1966- Gao Kun, a scientist in China and the "father of optical fiber", predicted that optical fiber would be used for communication.
1970- Corning Company of the United States has successfully developed an optical fiber with a transmission loss of only 20dm/km.
1977- First practical installation of telephone optical fiber network
1978-FORT installed optical fiber power for the first time in France.
1979—— Zhao Zisen drew the first practical optical fiber independently developed in China, and was known as "the father of optical fiber in China".
1990- optical fiber for LAN and other short-distance transmission applications
2000-FTTH = > FTTH to Desktop
In 2005, FTTH (fiber-to-the-home) optical fiber was classified into inorganic optical fiber and polymer optical fiber, and the former is widely used in industry at present. Inorganic fiber materials can be divided into single-component and multi-component types. The main raw materials are silicon tetrachloride, phosphorus oxychloride and boron tribromide. Its purity requires that the impurity content of transition metal ions such as copper, iron, cobalt, nickel, manganese, chromium and vanadium be lower than 10ppb. In addition, the requirement of OH- ion is lower than 10ppb. Timely fiber has been widely used. There are many multi-component raw materials, mainly silicon dioxide, boron trioxide, sodium nitrate, thallium oxide and so on. This material has not been popularized. Polymer optical fiber is an optical fiber made of transparent polymer, which consists of a core and a sheath. The core material is a fiber made of high purity and high transmittance polymethylmethacrylate or polystyrene, and the outer layer is a fluoropolymer or silicone polymer.
The research and application of optical communication is closely related to the low loss of optical fiber. Whether the loss of light energy can be greatly reduced depends on the improvement of material purity. The optical absorption caused by impurities in glass materials causes the greatest optical loss, among which transition metal ions are particularly harmful. At present, due to the high purity of glass materials, these impurities have little effect on the loss of optical fibers.
The advantage of timely glass fiber is low loss. When the wavelength of light is 1.0 ~ 1.7 μ m (about 14μm), the loss is only 1dB/km, and the lowest is 0.2dB/km at 1.55 μ m, so the optical loss of polymer fiber is high. 1982, Japan Telecom and Telegraph Company adopted deuterated methyl methacrylate as the core material, and the light loss rate was reduced to 20dB/km. The characteristics of polymer optical fiber are that it can make large-size optical fiber with large numerical aperture, high light source coupling efficiency, good flexibility, slight bending without affecting light guiding ability, easy arrangement and bonding, convenient use and low cost. But the light loss is large, so it can only be used in a short distance. Optical fiber with optical loss 10 ~ 100 dB/km can transmit hundreds of meters.
Optical fibers are mainly divided into the following two categories:
1) transmission point modulus class
The transmission point modulus can be divided into single-mode fiber and multimode fiber. The core diameter of single-mode fiber is very small, and it can only be transmitted in single mode at a given working wavelength, with wide transmission frequency band and large transmission capacity. Multimode fiber is an optical fiber that can transmit in multiple modes at a given working wavelength. Compared with single-mode fiber, multimode fiber has poor transmission performance.
2) refractive index distribution category
Refractive index distribution fiber can be divided into jump fiber and graded fiber. The refractive index of the core and the refractive index of the protective layer are constant. At the interface between the core and the protective layer, the refractive index changes gradually. The refractive index of graded fiber core decreases with the increase of radius, and decreases to the refractive index of protective layer at the junction of core and protective layer. The change of refractive index of fiber core is similar to parabola. Optical fiber structure, types and characteristics of light;
Light is an electromagnetic wave.
The wavelength range of visible light is 390~760nm (nanometer). The part above 760nm is infrared light and the part below 390nm is ultraviolet light. The applications in optical fiber are: 850, 1300, 1550.
2. Refraction, reflection and total reflection of light.
Because the propagation speed of light in different substances is different, when light is emitted from one substance to another, it will be refracted and reflected at the interface between the two substances. Moreover, the angle of refracted light will change with the angle of incident light. When the angle of incident light reaches or exceeds a certain angle, the refracted light will disappear and all the incident light will be reflected back, which is the total reflection of light. Different substances have different refraction angles for light with the same wavelength (that is, different substances have different refractive indexes), and the same substance has different refraction angles for light with different wavelengths. Optical fiber communication is based on the above principle.
