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Laser and laser technology

(I) what is laser and laser technology

laser is a kind of light with good directivity, high brightness, good monochromaticity and good coherence that does not exist in nature and is emitted due to excitation. Physicists trace the mechanism of laser production to the hypothesis put forward by Einstein when he explained the blackbody radiation law in 1917, that is, the absorption and emission of light can pass through three basic processes: stimulated absorption, stimulated emission and spontaneous emission. As we all know, the luminescence of any kind of light source is related to the motion state of particles inside the material. When a particle (atom, molecule or ion) at a low energy level absorbs external energy (light) of an appropriate frequency and is excited to transition to the corresponding high energy level (stimulated absorption), it always tries to transition to a lower energy level and release the excess energy in the form of photons. If light is spontaneously released without the action of external photons (spontaneous emission), the light released at this time is ordinary light (such as electric lights, neon lights, etc.), which is characterized by the fact that the frequency, direction and pace of light are very inconsistent. However, if the excess energy is released in the form of photons (stimulated radiation) during the transition from high-energy level to low-energy level under the direct action of external photons, the released photons are completely consistent with the external incident photons in terms of frequency, phase, propagation direction, etc., which means that the external light has been strengthened, which is called light amplification. Obviously, if the number of particles at the high energy level is more than that at the low energy level through stimulated absorption (particle number inversion), the amplification of this light will be more obvious, and then it is possible to form a laser

laser is known as magical light because it has four characteristics that ordinary light does not have at all

1. Good directivity: ordinary light sources (sun, incandescent lamp or fluorescent lamp) emit light in all directions, while the light direction of the laser can be limited to less than a few milliarc solid angles, which increases the illumination in the direction of illumination by ten million times. Laser collimation, guidance and ranging make use of the good directivity

2. High brightness: laser is the brightest light source in modern times. Only the strong flash at the moment of hydrogen bomb explosion can be compared with it. The brightness of sunlight is about 103 watts/(cm2. Sphericity), and the output brightness of a high-power laser is 7 ~ 14 orders of magnitude higher than that of sunlight. In this way, although the total energy of the laser is not necessarily large, due to the high concentration of energy, it is easy to produce high pressure and tens of thousands of degrees Celsius or even millions of degrees Celsius high temperature at a tiny point. Laser drilling, cutting, welding and laser surgery make use of this feature

3. Good monochromaticity: light is an electromagnetic wave. The color of light depends on its wavelength. The light emitted by ordinary light sources usually contains various wavelengths, which is a mixture of various colors of light. Sunlight includes visible light in seven colors: red, yellow, green, blue, blue and purple, and invisible light such as infrared light and ultraviolet light. The wavelength of a certain laser is only concentrated in a very narrow spectral band or frequency range. For example, the wavelength of He Ne laser is 632.8 nm, and its wavelength variation range is less than 1/10000 nm. Due to the good monochromaticity of laser, it provides a very favorable means for precision instrument to measure and stimulate some chemical reactions and other scientific experiments

4。 Good coherence: interference is an attribute of wave phenomenon. Because laser has the characteristics of high directivity and high monochromaticity, it must have excellent coherence. This characteristic of laser makes holography a reality

the so-called laser technology is the general term for exploring and developing various methods of generating laser and exploring and applying these characteristics of laser for the benefit of mankind. Since the United States successfully developed the world's first ruby laser in 1960, and China also successfully developed the first domestic ruby laser in 1961, laser technology is considered to be another major new scientific and technological achievement in the 20th century after quantum physics, radio technology, atomic energy technology, semiconductor technology and electronic computer technology. Over the past 30 years, laser technology has been developed by leaps and bounds. It has not only developed a variety of lasers with various characteristics, but also expanded the field of laser applications, and formed a series of emerging industries, such as laser disc player, laser medical treatment, laser processing, laser holography, laser and axial cloth, C-cloth and solid foam sandwich composite overlay Phototypesetting printing, laser printing, and laser weapons. The rapid development of laser technology makes it one of the "leading technologies" in today's new technological revolution

(II) all kinds of lasers - in the light source, the realization of energy level particle number inversion is the premise of optical amplification, that is, the prerequisite for laser generation. To realize the inversion of particle number, we need to use the power of external light to make a large number of particles that were originally at the low energy level jump to the high energy level. This process is called "excitation"

what we usually call a laser is a device that excites the particles in the light source to produce stimulated radiation transition, realizes the inversion of particle number, and then generates light amplification through stimulated radiation. Although there are many kinds of lasers, their mission is to obtain lasers through excitation and stimulated radiation. Therefore, the basic composition is usually composed of three parts: the activation medium (that is, the working material that can produce particle number inversion after being excited), the excitation device (that is, the energy source that can make the particle number inversion of the activation medium, the pump source) and the optical resonator (that is, the two planar mirrors that can make the beam oscillate repeatedly and be amplified many times) (Fig. 8-2)

