Light in glass
The switch works with electrical signals. A small plug-in module, an SFP+ transceiver (transmitter + receiver), turns them into light: its laser flashes brighter for a 1 and dimmer for a 0, about ten billion times a second.
The light enters a fibre: a strand of very pure glass 125 µm (millionths of a metre) thick, about as thick as a hair. Light travels in its centre, the core. Around it sits the cladding, then a protective coating and jacket.
The cladding glass bends light slightly less than the core (it has a lower refractive index). So light that meets the edge at a shallow angle cannot escape: it bounces back in. This is total internal reflection, and it keeps the light in the core.
The flashes race along the fibre to a building 10 km away. Light in glass moves at about two-thirds of its speed in a vacuum, roughly 200,000 km per second, so the trip takes about 50 microseconds.
In the far building, a second transceiver catches the light with a photodiode, a sensor that turns light into electric current, and rebuilds the 1s and 0s. Each link uses two fibres, one for each direction, so both ends can send at once.
Two kinds of fibre
Single-mode fibre has a tiny core, about 9 µm wide in 125 µm of cladding. Light can follow only one path, so pulses stay sharp for tens of kilometres. It uses infrared lasers at 1310 or 1550 nm (nanometres, the light's wavelength).
Multimode fibre has a wider core, 50 or 62.5 µm. Light takes many paths of different lengths, so each pulse smears out as it travels. That keeps runs short, about 300 m at 10 Gb/s, but its 850 nm light sources are cheaper.
Why fibre
A motor or power cable makes electrical noise that can corrupt the signal in a copper cable next to it. Fibre carries light in glass, which that noise cannot disturb, and it does not connect the two buildings electrically.
Fibre loses very little light: single-mode glass loses under 0.4 dB per km (less than 9% of the light). Copper Ethernet reaches only 100 m, while one single-mode 10 Gb/s link spans 10 km, and the same glass can carry far faster signals.
Connectors line the glass cores up exactly. LC is the small one, with a 1.25 mm ferrule (the ceramic tip that holds the fibre), used on SFP modules. SC is older and bigger, with a 2.5 mm ferrule. Keep the ends clean: dust blocks light.
The numbers
;; step 1/10 · transceiver SFP = small form-factor pluggable (SFF-8472, INF-8074i) SFP+ = the 10 Gb/s version (SFF-8431) inside: a laser (transmit) and a photodiode (receive) 10GBASE-LR line rate 10.3125 Gbaud (64b/66b code) a 1 = more light, a 0 = less light (not fully dark)
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. 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.