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Popularization of laser technology knowledge
Shenke Optoelectronics / 2012-11-01

   是20世纪以来,继原子能、计算机、半导体之后,人类的又一重大发明,被称为“最快的刀”、“最准的尺”、“最亮的光”和“奇异的激光”。 Laser is another major invention of mankind since atomic energy, computers, and semiconductors since the 20th century. It is called the "fastest knife", "the most accurate ruler", "the brightest light" and "strange laser light". . Its brightness is about 10 billion times that of sunlight.

The principle of laser was discovered by the famous American physicist Einstein as early as 1916, but it was not until 1960 that the laser was successfully manufactured for the first time. Laser was born under the background of the urgent need of theoretical preparation and production practice. As soon as it came out, it achieved an extraordinary rapid development. The development of laser not only gave birth to the ancient optical science and optical technology, but also caused new life. The emergence of an entire emerging industry. Lasers enable people to effectively use unprecedented advanced methods and means to obtain unprecedented benefits and results, thereby promoting the development of productivity.

The theoretical basis of laser light originated from the great physicist 'Einstein'. In 1917, Einstein proposed a new set of technical theories 'light and matter interaction'. This theory is that among the atoms that make up matter, different numbers of particles (electrons) are distributed at different energy levels. Particles at high energy levels are excited by certain photons and will jump from high energy levels to (transitions). At a low energy level, light with the same properties as the light that excites it will be radiated at this time, and in a certain state, a phenomenon that a weak light excites a strong light can occur. This is called "light amplification of stimulated radiation", or laser for short.

In 1958, American scientists Schawlow and Townes discovered a magical phenomenon: When they shined light from a neon light bulb on a rare earth crystal, the molecules of the crystal emit bright, Strong light that always gathers together. Based on this phenomenon, they proposed the "laser principle", that is, when a substance is excited by the same energy as its molecule's natural oscillation frequency, it will produce such a non-divergent strong light-laser. They published important papers for this purpose and won the Nobel Prize in Physics in 1964.

On May 15, 1960, Mayerman, a scientist at the Hughes Laboratory in California, USA announced that he had obtained a laser with a wavelength of 0.6943 microns, which was the first laser light ever obtained by human beings. As a result, Mayman became the first Scientists who introduced lasers into practical fields.

由诞生,梅曼的方案是,利用一个高强闪光灯管,来激发红宝石。 On July 7, 1960, Mayman announced the birth of the first laser in the world. Mayman's plan was to use a high-intensity flash tube to excite the ruby. Because ruby is actually just a type of corundum doped with chromium atoms, when ruby is stimulated, it emits a red light. A hole is drilled on the surface of a ruby plated with a reflector, so that red light can overflow from this hole, resulting in a rather concentrated slender red beam of light. When it hits a certain point, it can make it more than the surface of the sun. High temperature.

Former Soviet scientist Nicholas Basov invented the semiconductor laser in 1960. The structure of a semiconductor laser is usually composed of a p-layer, an n-layer, and an active layer forming a double heterojunction. Its characteristics are: small size, high p-coupling efficiency, fast response speed, wavelength and size adapt to the fiber size, direct modulation, and good coherence.

First, the principle of laser generation

To learn the principle of laser we need to understand these concepts

1.Energy level

Matter is composed of atoms, and atoms are composed of nuclei and electrons. The electrons move around the nucleus. The energy of an electron in an atom is not arbitrary. The quantum mechanics that describe the micro world tells us that these electrons will be at some fixed "energy levels". Different energy levels correspond to different electron energies, and the orbital energy that is farther away from the nucleus is higher. In addition, the maximum number of electrons that can be accommodated in different orbits is different. For example, the lowest orbit (which is also the closest atomic nucleus) can only accommodate up to 2 electrons, and the higher orbit can accommodate 8 electrons.

