In order for a semiconductor laser to generate a beam of light, the energy level of the electrons from the valence band must be raised so that they can bridge the band gap between the valence band and the conduction band and switch to the conduction band. This energy supply is provided by the pump laser with a stimulated emission at which the pump power, provided as light, is higher than the laser threshold of the laser. To obtain the highest possible pump energy, the pump beam is generated in a reflective resonator cavity. Once the electrons are at the conduction band level, they fall back in energy to the valence band level, where they recombine with the remaining holes, releasing photons.
Pump lasers are used to supply energy and raise the energy level of electrons from the valence band enough to cause stimulated light emission. The emitted wavelength is in the infrared range and stimulates emission by converting the pump energy into light.