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Common problem
Basic Working Principle of Laser
time:2019-07-05 10:58Click:
1. What is laser ?
Laser is a kind of light which does not exist in nature. It has the characteristics of good orientation, high brightness, good monochromaticity and good coherence.
 
In 1917, a group of theoretical physicists represented by the great scientist Einstein laid down one of the important disciplines of modern physics, quantum theory. Laser-LASER is the abbreviation of Light-Amplification-Stimulated Emission of Radiation in English. It means to be amplified by laser radiation. Quantum theory predicts the possibility of stimulated radiation.
 
After Einstein put forward the concept of stimulated radiation in 1917, it took 40 years. It was not until 1958 that two American microwave scientists, C.H. Townes and A.I. Schawlaw, broke the silence and published the famous paper " Infrared and Optical Laser ", pointing out the possibility of luminescence based on stimulated radiation and the necessary conditions for realizing "population inversion
". Their papers immediately excited scientists working in the field of optics, and proposed various experimental programs to achieve population inversion
, which opened up a new field of laser research.

In the same year, Soviet scientists Basov and Prokhorov published the paper "Suggestions for Three-Level Particle Inversion and Semiconductor Laser", and in September 1959, Thorns put forward the suggestion for manufacturing ruby lasers... On May 15, 1960, T. H. Maiman of Hughes Laboratory, California, made the first ruby laser in the world, and obtained a laser with a wavelength of 694.3 nm. Mayman uses Ruby as luminescent material and pulsed xenon lamp with high luminescent density as excitation source. In fact, his research began as early as 1957, and many years of efforts eventually led to the first laser beam in history. In 1964, Thomas, Basov and Prokhov shared the Nobel Prize in Physics for their contributions to laser research.
 
The first ruby laser in China was successfully developed in August 1961 at Changchun Institute of Optical Precision Machinery, Chinese Academy of Sciences. The structure of this laser has been improved much more than that of Mayman's. Especially at that time, the industrial level of our country was much lower than that of the United States. The conditions for developing this laser were very difficult. All the researchers designed and manufactured it by themselves. Since then, laser technology in China has also developed rapidly and has been widely used in various fields. In June 1987, Shenguang device, a 1000W high power pulsed laser system, was successfully developed by Shanghai Institute of Optical Precision Machinery, Chinese Academy of Sciences. It has made a great contribution to the research of laser fusion in China for many years.
 
When laser first appeared, Chinese scholars translated it into citicall according to the pronunciation of English LASER. citicall was often used in Taiwan, Hong Kong and other places in China. Later, after the proposal of the scientist Qian Xueshen, it was changed to laser and extended to the present. After more than 40 years of development, especially in the last decade, laser technology has developed rapidly. Today, there are many kinds of lasers and their applications are varied. Laser technology and products have penetrated into many fields, including our daily life.

2. The Principle of Laser Generation
 
Quantum theory holds that all matter consists of various microscopic "particles", such as molecules, atoms, protons, neutrons, electrons and so on. In the micro world, all kinds of particles have their inherent energy level structure. When a particle drops from a high level to a low level, according to the law of conservation of energy, it releases the energy of the difference between the two levels, usually in the form of light and heat.
 
The working principle of ordinary electric bulb gun is that the filament absorbs the energy of electricity, and all kinds of particles in the filament are pushed to a higher energy level by the electric energy. Because the high energy level is unstable, it has the tendency of downward energy level transition. When the particles in the high energy level transit to the low energy level, the filament will emit light and heat at the same time. The luminous emission of filaments is random. When particles at various levels transit downward, they radiate light of different wavelengths without correlation, which is called spontaneous emission.
 
However, lasers are different. Scientists carefully design the structure of the laser so that the radiation of one energy level is amplified, while the radiation of other energy levels is not amplified or suppressed. In the laser, when a particle radiates downward and emits photons. At this time, the photon will induce the other particles to radiate the same photon. This is a process of avalanche replication, so the light radiated by the laser is correlated. This process is called stimulated radiation.

