laser
Coherent radiation based on the principle of optical amplification of stimulated radiation. Laser has the following characteristics: ① Good directivity. The divergence solid angle of laser is extremely small, generally within the sphere of 10-5 ~ 10-8. The high directivity of laser means that the laser energy is concentrated in a very narrow beam. ② High brightness. The brightness of ordinary light source is very low, the brightness of the sun is about 103W/(cm2 sphericity), while the brightness of high-power laser is as high as101017w/(cm2 sphericity). ③ Good monochromaticity. The monochromaticity of laser is usually characterized by V/Δ V, where V is the frequency of the laser line center and Δ V is the line width. The better laser V/δV can reach10/kloc-0 ~1013. Good monochromaticity means good temporal coherence. ④ Good spatial coherence. The spatial coherence of ordinary light sources is very poor. When the optical path difference is several thousand times the wavelength, there is no interference. The laser is coherent in almost the whole wave field space.
The laser emitted by the laser device
Laser is widely used in ranging, radar, optical fiber communication, medicine and machining (welding, cutting, drilling, etc.). ), missile guidance and nuclear fusion test. High intensity laser has made a breakthrough in spectroscopy and opened up a new research field. The nonlinear effect caused by laser has created a new field of nonlinear optics. The excellent monochromaticity of laser provides a very favorable light source for accurate length measurement. The beat frequency technology of light wave can be developed by using good monochromaticity, and extremely slow velocity (about 1 micron/second) and angular velocity (about 10- 1 radian/second) can be measured. After the appearance of laser with good coherence, holography can enter the practical stage and be quickly applied to various fields. In the field of coherent information processing, laser has become an indispensable light source.
Laser material
Laser material
Materials that convert all kinds of pumping (electricity, light and radiation) energy into laser. Working substance of laser. Laser materials are mainly condensed substances, mainly solid laser materials. Solid-state laser materials can be divided into two categories. One kind is semiconductor laser material mainly excited by electricity, which generally adopts heterostructure and consists of semiconductor thin films, and is prepared by epitaxial method and vapor deposition method. Different doped semiconductor materials are used according to different laser wavelengths. Usually in the visible region, it is mainly composed of group compound semiconductors; In the near infrared region, group compound semiconductors are dominant; In the mid-infrared region, ⅳ-ⅵ compound semiconductors are dominant. The other is luminescent material, which absorbs the energy of optical pump and converts it into laser output through separate luminous centers. This kind of material is based on solid dielectric and can be divided into crystal and amorphous glass. The active ions in laser crystal are in the lattice of ordered structure, and the active ions in glass are in the network of disordered structure. Commonly used laser materials are mainly oxides and fluorides, such as silicate glass, phosphate glass, fluoride glass, alumina crystal, yttrium aluminum garnet crystal, yttrium lithium fluoride and so on. Oxide materials have good physical properties, such as high hardness, mechanical strength and good chemical stability; Fluoride materials have low phonon frequency, wide spectral transmission range and high luminous quantum efficiency.
laser ranging
laser ranging
Laser is used as a light source for ranging. According to the working mode of laser, it can be divided into continuous laser and pulsed laser. Gas lasers such as He-Ne, Ar-ion, Krypton, Cadmium, etc. work in continuous output state and are used for phase laser ranging. Double heterostructure GaAs semiconductor lasers for infrared ranging: solid-state lasers such as ruby and neodymium glass are used for pulsed laser ranging. Compared with photoelectric rangefinder, laser rangefinder can not only work day and night, but also improve the ranging accuracy and significantly reduce the weight and power consumption, making it possible to measure the distance to long-distance targets such as artificial earth satellites and the moon.
laser record
Laser disc
A circular sheet carrier for recording audio signals by laser recording. Laser digital records are also called compact records and pocket records. Laser recording and playback is the result of the development of digital recording in the late 1970s. The diameter of laser digital record is 1.20 mm, and it can record on one side, and can play stereo programs at 1 hour with a dynamic range of 90dB. This kind of sound track with extremely high recording density is composed of pits and lands between pits recorded by laser beam according to signal coding. They represent binary 0 and 1 respectively. When playing a record, the binary number is scanned and picked up by a laser beam, and the whole playback equipment adopts a very precise servo control system to ensure good tracking. This disc has been erased and recorded again. Because CD has the advantages of high recording density, good sound quality, small size and easy storage, it is gradually replacing ordinary records and tapes as the main carrier of audio signals in the future.
Laser crystal
The energy provided by the outside world can be converted into the crystal material of high parallelism and monochromatic laser which is coherent in space and time through the optical resonator. It is the working substance of crystal laser. The laser crystal consists of two parts: the luminescent center and the matrix crystal. The luminescent center of most laser crystals consists of activated ions, which partially replace the cations in the matrix crystals to form doped laser crystals. When the activated ions become part of the matrix crystal, they form self-activated laser crystals.
The active ions used in laser crystals are mainly transition metal ions and trivalent rare earth ions. The optical electrons of transition metal ions are 3d electrons in the outer layer, which are easily influenced by the surrounding crystal field in the crystal, so their spectral characteristics are very different in different crystal structures. 4f electrons of trivalent rare earth ions are shielded by 5s and 5p outer electrons, which weakens the role of crystal field. However, the perturbation of crystal field makes it possible for 4f electrons to jump, resulting in narrow-band absorption and fluorescence lines. Therefore, the spectral changes of trivalent rare earth ions in different crystals are not as great as those of transition metal ions.
