So! How do laser diodes work?

Laser diodes are Light-Emitting Diodes with two "mirrors" on the surface of the diode to create a laser cavity.

When the diode is forward-biased, charges are injected into the active area of the junction. Electrons and holes recombine in the junction creating spontaneous emission of photons.

These photons can cause other electron-hole pairs to recombine by stimulated emission. When the current is high enough, the device lases.