(1) directional luminescence
Ordinary light sources emit light in all directions. In order to make the emitted light spread in one direction, it is necessary to install a certain condensing device on the light source. For example, headlights and searchlights of automobiles are all equipped with reflectors with light gathering function, so that the radiated light can be collected and emitted in one direction. The laser emitted by the laser is naturally emitted in one direction, and the divergence of the beam is extremely small, only about 0.00 1 radian, which is close to parallel. 1962, man irradiated the moon with laser for the first time. The distance between the earth and the moon is about 380,000 kilometers, but the laser spot on the surface of the moon is less than two kilometers. If the focusing effect is good, the seemingly parallel searchlight beam will be aimed at the moon and cover the whole moon according to its spot diameter.
(2) Very high brightness
Before the invention of laser, the brightness of high-voltage pulsed xenon lamp in artificial light source is the highest, which is equivalent to the brightness of the sun, while the laser brightness of ruby laser can exceed tens of billions of times that of xenon lamp. Because the brightness of laser is extremely high, it can illuminate distant objects. The illumination produced by ruby laser beam on the moon is about 0.02 lux (illumination unit), the color is bright red, and the laser spot is obviously visible. If the strongest searchlight is used to illuminate the moon, the illumination produced is only about one trillionth lux, which is impossible for human eyes to detect. The main reason of high laser brightness is directional light emission. A large number of photons are emitted in a very small space, and the energy density is naturally extremely high.
(3) the color is extremely pure
The color of light is determined by the wavelength (or frequency) of light. A certain wavelength corresponds to a certain color. The wavelength distribution range of sunlight is about 0.76 micron to 0.4 micron, and the corresponding colors range from red to purple, so sunlight is not monochromatic. A light source that emits a single color light is called a monochromatic light source, and the light wave it emits has a single wavelength. Such as krypton lamp, helium lamp, neon lamp, hydrogen lamp, etc. All are monochromatic light sources, which only emit light of a certain color. Although monochromatic light source has a single wavelength, it still has a certain distribution range. For example, krypton lamp only emits red light, which is said to have the highest monochromaticity, and the wavelength distribution range is still 0.000 1 nm, so the red light emitted by krypton lamp still contains dozens of reds if carefully identified. It can be seen that the narrower the wavelength distribution range of optical radiation, the better the monochromaticity. The wavelength distribution range of the light output by the laser is very narrow, so the color is extremely pure. Take the He-Ne laser that outputs red light as an example, its wavelength distribution range can be as narrow as 2× 10-9 nm, which is two ten thousandths of the wavelength distribution range of red light emitted by krypton lamp. The monochromaticity of visible laser far exceeds that of any monochromatic light source. In addition, laser has other characteristics: good coherence. The frequency, vibration direction and phase of laser are highly consistent, so when laser light waves overlap in space, the light intensity distribution in the overlapping area will appear stable phenomenon of alternating strength. This phenomenon is called interference of light, so laser is coherent light. The light emitted by a common light source is called incoherent light because its frequency, vibration direction and phase are inconsistent. The flash time can be very short. Due to technical reasons, the flash time of ordinary light source can not be very short, and the flash time of photographic flash is about one thousandth of a second. The flash time of pulsed laser is very short, reaching 6 femtoseconds (1 femtosecond = 10- 15 seconds). The light source with extremely short flash time has important uses in production, scientific research and military affairs.
Application of laser technology
Laser machining technology is a technology that uses the characteristics of the interaction between laser beam and substance, and takes cutting, welding, surface treatment, drilling, micromachining materials and identifying objects as light sources. The most traditional application field is laser processing technology. Laser technology is a comprehensive technology involving optics, mechanics, electricity, materials, detection and other disciplines. Traditionally, its research scope can be generally divided into:
Laser processing system. Comprise a laser, a light guide system, a processing machine tool, a control system and a detection system.
2. Laser processing technology. Including cutting, welding, surface treatment, drilling, marking, marking, fine tuning and other processing technologies.
Laser welding: automobile body thick plates, automobile parts, lithium batteries, pacemakers, sealed relays and other sealing devices, as well as various devices that are not allowed to weld pollution and deformation. At present, the lasers used are YAG laser, CO2 laser and semiconductor pump laser.
Laser cutting: cutting all kinds of metal parts and special materials in automobile industry, computer, electrical cabinet and woodworking tool mould industry, circular saw blade, acrylic, spring washer, copper plate of electronic parts below 2mm, partial metal mesh plate, steel pipe, tinned iron plate, lead-plated steel plate, phosphor bronze, bakelite plate, thin aluminum alloy, timely glass, silicone rubber and alumina ceramic plate 1mm or less. The lasers used are YAG laser and CO2 laser.
Laser marking: widely used in various materials and almost all industries. At present, the lasers used are YAG laser, CO2 laser and semiconductor pump laser.
