Laser diode

A laser diode is an LED pushed to a higher discipline. Drive an ordinary LED and it sprays photons in every direction, in a jumble of slightly different colours and out-of-step phases — like a crowd all talking at once. A laser diode adds two tricks: it confines the light in a tiny waveguide between two mirror-like facets, and it runs at enough current that emitted photons trigger more photons identical to themselves (stimulated emission). The result is a beam where the waves march in lockstep: one pure colour, one direction, coherent — a disciplined chorus instead of a babble.

Below a sharp threshold current the device just glows like an LED; above it, the gain inside overcomes the losses, the chorus locks, and optical power shoots up steeply with current. That threshold (a few mA to tens of mA) is the laser diode's signature. Cheap, tiny, and efficient, these chips are everywhere: the red one reading a DVD, the infrared ones firing pulses down the world's fibre-optic backbone, the green/blue ones in laser projectors and lidar.