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Views: 0 Author: Site Editor Publish Time: 2025-07-15 Origin: Site
Fiber optics, as a high-bandwidth and high-security data transmission medium, is widely used in the construction of various medium and large-scale networks. Due to the high cost of cables and equipment, most fibers were originally only used in the main network, and were applied to the vertical backbone subsystem and building group subsystem for system wiring to connect buildings and floors. Currently, they are also used in the horizontal wiring subsystem with higher requirements for transmission rate and security.
The wavelength range of visible light is: 390 - 760 nm (nanometers). The part longer than 760 nm is infrared light, and the part shorter than 390 nm is ultraviolet light. The types of light used in optical fibers are: 850, 1300, and 1550.
The speed of light propagation in different substances is different. Therefore, when light travels from one substance to another, a refraction and reflection occur at the interface of the two substances. Moreover, the angle of the refracted light changes with the angle of the incident light. When the angle of the incident light reaches or exceeds a certain angle, the refracted light disappears, and the incident light is completely reflected back. This is called total reflection. Different substances have different refraction angles for the same wavelength light (that is, different substances have different light refractive indices), and the same substance has different refraction angles for different wavelengths of light. Optical fiber communication is formed based on the above principles.
The bare fiber of the optical fiber is generally divided into three layers: the central high-refractive-index glass core (the core diameter is usually 50 or 62.5 μm), the middle layer is the low-refractive-index silica glass cladding (the diameter is generally 125 μm), and the outermost layer is the resin coating used for reinforcement.
Multi-mode fibers:The central glass core is relatively thick (50 or 62.5 μm), and can transmit multiple modes of light. However, the inter-mode dispersion is relatively large, which limits the frequency of digital signal transmission and becomes more severe with the increase of distance. For example, a 600MB/KM fiber has only 300MB bandwidth at 2KM. Therefore, the transmission distance of multi-mode fibers is relatively short, usually only a few kilometers.
Single-mode fibers: The central glass core is relatively thin (the core diameter is generally 9 or 10 μm), and can only transmit one mode of light. Therefore, the inter-mode dispersion is very small, and it is suitable for long-distance communication. However, chromatic dispersion plays a major role, so single-mode fibers have higher requirements for the spectral width and stability of the light source, that is, the spectral width should be narrow and the stability should be good.
Conventional: Fiber manufacturers optimize the transmission frequency of the fiber at a single wavelength of light, such as 1300nm.
Dispersion-shifted: Fiber manufacturers optimize the transmission frequency of the fiber at two wavelengths of light, such as: 1300nm and 1550nm.
Abrupt type:The refractive index from the core to the glass cladding of the optical fiber is abrupt. It has low cost and high inter-mode dispersion. It is suitable for short-distance low-speed communication, such as industrial control. However, single-mode optical fibers use the abrupt type because the inter-mode dispersion of single-mode fibers is very small.
Gradual type optical fibers:The refractive index from the core to the glass cladding of the optical fiber gradually decreases, allowing high-mode light to propagate in a sine form. This can reduce inter-mode dispersion, increase the fiber bandwidth, and increase the transmission distance. However, it has a higher cost. Currently, most multi-mode optical fibers are of the gradual type.
Common optical fiber specifications:
Single mode: 8/125μm, 9/125μm, 10/125μm. Multi-mode: 50/125μm, European standard 62.5/125μm, American standard for industrial, medical and low-speed networks: 100/140μm, 200/230μm. Plastic: 98/1000μm, used for automotive control.
Fiber Manufacturing
The current methods for manufacturing fibers mainly include: in-tube CVD (chemical vapor deposition), in-rod CVD, PCVD (plasma chemical vapor deposition), and VAD (axial vapor deposition).
The main factors causing fiber attenuation are: intrinsic, bending, compression, impurities, unevenness, and splicing, etc.
Intrinsic: This is the inherent loss of the fiber, including: Rayleigh scattering, intrinsic absorption, etc.
Bending: When the fiber is bent, some of the light in the fiber will be lost due to scattering, resulting in attenuation.