2. jump

Electrons can transition from one energy level to another by absorbing or releasing energy. For example, when an electron absorbs a photon, it may transition from a lower energy level to a higher energy level. Similarly, an electron at a high energy level will jump to a lower energy level by emitting a photon. In these processes, the photon energy released or absorbed by the electron is always equal to the energy difference between the two energy levels. Since the photon energy determines the wavelength of light, the absorbed or released light has a fixed color.

3.Spontaneous radiation

Refers to the spontaneous migration of high-level electrons to low-level energy without external effects, and the light (electromagnetic wave) radiation is generated during the transition. The radiant photon energy is hv = E2-E1, which is the energy difference between the two energy levels. The characteristic of this radiation is that each electron transition is spontaneous and independent, and the process has no external influence and has no relationship with each other. So the states of the photons they emit are different. Such optical coherence is poor and the directions are scattered.

4, stimulated absorption

Stimulated absorption means that atoms in a low energy state absorb external radiation and transition to a high energy state. Electrons can transition from low to high energy levels by absorbing photons. The luminescence of common light sources (such as the light of electric lamps, flames, and the sun) is caused by the transition of low energy levels to the absorption of external energy by electrons in atoms when substances are exposed to external energy (such as light energy, electrical energy, and thermal energy) High energy levels, where atoms are excited. The process of excitation is a "stimulated absorption" process.

5, stimulated radiation

Stimulated radiation refers to electrons at high energy levels that are "stimulated" or "induced" by photons, transition to low energy levels, and radiate a photon with the same frequency as the incident photon. The biggest feature of stimulated radiation is that the photons produced by stimulated radiation have exactly the same state as the original photons that caused the stimulated radiation. They have the same frequency and the same direction, and the difference between the two cannot be distinguished at all. In this way, with a single stimulated radiation, one photon becomes two identical photons. This means that the light is strengthened, or that the light is amplified. This is the basic process of laser generation.

Photons incident on matter induce electrons to transition from high energy levels to low energy levels and release photons. The incident photon has the same wavelength and phase as the released photon, and this wavelength corresponds to the energy difference between the two energy levels. A photon induces an atom to emit a photon, which eventually becomes two identical photons.

6.The relationship between stimulated absorption and stimulated radiation

So, after the atoms absorb foreign photons, do they appear as stimulated absorption or stimulated radiation?

In an atomic system, some atoms are always at high energy levels, and some are at low energy levels. The photons generated by spontaneous radiation can either stimulate high-level atoms to generate stimulated radiation, or they can be absorbed by low-level atoms to cause stimulated absorption. Therefore, in the interaction of light and atomic systems, spontaneous radiation, stimulated radiation and stimulated absorption always coexist.

If you want to get more and more intense light, that is, to generate more and more photons, you must make more photons from stimulated radiation than photons absorbed by stimulated absorption. How can this be done? We know that photons treat all high and low level atoms equally. Under the action of photons, the opportunity for high-level atoms to generate stimulated radiation is the same as the opportunity for low-level atoms to generate stimulated absorption. In this way, whether the amplification of light can be obtained depends on the ratio of the number of atoms at high and low levels.

If there are far more atoms in the high energy state than atoms in the low energy state, we get highly amplified light. However, in atomic systems that are usually thermally balanced, the distribution of atomic numbers according to energy levels obeys the Boltzmann distribution law. Therefore, the number of atoms at high energy levels is always less than the number of atoms at low energy levels. In this case, in order to obtain the amplification of light, we must look for it in a non-thermal equilibrium system.

7, the number of particles is reversed

An induced photon can not only cause stimulated radiation, but it can also cause stimulated absorption, so only when the number of atoms at a high energy level is more than that at a low energy level, can the stimulated radiation exceed the stimulated absorption, and dominate . It can be seen that the key to causing a light source to emit laser light instead of ordinary light is that the number of light-emitting atoms is higher at higher energy levels than at lower energy levels. This situation is called particle inversion. However, under thermal equilibrium conditions, the atoms are almost at the lowest energy level (ground state).

