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Shandong Yibo Optoelectronics Technology Co., Ltd. tells you that optical fiber is widely used in various medium and large networks as a high-bandwidth, high-security data transmission medium. Due to the high cost of cables and equipment, most of the fiber is used only for the network backbone, that is, the system wiring for the vertical trunk subsystem and the building group subsystem, to achieve the connection between buildings and between the floors, and is also currently applied to the transmission. A horizontal cabling subsystem with high requirements for speed and safety.
Basic knowledge and system design of optical cable wiring
First, the fiber
1, light and its characteristics:
1) Light is an electromagnetic wave
The visible light wavelength range is: 390 to 760 nm (nanometer). The portion larger than 760 nm is infrared light, and the portion smaller than 390 nm is ultraviolet light. The application in the fiber is: 850, 1300, 1550.
2) Refraction, reflection and total reflection of light.
Since the speed of light propagation in different materials is different, when light is emitted from one substance to another, refraction and reflection occur at the interface between the two substances. Moreover, the angle of the refracted light varies with the angle of the incident light. When the angle of incident light reaches or exceeds an angle, the refracted light will disappear and the incident light will be reflected back. This is the total reflection of light. Different materials have different angles of refraction for the same wavelength of light (ie different materials have different refractive indices of light), and the same material has different angles of refraction for different wavelengths of light. Optical fiber communication is based on the above principles.
2. Fiber structure and type:
1) Fiber structure:
The fiber bare fiber is generally divided into three layers: a central high refractive index glass core (core diameter is generally 50 or 62.5 μm), a low refractive index silica glass cladding (typically 125 μm in diameter), and the outermost resin coating for reinforcement. Floor.
2) Numerical aperture:
The light incident on the end face of the fiber cannot be transmitted entirely by the fiber, but only the incident light within a certain angle range. This angle is called the numerical aperture of the fiber. The larger numerical aperture of the fiber is advantageous for the docking of the fiber. Optical fibers produced by different manufacturers have different numerical apertures (AT&T??CORNING).
3) Type of fiber:
A. According to the transmission mode of light in the optical fiber, it can be divided into single-mode fiber and multi-mode fiber.
Multimode fiber: The center glass core is thicker (50 or 62.5 μm) and can transmit multiple modes of light. However, the dispersion between the modes is large, which limits the frequency of transmitting digital signals, and is more serious as the distance increases. For example, a fiber of 600 MB/KM has a bandwidth of only 300 MB at 2 KM. Therefore, the distance traveled by multimode fiber is relatively close, usually only a few kilometers.
Single mode fiber: The center glass core is thinner (the core diameter is generally 9 or 10 μm) and can only transmit one mode of light. Therefore, the inter-mode dispersion is small, suitable for remote communication, but its chromatic dispersion plays a major role, so that single-mode fiber has high requirements on the spectral width and stability of the light source, that is, the spectral width is narrow and the stability is good. .
B. According to the optimal transmission frequency window: conventional single mode fiber and dispersion shifted single mode fiber.
Conventional: Fiber manufacturers optimize fiber transmission frequencies on a single wavelength of light, such as 1300 nm.
Dispersion-displacement type: The fiber production family optimizes the fiber transmission frequency to two wavelengths of light, such as 1300 nm and 1550 nm.
C. According to the refractive index distribution: mutant and graded fiber.
Mutant: The refractive index of the fiber core to the glass cladding is abrupt. The cost is low and the dispersion between the modes is high. Suitable for short-distance low-speed communication, such as: industrial control. However, single-mode fibers use a mutant type because of the small dispersion between modes.
Gradient fiber: The refractive index of the fiber core to the glass cladding is gradually reduced, which can make the high mode light propagate in a sinusoidal form. This can reduce the dispersion between modes, increase the fiber bandwidth, increase the transmission distance, but the cost is higher. Most of the mode fibers are graded fibers.
4) Common fiber specifications:
Single mode: 8/125μm, 9/125μm, 10/125μm
Multimode: 50/125μm, European standard 62.5/125μm, US standard
Industrial, medical and low speed networks: 100/140μm, 200/230μm
Plastic: 98/1000μm for automotive control
3. Optical fiber manufacturing and attenuation:
1) Fiber manufacturing:
At present, optical fiber manufacturing methods mainly include: intra-tube CVD (chemical vapor deposition), in-rod CVD, PCVD (plasma chemical vapor deposition) and VAD (axial vapor deposition).
2) Attenuation of the fiber:
The main factors causing fiber attenuation are: intrinsic, bending, extrusion, impurities, unevenness and docking.
Intrinsic: is the inherent loss of fiber, including: Rayleigh scattering, inherent absorption.
Bending: The loss of light in some of the fibers when the fiber is bent due to scattering.
Extrusion: Loss caused by tiny bends in the fiber when it is squeezed.
Impurities: Impurities in the fiber that absorb and scatter light propagating in the fiber.
