We all know that semiconductor lasers are widely used in daily life, so do you know what its main working principle is? Let's take a look at it together.

Generally speaking, semiconductor lasers are composed of three parts: active layer, mirror and electrode structure.

The most critical one is the "active layer", also called "active region" or "gain region". This layer of material is generally a multilayer structure grown on GaAs (gallium arsenide) or InP (indium phosphide) substrate, and its thickness is only a few microns, which is invisible to the naked eye, but it is the soul of the whole laser.

When working, we power the laser forward, that is to say, the current flows from the P region to the N region. As soon as the current passes, electrons and holes recombine in the active layer-in short, they "touch" and then release a photon. Ordinary LED is actually the same principle, but the light emitted by LED is chaotic, in all directions and phases. Unlike laser, it will form a resonant cavity between the mirrors at both ends of the chip. The function of this cavity is to "amplify" the light in the same direction and phase, and finally form a laser with very high coherence.

This "resonant cavity" is actually two reflective surfaces naturally formed at both ends of the chip. In most cases, it is a Fresnel reflection formed by the cross section of the crystal itself. Generally, one end has a high reflectivity and the other end is a little lower, so that photons can be reflected back and forth in the cavity, and part of the light is output to form laser. When the number of particles in the active layer accumulates to a certain extent, that is, "the number of particles is reversed", the amplification of light exceeds the loss, and it enters the lasing state. At this time, the light you see is the laser with stable output.

From the outside, the laser diode is usually a device packaged in a small metal shell, such as TO package or butterfly package. The chip inside is actually a few hundred microns in size, but the power it emits can reach hundreds of milliwatts or even several watts. Lasers for different purposes will choose different wavelengths, such as 1310nm and 1550nm； for communication; 808nm and 940nm； are commonly used for industrial marking or ranging; Common in medical beauty is 650nm or 980nm.

Some semiconductor lasers are edge-emitting (EEL) and some are surface-emitting (VCSEL). EEL structure is that light comes out from the edge of the chip with high power and is often used for pumping or communication; VCSEL, on the other hand, emits light vertically from the surface, with compact structure and fast modulation speed. Now it is particularly popular in products such as 3D sensing, lidar and Face ID.

So you see, the semiconductor laser is driven by current, produces light by energy band structure, and then enhances the coherence of light by reflecting cavity. In principle, it is precise, but from the engineering point of view, the most difficult thing is to keep it emitting light stably.