Therefore, how to achieve the inversion of the number of particles technically is a necessary condition for generating a laser. So how can we achieve the state of particle number inversion? This requires the use of activation media. The so-called activated medium (also called amplifying medium or amplifying medium) is a substance that can reverse the number of particles between two energy levels. It can be a gas or a solid or liquid. It is impossible to use a two-level system as the activation medium to achieve particle number inversion. To get the particle number inversion, a multilevel system must be used.

8, Boltzmann distribution

Under normal thermal equilibrium conditions, the atomic number density N2 at high energy level E2 is much lower than the atomic number density at low energy level. This is because the atomic number density N at energy level E increases with the increase of energy level E The exponential decrease is N∝exp (-E / kT), which is the well-known Boltzmann distribution law.

Therefore, the atomic number density ratio at the upper and lower energy levels is: N2 / N1∝exp {-(E2-E1) / kT} where k is the Boltzmann constant and T is the absolute temperature. Since E2> E1, N2 <N1. For example, it is known that the ground-state energy of a hydrogen atom is E1 = -13.6eV, and the energy of the first excited state is E2 = -3.4eV. At 20 ° C, kT≈0.025eV, then N2 / N1∝exp (-400) ≈0

It can be seen that at 20 ° C, almost all hydrogen atoms are in the ground state. To make the atoms emit light, the outside must provide energy to make the atoms reach the excited state. Therefore, the general broad-based light emission includes two processes: stimulated absorption and spontaneous radiation. Generally speaking, the energy of the light radiated by such a light source is not strong, and the energy is scattered in all directions.

Second, the laser generation process

Taking a ruby laser as an example, the atom first absorbs externally injected energy and transitions to an excited state (E3). The atom is in the excited state for a very short time, after about 10-7 seconds, it will fall into an intermediate state called metastable state (E2). Atoms spend a long time in the metastable state, about 10-3 seconds or longer. Atoms stay in the metastable state for a long time, resulting in more atoms in the metastable state than in the ground state. The state at this time is called the number of particles inversion. As a result, there are more atoms that are metastable back to the ground state (E1) by stimulated radiation than atoms that transition from the ground state to the metastable state by stimulated absorption, thereby ensuring that the number of photons in the medium can increase, thereby Form a laser. This is a typical laser three-level system.

When the particles are excited by external energy from E1 to E3, the energy level of E3 is short, and it is quickly transferred to E2. Because energy level E2 is metastable, the number of particles is reversed between E2 and E1. Since the lower energy level E1 is the ground state, a large number of particles are always accumulated. Therefore, in order to achieve the inversion of the number of particles, more than half of the ground state particles must be excited to E2. Therefore, the external excitation requires a strong ability.

The YAG laser system we use is a four-level system. As shown, energy level E1 is the ground state, and E2, E3, and E4 are the excited states. Under the condition of external excitation, a large number of particles in the ground state E1 are excited to E4, and then quickly transferred to E3. The energy level of E3 is metastable, and the lifetime is longer. However, the E2 energy level has a short life span, and the particles on E2 quickly transition to the ground state E1. Therefore, in a four-level system, the number of particles is reversed between E3 and E2.

In other words, the lower energy level of the laser capable of inverting the number of particles is E2, unlike the three-level system, which is the ground state E1. Because E2 is not the ground state, the number of particles at the E2 level is very small at room temperature. Therefore, particle number inversion is easier to achieve in a four-level system than in a three-level system. Among the common lasers, in addition to Nd3: YAG lasers, helium-neon lasers and carbon dioxide lasers are also four-level system lasers. It should be noted that the three-level system and the four-level system discussed above are both directly related to the energy levels during the operation of the laser, not that a certain substance has only three energy levels or four energy levels.