1. Optical fiber structure:
Bare optical fiber is generally divided into three layers: the middle high refractive index glass core (core diameter is generally 50 or 62.5μm), the middle low refractive index silica glass cladding (diameter is generally 125μm), and the outermost resin cladding is used for reinforcement.
2. Numerical aperture:
The light incident on the end face of the optical fiber can not be completely transmitted by the optical fiber, but only the incident light within a certain angle range. This angle is called the numerical aperture of the optical fiber. The larger numerical aperture of optical fiber is beneficial to the docking of optical fiber. Optical fibers produced by different manufacturers have different numerical apertures (at & amp； T corning)
3. Type of optical fiber:
A. According to the transmission mode of light in optical fiber, it can be divided into single-mode fiber and multimode fiber.
Multimode optical fiber: The central glass core is thick (50 or 62.5μm), which can transmit multiple modes of light. However, its large intermodal dispersion limits the frequency of transmitting digital signals, and it will become more serious with the increase of distance. For example, a 600MB/KM optical fiber has only 300MB bandwidth at 2KM. Therefore, the transmission distance of multimode fiber is relatively short, generally only a few kilometers. Single-mode fiber: The glass core in the center is thin (the core diameter is generally 9 or 10μm) and can only transmit one mode of light. Therefore, its intermodal dispersion is very small, which is suitable for long-distance communication, but its dispersion plays a major role. Therefore, single-mode fiber requires high spectral width and stability of light source, that is, narrow spectral width and good stability.
Single-mode fiber: generally, * * indicates fiber jumping, and the connector and protective sleeve are blue; Long transmission distance.
Multimode optical fiber: generally, optical fiber jumpers are indicated by orange, some are indicated by gray, and connectors and protection are indicated by beige or black; The transmission distance is short.
B according to the optimal transmission frequency window, it can be divided into conventional single-mode fiber and dispersion-shifted single-mode fiber.
Conventional type: optical fiber manufacturers optimize the transmission frequency of optical fiber at a single wavelength, such as 1300nm.
Dispersion shift type: optical fiber manufacturers optimize the transmission frequency of optical fiber at two wavelengths, such as 1300nm and 1550nm.
C. According to the refractive index distribution, it can be divided into abrupt fiber and graded fiber.
Abrupt type: The refractive index from the central core of the optical fiber to the glass cladding is abrupt. It has low cost and high dispersion between modules. Suitable for short-distance low-speed communication, such as industrial control. However, due to the small dispersion between modes, single-mode fibers are all abrupt.
Graded fiber: the refractive index decreases gradually from the central core of the fiber to the glass cladding, which can make the high-mode light propagate in sinusoidal form, reduce the intermodal dispersion, increase the bandwidth of the fiber and increase the transmission distance, but the cost is higher. Nowadays, multimode fibers are mostly graded fibers.
4. Commonly used optical fiber specifications:
Single mode: 8/ 125μm, 9/ 125μm,10/125μ m.
Multimode: 50/ 125μm, European standard.
62.5/ 125μm, American standard
Industrial, medical and low-speed networks: 100/ 140μm, 200/230μ m.
Plastic: 98/ 1000μm, the attenuation of optical fiber for automobile control is mainly caused by intrinsic, bending, extrusion, impurities, unevenness and butt joint.
Inherent: it is the inherent loss of optical fiber, including Rayleigh scattering and inherent absorption.
Bending: when the optical fiber is bent, some light in the optical fiber will be lost due to scattering, resulting in loss.
Squeeze: Loss caused by slight bending of optical fiber when squeezed.
Impurity: Loss caused by absorption and scattering of light propagating in optical fiber by impurities in optical fiber.
Non-uniformity: Loss caused by non-uniform refractive index of optical fiber materials.
Butt joint: Losses caused by fiber butt joint, such as: different axes (the coaxiality of single-mode fiber is required to be less than 0.8μm), the end face is not perpendicular to the axis, the end face is uneven, the butt joint diameter is not matched, and the welding quality is poor. 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 encrypted, 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. Optical fiber cable is usually called logarithmic cable, which is used for drainage and fire prevention, and is generally used for telephone. Logarithmic cables are generally divided into 3 types of logarithms and 5 types of logarithms, and further divided into 5 pairs 10 pairs, 20 pairs, 25 pairs, 30 pairs, 50 pairs 100 pairs, 200 pairs and 300 pairs.
Generally speaking, the large logarithm cable is often used as the voice backbone in weak current engineering.