Compression: The fiber experiences a slight bending when compressed, causing attenuation.
Impurities: The impurities in the fiber absorb and scatter the light propagating within it, resulting in loss.
Unevenness: The loss caused by the uneven refractive index of the fiber material.
Splicing: Losses generated during the docking of optical fibers, such as: non-coaxiality (the coaxiality requirement for single-mode optical fibers is less than 0.8μm), the end face is not perpendicular to the axis center, the end face is uneven, the core diameter of the docking does not match, and the splicing quality is poor, etc.
The transmission bandwidth of optical fibers is very wide. Theoretically, it can reach 30 billion megahertz.
There is no repeater section. It is only several tens to over 100 kilometers long, while copper wires are only a few hundred meters long.
It is not affected by electromagnetic fields and radiation.
Light in weight and small in size. For example, for a 900-pair twisted pair cable with 21,000 channels, its diameter is 3 inches and its weight is 8 tons per kilometer. While the optical cable with ten times the communication capacity has a diameter of 0.5 inches and a weight of 450 pounds per kilometer.
Optical fiber communication is not electrified. It is safe to use and can be used in flammable and explosive places.
The operating temperature range is wide.
Chemically resistant and has a long service life.
Fiber selection: Select fibers with excellent transmission characteristics and appropriate tension.
Fiber coloring: Use a standard full color spectrum for identification, requiring no fading or migration at high temperatures.
Secondary extrusion: Select a plastic with high elastic modulus and low coefficient of thermal expansion to extrude into a certain size tube. Insert the optical fibers and fill them with moisture-proof and water-proof gel, and store for several days (at least two days).
Cable twisting: Twist several extruded optical fibers together with the reinforcement unit.
Extrusion of outer protective sheath of the optical cable: Add an outer protective sheath to the twisted optical cable.
According to the laying method, there are: self-supporting overhead optical cables, pipeline optical cables, armored buried optical cables and submarine optical cables.
According to the optical cable structure, there are: bundled tube optical cables, layer-wound optical cables, tightly wrapped optical cables, belt-type optical cables, non-metallic optical cables and branchable optical cables.
According to the purpose, there are: long-distance communication optical cables, short-distance outdoor optical cables, mixed optical cables and optical cables for buildings.
Over the years, conducting optical cable construction has enabled us to develop a mature method and experience.
Outdoor construction of optical cables: For long-distance optical cable laying, the most important factor is to select a suitable path. The shortest path is not necessarily the best; one must also consider the land usage rights, the possibility of erection or underground laying, etc. Complete design and construction drawings must be available to ensure convenient and reliable construction and future inspection. During the construction, one must always be careful not to subject the optical cable to heavy pressure or be injured by hard objects. When the optical cable turns, its turning radius should be greater than 20 times the diameter of the optical cable.
Suspension cable support overhead method: This method is simple and inexpensive and is the most widely used in China. However, the installation of hooks and hanging is time-consuming.
Cable winding overhead method: This method is more stable and requires less maintenance work. However, a specialized winding machine is required.
Self-bearing overhead method: This method has high requirements for the cable trunk and is difficult to construct and maintain. It is costly and is currently rarely used in China.
When laying the optical cable overhead, a guiding device must be added at the end of the cable trunk, and the optical cable must not drag on the ground. Pay attention to reducing friction during cable traction. Each trunk should have a remaining section for expansion.
Pay attention to the reliable grounding of metal objects in the optical cable. Especially in mountainous areas, high-voltage power grids, and multiple areas, there should be 3 grounding points per kilometer, or non-metallic optical cables should be selected.
Before construction, the occupation of pipelines should be checked, plastic sub-pipes should be cleaned and placed, and traction lines should be put in at the same time.
Calculate the layout length accurately and make sure there is sufficient reserved length.
The length of each deployment should not be too long (generally 2 kilometers). When wiring, start pulling from the middle to both sides.
The traction force of the cable laying should generally not exceed 120kg, and the reinforcing core part of the optical cable should be pulled, and waterproof reinforcement treatment at the head of the optical cable should be done well.