Third, the structure of the laser

Three laser elements: working medium, excitation source, resonant cavity

1.Laser working medium

The generation of laser light must choose a suitable working medium, which can be gas, liquid, solid or semiconductor. The key is to be able to invert the number of particles in this medium to obtain the necessary conditions for generating a laser. Obviously, the existence of metastable energy levels is very beneficial to achieve the number of particles inversion.

2.Incentive source

In order to invert the number of particles in the working medium, a certain method must be used to stimulate the atomic system to increase the number of particles in the upper energy level. Generally, gas discharge can be used to excite the atoms of the medium with kinetic energy, which is called electrical excitation; pulsed light sources can also be used to irradiate the working medium, which is called light excitation; thermal excitation, chemical excitation, and so on. The various incentives are visually referred to as pumping or pumping. In order to continuously obtain the laser output, it must be continuously "pumped" to maintain that the number of particles at the upper level is higher than the lower level.

3.Resonant cavity

With the proper working substance and excitation source, the number of particles can be reversed, but the intensity of the stimulated radiation generated in this way is weak and cannot be practically applied. It is also necessary to amplify the radiated light, so people have thought of using an optical resonant cavity for amplification. The so-called optical resonant cavity is actually mounted on the two ends of the laser with two highly reflective lenses in parallel, one is a total reflection lens, and the other is a partially reflective lens with a small amount of transmission. The function of the total reflection lens is to reflect all the incident light back along the original path. The function of the partial reflection lens is to reflect the part of the photons whose energy has not reached a certain limit back to the original path, and the photons reaching a certain energy limit are transmitted out. In this way, the part of the photon transmitted out becomes what we need, the amplified laser; and the light reflected back to the working medium will continue to induce a new round of stimulated radiation, and the light will be gradually amplified. Therefore, the light oscillates back and forth in the resonant cavity, causing a chain reaction, amplifying like avalanche, and generating intense laser light until the energy reaches a certain limit and is output from the partially reflecting lens.

Laser types:

There are different classification methods for lasers, which are generally classified according to different working media. They can be divided into solid lasers, gas lasers, liquid lasers, and semiconductor lasers. In addition, according to the different laser output methods, it can be divided into continuous lasers and pulsed lasers. The peak power of pulsed lasers can be very large, and they can also be classified according to the frequency of light emission and the size of light emission power.

1.Solid laser

Generally speaking, solid-state lasers are characterized by small components, ruggedness, convenient use, and high output power. The working medium of this laser is uniformly doped with a small amount of activated ions in the crystal or glass as the matrix material. In addition to the ruby and glass described earlier, yttrium aluminum garnet (YAG) crystals are commonly used to incorporate trivalent neodymium Ion laser, which emits near-infrared laser light at 1060nm. The solid-state laser generally has a continuous power of more than 100W and a peak pulse power of 109W.

2.Gas laser

The gas laser has the advantages of simple structure and low cost, convenient operation, uniform working medium and good beam quality, and the ability to work continuously and stably for a long time. This is also a type of laser with the most variety and wide application at present, occupying about 60% of the market. Among them, the helium-neon laser is the most commonly used one.

3.Semiconductor laser

Semiconductor lasers use semiconductor materials as the working medium. At present, the more mature is a gallium arsenide laser, which emits a 840nm laser. There are also lasers such as aluminum doped gallium arsenide, chromium sulfide and zinc sulfide. Excitation methods include optical pumping and electrical excitation. This kind of laser is small in size, light in weight, long in life, simple in structure and sturdy, and is particularly suitable for use in aircraft, vehicles, and spacecraft. In the late 1970s, the development of semiconductor lasers was greatly promoted by the development of optical fiber communication and optical disc technology.

4.Liquid laser

Commonly used are dye lasers, which use organic dyes as the working medium. In most cases, organic dyes are used in solvents (ethanol, acetone, water, etc.), and they also work in the vapor state. Use different dyes to get different wavelength lasers (in the visible range). The dye laser generally uses a laser as a pump source, for example, an argon ion laser is commonly used. The working principle of liquid lasers is more complicated. The output wavelength is continuously adjustable, and the wide coverage is its advantage, which makes it also widely used.

