1, fiber optic transmission materials:
Comprehensive wiring system used in the optical fiber for the glass multimode 850nm wavelength of the LED, the transmission rate of 100M/bps, the effective range of about 20Km. Its core and cladding by the two kinds of optical properties of the media constitute different. The refractive index of the internal medium to light is higher than the refractive index of the medium surrounding it. As can be seen from physics, in the interface of the two media, when the light from the side of the refractive index of high refractive index into the side of the refractive index of high refractive index, as long as the angle of incidence is greater than a critical value, the phenomenon of reflection will occur, the energy will not be subject to loss. At this point the covering layer wrapped around the periphery acts like an opaque substance, preventing light from escaping from the surface during penetration. Only those light rays whose initial angle of incidence is small have refraction occurring and are absorbed cleanly by the outer material in a very short distance.
Optical fibers, both glass and plastic media, are currently produced to transmit all visible light and part of the infrared spectrum. Fiber optic cable made of optical fiber has a variety of structural forms. Short distance with the cable there are two main types of optical fiber, a layer of structural fiber optic cable is in the center of the steel wire or nylon filaments, the outer bundle of a number of optical fibers, the outside of a layer of plastic sheath; another is a high-density fiber optic cables, which have multiple layers of ribbons stacked together and become a layer of ribbons parallel to the laying of a row of optical fibers.
Fiber optic cable made of optical fiber has a variety of structural forms. There are two main types of fiber optic cables for short distances, a layer structure. Fiber optic cable is in the center of the steel wire or nylon wire, outside the bundle of a number of optical fibers, outside the addition of a layer of plastic sheath; another is a high-density fiber optic cable, which has a multi-layer ribbon stacked and become a layer of ribbons on each layer of parallel laying of a row of optical fibers.
2, fiber optic transmission process:
By the light-emitting diode LED or injected laser diode ILD issued by the optical signal propagation along the optical media, at the other end of the PIN or APD photodiode as a detector to receive the signal. Modulation of the optical carrier for the amplitude shift keying method, also known as brightness modulation (IntensityModulation). Typically, two binary digits are represented by the appearance and disappearance of light at a given frequency. The signals from light-emitting diode LEDs and injection laser diode ILDs can be modulated in this way, with the PIN and ILD detectors responding directly to the luminance modulation.
Power amplification - placing an optical amplifier before the optical transmitter to increase the optical power into the fiber. This allows the optical power of the entire line system to be increased. Online relay amplification - larger building groups or building spacing away from the farther, can play the role of relay amplification to improve the optical power. Pre-amplification - in the receiving end of the photoelectric detector will be amplified after the micro-signal to improve reception.
3, fiber optic transmission characteristics:
Fiber optic cable is not easy to branch, because the transmission of optical signals, so it is generally used for point-to-point connections. Fiber optic bus topology of the experimental multipoint system has been built, but the price is still too expensive. In principle, fiber optics can generally support a much larger number of taps than twisted-pair or coaxial cables because of their low power loss, reduced attenuation, and greater bandwidth potential. The current low-cost and reliable transmitters are light-emitting diode LEDs with a wavelength of 0.85um, which can support transmission rates of 100Mbps and LANs in the range of 1.5 to 2KM. Laser diode transmitters are more costly and do not meet the million-hour lifetime requirement.
Light-emitting diode detector PINs running at 0.85um wavelength are also low-cost receivers. Avalanche light-emitting diode signal gain than PIN, but to use 20 to 50V power supply, while the PIN detector only need to use 5V power supply. If you want to achieve longer distances and higher speeds, it can be used 1.3um wavelength system, this system attenuation is very small, but to be more expensive than the 0.85um wavelength system source. In addition, with the matching fiber optic connectors is also very important, the requirements of each connector connection loss of less than 25dB, easy to install, lower prices. Fiber optic core and aperture the larger, the more light received from the light-emitting diode LED, the better its performance. Core diameter of 100um, cladding diameter of 140um fiber, can provide quite good performance. Its received light energy than 62.5/125um fiber 4dB more than 50/125um fiber more than 8.5dB. running at 0.8um wavelength fiber attenuation of 6dB/Km, running at 1.3um wavelength fiber attenuation of 4dB/Km. 0.8um optical fiber bandwidth of 150MHz/Km, 1.3um optical fiber frequency bandwidth for the 500MHz/Km.
Comprehensive cabling system, the trunk line using optical fiber as a transmission medium is very appropriate and necessary.