At the points where optical cables are introduced and led out, guiding devices must be added and they must not be directly dragged on the ground.
Pipelines and optical cables should also be reliably grounded.
The depth of the direct-buried optical cable trench should be excavated according to the standards. The standards are as follows:
Laying area and soil type | Burial depth requirements (meters) | special case handling |
Ordinary soil, hard soil | ≥1.2 | - |
Semi-stony (gravel soil, weathered soil) | ≥1.0 | Lay 10cm of fine soil or sand at the bottom of the trench and above the optical cable respectively |
All-stone | ≥0.8 | Add 10cm of fine soil at the bottom of the ditch and spread 10cm of fine soil on top. When it cannot be achieved, the trench can be sealed with cement mortar (burial depth ≥0.4m). |
Quicksand | ≥0.8 | Measures for sand fixation need to be taken |
Suburbs and villages | ≥1.2 | Warning signs need to be added when crossing residential areas |
Urban sidewalk | ≥1.0 | Concrete protective plates need to be covered |
Cross the railway/highway | ≥1.2 | Protected by steel pipes or hard plastic pipes, at least 1 meter away from the roadbed surface |
Ditches, canals and ponds | ≥1.2 | Cement boards or cement sandbags are covered above the optical cables |
Farmland drainage ditch (ditch width ≤1m) | ≥0.8 | Irrigation soil extraction areas should be avoided |
High-grade highway shoulders/median strips | ≥0.8 | To avoid the posts of the anti-collision guardrail, the burial depth can be reduced to 0.6 meters (with the consent of the road authorities). |
River | ≥1.5 | When the riverbed is unstable, artificial armoring or the construction of a flood slope is required |
Cultivated land area | ≥1.0 | In cold regions, an additional 0.2 to 0.3 meters should be added to prevent the influence of the permafrost layer |
In areas where trenching is not possible, the cables can be laid overhead or pre-buried through drilling.
The bottom of the trench should be flat and stable. If necessary, some sand, cement or supports can be pre-filled.
During laying, manual or mechanical traction can be used, but attention should be paid to guidance and lubrication.
After laying, the soil should be quickly returned and compacted as soon as possible.
When laying the cables vertically, special attention should be paid to the load-bearing issue of the cables. Generally, the cables should be fixed once every two floors.
When the cables pass through walls or floors, protective plastic pipes with covers should be added, and the pipes should be filled with flame-retardant fillers.
In buildings, a certain amount of plastic pipes can also be pre-installed. When laying optical cables later, they can be pulled or vacuumed to lay the cables.
In addition to considering the number of fiber cores and the type of fiber, the selection of optical cables also depends on the usage environment.
For outdoor optical cables buried directly, armored optical cables are recommended. For overhead use, black plastic outer sheaths with two or more reinforcing ribs can be selected.
When choosing optical cables for buildings, attention should be paid to their flame retardancy, toxicity, and smoke emission characteristics. Generally, flame-retardant but with smoke-emitting types (Plenum) can be selected in pipes or in areas with forced ventilation, while flame-retardant, non-toxic, and smoke-free types (Riser) should be chosen in exposed environments.
For vertical cabling within buildings, layer-wound optical cables (DistributionCables) can be selected; for horizontal wiring, breakout optical cables (BreakoutCables) can be chosen.
For transmission distances within 2 km, multi-mode optical cables can be selected. For distances exceeding 2 km, relay or single-mode optical cables can be used.
The main methods include permanent connection, emergency connection, and active connection.
This connection is achieved by using an electric discharge method to melt and connect the connection points of the optical fibers. It is generally used for long-distance splicing, permanent or semi-permanent fixed connections. Its main feature is that the connection attenuation is the lowest among all connection methods, with typical values ranging from 0.01 to 0.03 dB/point. However, during the connection process, special equipment (fusion splicer) and professionals are required for operation, and the connection points also need to be protected by special containers.
