The light-emitting principle of laser diodes

- 2021-08-24-

The light-emitting principle of the laser diode: The P-N junction in the laser diode is formed by two doped gallium arsenide layers. It has two flat-end structures, mirrored parallel to one end (highly reflective surface) and one partly reflective. The wavelength of the light to be emitted is exactly related to the length of the connection. When the P-N junction is forward biased by an external voltage source, electrons move through the junction and recombine like a normal diode. When electrons and holes recombine, photons are released. These photons hit the atoms, causing more photons to be released. As the forward bias current increases, more electrons enter the depletion zone and cause more photons to be emitted. Eventually, some photons randomly drifting in the depletion zone illuminate the reflective surface perpendicularly and reflect back along their original path. The reflected photons are again reflected from the other end of the junction. This movement of photons from one end to the other is continuous multiple times. During the movement of photons, more atoms will release more photons due to the avalanche effect. This process of reflecting and generating more and more photons produces a very intense laser beam. Each photon produced in the emission process explained above is the same as the other photons in energy level, phase relationship and frequency. Therefore, the emission process gives a laser beam of a single wavelength. In order to produce a laser beam, the current of the laser diode must exceed a certain threshold level. The current below the threshold level forces the diode to behave as an LED, emitting incoherent light.