This document discusses the fundamentals of laser diodes, including: 1) Laser diodes use direct bandgap semiconductors where electron-hole recombination emits photons of light equal to the bandgap energy. Population inversion, needed for lasing, can be achieved through heavy doping of both p-type and n-type materials near the depletion layer. 2) Early laser diodes used homojunctions but now use double heterojunctions of GaAs for the active region surrounded by higher bandgap AlGaAs for better optical confinement. 3) Double heterojunction lasers have lower lasing thresholds than earlier designs due to reduced optical losses from improved carrier and light confinement, though