{"id":2190,"date":"2021-04-20T03:26:03","date_gmt":"2021-04-20T03:26:03","guid":{"rendered":"https:\/\/fclatbz2dc.wpdns.site\/?p=2190"},"modified":"2024-01-22T01:34:09","modified_gmt":"2024-01-22T01:34:09","slug":"the-physical-characteristics-of-laser","status":"publish","type":"post","link":"https:\/\/mydery.com\/ko\/the-physical-characteristics-of-laser\/","title":{"rendered":"\ub808\uc774\uc800\uc758 \ubb3c\ub9ac\uc801 \ud2b9\uc131"},"content":{"rendered":"\n

<\/span>Estimated reading time: <\/span>19<\/span> minutes<\/span><\/p>\n\n\n\n

Laser is a kind of optical radiation that uses the principle of stimulated radiation of atoms or molecules to excite the working substance. All photons in the same laser beam have the same frequency, the same phase, and the same polarization and propagation direction. Therefore, the laser is coherent light radiation with good monochromaticity, strong directionality, and extremely high brightness. The laser processing technology is a systematic engineering integrating light, mechanics, and electricity. It also intersects with many disciplines such as physics, materials, machinery, and automation. It is one of the frontier areas of scientific and technological development. Laser technology and equipment have developed rapidly in recent years, and have received increasing attention from countries all over the world.<\/p>\n\n\n\n

Features of laser<\/h3>\n\n\n\n
\"Laser
Figure 2.1 Laser beam<\/figcaption><\/figure>\n\n\n\n

As coherent light, the laser has many characteristics.<\/p>\n\n\n\n

Good Monochromaticity<\/h4>\n\n\n\n

The essence of light is a kind of electromagnetic wave radiation. For electromagnetic wave radiation, the longer the coherence length, the narrower the spectral line width, and the purer the color, that is, the better the monochromaticity of light. Taking the HeNe laser as an example, the laser coherence length is about 4×104<\/sup>m. Before the advent of lasers, the best monochromatic light source was a krypton lamp, which produced a coherent length of light radiation of about 0.78m. Visible excitation is the simplest light source in the world.<\/p>\n\n\n\n

High Brightness<\/h4>\n\n\n\n

High brightness is another outstanding feature of the laser. Generally, the light radiation intensity emitted by the unit light-emitting area \u0394S, the unit light radiation width \u0394\u03bd, and the divergence angle \u03b8 are defined as the monochromatic brightness B\u03bb<\/sub> of the light source.<\/p>\n\n\n\n

B\u03bb<\/sub> =P\/\u0394S\u0394\u03bd\u03b82<\/sup> (2.1)<\/p>\n\n\n\n

In the formula, P is the laser power.<\/p>\n\n\n\n

Although the total power emitted by the sun is high, the light radiation width \u0394\u03bd is very wide, the divergence angle \u03b8 is large, and the monochromatic brightness is still very small.<\/p>\n\n\n\n

Although the \u0394\u03bd and \u03b8 are small, the laser has high monochromatic brightness. It is reported that the laser monochromatic brightness B\u03bb<\/sub> produced by high-power lasers is even 100 trillion times higher than that of the sun.<\/p>\n\n\n\n

Strong Directionality<\/h4>\n\n\n\n

It can be seen from the mechanism of laser generation that under the condition of the uniform propagation medium, the divergence angle \u03b8 of the laser is only limited by the engagement;<\/p>\n\n\n\n

\u03b8=1.22\u03bb\/D (2.2)<\/p>\n\n\n\n

In the formula, \u03bb is the wavelength and D is the diameter of the light source spot.
The distance between the earth and the surface of the moon is about 3.8\u00d7105<\/sup>km. The laser beam reaches the moon with the best focus, and its spot diameter is only tens of meters.<\/p>\n\n\n\n

Good Coherence<\/h4>\n\n\n\n

The longest time interval during which light produces coherence is called the coherence time \u03c4. In the coherence time, the farthest distance the light travels is called the coherence length Lc.<\/p>\n\n\n\n

Lc<\/sub> =c\u03c4=\u03bb2<\/sup>\/\u0394\u03bb (2.3)<\/p>\n\n\n\n

In the formula, c is the speed of light.
Because the laser bandwidth \u0394\u03bb is very small, the coherence length Lc is very long. In fact, if the monochromaticity is good, the coherence is good, and the coherence length is also longer.<\/p>\n\n\n\n

