Principle of Laser Generation

Why do we need to know the principle of lasers?

Knowing the differences between common semiconductor lasers, fibers, discs, and YAG laser can also help to gain a better understanding and engage in more discussions during the selection process.

The article mainly focuses on popular science: a brief introduction to the principle of laser generation, the main structure of lasers, and several common types of lasers.

Firstly, the principle of laser generation

Laser is generated through the interaction between light and matter, known as stimulated radiation amplification; Understanding stimulated radiation amplification requires understanding Einstein’s concepts of spontaneous emission, stimulated absorption, and stimulated radiation, as well as some necessary theoretical foundations. 

Theoretical Basis 1: Bohr Model

The Bohr model mainly provides the internal structure of atoms, making it easy to understand how lasers occur. An atom is composed of a nucleus and electrons outside the nucleus, and the orbitals of electrons are not arbitrary. Electrons only have certain orbitals, among which the innermost orbital is called the ground state; If an electron is in the ground state, its energy is the lowest. If an electron jumps out of an orbit, it is called the first excited state, and the energy of the first excited state will be higher than that of the ground state; Another orbit is called the second excited state;

The reason why laser can occur is because electrons will move in different orbits in this model. If electrons absorb energy, they can run from the ground state to the excited state; If an electron returns from the excited state to the ground state, it will release energy, which is often released in the form of a laser.

Theoretical Basis 2: Einstein’s Stimulated Radiation Theory

In 1917, Einstein proposed the theory of stimulated radiation, which is the theoretical basis for lasers and laser production: the absorption or emission of matter is essentially the result of the interaction between the radiation field and the particles that make up matter, and its core essence is the transition of particles between different energy levels. There are three different processes in the interaction between light and matter: spontaneous emission, stimulated emission, and stimulated absorption. For a system containing a large number of particles, these three processes always coexist and are closely related.

Spontaneous emission:

As shown in the figure: an electron on the high-energy level E2 spontaneously transitions to the low-energy level E1 and emits a photon with an energy of hv, and hv=E2-E1; This spontaneous and unrelated transition process is called spontaneous transition, and the light waves emitted by spontaneous transitions are called spontaneous radiation.

The characteristics of spontaneous emission: Each photon is independent, with different directions and phases, and the occurrence time is also random. It belongs to incoherent and chaotic light, which is not the light required by the laser. Therefore, the laser generation process needs to reduce this type of stray light. This is also one of the reasons why the wavelength of various lasers has stray light. If controlled well, the proportion of spontaneous emission in the laser can be ignored. The purer the laser, such as 1060 nm, it is all 1060 nm, This type of laser has a relatively stable absorption rate and power.

Stimulated absorption:

Electrons at low energy levels (low orbitals), after absorbing photons, transition to higher energy levels (high orbitals), and this process is called stimulated absorption. Stimulated absorption is crucial and one of the key pumping processes. The pump source of the laser provides photon energy to cause particles in the gain medium to transition and wait for stimulated radiation at higher energy levels, emitting the laser.

Stimulated radiation:

When irradiated by the light of external energy (hv=E2-E1), the electron at the high energy level is excited by the external photon and jumps to the low energy level (the high orbit runs to the low orbit). At the same time, it emits a photon that is exactly the same as the external photon. This process does not absorb the original excitation light, so there will be two identical photons, which can be understood as the electron spits out the previously absorbed photon, This luminescence process is called stimulated radiation, which is the reverse process of stimulated absorption.

After the theory is clear, it is very simple to build a laser, as shown in the above figure: under normal conditions of material stability, the vast majority of electrons are in the ground state, electrons in the ground state, and laser depends on stimulated radiation. Therefore, the structure of the laser is to allow stimulated absorption to occur first, bringing electrons to the high energy level, and then providing an excitation to cause a large number of high energy level electrons to undergo stimulated radiation, releasing photons, From this, laser can be generated. Next, we will introduce the laser structure.