Uneven: loss due to uneven refractive index of the fiber material.
Docking: Loss caused when the fiber is docked, such as: different axes (single mode fiber coaxiality requirement is less than 0.8μm), the end face is not perpendicular to the axis, the end face is not flat, the butt diameter is not matched, and the welding quality is poor.
4, the advantages of fiber:
1) The pass band of the fiber is very wide. The theory can reach 3 billion megahertz.
2) No relay section length. From tens to more than 100 kilometers, the copper wire is only a few hundred meters.
3) Not affected by electromagnetic fields and electromagnetic radiation.
4) Light weight and small size. For example: 900 pairs of twisted pairs with 21,000 channels, 3 inches in diameter and 8 tons / KM. The cable with ten times the communication capacity is 0.5 inches in diameter and weighs 450P/KM.
5) Optical fiber communication is not charged, and it can be used for flammable and explosive places.
6) The ambient temperature range is wide.
7) Chemical corrosion, long service life.
Second, the optical cable
1. Manufacturing of optical cable:
The manufacturing process of optical cable is generally divided into the following processes:
1) Screening of optical fibers: Fibers with excellent transmission characteristics and acceptable tension are selected.
2) Dyeing of optical fiber: It is marked by the standard full chromatogram, and it is required that the high temperature does not fade and does not migrate.
3) Secondary extrusion: plastics with high elastic modulus and low linear expansion coefficient are extruded into pipes of a certain size, and the optical fibers are incorporated into the moisture-proof and waterproof gel, and finally stored for several days (not less than two days). .
4) Cable stranding: stranding a number of extruded fibers with a reinforcing unit.
5) Extruded cable outer sheath: a sheath is attached to the stranded cable.
2. Types of optical cable:
1) According to the laying method: self-supporting overhead optical cable, pipeline optical cable, armored buried optical cable and submarine optical cable.
2) According to the optical cable structure: bundle tube type optical cable, layer twisted optical cable, tightly held optical cable, ribbon optical cable, non-metallic optical cable and branchable optical cable.
3) According to the purpose: long-distance communication optical cable, short-distance outdoor optical cable, hybrid optical cable and optical cable used in buildings.
3. Construction of optical cable:
Over the years, we have had a mature methodology and experience in cable construction.
Outdoor construction of fiber optic cable:
The most important thing for a longer cable installation is to choose a suitable path. The shortest path here is not the best, but also pay attention to the right to use the land, the possibility of erecting or burying.
Must have a very complete design and construction drawings, so that construction and future inspections are convenient and reliable. During construction, be careful not to subject the cable to heavy pressure or to be damaged by hard objects.
When the cable is turned, its turning radius is 20 times larger than the diameter of the cable itself.
1) Outdoor overhead cable construction:
A. Hanging wire support hanging overhead mode, this method is simple and cheap, China is the most widely used, but it is more time-consuming to hook up and hang.
B. Hanging wire winding overhead mode, this method is more stable and less maintenance work. But a special entanglement machine is needed.
C. Self-supporting overhead type, high requirements for line drying, difficult construction and maintenance, high cost, and rarely used in China.
D. When overhead, the cable should be connected to the trunk of the cable to be guided, and the cable should be avoided. Pay attention to reduce friction when pulling the cable. A cable for expansion and contraction is left on each of the stems.
E. Pay attention to the reliable grounding of metal objects in the cable. Especially in mountainous areas, high-voltage power grid areas and many areas, there are generally 3 grounding points per kilometer, and even non-metallic optical cables are used.
2) Outdoor pipeline cable construction:
A. Check the pipe occupation before construction, clean and place the plastic sub-tube, and put it into the traction line.
B. Calculate the length of the layout, there must be enough reserved length.
C. Do not lengthen the length of one time (usually 2KM). When wiring, it should be pulled from the middle to both sides.
D. The traction force of the cable is generally not more than 120kg, and the reinforcing part of the cable should be pulled, and the waterproofing of the cable head should be made.
E. The introduction and exit of the cable must be added with a directing device, which cannot be directly mopped.
F. Pipeline cable should also pay attention to reliable grounding.
3) Directly laying the cable:
A. The depth of the directly buried cable trench should be excavated according to the standard. The standard is shown in the following table:
B. Where trenches cannot be dug, it can be laid or drilled in pre-buried pipelines.
C. The bottom of the ditch should be kept flat and strong, and some sand, cement or support can be pre-filled when needed.
D. Manual or mechanical traction can be used during laying, but attention should be paid to guiding and lubrication.
E. After the laying is completed, it should be returned to the soil as soon as possible and compacted.
4) Laying of optical cables in buildings:
A. When laying vertically, pay special attention to the bearing capacity of the optical cable. Generally, the optical cable should be fixed once every two layers.
B. When the cable passes through the wall or wears the floor, it is necessary to add a protective plastic tube with a protective cover, and fill the tube with a flame-retardant filler.