Highly Concentrated Energy<\/h4>\n\n\n\n

Some military, aviation, medical, and industrial lasers can produce high laser energy. For example, the output power of lasers <\/a>for nuclear fusion can be as high as 1018<\/sup>W. It can overcome the repulsive force between nuclei and realize nuclear fusion reaction. With the development of laser ultrashort pulse technology, people can use pulse amplification technology to obtain lasers. With peak power up to 1015<\/sup>W from Ti-doped sapphire laser devices used to generate extremely short laser pulses.<\/p>\n\n\n\n

The Basic Principle of Laser Generation<\/h3>\n\n\n\n

The Interaction of Light and Matter<\/h4>\n\n\n\n

Basic Assumptions of Atomic Theory<\/h5>\n\n\n\n

Assumption of atomic stationary state All matter is composed of atoms. The atomic system is in a series of discontinuous energy states. Around the nucleus, the orbit of electrons is discontinuous, and the atom is in a stable state with constant energy. It is called the stationary state of the atom, and the state corresponding to the lowest energy of the atom is called the ground state.<\/p>\n\n\n\n

If the electron in the outer orbit of the atom obtains a certain amount of energy from the outside. The electron will jump to the outer orbital motion. The energy of the atom increases, and at this time the atom is called an atom in an excited state.<\/p>\n\n\n\n

Frequency conditions The atom transitions from one stationary state E1<\/sub> to another stationary state E2. The frequency \u03bd is determined by the following formula.<\/p>\n\n\n\n

h\u03bd= E2<\/sub> – E1<\/sub> (2.4)<\/p>\n\n\n\n

A kind of monochromatic light corresponds to a photon produced by the same transition of an atom. h\u03bd is the energy of a photon.<\/p>\n\n\n\n

The interaction between the radiation field and matter, especially the co-resonance interaction, laid the physical foundation for the advent and development of lasers. When the frequency of the incident electromagnetic wave is consistent with the resonance frequency of the medium, resonance absorption (or gain) will occur. The generation of and the interaction of light and matter will involve the cooperation of the field and the medium.<\/p>\n\n\n\n

Stimulated Absorption<\/h5>\n\n\n\n

Assuming that the two energy levels of the atom are E1 <\/sub>and E2<\/sub>, and E1<\/sub>< E2<\/sub> if there is a photon with energy satisfying formula (2.4) irradiated, the atom may absorb the energy of this photon and transition from the low-level E1<\/sub> state to the high-level The E2<\/sub> state. This kind of atomic absorption of photons and the transition from a low energy level to a high energy level is called the stimulated absorption process of the atom (Figure 2.2).<\/p>\n\n\n\n

Spontaneous Radiation<\/h5>\n\n\n\n

The state of an atom at a high energy level after being excited is unstable. Generally, it can only stay on the order of 10-8<\/sup>s. It will spontaneously return to a low energy state without external influence, and at the same time radiate energy to the outside world. For the photon with h\u03bd = E2<\/sub>-E1<\/sub>, this process is called the spontaneous emission process of the atom. Spontaneous radiation is random, the emission direction and initial phase of each photon of radiation are different, and the radiation of each atom is independent of each other, so the light of spontaneous radiation is incoherent Figure 2.1.<\/p>\n\n\n\n

\"Stimulated
Figure 2.2 Stimulated absorption, spontaneous emission, and stimulated emission<\/figcaption><\/figure>\n\n\n\n
Stimulated Emission and Optical Amplification<\/h5>\n\n\n\n

An atom at an excited state energy level, if it is excited by a photon with external energy h\u03bd and satisfying formula (2.4) before it emits spontaneous emission, it may transition from a high-energy state to a low-energy state, and at the same time emit a Photons with the same frequency, same phase, same direction, and even the same polarization state with external photons. This process is called stimulated emission of atoms [Figure 2.2]<\/p>\n\n\n\n

