Laser Diode Principle and Structure

Laser diode is a light source semiconductor laser invented in the 1960s, also known as Laser Diode. It includes single heterojunction (SH), double heterojunction (DH), and quantum well (QW) laser diodes. Quantum well laser diodes have the advantages of low threshold current and high output power, and are the mainstream products in the market. High coherence is the most important feature because it can produce light with identical properties such as wavelength and phase. Here is a look at its principle and structure.

Working Principle

The P-N junction in a laser diode is formed by two doped gallium arsenide layers. It has two flat-ended structures, parallel to a mirror image at one end (highly reflective surface) and a partial reflection. The wavelength of the light to be emitted is exactly related to the length of the junction. When the P-N junction is forward biased by an external voltage source, the electrons move through the junction and recombine as in a normal diode. When the electrons are compounded with holes, 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 region and cause more photons to be emitted. Eventually, some of the photons randomly drifting in the depletion region strike the reflecting surface vertically, thus reflecting back along their original path.

The reflected photons are again reflected back from the other end of the junction. This movement of photons from one end to the other is continuous several times. During the photon motion, more atoms release more photons due to the avalanche effect. This process of reflection and production of more and more photons produces a very intense laser beam. Each photon produced in the emission process explained above is identical to the other photons in terms of energy level, phase relationship and frequency. Thus, the emission process gives a laser beam of a single wavelength. In order to produce a laser beam, the current in the laser diode must be made to exceed a certain threshold level. Currents below the threshold level force the diode to behave as an LED, emitting incoherent light.

Internal structure

Laser diode LASER includes four parts: the first part is the laser emitting part (available LD representation), its role is to launch the laser; the second part is the photodiode, is the laser receiving feedback (available PD representation), its role is to receive, monitor the LD emitted laser (of course, if you do not need to monitor the output of the LD, the PD part can not be used), in addition to these two parts also a common electrode. Therefore, usually laser diodes have three electrodes; the third part is the glass cover sheet to do the lens, its role is dustproof and resonant cavity purposes; the fourth part is the metal shell, mainly fixed, shielding the role of external interference signals and heat dissipation.

The above is the principle and structure of the laser diode is introduced. As the laser diode emitted laser light may cause harm to the human eye, so the diode in the work, it is strictly prohibited to look directly at its end, not through the lens directly into the laser, but also not through the mirror to observe the laser.