Definition: The abbreviation of optical fiber is a light transmission tool based on the principle of total reflection of light in glass or plastic fibers. Optical fiber was invented by Gao Kun, former president of the Chinese University of Hong Kong.
The tiny optical fiber is encapsulated in a plastic sheath, so it can be bent without breaking. Generally, a transmitting device at one end of an optical fiber uses a light emitting diode (LED) or a laser beam to transmit light pulses to the optical fiber, while a receiving device at the other end of the optical fiber uses a photosensitive element to detect the pulses.
In daily life, optical fiber is used for long-distance information transmission because the transmission loss of light in optical fiber is much lower than that of electricity in wires.
Generally, the terms optical fiber and optical cable are confused. Most optical fibers are coated with several layers of protective structures before use, and the coated optical cable is called optical cable. The protective structure of the outer layer of the optical fiber can prevent the surrounding environment from damaging the optical fiber, such as water, fire and electric shock. Optical fiber can be divided into optical fiber, buffer layer and coating. Optical fiber is similar to coaxial cable, but there is no mesh shielding layer. The center is the glass core of light propagation. In multimode fiber, the diameter of the fiber core is 1.5 μ m ~ 50 μ m, which is roughly equivalent to the thickness of human hair. The diameter of single-mode fiber core is 8 μ m ~ 10 μ m, and the fiber core is wrapped by glass cladding with lower refractive index than the fiber core, so that the fiber is kept in the fiber core. Outside is a thin plastic coat to protect the envelope. Optical fibers are usually bundled and protected by a housing. The fiber core is usually a double-layer concentric cylinder with a small cross-sectional area, which is made of timely glass, fragile and easy to break, and needs a protective layer.
The invention and use of optical fiber;
/kloc-one day in 0/870, the British physicist Tindal went to the lecture hall of the Royal Society to talk about the principle of total reflection of light. He did a simple experiment: drill a hole in a wooden bucket filled with water, and then use a lamp to illuminate the water surface from the top of the bucket. The result surprised the audience. People see sparkling water flowing out of the small hole in the bucket. The water bends and the light bends, and the light is actually captured by the winding water.
It has been found that light energy propagates along the stream of wine ejected from the barrel; It has also been found that light energy propagates along a curved glass rod. Why is this? Isn't the light coming straight in? These phenomena have attracted the attention of Tindal. After his research, he found that this is the effect of total reflection, that is, when the incident angle is greater than a certain angle, the refracted light disappears and all the light is reflected back into the water. On the surface, light seems to bend forward in the current. In fact, in the curved water flow, the light still travels in a straight line, but it has been totally reflected on the inner surface for many times, and the light travels forward after many times of total reflection.
Later, people made a kind of glass fiber with high transparency and thickness like spider silk-glass fiber. When light enters the glass fiber at an appropriate angle, the light travels along the wound glass fiber. This kind of fiber is called optical fiber because it can be used to transmit light.
Optical fiber can be used in communication technology. 1979 September, Beijing built a 3.3km 120 optical cable communication system. A few years later, optical cable lines were laid in Shanghai, Tianjin, Wuhan and other places, and optical fibers were used for communication.
Communication using optical fiber is called optical fiber communication. A pair of metal telephone lines can only transmit more than 1000 calls at the same time, but according to theoretical calculation, a pair of optical fibers as thin as spider silk can transmit1000 billion calls at the same time! Laying 1000 km coaxial cable requires about 500 tons of copper, and it only takes a few kilograms to switch to optical fiber communication. It is contained in the sand in time, and the sand is almost inexhaustible.
In addition, the endoscope made of optical fiber can help doctors check the diseases of stomach, esophagus and duodenum. Fiber optic gastroscope is a hose composed of thousands of glass fibers. It has the ability to transmit light and images, and it is soft, elastic and can be bent at will. It can be inserted into the stomach through the esophagus. Optical fiber transmits images in the stomach, so that doctors can see the situation in the stomach, and then make diagnosis and treatment according to the situation.
Application of optical fiber system
The optical cable made of multi-fiber can be used for communication, with good conductivity and large information transmission capacity, and one channel can accommodate one billion people at the same time. It can transmit thousands of TV programs at the same time for free choice. Fiber optic endoscope can be introduced into heart and ventricle to measure heart blood pressure, oxygen saturation and body temperature. The laser scalpel connected with optical fiber has been applied in clinic and can be used for the treatment of photosensitive cancer.
Optical fiber can send sunlight to every corner and can also be processed. Computer, robot, automobile switchboard, etc. Optical fibers have also been successfully used to transmit light sources or images. If it is combined with sensitive elements or uses its own characteristics, it can be made into various sensors to measure pressure, flow, temperature, displacement, luster, color and so on. It is also widely used in energy transmission and information transmission.