Currently using a light wave wavelength division multiplexing technology WDM (WAVELENGTH DIVISION MULTI-PLEXING), can be multiplexed on a single line, sending, transmitting multiple bits, generally according to a byte of eight bits transmitted in parallel, for each bit stream to use a different wavelength, so it requires support circuitry can be operated at a low rate. fiber optic WDM links are suitable for byte-width device interfaces and are a new data transmission system.
(l) Laser communication
Transmission of information by light is very common today. For example, ships use light language communication, traffic lights with red, yellow and green color scheduling. But all these ways of transmitting information with ordinary light can only be limited to short distances. If you want to transmit information directly to a distant place through light, you can not use ordinary light, but only move the laser.
So how do you transmit a laser? We know that electricity can be transported along copper wires, but light cannot be transported along ordinary metal wires. For this reason, scientists have developed a thin wire that can transmit light, called an optical fiber, or fiber optics for short. Optical fibers are made of a special glass material, thinner than a human hair, usually 50 to 150 microns in diameter, and very soft.
In fact, the inner core of the fiber is a high refractive index of transparent optical glass, while the outer cladding is made of low refractive index glass or plastic. Such a structure, on the one hand, can make the light refracted along the inner core forward, just like water in the tap water pipe flow forward, electricity in the wire forward transmission, even if a thousand times around a hundred fold has no effect. On the other hand, the low-refractive-index cladding prevents light from escaping, just as water pipes don't seep, and the insulation of electrical wires doesn't conduct electricity.
The emergence of optical fibers solves the problem of transmitting light, but it doesn't mean that with it any light can be transmitted far, far away. Only high brightness, pure color, good directionality of the laser, is the most ideal light source for transmitting information, which is input from one end of the fiber, with little loss and output from the other end. Therefore, optical communication is essentially laser communication, which has the advantages of large capacity, high quality, wide source of materials, strong confidentiality, durability, etc. It has been hailed by scientists as a revolution in the field of communication, and is one of the most brilliant achievements in the technological revolution.
Where is laser communication advanced? The advantages of laser communication are first of all the large capacity. How much capacity does it have? When we usually talk on the phone, talking sometimes strings in disjointed speech. This fighting phenomenon is due to a pair of telephone lines can only pass through a phone, if another string into a phone, the normal call will be interfered with both sides. If there are 10 pairs of people talking at the same time with a pair of telephone lines, it is equal to 20 people talking at the same time, it is simply impossible to talk. In order to solve this problem, it is necessary to use the carrier wave and other methods, so that the road phone in each frequency band. As the ordinary telephone frequency range of 300 to 400 Hz, and in a pair of telephone lines on the highest frequency of only 1500 kHz, so in a pair of telephone lines can only be passed at the same time more than a dozen ways to telephone. Obviously, such telecommunication capacity is far from meeting the requirements of today's information society.
If we compare the amount of information transmitted by an ordinary telephone to that of a trolley, then laser communication is a car. Since the frequency of laser is much higher than that of radio waves, the information capacity of laser communication is one billion times larger than that of electrical communication. A single optical fiber, which is thinner than a strand of hair, can transmit tens of thousands of telephone calls or thousands of television programs. A fiber-optic cable made up of 20 fibers is only as thick as a pencil and can make 76,200 calls per day. By contrast, a cable made up of 1,800 copper wires, about 7.6 centimeters in diameter, can make only 900 calls a day.
Particularly surprising is the fact that fiber-optic communications are particularly well suited to the transmission of television, images and numbers. A pair of optical fibers can reportedly deliver a full set of the Encyclopedia Britannica in less than a minute.
In addition, the material used to make optical fibers is quartz, a sand found everywhere on Earth, and it only takes a few grams of quartz to make a 1-kilometer-long fiber. In this way, not only is the raw material inexhaustible, inexhaustible, but also can greatly save copper and aluminum. Because of this, the world's developed countries are now competing to study laser communications. So laser communication has become the favorite of the competition for development.
In the history of communications technology, the rapid development of fiber optic communications technology is unprecedented. Take a few milestones in the history of communications technology, the telephone from invention to application, it took about 60 years, and telephone communication is still a large number of common use. Radio technology (e.g., the telegraph) also took about 30 years from invention to application. Television technology, while more rapidly developed, was still gestating for about 14 years. Laser communication, on the other hand, from the birth of the first low-loss optical fiber to its application, the total *** only 5 years. Now laser communication is not only widely used, but also formed a huge fiber market.
