Femtosecond laser has the characteristics of ultra-high speed and high resolution, so it plays an irreplaceable role in physics, biology, chemical reaction control, optical communication and medicine.
The main uses of femtosecond laser are:
1, early diagnosis of human diseases;
2. Medical imaging;
3. surgery;
4. In vivo detection;
5. Nanosatellites;
6. Ultrafine machining;
7, high-density information recording and storage. The following are the uses of lasers:
(1) laser communication
It is very common to transmit information by light today. For example, ships communicate in light, and traffic lights use red, yellow and green. But all these ways of transmitting information by ordinary light can only be limited to a short time interval. If you want to transmit information directly to a distant center by light, you can't use ordinary light, you can only use laser.
Where is the advanced laser communication? The advantage of laser communication is firstly large capacity. What's its capacity? When we usually make a phone call, sometimes there will be irrelevant voices. This kind of fighting phenomenon is because a pair of telephone lines can only pass through one telephone line. If another telephone line is connected in series, it will interfere with normal calls. Suppose there are 10 pairs of people talking on a pair of telephone lines at the same time, that is to say, there are 20 people talking at the same time, so it is basically impossible to talk. In order to solve this problem, it is necessary to use carrier wave and other methods to make every mobile phone in every frequency band. Because the frequency range of ordinary telephone is 300 ~ 400 Hz, and the highest frequency on a pair of telephone lines is only 1500 kHz, so a pair of telephone lines can only pass through more than a dozen telephones at the same time. Obviously, such telecom capacity is far from meeting the requirements of today's information society.
Surprisingly, optical fiber communication is especially suitable for TV, image and digital transmission. It is reported that a pair of optical fibers can transmit a complete set of Encyclopedia Britannica in one minute.
In addition, the material for making optical fibers is timely, a kind of sand that can be seen everywhere on the earth. It takes only a few grams of time to manufacture an optical fiber with a length of 1km. In this way, not only raw materials are inexhaustible, but also copper and aluminum can be greatly saved. Because of this, all the prosperous countries in the world are competing to discuss laser communication. Therefore, laser communication has become the darling of competing development.
In the history of communication technology, the development of optical fiber communication technology is unprecedented. Looking at several milestones in the history of communication technology, it took about 60 years for the telephone to be produced and applied, and telephone communication is still widely used. Radio technology (such as telegraph) also took about 30 years from creation to application. Although TV technology has developed rapidly, it was born about 14 years ago. Laser communication, from the birth of the first low-loss optical fiber to its application, only takes five years. Nowadays, laser communication is not only widely used, but also constitutes a huge optical fiber market.
In a word, industrialized countries have established nationwide optical fiber communication networks to completely replace the current copper wires and cables. This huge technical project is expected to be completed in 2000. By then, laser communication will bring great changes to our planet. For example, in seclusion, you can use fiber-optic network to handle documents or meetings at home; Or connect the optical fiber network at home to the shopping center, just like in a supermarket, you can buy the goods you need at home, and the payment only needs to be settled by e-finance shopping system. Medical centers all over the country can also check the patient's condition and laboratory reports from the screen, and prescribe prescriptions accordingly, so as to truly achieve "a scholar can know what's going on in the world without going out" and "strategize and win a thousand miles".
Laser and optical fiber can also transmit images. First of all, a single optical fiber with a diameter smaller than a human hair should be combined into an optical fiber bundle. In the process of transmitting information, there are two kinds of commonly used optical fiber bundles: one is called light beam and the other is called image beam. The task of transmitting light beam is to transmit light from one end to the other. The structure of the beam is relatively simple. It is made by gluing a plurality of monofilaments, then polishing and grinding its end face to reduce the reflection and scattering loss when light enters the optical fiber, and then putting a plastic sheath on the outside of the light beam.
Because one optical fiber can only transmit one light spot, to transmit the whole image, it is necessary to arrange the optical fibers one by one, and the formed optical fiber bundle is called image transmission bundle.
In the image beam, all the optical fibers are arranged in a row, and the positions of the two ends are in strict one-to-one correspondence, which is not messy at all, just like a uniform chopstick. For example, one end of the optical fiber is in the eighth row and the eighth column in the image beam, so the other end is also in the eighth and eighth positions.
When an image beam transmits an image, the image is first divided into grids, that is, an image is composed of countless optical fibers and then transmitted. One optical fiber is used to transmit one pixel, and countless optical fibers can transmit the whole image to the other end. If you want to transmit images clearly, you should use optical fibers with smaller diameters as much as possible. Because the thinner the optical fiber, more light beams can be accommodated in an image beam, so more pixels can be transmitted. Obviously, the more pixels, the clearer the image.