4. Detailed explanation of various lasers

According to the laser-excited material classification, the main classification methods are:

1.Solid state laser

2.Liquid laser

3.Gaseous laser

4.Chemical laser

5.Semiconductor laser

6.Color-center laser

7.Free electron laser

8.Frequency double laser

1.Solid state laser

The laser medium of this laser is an insulating solid doped with impurities, including crystal laser, glass laser and fiber laser. Crystals, glass, and optical fibers are host materials. The "impurity" ions that replace some host ions are the active media for lasers. For example, the host of ruby is alumina trioxide (Sapphire; often translated as "sapphire"), and the active medium is chromium [chrome read as "each"] ion (Cr3 +). In the following, a symbol such as "Cr3 +: Al2O3" will be used to indicate the active medium and host, and a symbol in the form of "chrome-alumina laser" will indicate the laser name.

An active medium that can be incorporated into several crystals, glass, and optical fibers. Due to the difference in the electron distribution and symmetry in each host, the energy level structure and energy level difference of the same ion in them are different. The wavelength of the laser light generated may be similar, but not exactly the same. For example, lasers of neodymium [pronounced "female"] ions (Nd3 +) have wavelengths close to 1 μm but not equal. Other differences in using different materials as hosts are gain, heat dissipation, available crystal length, and so on. These factors will affect the power that the laser can reach, and therefore the product specifications and price.

a.Ruby laser

A ruby laser can produce 694.3 nm and 692.8 nm laser light, but the latter has a lower gain and generally takes its output of 694.3 nm.

b.Laser of neodymium ion (Nd3 +)

Of the neodymium ion lasers, neodymium-yak (Nd-YAG) lasers are the most well-known. Jacques is a transliteration of YAG, which stands for Yttrium aluminum garnet (Y3Al5O12). The active medium is a neodymium ion that replaces about 1% yttrium ion (Y3 +) in YAG crystals. (Related wavelength of neodymium ion)

c. Titanium-sapphire laser and other variable-frequency solid-state lasers

The active medium of a titanium-sapphire laser is Ti3 + ions doped in Al2O3 crystals, which replaces aluminum ions. It has two characteristics: (1) the output laser light frequency can be adjusted between 660 and 1,180 nm; (2) it can generate ultra-short pulse waves shorter than 100 fs. (Tunable wavelength solid-state laser)

d. Other solid-state lasers

Other solid-state lasers

Liquid laser

Liquid lasers use dyes dissolved in solvents as the active medium, and are commonly referred to as dye lasers. Dye molecules have a complex structure containing multiple benzene rings. Among their energy levels, corresponding to each electronic energy level, there are many finely spaced vibrational energy levels, which are distributed in a band shape, so that frequencies within a range can all transition and generate laser light.

On the other hand, the energy level structure of the dye also enables it to absorb excitation light with a wide range of frequencies, most of which are in the ultraviolet and visible light bands. Irradiating a dye with different excitation wavelengths will also produce different laser light wavelengths. (Rose Red 6G Laser Light Wavelength Data)

A device that can tune the wavelength of laser light can generate a variety of required laser light, so it has a wide range of uses. However, solid lasers have many advantages over dye lasers in terms of volume, power consumption, voltage requirements, cooling requirements, safety, and stability. These advantages enable solid-state lasers to take advantage of miniaturization and portability. Since the applicable wavelength range of solid frequency-converted lasers is mostly in the infrared region, although high-quality beams with short wavelengths can be generated by amplifiers and non-linear frequency conversion devices, many wavelength ranges are still unattainable, so dye lasers are still available at present Existence and use of space.

