → 490 µs; round trip 0.98 ms |
| Pulse length in the fibre | c · τ / (2 · n) | 100 ns → 10.2 m, the floor of the event dead zone |
| Distance with a corrected index | d_true = d_shown · n_set / n_true | an index error of 0.0015 shifts distances by 0.1 % |
| Usable OTDR range | (dynamic range − 6 dB − event losses) / attenuation | 40 dB, 3 dB of events, 0.2 dB/km → 155 km |
c = 299 792.458 km/s. Light in fibre travels at about c / 1.468 ≈ 204 000 km/s — 4.9 µs per kilometre one way.
Group index
| Fibre | Group index |
|---|---|
| G.652.D single-mode, 1310 nm | 1.4675 |
| G.652.D single-mode, 1550 nm | 1.4680 |
| OM3/OM4 multimode, 850 nm | 1.482 |
| OM3/OM4 multimode, 1300 nm | 1.477 |
Take the value from the fibre's data sheet when you have it: an OTDR set to the wrong index is wrong about every distance by the same percentage. The route to a fault is also shorter than the fibre: loose-tube cable holds 0.1–0.5 % more fibre than its length, and closures store slack.
More in the documentation: Reading an OTDR trace, Formulas and worked calculations, Fibre characterization.