Laser is another major invention of humanity in the 20th century after nuclear power, the computer, and semiconductor. The English name Light Amplification by Stimulated Emission of Radiation means “expansion of light by stimulated radiation.” The full name of the laser has fully expressed the primary process of manufacturing the laser. The famous Jewish physicist Albert Einstein discovered the principle of the laser as early as 1916.
The light from an atom is excited and radiated, hence the name “laser.” Laser: When an electron in an atom absorbs energy and jumps from a low energy level to a high energy level, and then falls back to a low energy level, the energy released is in the form of a photon. The induced (excited) photon beam (laser), in which the optical properties of the photons are highly uniform. Therefore, lasers are monochromatic, directional, and brighter than ordinary light sources.
Laser applications are widespread, including laser marking, laser welding, laser cutting, fiber optic communication, laser ranging, laser radar, laser weapons, laser recordings, laser vision correction, laser cosmetology, laser scanning, laser mosquito exterminator, LIF non-destructive testing technology, etc. Laser systems can be divided into continuous wave lasers and pulsed lasers.
The theory of laser
The interaction of light with matter is essentially a manifestation of the microscopic particles that make up matter, absorbing or radiating photons while changing their motion.
Microscopic particles have specific energy levels (usually, these energy levels are discrete). At any moment, a particle can only be in a state corresponding to a certain energy level (or expressed as being at a certain energy level). When interacting with a photon, the particle jumps from one energy level to another and absorbs or radiates the photon accordingly. The energy value of the photon is the energy difference between the two energy levels △E, and the frequency is ν = △E/h (h is Planck’s constant).
1. Stimulated absorption (referred to as absorption)
In the lower energy level of the particle in the external excitation (that is, with other particles with energy exchange interactions, such as inelastic collisions with photons), absorbed energy jumped to the higher energy level corresponding to this energy. This jump is called excited absorption.
2. Spontaneous radiation
The excited state into which the particle is excited, not the stable state of the particle, such as the existence of lower energy levels that can accept particles, even if there is no external action, the particle has a certain probability, spontaneously from the high-energy excited state (E2) to the low-energy ground state (E1) leap, while radiation energy (E2-E1) photon, photon frequency ν = (E2-E1)/h. This radiation process is called spontaneous radiation. The light emitted by many atoms by spontaneous radiation does not have the same phase, polarization state, or propagation direction; it is physically incoherent light.
3. Excited radiation, laser
In 1917, Einstein pointed out theoretically that in addition to spontaneous radiation, particles at high energy level E2 can also jump to lower energy levels in another way. He pointed out that when the frequency of ν = (E2-E1)/h, photon incident will also trigger a particle with a certain probability, rapid jump from energy level E2 to energy level E1. In contrast, the radiation of two and foreign photon frequency, phase, polarization state, and propagation direction are identical photons; this process is called exciting radiation.
It is conceivable that if a large number of atoms are at high energy level E2 when a photon with frequency ν=(E2-E1)/h is incident, which excites the atoms on E2 to produce exciting radiation, two photons with the same characteristics are obtained. These two photons then excite the atoms on energy level E2 to produce exciting radiation; four photons with the same characteristics can be obtained, which means that the original light signal is amplified. This means that the original light signal is amplified. The light produced and amplified during the exciting radiation is the laser.
The history of laser
Einstein proposed exciting radiation in 1917, but lasers were introduced in 1960, a gap of 43 years; why? The main reason is that the probability of particles in an ordinary light source producing exciting radiation is minimal. When the light of a specific frequency is injected into an active substance, two processes exist: excited radiation increases the number of photons. In contrast, excited absorption decreases the number of photons. When the substance is in thermal equilibrium, the distribution of particles at each energy level follows the law of statistical distribution of particles in equilibrium. According to the statistical distribution law, the number of particles in the lower energy level E1 must be greater than the number of particles in the higher energy level E2. This is because this light, through the work of matter, light energy will only weaken, not strengthen. Therefore, to make the exciting radiation dominant, the number of particles in the higher energy levels E2 must be greater than the number of particles in the lower energy levels E1. This distribution is the opposite of the particle distribution in equilibrium, called the particle number inversion distribution or particle number inversion. The technical realization of the particle number inversion is necessary for the generation of lasers.
