9/2/2023

Laser (light amplification by stimulated erission of radiation, referred to as laser) is regarded as the “new four great inventions” of the 20th century together with atomic energy, computers, and semiconductors. Laser is light produced by artificially exciting a specific substance with strong energy such as light or electricity. The principle is that the electrons in the atom absorb energy and jump from a low energy level to a high energy level, and then fall back from a high energy level to a low energy level, the released energy is released in the form of photons, producing a collimated, monochromatic, coherent directional beam.

The theoretical basis of the laser originated from the physicist Einstein. In 1917, Einstein proposed the theory of “stimulated radiation”. In 1958, American scientists Towns and Sholow proposed the “laser principle”, that is, when a substance is excited by the energy with the same oscillation frequency as its molecule, it will produce non-divergent Strong light-laser, they won the Nobel Prize in Physics in 1964 and 1981 respectively. In 1960, the laser was successfully manufactured for the first time, and Maiman of the Hughes Laboratory used a ruby rod with a diameter of 6mm and a length of 45mm to generate a wavelength The beam is 0.6943um, which is the first laser ever obtained by human beings. In the same year, Maiman successfully developed the world’s first ruby laser (ruby laser), which was highly valued by the laser scientific research field and created a breakthrough in the laser field. new era.

In the 21st century, the application of laser technology is on the fast track of development, and the laser industry has formed an industrial chain of a certain scale.

China has a large number of laser equipment integrators, and it is also one of the most important markets for laser applications. In addition, the country continues to vigorously promote the development of the intelligent manufacturing industry, and effectively promotes industry development and technological upgrading through the release of policy plans and industry standards. With the continuous acceleration of the process of replacing humans with machines and the continuous innovation of laser technology, the new industrial processing technology formed by industrial robots combined with laser technology will help expand the field of laser applications and provide new development momentum for the laser industry.

Generation of laser light

Microscopic particles have a specific series of energy levels (usually these energy levels are discrete), and particles can only be in the state corresponding to a certain energy level (that is, at a certain energy level) at any time. When the particle obtains the excitation energy equivalent to the difference between the two energy levels due to heat, collision or radiation, etc., it will transition from the initial state with lower energy to the excited state with higher energy, but it is not stable and has A tendency to spontaneously return to a steady state. After releasing the corresponding energy, the particle automatically returns to the initial state. Figure 1:1 shows the transition types of particles. There are three types of particle energy transitions—spontaneous absorption, spontaneous emission, and stimulated emission.

• Spontaneous absorption: When a particle at a lower energy level is excited by the outside world (that is, interacts with other particles with energy exchange, such as an inelastic collision with a photon), when it absorbs energy, it transitions to a state corresponding to this energy. Corresponding to the higher energy level, this transition is called spontaneous absorption.
• Spontaneous emission: The excited state that the particle enters after being excited is not the stable state of the particle. If there is a lower energy level that can accept particles, even if there is no external action, the particles have a certain probability to spontaneously transition from the high-energy excited state (E2) to the low-energy ground state (E1), and at the same time radiate energy E2-E1 Photon, photon frequency v=(E2-E1)/h (where h is Planck’s constant), this radiation process is called spontaneous radiation. The light emitted by many atoms with spontaneous radiation does not have the consistency in phase, polarization state, and propagation direction, which is what is called non-phase light in physics.
• Stimulated radiation: In 1917, Einstein theoretically pointed out that in addition to spontaneous radiation, particles at a high energy level E2 can also jump to a lower energy level E1 in another way. When a photon with a frequency of v=(E2-E1)/h is incident, it will also cause the particle to rapidly transition from the energy level E2 to the energy level E1 with a certain probability, and at the same time radiate a photon with the frequency, phase, and polarization of the external photon Photons with the same state and the same direction of propagation, this process is called stimulated emission.

Laser light is formed by stimulated emission of atoms and resonance amplification. Atoms have discontinuous distribution of energy electrons. These energy electrons are stable when they rotate on the orbit closest to the nucleus. At this time, the energy level of the atom is Ground state. When there is external energy to follow, the orbital radius of electrons expands, and the internal energy of atoms increases. When excited to a higher energy level, it is called an excited state or a high energy state. Atoms excited to a higher energy level are unstable and always try to return to a lower energy level. When atoms are transitioning, their energy is best radiated in the form of light. This is atomic luminescence, which is spontaneously radiated light, also known as fluorescence. If the atom is induced by an external photon during the transition of the atom, the atom will emit a photon with the same frequency, phase, propagation direction, and polarization state as the photon emitted by the human, which is the light of stimulated emission.