The main matrix crystals used in laser crystals are oxides and fluorides. As a matrix crystal, it requires stable physical and chemical properties, easy growth of large-size crystals with good optical uniformity and low price, but its adaptability to activated ions should be considered, such as the radius, electronegativity and valence of matrix cations and activated ions should be as close as possible. In addition, the influence of matrix crystal field on the spectrum of activated ions should also be considered. For some matrix crystals with special functions, laser with certain characteristics can be directly generated by doping active ions. For example, in some nonlinear crystals, the laser generated by activated ions can be directly converted into harmonic output through the matrix crystal.
laser radar
A radar that uses a laser as a radiation source. Lidar is a combination of laser technology and radar technology. It consists of transmitter, antenna, receiver, tracking frame and information processing. Emitters are various types of lasers, such as carbon dioxide lasers, Nd-doped yttrium aluminum garnet lasers, semiconductor lasers and solid-state lasers with adjustable wavelengths. The antenna is an optical telescope; The receiver adopts various forms of photodetectors, such as photomultiplier tubes, semiconductor photodiodes, avalanche photodiodes, infrared and visible light multi-detector devices, etc. Lidar works in two modes: pulse or continuous wave. Detection methods are divided into direct detection and heterodyne detection.
Lidar can be used to measure the trajectories of various military flying targets. Such as tracking and measuring the initial stage of missiles and rockets, tracking and measuring the low elevation angle of aircraft and cruise missiles, and accurately determining the orbit of satellites. Laser radar is combined with infrared, television and other photoelectric equipment to form ground, shipboard and airborne fire control systems to search, identify, track and measure targets. Because lidar can obtain the three-dimensional image and velocity information of the target, it is beneficial to identify the stealth target. Lidar can monitor the atmosphere, measure the pollution and poison in the atmosphere by remote sensing, and also measure the temperature, humidity, wind speed, visibility and cloud height of the atmosphere.
Laser recording
The process of recording encoded image and sound information on a circular sheet carrier with a laser beam through an optical modulator. The FM video signal is limited by the audio signal, which is engraved on the original disc by the laser beam through the optical modulator to form a pit train for recording the modulated video signal and the audio signal. The pits are spirally arranged on the disk from the inside out. Then the original disc is used to make the die of the record, and the record material is transparent PVC plastic. In order to reflect the laser beam, the aluminum layer is evaporated after molding and a protective film is added. Finally, two such records are glued together back to back to become a double-sided record. The He-Ne laser of laser tv record player emits a laser beam, which shines on the track engraved with pits through the objective lens. When the laser beam is reflected by the evaporated aluminum layer in the pit, the light intensity modulation is generated due to diffraction, and the corresponding electrical signal is generated after entering the photodiode. Laser tv video technology is widely used, not only for recording TV signals, but also as a part of computer system, with the characteristics of high recording density and easy retrieval. The development direction of laser recording is to increase the recording density and reduce the recording size, so that recording can be played while erasing and re-recording.
② UV or visible laser photolysis reaction. In this reaction, the reactant molecules are excited to the excited state of electrons. Because the dissociation energy of most molecules is between 60 ~ 752.4 kJ/mol or 3 ~ 7 eV, ultraviolet irradiation with wavelength of 400 ~ 140 nm is needed. In principle, any molecule can be photolyzed as long as the laser with appropriate wavelength is selected. For the same molecule, lasers with different wavelengths can be photolyzed in different ways. For example, vinyl chloride (C2H3Cl) is produced by laser:
C2H4ClC2H4Cl +Cl
C2H4Cl2+Cl →C2H3Cl2 +HCl
C2H3 Cl 2 C2H3 Cl+Cl This is a free radical chain reaction induced by ultraviolet laser, and the key point is that dichloroethane is initiated by excimer laser photolysis. Laser-induced chemical reactions have been used to separate 10 isotopes.
Laser glaze laser energy;
Laser can also be applied to nuclear power generation. The nuclear fuel used in nuclear power plants built in the world is uranium, and the research on using tritium nuclear fuel has not been successful. From the research, tritium nuclear fuel is more "burn-resistant" than uranium nuclear fuel, and the energy generated by burning 1 kg tritium nuclear fuel is more than three times higher than that of uranium nuclear fuel. What is even more attractive is that tritium nuclear fuel is stored in a large amount on the earth. 1 kg seawater contains 0.03 g of tritium, and the oceans on earth contain tritium 102 1 kg seawater; That is to say, 10 17 kg of tritium is stored in the ocean of the earth. If it is developed as a fuel, it is equivalent to providing us with 10 trillion (10/7) tons of coal, which is enough for human use for hundreds of millions of years. Since tritium nuclear fuel is so good, why not use it now? The problem is that it is not easy to ignite. Draw the burning temperature of a match and you can ignite paper and gasoline. To make this nuclear fuel catch fire, a high temperature of 1 100 million degrees is needed. Laser is the technology that can reach this ignition temperature at present.