Laser drilling: Laser drilling is mainly used in aerospace, automobile manufacturing, electronic instruments, chemical industry and other industries. The rapid development of laser drilling is mainly reflected in the increase of the average output power of YAG laser for drilling from 400w five years ago to 800w, reaching1000 W. At present, the more mature application of laser drilling in China is in the production of synthetic diamond and natural diamond wire drawing dies, as well as the production of gem bearings in industries such as clock instruments, aircraft blades and multilayer printed circuit boards. At present, most of the lasers used are YAG lasers and CO2 lasers, but there are also some excimer lasers, isotope lasers and semiconductor pump lasers.
Laser heat treatment: widely used in automobile industry, such as heat treatment of cylinder liner, crankshaft, piston ring, commutator, gear and other parts, as well as aerospace, machine tool industry and other mechanical industries. Laser heat treatment is widely used in China than abroad. At present, most of the lasers used are YAG laser and CO2 laser.
Laser rapid prototyping: it is formed by combining laser processing technology with computer numerical control technology and flexible manufacturing technology. Mostly used in mold and model industry. At present, most of the lasers used are YAG laser and CO2 laser.
Laser coating: widely used in aerospace, mold and electromechanical industries. At present, most of the lasers used are high-power YAG lasers and CO2 lasers.
Application of laser in medicine
Laser system applied to dentistry
According to the different functions of laser in dental application, it can be divided into several different laser systems. An important feature of distinguishing lasers is that the wavelength of light has different effects on tissues. Visible light and near infrared spectrum have low light absorption rate and strong penetration, and can penetrate deep tooth tissues, such as argon ion laser, diode laser or Nd: YAG laser (as shown in figure 1). However, Er: YAG laser and CO laser have poor light penetration, which can only penetrate the tooth tissue by about 0.01mm.. The second important feature that distinguishes laser is the intensity (i.e. power) of laser. For example, diode laser used for diagnosis is only a few milliwatts, and sometimes it can also be used for laser display.
The laser used for treatment is usually a moderate intensity laser of several watts. The effect of laser on tissue also depends on the way of laser pulse emission. Typical continuous pulse emission methods are argon ion laser, diode laser, CO2 laser and laser. Some Er: YAG lasers or many Nd: YAG lasers emit in short pulses. The intensity (i.e. power) of short-pulse laser can reach more than 1 1,000 watts. These high intensity and high light absorption lasers are only suitable for removing hard tissues.
Application of laser in the diagnosis of dental caries
1. Demineralization and shallow caries
2. Recessive dental caries
Application of laser in treatment
1. Cut
2. Filler polymerization and pit treatment
Laser cosmetic surgery
(1) Laser is widely used in beauty industry. Laser produces monochromatic light with high energy, accurate focus and certain penetrating power, which acts on human tissues to produce high fever, thus cutting or destroying target tissues. Pulsed laser with different wavelengths can treat various vascular dermatoses and pigmentation, such as nevus OTA, nevus flammeus, freckles, senile plaque, telangiectasia, tattoo removal, eyeliner washing, eyebrow washing and scar treatment. In recent years, some new laser instruments, such as high-energy ultra-pulse CO2 laser, erbium laser, etc., have achieved good results in wrinkle removal, peeling and skin replacement, snoring treatment, tooth whitening and so on, which has opened up more and more broad fields for laser surgery.
(2) Laser surgery has incomparable advantages over traditional surgery. First of all, laser surgery does not require hospitalization, with small incision, no bleeding during operation, light trauma and no scars. For example, the traditional surgical treatment of bags under the eyes has many shortcomings, such as wide peeling range, more bleeding during operation, slow healing after operation and easy scarring. The application of high-energy ultra-pulse CO2 laser in the treatment of eye bags has the advantages of no bleeding, no suture, no influence on normal work, light edema at the surgical site, quick recovery and no scar left, which is incomparable to traditional surgery. Some endoscopic operations that cannot be performed due to excessive bleeding can be completed by laser cutting instead. (Note: There is a certain range of adaptation)
(3) Laser has achieved remarkable results in the treatment of vascular dermatosis and pigmentation. The treatment of port wine nevus with pulsed dye laser has obvious curative effect, little damage to surrounding tissues and almost no scar. Its appearance has become a revolution in the history of treatment of port wine stains, because in the history of treatment of port wine stains, radiation, freezing, electrocautery, surgery and other methods have a high incidence of scars, and pigment loss or deposition often occurs. Laser treatment of vascular dermatosis is the selective absorption of oxygen-containing hemoglobin to a certain wavelength of laser, which leads to the high destruction of vascular tissue. It has high accuracy and safety, and will not affect the surrounding adjacent organizations. Therefore, laser treatment of telangiectasia is also effective.
In addition, due to the appearance of variable pulse laser, great breakthroughs have been made in removing unsatisfactory tattoos and treating various pigmented skin diseases such as OTA nevus and senile plaques. According to the theory of selective photothermal effect (that is, lasers with different wavelengths can selectively act on skin lesions of different colors), this kind of laser uses its powerful instantaneous power, highly concentrated radiation energy, pigment selectivity and extremely short pulse width to concentrate laser energy on pigment particles, vaporize and crush them directly, and excrete them through lymphatic tissues without affecting the surrounding normal tissues, which is deeply rooted in people's hearts because of its exact curative effect, safety and reliability, no scar and little pain.