The image beam used now is composed of tens of thousands of optical fibers, and it is not easy to arrange so many optical fibers evenly. After the arrangement, use an organic adhesive called epoxy resin to stick the two ends together, so that the optical fibers can be bonded and fixed, ensuring that the optical fibers at both ends correspond to each other one by one. Both ends should be polished. As for the middle part, it doesn't need to be glued firmly, but it is as loose as the erhu string. Just add a plastic sleeve for maintenance outside, so that the image beam is both soft and flexible.
In addition to transmitting images, image beams can also transmit ordinary symbols or numbers to enlarge or reduce images.
If you want to enlarge the image, you can make one end of the image beam big and the other end small, just like a cone. When pixels are transmitted from small end to large end, the whole image is enlarged. On the contrary, if the image is sent from the big end to the small end, the whole image will be reduced.
In addition, the application of optical fiber can also change the image. If the arrangement of optical fibers is intentionally disturbed according to the demand, then the pixels at the exit end cannot fall on the original corresponding points, but on the objective imaginary points, so the image is changed. If the optical fiber at the entrance end of the pixel is square and the optical fiber at the exit end is round, then the square pixel can be turned into a round pixel.
In a word, optical fiber image bundle has great potential, and it will play an increasingly important role in the future optical information processing technology.
(2) Data processing
Drilling, cutting, welding and quenching are the most common operations when processing metal data. Since the introduction of laser, a new scene has been created in the intensity, quality and scope of processing. In addition to metal data, laser can also process a lot of non-metal data.
Laser drilling machine Before the emergence of laser drilling machine, all kinds of mechanical parts were drilled by electric drilling machine or punch. But mechanical drilling is not only inefficient, but also the appearance of drilling is not bright enough.
The principle of laser drilling is to use the convergence of laser beams to make the temperature of the metal surface focus rise rapidly, and the temperature rise can reach/kloc-0.00 million degrees per second. Before the heat dissipates, the light beam melts the metal until it evaporates, leaving a small hole. Laser drilling is not limited by the hardness and brittleness of processing data, and the drilling speed is extremely fast, and small holes can be drilled as fast as thousands or even millions of seconds.
For example, it is necessary to drill hundreds of imperceptible micropores in a metal plate, which is obviously not competent with an electric drilling machine, but it can be completed with a laser drilling machine 1 ~ 2 seconds. If we carefully examine these micropores with a magnifying glass, we can find that their surfaces are very uniform and bright.
Laser drilling can also be used to process watch diamonds. It can drill 20 ~ 30 holes per second, which is hundreds of times more efficient than mechanical processing and has high quality. At the same time, laser drilling is the same as the laser cutting we will talk about below, and the processing process is non-contact, that is, unlike mechanical processing, steel drills are not used to drill through metal data step by step. Therefore, laser operation can play a role in automatic continuous machining or ultra-clean and vacuum special environment.
When the laser cutting machine knows the principle of laser drilling, it is easy to understand why laser can cut metal data: as long as the workpiece or laser beam is moved and the drilled holes are lined up, the data can be cut naturally. Moreover, no matter what kind of material, such as steel plate, titanium plate, ceramic, timely, rubber, plastic, leather, chemical fiber, wood and so on. The laser is like a lightsaber that cuts iron like mud and wood like ash with a bright blade.
Laser welding machine Laser can be used for welding because of its high power density. The so-called high power density means that extremely high energy can be concentrated per square centimeter. How high is the power density of the laser? We can make an analogy: the acetylene flame usually used for welding in factories can weld two steel plates together, and the power density of this flame can reach per square centimeter 1000 watt; The power density of argon arc welding equipment is higher, which can reach per square centimeter 10000 watt. But these two kinds of welding flames can't be compared with laser, because the power density of laser is ten million times higher than them. Such a high power density can weld not only ordinary metal materials, but also hard and brittle ceramics.
The traditional laser quenching method is very simple. Burn the blade red first, and then suddenly immerse it in cold water. After this cold and hot treatment, the hardness of the blade is greatly improved. But such quenching is obviously not easy, and the effect is not necessarily ideal.
Laser quenching is to use laser to scan the parts that need to be quenched, so that the temperature of the scanned area rises, while the parts that are not scanned remain at room temperature. Due to the rapid heat dissipation of metal, the temperature of this part drops sharply when the laser beam is swept away. The faster the temperature drops, the higher the hardness. If the scanned parts are sprayed with rapid cooling agent, the hardness can be much better than that of ordinary quenching.
