A high-power laser diode is a laser that produces high-power beams of light using a semiconductor as the laser's gain medium. A laser's power, measured in units called watts, is the rate at which the laser converts energy. High-power is not a precisely defined term, and its meaning varies according to the type of laser and the laser's application, but a high-power laser diode is usually a laser with an output in the watts or kilowatts (kW), distinguishing it from the milliwatt-range laser diodes used for applications such as laser pointers and optical disc readers. High-power laser diodes are used for industrial applications such as heating and welding and are also used to optically pump high-powered solid-state lasers.
A laser diode produces a beam of light by pumping energy in the form of electricity into a semiconductor, which serves as the laser gain medium. The semiconductor is usually composed of substances such as indium phosphide (InP) or gallium compounds such as gallium nitride (GaN) and gallium arsenide (GaAs). The electricity temporarily raises the energy level of the semiconductor's electrons, and when they drop back to their original level the energy is emitted in the form of photons, which are then focused into a beam. Laser diodes are often grouped into arrays of side-by-side individual laser emitters called diode bars, which are in turn sometimes further combined into a two-dimensional array called a diode stack.
Power is energy output over time, with the watt defined as one joule of energy per second. A laser's power can refer to either the maximum number of watts it can produce during a short pulse, called peak power, or the average number of watts it can output over an extended period, called average power. Lasers face a trade-off between beam power and beam quality, which refers to how focused the beam is. This affects how precise the beam can be and how well it is able to delivered focused energy at greater distances. Beam quality can be improved through various design features of the laser, but more powerful lasers have lower beam quality than less powerful lasers, all other things being equal.
A high-power laser diode usually has relatively poor beam quality and produces a beam with high divergence, meaning that the photons in the beam spread rapidly as they travel forward. Consequently, they are most commonly used for applications that can be done at short range and do not require extreme precision. High-power laser diodes are commonly used in industry for purposes such as soldering, welding, and heat treatment.
A high-power laser diode can also be used as an optical pump, a light source that powers other lasers. In a diode-pumped solid-state laser, a high-power laser diode fires a beam of light into the gain medium of the solid-state laser. The solid-state gain medium is usually a glass or crystalline material, such as corundum or yttrium aluminum garnet doped with metal ions. The light from the diode laser energizes the solid-state gain medium, which in turn begins to produce light that can be focused into a beam with higher beam quality than a high-power laser diode could produce on its own.