2.1. Luminescence of Common Light Source: Stimulated Absorption and Spontaneous Emission
 
The illumination of ordinary common light sources (such as electric lights, flames, sun, etc.) is caused by the fact that when the material is subjected to external energy (such as light energy, electric energy, thermal energy, etc.), the electrons in the atom absorb the external energy and transition from the low energy level To the high energy level, the atom is excited. The process of excitation is a process of “stimulated absorption”. The electrons at the high energy level (E2) have a very short lifetime (generally 10-8 to 10-9 seconds). They spontaneously transition to the low energy level (E1) without external influences, and generate light (electromagnetic wave) radiation during the transition. . Radiation photon energy is
hυ=E2-E1
The processes are independent and unrelated, that is, the radiated light is irregular in all directions in the direction of emission, and the unphased and polarized states are also different. Because the excitation level has a width, the frequency of light emission is not single, but has a range.

Under normal thermal equilibrium conditions, the atomic number density N2 at high level E2 is much lower than that at low level. This is because the atomic number density N at energy level E decreases exponentially with the increase of energy level E, that is, N_exp(-E/kT), which is the famous Boltzmann distribution law. So the atomic number density ratio at the upper and lower levels is
N2/N1∝exp{-(E2-E1)/kT}
In the formula, K is Boltzmann constant and T is absolute temperature. Because E2 > E1, N2 "N1". For example, if the ground state energy of hydrogen atom is known to be E1=-13.6eV, the first excited state energy is E2=-3.4eV, and at 20 C, kT_0.025eV, then
N2/N1∝exp(-400)≈0
It can be seen that almost all hydrogen atoms are in the ground state at 20 C. In order to make the atoms emit light, it is necessary to provide energy from outside to make the atoms reach the excited state. Therefore, the general broad sense of luminescence includes two processes: stimulated absorption and spontaneous emission. Generally speaking, the energy of the light radiated by this kind of light source is not strong. In addition, the energy is dispersed when it is emitted in all directions.

2.2. Stimulated Radiation and Light Amplification
 
Quantum theory tells us that the transition of electrons from high-energy state to low-energy state can only occur between two states with l quantum number difference (±1), which is a selection rule. If the selection rules are not satisfied, the probability of transition is very small, even close to zero. There may be some energy levels in the atom. Once the electron is excited to the energy level,it will have a long life in such an energy level because it does not meet the selection rules of transition, so it is not easy to spontaneously transition to a lower energy level.This level is called metastable level. However, induced and stimulated by external light, it can rapidly transition to low level and emit photons. This process is stimulated, so it is called stimulated radiation.

2.3. Population Inversion
 
An induced photon can not only cause stimulated radiation, but also stimulated absorption. Therefore, only when the number of atoms in the high level is more than that in the low level, the stimulated radiation transition can exceed the stimulated absorption. Thus, in order to make the light source emit laser, rather than ordinary light, the key is that the number of luminous atoms in the high level is more than that in the low level, which is called particle number inversion. However, under the condition of thermal equilibrium, almost all atoms are in the lowest energy level (ground state). Therefore, how to technically realize the inversion of the number of particles is a necessary condition for the generation of lasers.
 
Laser usually consists of three parts: excitation source, working substance and  resonant cavity. By absorbing the energy of the excitation source, the working substance raises its internal "particles" to a high energy level. The role of the resonant cavity is to amplify the radiation of a pair of energy levels to achieve stimulated radiation and output laser.

3. Characteristics of Laser (Difference from Ordinary Light Source)
 
Laser is known as magical light because it has four characteristics that ordinary light does not possess.
 
3.1. Good Direction
 
Ordinary light sources (sun, incandescent lamp or fluorescent lamp) emit light in all directions, while the direction of laser emission can be limited to less than a few milliradian stereo angles (Fig. 8-9), which makes the illumination in the direction of irradiation increased by tens of millions of times. Laser collimation, guidance and ranging are based on the good directivity.
 
3.2. High brightness
 
Laser is the brightest light source in contemporary times. and only the intense flash of a hydrogen bomb can match it. The brightness of the sun is about 103 W/(centimeter 2.sphericity), and the output brightness of a high-power laser is 7~14 orders of magnitude higher than that of the sun. In this way, although the total energy of the laser is not necessarily very large, it is easy to produce high pressure and tens of thousands of degrees Celsius or even millions of degrees Celsius high temperature at a small point because of the high concentration of energy. Laser drilling, cutting, welding and laser surgery take advantage of this feature.

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