after more than 30 years of development, more than 200 kinds of practical lasers have been developed in various countries. There are many kinds, different characteristics and different uses. There are various classification methods for lasers: according to the working substances, there are gas, glass, crystal, liquid, semiconductor, excimer and other lasers, as well as chemical lasers (formed by chemical reaction) and free electron lasers; According to the wavelength, the wavelength range covered includes far infrared, infrared, visible light, ultraviolet and far ultraviolet. Recently, X-ray lasers and γ X-ray apparatus; According to different excitation methods, there are light excitation (light source or ultraviolet excitation), gas discharge excitation, chemical reaction excitation, nuclear reaction excitation, etc; According to different output modes, there are continuous, single pulse, continuous pulse and ultrashort pulse, etc; In terms of power output, the continuous output power is as small as micro watt level and as large as megawatt level; The energy output of the pulse can range from micro joules to more than 100000 joules, and the pulse width ranges from milliseconds to picoseconds and even femtoseconds (1/1000 trillion)

the emergence of various lasers mentioned above is mainly to meet different application purposes. For example, laser processing and some military lasers require high-power lasers or high-energy lasers (the so-called strong lasers). Some hope to shorten the pulse time as far as possible in order to engage in the research of some ultrafast processes. Some also put forward high requirements for improving the monochromaticity of light, improving the mode of output light, improving the light intensity distribution of light spot and requiring wavelength adjustment, so that the exploration depth and application breadth of lasers have been unprecedented development. The application of lasers has penetrated into various fields and is miraculously changing our world

(III) booming laser applications

laser is not only one of the most important inventions of mankind in the 20th century, but also the application of laser technology has been widely applied to all aspects of industry, agriculture, military, medicine and even society, and is playing an increasingly important role in the progress of human society

1. Application of laser in information field: semiconductor laser and fiber amplifier are two key technologies of fiber communication. The laser emitted by the semiconductor laser is not only monochromatic and coherent, but also the light wave frequency is 10000 times higher than the microwave frequency. Therefore, the optical fiber communication with laser as the carrier of information transmission and optical fiber as the information transmission line has not only good communication quality, strong anti-interference ability and good confidentiality, but also the communication capacity is 10000 times higher than that of microwave communication. An optical fiber thinner than hair can transmit tens of thousands or thousands of TV programs at the same time, so that communication can truly become a new era leading to thousands of households

the use of laser technology for optical storage has revolutionized the storage of information. The recording density of a CD audio disc is equivalent to 10million bits/cm2, and it can record 78 minutes of music programs, which is several orders of magnitude larger than that of a compact disc. A CD-ROM with a disk diameter of 5 inches for a computer can have a capacity of 650 megabits. An LD (laser video disc), or the most popular VCD (laser video disc, Gu called small video disc) in recent years, and the new generation of video disc DVD (digital video disc) after VCD, the signal contained in its video is thousands of times higher than that of CD, and it can record 100 minutes of high-definition video programs. CD, VCD and ld not only occupy a considerable share in the playback equipment market, but also can be played on computers equipped with laser drivers

in addition, laser printers, laser fax machines, laser Phototypesetting, laser large screen color TV, fiber-optic cable TV and atmospheric laser communication have been widely used

2. Application of laser in the field of holography: light is a wave phenomenon, and its physical quantities include wavelength (Jinan new era Gold Testing Instrument Co., Ltd. sincerely escorts your work, which is related to color), amplitude (which is related to the intensity of light) and phase (which represents the relationship between the starting point of wave motion and the reference time). People can only record the wavelength and amplitude by using the photosensitive photography method, so no matter how realistic it is, looking at photos and looking at real scenery are always different. Laser has high coherence, which can obtain all the information of interference wave space, including phase. Therefore, using laser for holography, all the information of the photographed object is recorded on the negative. Through the diffraction of light, the lifelike three-dimensional image of the photographed object can be reproduced. Up to now, on the basis of holography, new holographic techniques such as holographic interferometry, color holography, rainbow holography and periscopic holography have been further developed

holography has the characteristics of three-dimensional imaging, which can be recorded repeatedly, and each small piece of holographic negative can reproduce the complete three-dimensional image of the object, which is widely used. It can be widely used in scientific research such as precision interferometry, nondestructive testing, holographic photoelasticity, micro strain analysis and vibration analysis. Holographic interferometry has been widely used to study the combustion process of gas, the vibration mode of mechanical parts, the bonding quality of honeycomb structure and the inspection of subcutaneous defects of automobile tires. Hologram has become an industry as an anti-counterfeiting mark for commodities and credit cards. Photographing precious works of art with hologram is not only enjoyable, but also provides a reliable and realistic basis for the restoration of works of art. The developing holographic TV will also add a new kind of life enjoyment to people

II. Space technology

(I) what is space technology: space technology, as the name suggests, is the technology of exploring, developing and utilizing space. At this stage, space technology is also known as space technology. However, at present, experts have two understandings of "Tian":

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