Laser structure:

Match the laser structure with the laser generation conditions mentioned earlier one by one:

Condition of occurrence and corresponding structure:

1. There is a gain medium that provides amplification effect as the laser working medium, and its activated particles have an energy level structure suitable for generating stimulated radiation (mainly able to pump electrons to high-energy orbitals and exist for a certain period of time, and then release photons in one breath through stimulated radiation);

2. There is an external excitation source (pump source) that can pump electrons from the lower level to the upper level, causing particle number inversion between the upper and lower levels of the laser (i.e., when there are more high-energy particles than low-energy particles), such as the xenon lamp in YAG lasers;

3. There is a resonant cavity that can achieve laser oscillation, increase the working length of the laser working material, screen the light wave mode, control the propagation direction of the beam, selectively amplify the stimulated radiation frequency to improve monochromaticity (ensuring that the laser is outputted at a certain energy).

The corresponding structure is shown in the above figure, which is a simple structure of a YAG laser. Other structures may be more complex, but the core is this. The laser generation process is shown in the figure:

Laser classification: generally classified by gain medium or by laser energy form

Gain medium classification:

Carbon dioxide laser: The gain medium of carbon dioxide laser is helium and CO2 laser, with a laser wavelength of 10.6um, which is one of the earliest laser products to be launched. The early laser welding was mainly based on carbon dioxide laser, which is currently mainly used for welding and cutting non-metallic materials (fabrics, plastics, wood, etc.). In addition, it is also used on lithography machines. Carbon dioxide laser cannot be transmitted through optical fibers and travels through spatial optical paths, The earliest Tongkuai was done relatively well, and a lot of cutting equipment was used;

YAG (yttrium aluminum garnet) laser: YAG crystals doped with neodymium (Nd) or yttrium (Yb) metal ions are used as the laser gain medium, with an emission wavelength of 1.06um. The YAG laser can output higher pulses, but the average power is low, and the peak power can reach 15 times the average power. If it is mainly a pulse laser, continuous output cannot be achieved; But it can be transmitted through optical fibers, and at the same time, the absorption rate of metal materials increases, and it is beginning to be applied in high reflectivity materials, first applied in the 3C field;

Fiber laser: The current mainstream in the market uses ytterbium doped fiber as the gain medium, with a wavelength of 1060nm. It is further divided into fiber and disc lasers based on the shape of the medium; Fiber optic represents IPG, while disc represents Tongkuai.

Semiconductor laser: The gain medium is a semiconductor PN junction, and the wavelength of the semiconductor laser is mainly at 976nm. Currently, semiconductor near-infrared lasers are mainly used for cladding, with light spots above 600um. Laserline is a representative enterprise of semiconductor lasers.

Classified by the form of energy action: Pulse laser (PULSE), quasi continuous laser (QCW), continuous laser (CW)

Pulse laser: nanosecond, picosecond, femtosecond, this high-frequency pulse laser (ns, pulse width) can often achieve high peak energy, high frequency (MHZ) processing, used for processing thin copper and aluminum dissimilar materials, as well as cleaning mostly. By using high peak energy, it can quickly melt the base material, with low action time and small heat affected zone. It has advantages in processing ultra-thin materials (below 0.5mm);

Quasi continuous laser (QCW): Due to high repetition rate and low duty cycle (below 50%), the pulse width of QCW laser reaches 50 us-50 ms, filling the gap between kilowatt level continuous fiber laser and Q-switched pulse laser; The peak power of a quasi continuous fiber laser can reach 10 times the average power under continuous mode operation. QCW lasers generally have two modes, one is continuous welding at low power, and the other is pulsed laser welding with a peak power of 10 times the average power, which can achieve thicker materials and more heat welding, while also controlling the heat within a very small range;

Continuous Laser (CW): This is the most commonly used, and most of the lasers seen on the market are CW lasers that continuously output laser for welding processing. Fiber lasers are divided into single-mode and multi-mode lasers according to different core diameters and beam qualities, and can be adapted to different application scenarios.