After an atom is excited to a high energy level, it will quickly jump back to a low energy level, and the time it stays at a high energy level is called the average lifetime of the atom at this energy level. When atoms are excited by external energy, the number of atoms in the high energy level is greater than the number of atoms in the low energy level. This state is called population inversion. At this time, under the stimulation of external photons, the atoms generate stimulated radiation light, and these photons are amplified by the action of the optical resonant cavity, the stimulated radiation becomes stronger and stronger, and the beam density continues to increase, forming a laser.

From the above laser principle, it can be seen that any type of laser must include three basic elements: a working substance that can be excited, a working substance that needs to achieve particle number inversion (that is, an excitation energy source), and an optical resonant cavity.

The photons of ordinary light sources are different and spread out in all directions. The photons in the laser are induced to emit, the optical characteristics and pace are extremely consistent, and the energy is concentrated, which has powerful power. Because stimulated radiation is the main source, and the light waves have the same frequency, propagation direction, polarization state and phase, the laser has high brightness (concentrated energy, extremely high temperature), high directivity (very small divergence, power density High), high monochromaticity (very narrow wavelength distribution range, extremely pure color) and high coherence (basically consistent in wavelength, frequency, and polarization direction) and many other excellent characteristics.

Principle and structure of laser

Figure 1.2 shows the schematic diagram of the principle and structure of the laser. A laser is a laser generating device, mainly composed of a gain medium, a pump source, and an optical resonator. The gain medium refers to the working substance that can amplify light, the pump source is the excitation source of the laser, and the optical resonant cavity is the loop between the pump source and the gain medium. In the working state, the gain medium transitions from the ground state to the excited state by absorbing the energy generated by the pump source. Since the excited state is an unstable state, at this time, the gain medium will release energy and return to the ground state. In the process of energy release, the gain medium produces photons, and these photons are highly consistent in energy, wavelength, and propagation direction. They are continuously reflected in the optical resonant cavity, reciprocating, and finally exit the laser through the half-reflecting mirror to form a laser. bundle.

Classification of laser

There are many types of lasers, and there are four common classification methods, that is, they can be divided into various types according to different classifications of gain medium, output power, output wavelength, and working mode.

Different gain media determine the laser wavelength, output power and application fields. According to different gain media, they can be divided into gas, liquid, solid-state lasers, etc., as shown in Table 1.1.

Among them, fiber laser refers to the laser doped with rare earth element glass fiber as the gain medium, which is a kind of solid-state laser. Because the gain medium has a special shape and has typical technical and industrial advantages, it is generally carried out separately from other solid-state lasers in the industry. Research.

According to the different output power, it can be divided into low power (0~100 W), medium power (100~1 000 W), high power Power (above 1000 W) lasers, lasers with different powers are applicable to different scenarios.

According to different output wavelengths, it can be divided into infrared lasers, visible light lasers, ultraviolet lasers, etc. The wavelength range of light that can be absorbed by substances with different structures is different.

According to different working methods, it can be divided into continuous laser and pulse laser. The continuous laser can output continuously for a long period of time, with stable operation and high thermal effect. Pulsed lasers output in the form of pulses, and their main features are high peak power and small thermal effects. According to the pulse time, pulsed lasers can be further divided into millisecond, microsecond, nanosecond, picosecond and femtosecond. Generally speaking, the shorter the pulse time, the higher the single pulse energy, the narrower the pulse width, and the higher the machining accuracy.

Fiber lasers have the characteristics of high photoelectric conversion efficiency, simple structure, and good beam quality. At present, they have become the mainstream direction of laser technology development and the main force in laser industry applications. A typical fiber laser is mainly composed of four parts: optical system, power system, control system and mechanical structure. Among them, the optical system is formed by fusion and pressing of optical components such as pump source, gain fiber, fiber grating, pump beam combiner and laser transmission cable, and realizes laser output under the drive and monitoring of the power supply system and control system.