(3) Laser phototypesetting
Phototypesetting exercises introduce the principle of optical photography. When using movable type typesetting, it is necessary to detect the types and symbols of various sizes and fonts according to the original manuscript and stop typesetting. On the other hand, photo typesetting is much more troublesome. It changes the size and shape of the text through the lens on the typesetting machine. As for why you can change the size and shape of words with lenses, this is actually equivalent to taking a "ha ha mirror".
In phototypesetting, only the required characters and symbols need to be imaged on photographic paper through a lens, and then developed and fixed to form a photographic negative. Then, print it out like a photo.
Phototypesetting can use two kinds of light sources, just ordinary light sources. In contrast, laser typesetting saves time and effort. Because of the high brightness and good color of the laser, the clarity of the image can be greatly improved, and the quality of the printed book is naturally high. What is its principle? First, the computer turns the text into dots, and then the point-controlled laser scans the photosensitive film, so that a real hologram can be taken.
Holography and plane photography are two different things. Although the plane color photo looks bright, clear and full of plane sense, it is still a single-sided image, and even the best plane photo can't replace the real thing. For example, no matter how we change our perspective, we can only see the picture in the photo, but the hologram is different. By changing the perspective, we can see six aspects of this square. Because holographic technology can record all the geometric features of objects on the negative, which is one of the most important characteristics of holography.
The second important feature of holography is that you can know the whole leopard at a glance. When the hologram is damaged, even if most of it is damaged, we can still see the whole picture of the original object in this hologram from the remaining half. This is not the case for ordinary photos. Even if a corner is lost, the picture on that corner will not be seen.
The third feature of holographic photos is that multiple holographic photos can be recorded in layers on a holographic negative without interfering with each other. It is this layered recording that enables holograms to store a large amount of information. The negative of laser holography can be special glass, latex, crystal or thermoplastic. A small special glass can store all the contents of millions of books in a large library. Holography is becoming more and more popular.
Holography can record precious historical relics. In case the cultural relics are seriously damaged, even if nothing is left, we can still reconstruct them according to holography. For example, places of interest such as the Yuanmingyuan in Beijing were burned down by Eight-Nation Alliance in those days, but now they have to be rebuilt. Because they don't know the original appearance, it is difficult to completely restore it. If holography was created 100 years ago, things would be simple.
Holography can also be used for nondestructive testing in industry. What is nondestructive testing? In other words, using laser holographic technology can not only check whether there are tiny defects in products, but also won't damage these products at all. More interestingly, at present, holography is also used to shoot holographic films and television, and soon the audience will see real-life images. That is, the laser "hits" the photosensitive coating on the negative film, leaving countless corresponding points, which are developed and fixed and then become words or images again. Here the laser beam is equivalent to the electron beam, and the photosensitive film is equivalent to the TV screen. Next, you can print books, newspapers and magazines with negatives containing words and images. The color TV can display red, green and blue because the screen is coated with tricolor phosphor, which will appear three colors under the impact of electrons. Laser phototypesetting can also use similar principles to print beautiful color pictures.
(4) The application of laser in medicine.
There have been many achievements in the application of laser in the field of medical devices, which can play a variety of roles, such as drill, scalpel, welding torch and so on.
Welding torch and electric drill in ophthalmology, laser is mainly used to treat retinal detachment. Retinal detachment is a very difficult disease. The patient's retina is separated from the inner wall of the eyeball and cannot produce vision. Before the laser appeared, I'm afraid the patient would be blind.
Now the doctor can aim the laser at the patient's fundus, let the laser emit a beam of laser, and then heat it to reunite the retina with the inner wall of the eyeball. The whole process takes less than a few minutes, and the laser beam welds the patient's retina like a welding gun.
Besides welding, laser welding guns can also be used for cutting.
Cataract is a common disease in the elderly. The convex lens at the front of the patient's eyeball, that is, the lens, has gradually become turbid and inelastic from the original transparent elastic body, so that light cannot pass through the lens and fall on the retina of the fundus, and the patient gradually cannot see. The traditional method of treating cataract is to cut an incision in front of the eyeball and then insert a thin metal needle through the incision. The temperature of this metal needle is extremely low, and the turbid lens is frozen and stuck to the needle, and then taken out of the small hole together. Obviously, the whole operation is more troublesome.
If medical laser is used for treatment, it is not only convenient but also effective. As long as the laser beam is aimed at the front or back of the intraocular lens, the chaotic film on the lens surface can be quickly removed.