At the beginning of the development of polymer optical fiber, it was only used for the control and decoration of automobile lighting. Now it is mainly used in medicine, decoration, automobiles, ships and other aspects, mainly display components. In communication and image transmission, the application of polymer optical fiber is increasing, which is used in light guide, display panel, logo, switch dimming, optical sensor and so on. And decorative display and advertising display.
The history of optical fiber
1880-Alexandra Graham Bell invented light beam call transmission.
1960- the invention of electricity and optical fiber
1977- First practical installation of telephone optical fiber network
1978-FORT installed optical fiber power for the first time in France.
1990- optical fiber for LAN and other short-distance transmission applications
2000-FTTH = > FTTH to Desktop
In 2005, FTTH (Fiber to the Home) fiber directly to the home.
Advantages of optical fiber transmission
Until 1960, American scientist Maiman invented the world's first laser, which provided a good light source for optical communication. In the next two decades, people have overcome the key problems in optical transmission media and finally made low-loss optical fibers, thus laying the foundation stone of optical communication. Since then, optical communication has entered a stage of rapid development.
Optical fiber transmission has many outstanding advantages:
1。 frequency band width
The width of the frequency band represents the transmission capacity. The higher the carrier frequency, the wider the signal frequency band that can be transmitted. In VHF band, the carrier frequency is 48.5 MHz ~ 300 MHz. The bandwidth is about 250MHz, and only 27 TV sets and dozens of FM broadcasts can be transmitted. The frequency of visible light reaches 100000GHz, which is more than one million times higher than VHF band. Although the bandwidth is affected by the loss of light with different frequencies, the bandwidth of the lowest loss region can also reach 30000GHz. At present, the bandwidth of a single light source only accounts for a small part of it (the frequency band of multimode fiber is about several hundred megahertz, and a good single-mode fiber can reach more than 10GHz). Advanced coherent optical communication can arrange 2000 optical carriers for wavelength division multiplexing in the range of 30000GHz, which can accommodate millions of channels.
2. Low loss
In the system composed of coaxial cables, the best cable has a loss of more than 40dB per kilometer when transmitting 800MHz signals. In contrast, the loss of optical fiber is much smaller. The loss per kilometer is below 0.35dB when transmitting 1 and 3 1um light. If transmitting 1.55um light, the loss per kilometer is even smaller, reaching below 0.2dB, which is less than the power loss of coaxial cable by 1 billion times. In addition, optical fiber transmission loss has two characteristics. First, its loss in all cable TV channels is the same, and it doesn't need to attract an equalizer like cable trunk. Second, its loss hardly changes with temperature, so there is no need to worry about the fluctuation of trunk line level caused by environmental temperature change.
3. Lightweight
Because the fiber is very thin, the core diameter of single-mode fiber is generally 4 um ~ 10 um, and the outer diameter is only 125um. With waterproof layer, stiffener and sheath, the diameter of the optical cable composed of 4 ~ 48 optical fibers is less than 13mm, which is much smaller than that of the standard coaxial cable. In addition, the optical fiber is glass fiber with small specific gravity.
4. Strong anti-interference ability
Because the basic component of optical fiber is timely, it only transmits light, is not conductive, and is not affected by electromagnetic field, and the optical signal transmitted in it is not affected by electromagnetic field, so optical fiber transmission has strong ability to resist electromagnetic interference and industrial interference. Because of this, the signal transmitted in optical fiber is not easy to be eavesdropped, which is beneficial to confidentiality.
5. high fidelity
Because optical fiber transmission generally does not need relay amplification, it will not introduce new nonlinear distortion due to amplification. As long as the linearity of the laser is good, TV signals can be transmitted with high fidelity. The actual test shows that the three-beat ratio C/CTB of a good AM optical fiber system is above 70dB, and the intermodulation index cM is above 60dB, which is much higher than the nonlinear distortion index of the general cable trunk system.
6. Reliable working performance
We know that the reliability of the system is related to the number of devices that make up the system. The more devices, the greater the chance of failure. Because the optical fiber system contains a small number of devices (unlike wired systems that need dozens of amplifiers), the reliability is naturally high. In addition, the life of optical fiber devices is very long, and the trouble-free working time is 500,000 ~ 750,000 hours. Among them, the laser in the optical transmitter has the shortest life, with the minimum life exceeding 65438+ million hours. Therefore, the working performance of a well-designed and correctly installed optical fiber system is very reliable.
7. The cost is decreasing.
At present, some people have put forward a new Moore's law, also called the law of optics. The law points out that the bandwidth of optical fiber transmission information increases by 1 times every six months, while the price decreases by 1 times. The development of optical communication technology has laid a very good foundation for the development of Internet broadband technology. This clears the last obstacle for large-scale cable TV system to adopt optical fiber transmission mode. Because the materials for making optical fibers are very rich, with the progress of technology, the cost will be further reduced; However, the copper raw material needed for cables is limited, and the price will be higher and higher. Obviously, in the future, optical fiber transmission will occupy an absolute advantage and become the most important transmission means to establish the cable TV network in the whole province and even the whole country.