C. A certain amount of plastic pipes can also be pre-applied in the building. When the fiber optic cable is to be applied later, the cable is pulled by traction or vacuum.
4, the choice of optical cable:
In addition to the number of fiber cores and the type of fiber, the cable is selected according to the environment in which the cable is used.
1) When the outdoor optical cable is directly buried, the armored optical cable should be selected. For overhead, an optical cable with a black plastic outer jacket with two or more ribs is available.
2) The optical cable used in the building should pay attention to its characteristics of flame retardant, poison and smoke when selecting. Flame retardant but smoked types (Plenum) are generally available in pipelines or in forced air, and flame retardant, non-toxic and smokeless types (Riser) should be used in exposed environments.
3) When the vertical cable is installed in the building, the layered cable (DistributionCables) can be used; when the horizontal wiring is used, the branch cable can be used.
4) If the transmission distance is within 2km, multimode fiber optic cable can be selected, and more than 2km can be used for relay or single mode fiber optic cable.
Third, connection and detection
1. Cable connection:
The methods mainly include permanent connection, emergency connection, and active connection.
1) Permanent fiber connection (also known as hot melt):
This connection is to melt and join the joints of the connected fibers by means of electrical discharge. Generally used in long-distance connection, permanent or semi-permanent fixed connection. Its main feature is that the connection attenuation is the lowest among all connection methods, with a typical value of 0.01~0.03dB/point. However, when connecting, special equipment (welding machine) and professional personnel are required to operate, and the connection point also needs special container protection.
2) Emergency connection (also called) cold melting:
The emergency connection is mainly to fix and bond the two fibers together by mechanical and chemical methods. The main feature of this method is that the connection is fast and reliable, and the typical attenuation of the connection is 0.1~0.3dB/point. However, the long-term use of the connection point will be unstable, and the attenuation will be greatly increased, so it can only be used for emergency use in a short time.
3) Activity connection:
Active connections are a way to connect a site to a site or site to a fiber optic cable using a variety of fiber optic connectors (plugs and sockets). This method is flexible, simple, convenient, and reliable, and is often used in computer network wiring in buildings. Its typical attenuation is 1dB/connector.
2. Fiber inspection:
The main purpose of fiber inspection is to ensure the quality of the system connection, reduce the failure factor and identify the failure point of the fiber when the fault occurs. There are many detection methods, which are mainly divided into manual simple measurement and precision instrument measurement.
1) Manual and simple measurement:
This method is generally used to quickly detect the on and off of the fiber and to resolve the fiber made during construction. It uses a simple light source to convert visible light from one end of the fiber and observe which one is illuminated from the other end. Although this method is simple, it cannot quantitatively measure the attenuation of the fiber and the breakpoint of the fiber.
2) Precision instrument measurement:
Quantitative measurement of the fiber using an optical power meter or an optical time domain reflectometry (OTDR) can measure the attenuation of the fiber and the attenuation of the connector, and even the breakpoint position of the fiber. This measurement can be used to quantify the cause of failures in fiber optic networks and to evaluate fiber optic network products.
Fourth, the application of fiber and system design
1. Application of optical fiber:
Human society has now developed into the information society, and the exchange of information such as sound, images and data is very large. Previous communication methods have been unable to meet the current requirements, and optical fiber communication has been widely used due to its advantages of large information capacity, good confidentiality, light weight and small size, and long distance without hops. Its applications span the communications, transportation, industrial, medical, education, aerospace and computer industries, and are moving toward a broader and deeper level. The application of light and fiber is bringing profound impact and change to human life.
2, fiber network system design:
The design of a fiber optic system generally follows the following steps:
1) First, figure out what kind of network is designed, what is the status quo, and why use fiber.
2) According to the actual situation, it is suitable for fiber network equipment, optical cable, jumper and other items for connection. Selection should be based on availability and then based on performance, price, service, origin and brand.
3) Determine the route of the line according to the customer's requirements and network type, and draw the wiring diagram.
4) When the route is long, the attenuation margin of the system needs to be calculated. The accounting can be carried out according to the following formula:
Attenuation margin = transmitted optical power - acceptance sensitivity - line attenuation - connection attenuation (dB) where line attenuation = cable length x unit attenuation;
The unit attenuation has a great relationship with the fiber quality, generally 0.4~0.5dB/km for single mode and 2~4dB/km for multimode.
The connection attenuation includes the attenuation of the fusion attenuating joint. The welding attenuation is related to the welding means and the quality of the personnel. Generally, the hot melt is 0.01~0.3dB/point; the cold melting is 0.1~0.3dB/point; the joint attenuation has a great relationship with the quality of the joint. Usually 1dB / point. The system attenuation margin is generally not less than 4 dB.
5) When the accounting is unqualified, the design should be revised according to the situation and then calculated. This situation may sometimes be repeated several times.
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