In dentistry, laser can replace dental drill. According to the statistics of the World Health Organization, the incidence of dental caries in children is quite high, reaching about 75%. Using laser to treat teeth, patients will not feel uncomfortable at all, as long as they are not inflamed, they can deal with the problem in one treatment. Dental laser is the youngest brother in laser. Its power is very small, as long as 3 watts, equivalent to energy-saving lamps, almost no heat. Its transmitting end is actually an optical fiber as thin as a hair.
During treatment, just put the emitting end of the optical fiber close to the caries focus, emit a laser beam, and the caries tissue will be synthesized, and then rinse it with clean water. If dental caries is only the surface damage of tooth enamel, the laser beam will seal the tiny pores in the damaged area one by one, thus preventing the corrosion of lactic acid on tooth dentin. If dental caries have appeared, after drilling and cleaning with laser beam, the artificial enamel data can be filled into the gap, and then the joint can be heated with laser, so that the artificial enamel data and tooth enamel can be integrated. Laser treatment of teeth is not only painless and rapid, but also effective after treatment.
If you want to operate on the patient's bladder, heart, liver, stomach, intestines and other important internal organs, it will be more difficult. How does laser enter human viscera? This depends on a treasure in the doctor's hand, that is, the laser fiber endoscope.
The so-called endoscope is an optical device used by doctors to insert into the human body to directly examine organs. The usual endoscope is relatively large and rough, and can only be inserted into the stomach from the patient's mouth along the esophagus. Inserting into the stomach is very uncomfortable, and the patient will feel very painful. Laser fiber endoscope is completely different. Endoscopes made of optical fibers are soft, thin and flexible. When it is inserted into the patient's stomach, there will be no pain. In addition to the stomach, fiber optic endoscope can also enter other important organs. On the one hand, laser fiber optic endoscope can be used to check whether the patient's organs are diseased, and more importantly, it can input laser energy to internal organs to prevent the reflection of diseased tissues, that is, to remove diseased tissues and play the role of scalpel. Moreover, with laser knife cutting, the wound can stop bleeding automatically, and there is no need to puncture the bleeding point, which greatly shortens the operation time and the wound will not be inflamed. If the malignant tumor is removed with a laser knife, the spread of cancer cells can also be avoided.
(5) Laser weapons
During the Laser Missile Gulf War, the multinational forces led by the United States launched a large-scale air strike against Iraq, which destroyed many important military purposes in Iraq. Finally, the war ended in the failure of Iraq. Some people say that the Gulf War is a competition of advanced weapons, which is true.
American planes are equipped with laser sights, which can emit infrared lasers. When a reconnaissance plane finds an air target in the air, it rotates in the air and emits a laser beam to the target from time to time with a laser sight. This kind of laser beam actually plays a guiding role. At this time, other planes carrying out the attack mission then flew in and dropped laser-guided missiles at the destination. These laser-guided missiles are equipped with an automatic tracking system. This automatic tracking system is equivalent to the eyes of a missile. When the missile flies to the target, it can correct the course in flight from time to time according to the guided laser reflected by the target, so as to hit the target accurately.
In fact, this laser-guided missile was used by the United States in the Vietnam battlefield as early as the 1970s. Now there are not only air-to-surface missiles, but also ground-to-ground, air-to-air and ground-to-air laser missiles.
Today, people can separate the radio search radar from the laser radar to form a combat system. For example, when a radio radar finds an air target (enemy plane or missile), it can accurately measure the height, orientation and speed of the target. As long as the target enters a certain range, the lidar will be turned on, emitting a very fine laser beam, closely watching and accurately measuring the position of the target, and then the launched laser missile will accurately hit the target and destroy it according to the guiding laser beam provided by the lidar. This kind of laser missile can be easily deployed on trucks and converted into anti-tank missiles.
The anti-tank laser missile developed at present can be launched from the air or from a helicopter. The missile is equipped with a semiconductor laser, which plays the role of automatic tracking, so that the missile hits the tank in every shot.
Although lidar has high accuracy, small size, clever operation and easy transfer, it also has some defects, that is, it is easily limited by climatic conditions and is not suitable for large-scale search. Therefore, it is usually used in conjunction with radio radar to foster strengths and avoid weaknesses.
Laser guns and laser cannons The so-called laser guns and laser cannons belong to laser tactical weapons. They are shaped like guns and cannons, but instead of bullets and shells, they emit laser beams, causing enemy casualties or blindness. The ability of this kind of gun is related to its own energy and firing interval. Now the effective range of laser gun and laser gun is not far away, so the ability of death ray is limited.