Fibre optics: light in glass
Long links between buildings, cities and continents run on light. Follow a flash from a switch's SFP+ transceiver into a glass fibre, see why total internal reflection keeps it inside the core, and watch it reach a building 10 km away about 50 µs later. Then compare single-mode and multimode fibre, and see why fibre beats copper.
5 quick questions
Pick an answer to see why. Your score saves with your progress.
01What keeps the light inside the core of a fibre?
Light meeting a boundary into a lower-index glass at a shallow enough angle is reflected completely. The cladding is that lower-index glass.
02Which fibre would you pick for a 10 km link between two buildings?
Single-mode carries one sharp path for kilometres: 10GBASE-LR reaches 10 km. Multimode at 10 Gb/s reaches about 300 m, and copper Ethernet 100 m.
03About how long does light take to cross 10 km of fibre?
Light in glass moves at about two-thirds of its vacuum speed, roughly 5.0 µs/km. 10 km × 5 µs ≈ 50 µs.
04Why does a pulse smear out in multimode fibre?
Steep zig-zag paths are longer than the straight path down the middle. Over distance the arrival times spread and neighbouring pulses blur together.
05What does an SFP+ transceiver do?
Transceiver = transmitter + receiver. It plugs into the switch port; the fibre plugs into it with a duplex LC connector.
Glossary
- Optical fibre
- A thin strand of very pure glass that carries data as flashes of light.
- Core
- The centre of the fibre, where the light travels: about 9 µm in single-mode, 50 or 62.5 µm in multimode.
- Cladding
- The glass around the core, 125 µm across, with a slightly lower refractive index so light stays in the core.
- Total internal reflection
- Light that meets the edge of a lower-index material at a shallow angle bounces back completely instead of escaping.
- Refractive index
- How much a material slows and bends light; the core's is a little higher than the cladding's.
- Single-mode fibre
- Fibre with a tiny core that lets light follow one path, for links of many kilometres, using 1310 or 1550 nm lasers.
- Multimode fibre
- Fibre with a wider core that lets light take many paths; cheaper 850 nm optics, but runs of a few hundred metres.
- Transceiver (SFP/SFP+)
- A plug-in module that turns electrical signals into light and back. SFP+ is the 10 Gb/s version.
Notes and sources
- Speed in fibre is taken as 2/3 of c (199,862 km/s, 5.00 µs per km), as in the Bits on the wire lesson; real single-mode fibre is about c / 1.47.
- The refractive indices 1.45 and 1.445 are illustrative values typical of single-mode fibre. The signal speed follows the group index (about 1.47), a slightly different number.
- Reaches are from IEEE 802.3: 1000BASE-T 100 m (Clause 40), 10GBASE-SR 300 m on OM3 and 10GBASE-LR 10 km (Clause 52). Loss limits are ITU-T G.652.D cable values.
- The cable is drawn far thicker than life and its core wider than a real single-mode core; the two round slices in front are to scale with each other.
- The 10 km building link and the motor are example values. SFP and SFP+ are industry agreements (SFF-8472, SFF-8431); LC and SC are IEC 61754-20 and 61754-4.
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