Structure Principle Optical fiber consists of two layers of glass with different refractive indexes. The inner layer is an optical core with a diameter of several microns to tens of microns, and the outer layer has a diameter of 0.1~ 0.2 mm. Generally, the refractive index of the inner core glass is higher than that of the outer layer glass 1%. According to the principle of refraction and total reflection of light, when the angle of light incident on the interface between the inner core and the outer layer is greater than the critical angle of total reflection, the light cannot penetrate the interface and is totally reflected. At this time, the light passes through countless total reflections at the interface, propagates forward in a zigzag route in the inner core, and finally reaches the other end of the optical fiber. This kind of optical fiber belongs to the sheath-core structure. If the refractive index of the inner glass is uniform, it suddenly changes at the interface and decreases to the refractive index of the outer glass, which is called step structure. For example, the cross-sectional refractive index of the inner core glass changes outward from the center to the outer glass with low refractive index, which is called gradient structure. The outer glass has optical insulation to prevent the inner glass from being polluted. There is also a kind of optical fiber called self-focusing structure, which seems to be composed of many micro-lenticular lenses, forcing the incident light to converge to the center gradually and automatically. The refractive index of this kind of fiber is the highest in the center, and decreases continuously and uniformly around until the edge is the lowest.
Optical fiber communication:
Optical fiber communication technology stands out from optical communication, becomes one of the main pillars of modern communication, and plays an important role in modern telecommunication network. As a new technology, the rapid development and wide application of optical fiber communication in recent years is rare in the history of communication, and it is also an important symbol of the world's new technological revolution and the main transmission tool of various information in the future information society.
Optical fiber is short for optical fiber. Optical fiber communication is a communication mode with light wave as information carrier and optical fiber as transmission medium. In principle, the basic material elements of optical fiber communication are optical fiber, light source and optical detector. In addition to classifying optical fibers according to manufacturing process, material composition and optical characteristics, optical fibers are often classified according to their applications, which can be divided into optical fibers for communication and optical fibers for sensing. Transmission medium optical fiber can be divided into general purpose and special purpose, and functional device optical fiber refers to the optical fiber used to complete the functions of light wave amplification, shaping, frequency division, frequency doubling, modulation and optical oscillation, and often appears in the form of some functional device. The rapid development of optical fiber communication is mainly due to its following characteristics:
(1) has large communication capacity and long transmission distance; The potential bandwidth of optical fiber can reach 20 Hz. With this bandwidth, it only takes about one second to transmit all human written materials, ancient and modern, Chinese and foreign. At present, 400Gbit/s system has been put into commercial use. The loss of optical fiber is extremely low. When the wavelength of light is about 1.55μm, the loss of optical fiber can be lower than 0.2dB/km, which is lower than that of any transmission medium at present. Therefore, the transmission distance without relay can reach tens or even hundreds of kilometers.
(2) Small signal crosstalk and good safety performance;
(3) Anti-electromagnetic interference and good transmission quality. Electric communication can't solve all kinds of electromagnetic interference problems, only optical fiber communication is free from all kinds of electromagnetic interference.
(4) The optical fiber is small in size and light in weight, which is convenient for laying and transportation;
(5) The raw materials are rich in sources and have good environmental protection, which is beneficial to saving nonferrous metal copper.
(6) Without radiation, it is difficult to eavesdrop, because the light wave transmitted by optical fiber cannot run out of the optical fiber.
(7) The optical cable has strong adaptability and long service life.
(8) Brittle texture and poor mechanical strength.
(9) Cutting and splicing of optical fibers require certain tools, equipment and technology.
(10) The diverter and coupling are not flexible.
(1 1) The bending radius of optical cable should not be too small (>; 20 cm)
(12) Power supply is difficult.
A communication method that uses light waves to transmit information in optical fibers. Because laser has obvious advantages such as high directivity, high coherence and high monochromaticity, the light wave in optical fiber communication is mainly laser, so it is also called laser-optical fiber communication.
The principle of optical fiber communication is: at the transmitting end, the transmitted information (such as voice) should be converted into an electrical signal, and then modulated on the laser beam emitted by the laser, so that the intensity of light changes with the amplitude (frequency) of the electrical signal, and then sent out through the optical fiber; At the receiving end, the detector converts the optical signal into an electrical signal and recovers the original information after demodulation.
Optical fiber communication is the main transmission means of modern communication network. Its development history is only one or two decades, and it has experienced three generations: short wavelength multimode fiber, long wavelength multimode fiber and long wavelength single mode fiber. The adoption of optical fiber communication is a major change in the history of communication. More than 20 countries, including the United States, Japan, Britain and France, have announced that they will no longer build wired communication lines and are committed to developing optical fiber communication. China optical fiber communication has entered the practical stage.