Erbium

Silica fibre happens to be most transparent near 1550 nm, and Er³⁺ happens to have a laser transition at exactly that wavelength. The erbium-doped fibre amplifier, demonstrated at Southampton in 1987, exploits that coincidence: a few metres of erbium-doped fibre pumped by a cheap 980 nm diode boosts a signal by 30 dB without ever converting it back to electricity. Before the EDFA, transoceanic cables needed an electronic repeater every few tens of kilometres; after it, the whole optical spectrum could be amplified at once, which is what made wavelength-division multiplexing and therefore the modern internet economically possible. Erbium also gives glass a pink tint and drives the 2940 nm laser used in dermatology.