2. Physical principles

In the 1960s, once it had been demonstrated that certain semiconductors, such as gallium arsenide (GaAs), could spontaneously emit photons by recombination of electron-hole pairs in a p-n junction, several experimenters observed light amplification by stimulated emission in junctions through which a high current flowed. The threshold of stimulated emission appeared for a quantity of electron-hole pairs satisfying a condition analogous to that of population inversion in atomic systems. Then, following the example of gas lasers, by inserting this amplifying medium into a Fabry-Pérot-type optical cavity, we multiplied light amplification and, in 1962, created the first oscillator, or semiconductor laser, emitting at 0.9 µm.

Since then, diode structures and properties have evolved considerably. This progress has been achieved thanks to the discovery of new semiconductors,...