physics notes

Like the other lasers described so far, a semiconductor diode laser generates a beam when spontaneous emission triggers a cascade of stimulated emission from a population inversion inside a resonant optical cavity. As in a gas laser, the excitation energy comes from an electric current passing through the laser material, but in a semiconductor diode laser the laser material is a solid with a particular internal structure that produces and traps a population inversion.
The structure is called a diode and it is formed within the semiconductor by depositing layers with different compositions. We will get into the details later. It is possible to produce stimulated emission from semiconductors in other ways, such as stimulated Raman scattering in silicon (described in Chapter 10) but virtually all semiconductor lasers now in use rely on an internal diode to produce a population inversion. Thus, you really do not have to call them semiconductor diode lasers; you can call them semiconductor lasers, diode lasers, or laser diodes. Those terms are interchangeable. Sometimes, the devices are just called diodes in a context in which it is clear that they are lasers, such as a diodepumped solid-state laser.
Diode lasers are close cousins of the light-emitting diodes or LEDs. LEDs came first, initially...