Theoretical studies have shown that any working material, under appropriate excitation conditions, can achieve particle number inversion between specific high and low energy levels of the particle system. If a microscopic particle such as an atom or a molecule has a high energy level E2 and a low energy level E1, and the Buju number densities on the E2 and E1 energy levels are N2 and N1, there are three processes such as spontaneous emission leap, excited emission leap and excited absorption leap between the two energy levels. The excited emitted light produced by the excited emission leap has the same frequency, phase, propagation direction, and polarization direction as the incident light. Therefore, the excited emission light produced by many particles excited by the same coherent radiation field is coherent. Both the excited emission leap chance and the excited absorption leap chance are proportional to the monochromatic energy density of the incident radiation field. When the statistical weights of the two energy levels are equal, the odds of both processes are equal. In the thermal equilibrium case N2 < N1, the excited absorption leap is dominant, and the exciting absorption usually attenuates the light passing through the matter. The excitation of external energy can disrupt the thermal equilibrium and make N2>N1, called the particle number reversal state. In this case, the excited emission leap predominates. The light intensity increases by eGl after passing through a section of laser working material (activated material) of length l. G is a factor proportional to (N2-N1), called the gain factor, whose magnitude is also related to the nature of the laser working material and the frequency of the light wave. A segment of the activating substance is a laser amplifier. If, for example, a segment of the activated matter is placed in an optical resonant cavity formed by two mirrors parallel to each other (at least one of which is partially transmitted) (Figure 1), particles at high energy levels produce spontaneous emission in various directions. Among them, the non-axially propagating light waves quickly escape outside the resonant cavity: the axially propagating light waves can travel back and forth inside the cavity and grow in intensity as it propagates through the laser-matter. If the one-way small signal gain G0l in the resonant cavity is more significant than the one-way loss δ (G0l is the small signal gain coefficient), self-excited oscillations can be generated. The motion states of atoms can be divided into different energy levels. When atoms jump from higher to lower energy levels, photons of corresponding energy are released (so-called spontaneous radiation).
The theoretical basis of the laser originated with the physicist Albert Einstein, who in 1917 proposed an entirely new technical theory of ‘light-matter interaction.’ This theory is that in the atoms of matter, there are different numbers of particles (electrons) distributed at different energy levels; the particles in the higher energy levels, by the excitation of some photons, will jump (jump) from the higher energy levels to the lower energy levels, then will be radiated with the exact nature of the light to excite it. In some states, weak light controls the light phenomenon. This is called “light amplification by exciting radiation,” or laser for short.
In 1951, the American physicist Charles Hardt-Towns envisioned that radio waves of sufficiently small wavelengths could be obtained using molecules instead of electronic circuits. Molecules have various vibrations, some of which are the same as radiation in the microwave band range. The problem is how to convert these vibrations into radiation. Under the right conditions, the ammonia molecule vibrates 24,000,000,000 times per second (24 GHz), so it is possible to emit microwaves with a wavelength of 1.25 cm. He envisions pumping energy into the ammonia molecules by thermal or electrical means to put them in an “excited” state. Then, imagine that these excited molecules are in a microwave beam with the same intrinsic frequency as the ammonia molecules – the energy of this microwave beam can be fragile. A single ammonia molecule will be subject to the action of this microwave beam to the same wavelength in the form of a beam wave to release its energy; this energy will then act on another ammonia molecule so that it also emits energy. This feeble incident microwave beam is equivalent to the starting point of an avalanche of stimulating effects and will produce a powerful microwave beam. The energy initially excites the molecules is then transformed into a special kind of radiation.
In December 1953, Towns and his student Arthur Shallow finally built a device that worked according to the above principle and produced the desired microwave beam. This process is known as “microwave amplification of exciting radiation.” Its acronym in English is M.A.S.E.R, from which the word “maser” was coined (such words are acronyms, which are increasingly common in technical language).
In 1958, American scientists Schawlow and Townes discovered a miraculous phenomenon: when they shone the light emitted by a neon bulb on a rare-earth crystal, the crystal’s molecules emitted a bright, always-concentrated, intense light. Based on this phenomenon, they proposed the “laser principle,” which states that matter, when excited by energy at the same frequency as the inherent oscillation of its molecules, will produce this non-dispersive light, a laser. They published an important paper on this subject and were awarded the Nobel Prize in Physics in 1964.
On May 15, 1960, Meyman, a scientist at Hughes Laboratories in California, USA, announced that he had obtained a laser with a wavelength of 0.6943 microns, the first laser ever obtained by man. Thus Meyman became the first scientist in the world to introduce laser light into practical fields.
On July 7, 1960, Theodore Meyman announced the birth of the world’s first laser. Neyman’s solution was to use a high-intensity flash tube to excite a ruby. Since ruby is physically just corundum doped with chromium atoms, it emits a red light when stimulated. A hole is drilled in the surface of a ruby coated with a reflector so that the red light can spill out of this hole, thus producing a relatively concentrated column of slim red light that, when directed at a certain point, can bring it to a temperature higher than that of the surface of the sun.
The Soviet scientist Nikolai Basov invented the semiconductor laser in 1960. The structure of a semiconductor laser usually consists of a player, an n-layer, and an active layer forming a double heterojunction. It is characterized by small size, high coupling efficiency, fast response, wavelength and size adapted to the size of the fiber, direct modulation, and good coherence.