3.Gaseous laser

Among the lasers using gas as the active medium are neutral atom lasers, ion gas lasers, metal vapor lasers, molecular gas lasers, and excimer lasers. They are introduced as follows:

(1) Neutral atom laser and ion gas laser for rare gas elements

a, neutral atom laser

The difference between a neutral atom laser and an ion gas laser is that the laser light of the former comes from the transition between the neutral atom energy level (as shown in Figure 2), and the latter is the energy level of ions (such as Ar +, Kr +). Therefore, the argon laser is different from the argon ion laser. The wavelength of the laser light in the former belongs to the infrared band, while the latter is in the visible and ultraviolet regions. However, argon lasers are rare on the one hand, and argon ion lasers are often referred to simply as argon lasers for the sake of convenience. Not only the Chinese description, but the English literature also sees this usage, so when it is not confusing, the argon laser can still be called the argon ion laser.

Of the neutral atom lasers, the most common is a helium-neon laser. Its red light is especially familiar to everyone. Its light color is remarkable, so it is often used as a guiding beam in a non-visible laser. Its excellent coherence and convenient operating conditions (only air-cooled, 110V voltage, and relatively low price) make it widely used for scanning code reading devices and holographic images.

b.Ion laser

Among the rare gas ion lasers, argon ion laser and krypton ion laser are the most common. In addition to direct use, argon ion lasers often excite dye lasers with their ultraviolet and blue-green light. The 20W model can produce some laser light from 275.4 to 1090.0 nm. The main visible light wavelengths are 514.5, 501.7, 496.5, 488.0, 476.5, 472.7, 465.8, 457.9, 454.5 nm. Among the blue-green light bands, the strongest and most commonly used are 514.5 and 488 nm.

Krypton ion lasers can produce 337.4 to 799.3 nm, the strongest being 647.1 nm, followed by 413.1 and 530.9 nm. There are models that mix argon and krypton.

(2) Neutral atom laser and ion laser of metal vapor

a, neutral atom laser

The vapors of Au, Cu, Ba, Sn, Pb, Zn and other metals are all active media for neutral atom lasers. In their steam, low-pressure inert gas is often mixed to improve the discharge efficiency. Copper vapor laser products can produce more than 100W of green light (511 nm) and yellow light (578 nm), and gold vapor laser can get tens of watts of red light (628 nm).

These two lasers have many uses. For example, the peak of the absorption spectrum of a hemoporphyrin derivative is about 628 nm, and cancer cells can absorb this substance. This substance decomposes when exposed to laser light at 628 nm, producing a substance that can kill cancer cells. However, a copper vapor laser excites a dye laser and can get this light without having to rely on a gold vapor laser. In addition, the 578 nm laser can remove some birthmarks, which is better than using an argon ion laser.

b.Ion laser

Among metal vapor ion lasers, helium-cadmium (He-Cd) lasers are the most important, and helium-selenium (He-Se), helium-zinc (He-Zn) lasers are members of this family. He-Cd lasers with 325 nm UV and 441.6 nm blue light are the most common outputs. When combined with a special design, it can simultaneously produce red light (635.6 and 636.0 nm) and green light (533.7 and 537.8 nm). Its short wavelength component is useful in information processing. Properly adjusting the output of each wavelength can produce almost all the colors of visible light, so its white laser products are also famous. The increase of storage density and identification ability makes it have many applications in measurement, inspection, recording, printing and so on.

(3) molecular gas

Carbon dioxide laser and nitrogen laser are the most common molecular gas lasers. The main laser light is infrared (10,640 nm) and ultraviolet (337 nm). The moisture in biological tissue will absorb its 10,640 nm laser light, so it can be used for surgery, and the laser light power required is about 50W. In addition, the processing of non-metallic materials, heat treatment of metal surfaces, spectroscopy and photochemical research, environmental telemetry, ranging, excitation of other lasers, generation of plasma (commonly known as plasma; Plasma), etc. can also be performed with carbon dioxide lasers.

