Tool · Fiber and telecom

Fiber Latency Calculator

How long does light take to cross a fiber link, and how much of that would hollow-core fiber or a straight path save? From the route length and the group index, the calculator gives the delay per kilometre, the one-way and round-trip delay, the data in flight at a given bit rate, the length an OTDR reports with a given index setting, and the arrival-time spread from chromatic dispersion. Background: speed of light in fiber, group index, hollow-core fiber, OTDR, and chromatic dispersion.

Route

Fiber length (km)

Group index of the fiber n_g

Straight-line distance between the ends (km, 0 to leave out)

Bit rate (Gb/s)

Comparisons

Hollow-core group index

OTDR group-index setting

Chromatic dispersion (ps/(nm·km))

Wavelength difference (nm)

The default group index, 1.4682, is Corning’s figure for SMF-28 Ultra at 1550 nm; at 1310 nm it is 1.4676. The fiber length is the cable route, which is usually longer than the straight line between the ends. 0.8 nm is the spacing of two channels on the 100 GHz DWDM grid.

Presets

Readouts

One-way delay against distance

solid fiberhollow corevacuum

Learn with it

Three short experiments. Each one sets the inputs, says where to look, and asks for a prediction before it shows the result.

Checked against

These checks run in your browser on every load. The delay per kilometre, group velocity, route delays, hollow-core saving, OTDR length error, dispersion skew and data in flight are compared with values worked out by hand and with the figures in the site’s speed of light in fiber entry, and the number parser is checked on decimal and thousands commas.

Check	Expected	Computed	Tolerance

The speed of light is the defined SI value, 299,792,458 m/s. The tolerance is the largest difference from Expected that still passes, relative to Expected or to 1, whichever is larger.

The model

A signal travels at the group velocity $v_g = c/n_g$ , where $n_g$ is the group index of the guided mode and $c$ = 299,792,458 m/s. Over a length $L$ the one-way delay is

t = \frac{L\, n_g}{c}

and the round trip is twice that. An OTDR measures the round-trip time to an event and divides by its own index setting $n_\text{set}$ , so it reports a length $L\, n_g / n_\text{set}$ . Two wavelengths $\Delta\lambda$ apart in fiber with dispersion $D$ arrive $D\,\Delta\lambda\, L$ apart. The delays are for the fiber alone; transceivers, amplifiers, switching and forward error correction add their own, and in standard fiber the delay changes with temperature by a few tens of picoseconds per kilometre per kelvin.

Worked example

Standard single-mode fiber with a group index of 1.4682 at 1550 nm carries light at 204,190 km/s, a delay of 4.8974 µs per kilometre. A 10 km link takes 48.97 µs one way and 97.95 µs round trip, and at 100 Gb/s holds 4.897 Mb in flight. Over 100 km, hollow-core fiber with a group index of 1.0003 takes 333.7 µs against 489.7 µs, a saving of 31.9 %.

References: Corning SMF-28 Ultra optical fiber product information (effective group index). BIPM, The International System of Units (SI brochure), 9th ed. (2019). G. P. Agrawal, Fiber-Optic Communication Systems, 4th ed. (Wiley, 2010), ch. 2.