In May 1977, the United States has a large company called Telegraph and Telephone Company, which is in the city of Chicago between the two telephone offices, laid the world's first short-distance optical fiber communication lines, and since then in the whole of the United States in nearly a hundred places to establish a total of several hundred kilometers of short-distance laser communication lines. This means that in the short distance, laser communication has begun to replace ordinary electrical communication. By 1983, the 600-kilometer-long fiber-optic communication between New York and Boston in the United States had been put into use.
Following the United States behind is Japan. 1984, Japan completed from Hokkaido's Sapporo to Fukuoka, Kyushu, long-distance fiber-optic communications trunk line, the total length of 2,800 kilometers, the middle of the link with more than 30 cities. 1993 December, China and Japan across the East China Sea fiber-optic cables have been laid successfully. A 10,000-kilometer-long undersea fiber-optic cable across the Pacific Ocean between Japan and the United States is also being designed.
As a result of the booming development of optical fiber communications, the United States, Japan, Britain, France and other industrially developed countries have set up optical fiber, optical fiber cable production enterprises. The world's three famous fiber optic cable company - the United States of America's Western Electric Company, Corning Incorporated and Japan's Sumitomo Corporation, optical fiber production is more than 120,000 kilometers per year.
In short, industrially developed countries have established a national fiber-optic communication network in order to completely replace the current copper wires and cables, this vast technical project is estimated to be completed by the year 2000. By that time, laser communication will bring great changes to our planet. For example, without leaving home, you can use fiber-optic network at home to deal with documents or participate in a meeting; or home fiber-optic network and shopping centers are connected, as in the supermarket, sitting at home to buy the goods you need, the payment must be settled only with the electronic financial shopping system. Medical centers around the world can also view the patient's condition and laboratory reports from the screen, and accordingly issued a prescription order, so as to truly achieve the "showman does not go out, you can know the world", "planning in a tent, the victory in a thousand miles away.
Lasers and optical fibers can also transmit images. First of all, the diameter of the single optical fiber than the human hair is thin enough to group into fiber bundles. In the process of transmitting information, there are two types of fiber bundles commonly used: one is called a light-transmitting bundle and the other is called an image-transmitting bundle. The task of the beam is to transmit light from one end to the other. The structure of the transmission beam is relatively simple, it is made of multiple monofilaments glued together, and then its end faces are polished and ground in order to reduce the reflection and scattering losses when light enters the optical fiber, and then a plastic sheath is applied to the outside of the transmission beam.
Because an optical fiber can only transmit a point of light, to transmit the entire image must be optical fiber one by one neatly arranged, so that the composition of the optical fiber bundle is called the transmission beam.
In the image beam, all the fibers are neatly arranged, the two ends of the position are strictly corresponding to one by one, there is no confusion, like a neat chopsticks. For example, one end of an optical fiber is in the eighth row and eighth column of the beam, and the other end of the fiber is also in the eighth row and eighth column of the beam.
The beam transmits the image, first splitting the image into a mesh, i.e., an image is broken down by countless optical fibers into countless pixels, and then transmitted. One fiber is responsible for transmitting one image element, and countless fibers will be able to transmit the entire image to the other end. If you want to make the image transmitted clearly, we must choose as much as possible, the diameter of the thin fiber, because the finer the fiber, in a certain image beam can accommodate into more light beams, so that it can transmit more like elements. Obviously, the more pixels, the clearer the image.
The beams used today are made up of tens of thousands of optical fibers, and it's not easy to align them neatly. Arranged, and then an organic adhesive called epoxy resin will be glued to the ends, so that the fiber bonding fixed, to ensure that the two ends of the fiber one to one. The two end surfaces should also be smoothed and polished. As for the middle part does not have to be glued, but as loose as the strings of the erhu, only to be added to the outside of the protection of the plastic sleeve, so that the transmission beam is both soft, but also can be bent arbitrarily.
In addition to transmitting images, the beam can also transmit general symbols or numbers, as well as zoom in and out of the image.
To enlarge an image, the beam can be made large at one end and small at the other, like a cone. When an image element is passed from the small end to the large end, the entire image is enlarged. Conversely, if the image is sent from the large end to the small end, the entire image is reduced.
In addition, images can be altered using optical fibers. If you need to intentionally disrupt the arrangement of optical fibers, you can make the exit end of the image element does not fall on the original corresponding point, but falls on the subjective conception of the point, so the image is changed. If the fiber at the inlet end of the image element is made square and the fiber at the outlet end is made circular, the square image element can be turned into a circular image element.
In short, the fiber optic imaging beam has great potential for development, in the future of optical information processing technology will increasingly show its unique role.