The ultraviolet laser light of the nitrogen laser is suitable for exciting the dye laser and making a variety of substances generate fluorescence, and can be used for inspection and research work. The disadvantage is that the efficiency and power are low, the energy of each pulse is only about 10mJ, and the average power is about several hundred mW.

(4) Excimer laser

The original meaning of the term excimer is "a molecule composed of two atoms of the same type, which exists only in the excited state", such as the rare gas molecule He2, Ar2, Xe2, etc .; its original English name was a term synthesized by Excited Dimer. Its scope has now been broadened to include "any diatomic molecule (sometimes including triatomic molecules) that does not exist in the ground state but only appears in an excited state." Important excimer lasers use halides of rare gases as active media, such as ArF, KrF, XeF, KrCl, XeCl, etc. Because the excited state is often indicated by an asterisk (*) superscript, some data are written as ArF *, etc.

Excimers do not occur naturally, but are formed when they are discharged in a gas mixture. In addition, excimers can also be caused by impingement with an electron beam or microwave excitation in a waveguide type device. Its laser light comes from the electronic transition of excimer dissociation into atoms, so its laser light is ultraviolet, and is used in fine etching (such as circuit manufacturing), chemical vapor deposition, chemical reaction research, and medical applications. . Some of these applications are performed after exciter lasers excite variable frequency lasers.

Commodities are mostly ArF, KrF, XeCl, XeF and other excimers. The laser light frequencies are 193, 249, 308, and 350 nm, respectively.

4.Chemical laser

A laser that causes a reversed population by a chemical reaction is called a chemical laser.

In the fields of chemistry, military, materials research and biomedicine, chemical lasers have their strengths. For example, the beam of a hydrogen fluoride laser may be required for orthopedic surgery. The reaction in a hydrogen fluoride laser can be expressed as 2F2 + H2 → 2HF * + F2. In fact, its detailed reaction is a chain reaction: F + H2 → HF * + H, F2 + H → HF * + F, and the reaction can be discharged to discharge. start up. Another example is C2N2 + O2 → 2CO + N2 + 127 kcal. DF, HCl, HBr, etc. have similar effects.

Chemical laser wavelength

5.Semiconductor laser

A semiconductor laser is made of a semiconductor, and its structure and electrical properties are diodes, that is, it has two external circuit terminals, p-type and n-type portions located therein, with a junction in between. Therefore, the semiconductor laser is also referred to as a semiconductor diode laser or a diode laser.

When the current is low, it becomes a light emitting diode (LED), which emits self-emitting light; when the current is large enough, the population of free electrons can be reversed. On the other hand, the appropriate steps in the manufacturing process make the diode have a relatively high reflectance at both ends, and the conditions required for the laser are available.

The development of semiconductor laser technology has made semiconductor lasers a very efficient laser, but heat dissipation is still an important issue. In addition, the development of end-fire and surface-fire laser arrays has greatly improved the energy and control of the beams generated by the system. As the types of semiconductors expand, the wavelengths that semiconductor lasers can generate also continue to increase. The following table lists the wavelength data of several semiconductor lasers operating at room temperature.

Examples of semiconductor lasers:

Semiconductor lasers have a small dielectric volume (typical size is about 10 μm × 300 μm × 500 μm), high efficiency, high power, low operating current and voltage, and low energy consumption, so they are welcome to use. Exciting other solid-state lasers with many semiconductor lasers is a valuable application.

6.Color-center laser

Some impurities in the halide crystals of alkali metals will show color after being irradiated with appropriate radiation. Examples of the halide are KCl, RbCl, LiF, KF, and the like, and impurities are Li, Na, and the like. These impurities are called "color centers."

7.Free electron laser

Free electron laser (FEL) uses high-speed free electrons in an extremely high vacuum as the active medium. Its wavelength can be adjusted from microwave to soft X-ray. So a device can perform multiple functions. One of the current research directions is to develop desktop models to expand applications and reduce prices.