(4) Laser surgery initiated a new era of medical beauty. High-energy ultra-pulse CO2 laser peeling and skin changing has opened up a new technology in cosmetic surgery. It uses high-energy, ultra-short pulse laser to vaporize the damaged skin tissue instantly, without damaging the surrounding tissues, with almost no bleeding during the treatment, and can accurately control the depth of action. Its effect has been fully affirmed by the international medical plastic surgery field, and it is known as "creating a new era of medical beauty"; In addition, there is a high-energy ultra-pulse CO2 laser instrument to treat bags under the eyes, snoring, and even laser whitening teeth. With its safe and accurate curative effect and simple and quick treatment, it has created one miracle after another in the field of medical beauty. Laser cosmetology has made medical cosmetology take a big step forward and given the connotation of medical cosmetology renewal.
Laser cooling
Laser cooling is to use the interaction between laser and atoms to slow down the motion of atoms, so as to obtain ultra-low temperature atoms. In the early days, the main purpose of this important technology was to accurately measure various atomic parameters for high-resolution laser spectrum and ultra-high-precision quantum frequency standard (atomic clock), but later it became the key experimental method to realize atomic Bose-Einstein condensation. Although people noticed that light had radiation pressure on atoms as early as the beginning of the 20th century, it was not until the invention of laser that the technology of changing the speed of atoms by using light pressure was developed. It is found that when an atom moves in a pair of laser beams with a frequency slightly lower than the energy level difference of the atomic transition and propagating in opposite directions, due to the Doppler effect, the atom tends to absorb photons in the opposite direction to the atom, but the probability of absorbing photons propagating in the same direction is small. The absorbed photons will spontaneously radiate isotropically. On average, the net effect of two lasers is to produce a damping force opposite to the direction of atomic motion, thus slowing down the movement of atoms (that is, cooling down). 1985, Phillips of the National Institute of Standards and Technology and Steven Chu of Stanford University first realized the experiment of laser cooling atoms, and obtained the extremely low temperature (24μK) sodium gas. They further used a three-dimensional laser beam to form magneto-optical theory, trapped atoms in a small area of space and cooled them, and obtained "optical viscose" with lower temperature. Since then, many new laser cooling methods have appeared. Among them, the most famous ones are "velocity selective coherent population limitation" and "Raman cooling". The former was put forward by Claude Cohen-Tannoji of Paris Teachers College, while the latter was put forward by Zhu. They used this technology to obtain extremely low temperatures below the photon recoil limit. Since then, people have developed a series of cooling technologies combining magnetic field and laser, including polarization gradient cooling and magnetic induction cooling. Zhu, Cohen Danoki and Phillips also won the 1997 Nobel Prize in Physics. Laser cooling has many applications, such as atomic optics, atomic etching, atomic clock, optical lattice, optical tweezers, Bose-Einstein condensation, atomic laser, high-resolution spectroscopy, and basic research on the interaction between light and matter.
Laser spectrum
Laser spectroscopy is a spectral technology with laser as light source. Compared with ordinary light sources, laser light source has the characteristics of good monochromaticity, high brightness, strong directivity and strong coherence, and is an ideal light source to study the interaction between light and matter, so as to identify the structure, composition, state and change of matter and its system. The appearance of laser greatly improves the sensitivity and resolution of the original spectral technology. Because of the extremely high intensity and narrow pulse width of laser, it is possible to observe the multiphoton process, nonlinear photochemical process and relaxation process of molecules after excitation, and they have developed into new spectral technologies respectively. Laser spectroscopy has become a research field closely related to physics, chemistry, biology and materials science.
Laser sensor
Laser sensor is a kind of sensor that uses laser technology to measure. It consists of a laser, a laser detector and a measuring circuit. Laser sensor is a new type of measuring instrument, which has the advantages of non-contact long-distance measurement, high speed, high precision, large measuring range and strong anti-photoelectric interference ability.
laser radar
Lidar refers to a radar that uses 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.
Laser beam weapon
Laser weapon is a kind of directional energy weapon, which directly destroys or paralyzes the target by using directional laser beam. According to different operational purposes, laser weapons can be divided into tactical laser weapons and strategic laser weapons. The weapon system is mainly composed of laser, tracking, aiming and launching devices. At present, the commonly used lasers are chemical lasers, solid-state lasers and CO2 lasers. Laser weapons have the advantages of fast attack speed, flexible steering, accurate attack and no electromagnetic interference, but they are also vulnerable to weather and environment. Laser weapon has a history of more than 30 years, and its key technology has also made a breakthrough. The United States, Russia, France, Israel and other countries have successfully carried out various laser shooting experiments. At present, low-energy laser weapons have been put into use, mainly used for short-range interference and blinding photoelectric sensors, as well as attacking human eyes and some enhanced observation equipment; High-energy laser weapons mainly use chemical lasers. According to the current level, it is expected to be deployed and used on ground and air platforms in the next 5- 10 years for tactical air defense, theater anti-missile and anti-satellite operations.