If an incident photon triggers stimulated emission and add one photon, these two photons continue to trigger stimulated emission and add two more photons, and then four photons multiply into eight photons\u2026 and so on, under the action of one incident photon, the atomic system may obtain a large number of photons with exactly the same state and characteristics. This phenomenon is called optical amplification. Therefore, the stimulated emission process causes the atomic system to radiate a large number of photons with the same frequency, the same phase, the same propagation direction, and the same polarization state as the incident light, that is, identical photons. Light amplification caused by stimulated radiation is an important basic concept in the mechanism of laser generation.<\/p>\n\n\n\n

Population Reversal<\/h5>\n\n\n\n

From the definition of spontaneous emission and stimulated emission, it can be seen that the spontaneous emission of the light-emitting mechanism of ordinary light sources is dominant, but the emission of lasers is mainly stimulated emission of atoms. In order to make the stimulated radiation dominate the atomic system and make it continue to emit lasers. We should try to change the distribution of the atomic system when it is in thermal equilibrium so that the number of atoms at high energy levels continues to exceed the number of atoms at low energy levels, that is, the number of particles is achieved. <\/p>\n\n\n\n

In order to achieve population inversion, energy must be input into the system from the outside. So that as many particles in the system as possible absorb energy. Then transition from a low energy level to a high energy level. This process is called the excitation or pumping process. The excitation methods generally include light excitation, gas discharge excitation, chemical excitation, and even nuclear energy excitation. For example, ruby lasers use optical excitation, helium-neon lasers use electrical excitation, and dye lasers use chemical excitation.<\/p>\n\n\n\n

Laser Production<\/a> Conditions<\/h4>\n\n\n\n

In a working substance that has achieved population inversion (such as light excitation or electrical excitation), stimulated radiation can be dominated, but the photon that first triggers stimulated radiation is generated by spontaneous radiation, and spontaneous radiation is Random. Therefore, the light amplification achieved by stimulated radiation is also random and disordered on the whole. It requires a series of devices to be added.<\/p>\n\n\n\n

Optical Cavity<\/h5>\n\n\n\n

Two mirrors that are parallel to each other are installed at the two ends of the working material. An optical resonant cavity is formed between the two mirrors. One of which is a total reflection mirror and the other is a partial reflection mirror.<\/p>\n\n\n\n

Among the photons emitted in all directions, except the photons propagating along the axial direction. They all leave the optical resonant cavity quickly, and only the light along the axial direction is continuously amplified, forming oscillations in the cavity back and forth. Therefore, in the laser tube, the step-adjusted light is continuously amplified to form light with greater amplitude. In this way, the light is reflected back and forth between the mirrors that are parallel to each other at both ends of the tube. Then the fully amplified light passes through a partial mirror to emit monochromatic light with the same phase.<\/p>\n\n\n\n

Threshold Condition of Light Oscillation<\/h5>\n\n\n\n

From the energy point of view, although the light oscillation increases the light intensity, the absorption, deflection, and projection of the light on the two end faces and the medium at the same time will weaken the light intensity. Only when the gain is greater than the loss, can the laser be output. It requires the working substance and the resonant cavity to meet the condition of “gain greater than the loss”, It also called the threshold condition.<\/p>\n\n\n\n

Frequency Conditions<\/h5>\n\n\n\n

The role of the optical resonant cavity not only increases the effective length L of light propagation. But also forms a light standing wave between the two mirrors. In fact, the only light that satisfies the standing wave conditions can be amplified by stimulated radiation.<\/p>\n\n\n\n

From L=k\u03bbn<\/sub>\/2(k=1,2,3\u2026), \u03bbn<\/sub>=c\/n\u03bd, we have<\/p>\n\n\n\n

\u03bd=kc\/2nL or \u0394\u03bd=c\/2nL (2.5)<\/p>\n\n\n\n

In the formula, n is an integer and c is the speed of light.
The frequency \u03bd generated by stimulated radiation in the laser tube can be obtained from equation (2.4)<\/p>\n\n\n\n

\u03bd=( E2<\/sub> – E1<\/sub> )\/h (2.6)<\/p>\n\n\n\n

In the formula, h is Planck’s constant.<\/p>\n\n\n\n

To make the frequency meet the formula (2.5) and formula (2.6). The cavity length of the resonant cavity needs to be adjusted. In summary, the basic conditions for forming a laser are as follows.<\/p>\n\n\n\n