8.Frequency double laser

Frequency doubling lasers are actually lasers with frequency doubling crystals, but some data call them crystals that cause frequency doubling effects, such as KDP (Potassium dihydrogen phosphate) lasers and KTP (Potassium titanyl phosphate) lasers. Therefore, this article lists this item to draw the reader's attention to the fact that KDP and KTP are not active media for lasers, and introduces such crystals slightly. Frequency doubling is one of the functions of Nonlinear optical crystal. When a crystal is selected, the applicable frequency band and conversion efficiency for generating an octave wave are important indicators. Other items to note include whether it will deliquesce, the light intensity that will cause damage, the crystal cutting direction, and the coating condition.

KDP is potassium dihydrogen phosphate (KH2PO4), which is the earliest frequency-doubling crystal used. The conversion efficiency is very high under certain conditions, but it will deliquesce, so you must take precautions. KTP is potassium titanyl phosphate (KTiOPO4). Its performance is similar to KDP, but it does not deliquesce. Beta barium borate (β-BaB2O4; BBO) is a non-linear optical crystal that has received much attention. It has some excellent properties, but it is the same as other crystals and is not suitable for all occasions. For example, some crystals are suitable for generating double frequency, but not for triple frequency.

Five, the characteristics of laser

1.Good coherence

2.Strong Directionality

3.Good monochrome

1.Good coherence

A tens of watt light bulb can only be used for general lighting. If its energy is concentrated in a 1m diameter ball, a very high optical power density can be obtained, and the steel plate can be penetrated with this energy. However, the light from ordinary light sources is emitted in all directions, and the light energy cannot be highly concentrated. Light emitted from different points on an ordinary light source is chaotic in different directions and at different times, and it is impossible to condense on one point after passing through the lens.

Lasers are very different from ordinary light. Because its frequency is very simple, the light emitted from the laser can propagate in the same direction in a consistent manner. You can use a lens to focus them to a point and concentrate the energy highly. This is called high coherence. The brightness of a giant pulsed ruby laser can reach 1015w / cm2? Sr, which is several times higher than the brightness of the sun's surface. After the laser beam with high brightness is focused by the lens, it can generate a high temperature of thousands of degrees or even tens of thousands of degrees near the focal point, which makes it possible to process almost all materials.

2.Strong Directionality

The directivity of the laser is much better than all other light sources today, it is almost a bunch of parallel lines. If the laser is emitted to the moon, after a distance of 384,000 kilometers, there will be only a light spot with a diameter of about 2km.

3. Good monochromaticity:

Stimulated radiation (laser) is the light released by atoms when stimulated radiation occurs. Its frequency composition range is very narrow. In simple terms, the monochromaticity of stimulated radiation is very good, and the "color" of the laser is very pure. (Different colors are actually different frequencies). 的重要因素。 The monochromaticity of the laser is an important factor in achieving laser processing . We can illustrate this problem through simple physical experiments.

We use a triangular prism to decompose a beam of sunlight into seven-color spectral bands. The principle is that daylight is actually a multi-color light that is mixed with light of multiple wavelengths. When light of different wavelengths passes through the same medium, due to the refractive index in the medium The difference in the propagation direction of the light of different colors causes different degrees of deflection, so when they leave the prism, they are dispersed separately to form a spectral band.

Principle of a typical lamp-pumped YAG laser

In a cavity with an elliptical cross-section, a laser rod and a chirped lamp are placed at the two focal points, and a certain wavelength of light is emitted at one focal point (chrysanthemum lamp). After reflection by the inner wall of the reflective cavity, it converges in the other cavity. At a focal point (laser rod), the particles in the working substance are excited. After the particles are stimulated and absorbed, the atoms in the low energy state undergo energy level transitions due to absorption of external radiation, and then the laser is released, and the generated laser is totally reflected The lens and the partially reflecting lens oscillate back and forth. When the energy reaches a certain value, it can pass through the partially reflecting lens, which realizes the laser output.

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