The emergency connection mainly uses mechanical and chemical methods to fix and bond two optical fibers together. The main feature of this method is that the connection is rapid and reliable, with typical attenuation ranging from 0.1 to 0.3 dB per point. However, the connection point will become unstable over time and the attenuation will increase significantly, so it can only be used for emergency purposes for a short period of time.
Active connection is a method of connecting sites to sites or sites to optical cables by using various optical fiber connection devices (jacks and sockets). This method is flexible, simple, convenient, and reliable, and is mostly used in computer network wiring within buildings. Its typical attenuation is 1 dB per connector.
The main purpose of fiber optic detection is to ensure the quality of system connections, reduce failure factors, and identify the fault points of the fiber optic during faults. There are many detection methods, mainly including manual simple measurement and precise instrument measurement.
Manual Simple Measurement: This method is generally used for rapid detection of the connection status of the fiber optic and for distinguishing the fiber optic during construction. It uses a simple light source to inject visible light into one end of the fiber optic, and then observes which end emits light to achieve this. Although this method is simple, it cannot quantitatively measure the attenuation of the fiber optic and the fault points of the fiber optic.
Precise Instrument Measurement: Using an optical power meter or an optical time domain reflectometer (OTDR) for quantitative measurement of the fiber optic can measure the attenuation of the fiber optic and the attenuation at the connector, and even the position of the fault point of the fiber optic. This measurement can be used to quantitatively analyze the causes of faults in the fiber optic network and evaluate the fiber optic network products.
The human society has now entered the information age, with a huge volume of information exchanges such as voice, images, and data. The previous communication methods can no longer meet the current requirements, and fiber optic communication has been widely applied due to its advantages such as large information capacity, good confidentiality, light weight, small size, no intermediate sections, and long distance. Its application fields cover communications, transportation, industry, healthcare, education, aerospace, and computer industries, and are developing towards broader and deeper levels. The application of light and fiber optics is bringing profound impacts and changes to human life.
The design of a fiber optic system generally follows the following steps:
First, clarify what kind of network is to be designed, its current status, and why fiber optics should be used.
Based on the actual situation, select appropriate fiber optic network equipment, optical cables, patch cords, and other items for connection. When choosing, it should be based on availability, and then determine according to performance, price, service, origin, and brand.
Determine the routing of the lines according to the customer's requirements and the type of network, and draw a wiring diagram.
When the route is long, it is necessary to calculate the attenuation margin of the system. The calculation can be carried out according to the following formula: Attenuation Margin = Transmitting Optical Power - Accepting Sensitivity - Line Attenuation - Connection Attenuation (dB). Where line attenuation = Optical Cable Length × Unit Attenuation; Unit Attenuation is closely related to the quality of the fiber optic, generally single-mode is 0.4 - 0.5 dB/km; multi-mode is 2 - 4 dB/km. Connection attenuation includes fusion splicing attenuation and connector attenuation. Fusion splicing attenuation is related to the splicing method and the quality of the personnel, generally 0.01 - 0.3 dB/point; cold fusion 0.1 - 0.3 dB/point; connector attenuation is closely related to the quality of the connector, generally 1 dB/point. The system attenuation margin should generally be no less than 4 dB.
When the calculation is not qualified, modify the design according to the situation and then calculate again. This situation may sometimes repeat several times.
The renovation of a campus network: Based on the situation, on one side of the existing twisted-pair network, a three-port repeater (twisted-pair-fiber-coaxial cable) was used, and on the other side, a twisted-pair HUB with an optical fiber backbone was employed. In the middle, a suspended or buried conduit-type 4-core outdoor multimode optical cable was used, which was then fused to form indoor patch cords with ST connectors (as the optical fiber interface of the equipment is ST type).
Attenuation calculation: (Generally, for multi-mode equipment, no calculation is needed within 2km. Here, this is just an example)
Transmit power: -16dBm
Reception sensitivity: -29.5dBm
Line attenuation: 1.5km × 3.5dB/km = 5.25dB
Connection attenuation: 2 connectors result in attenuation of 2 points × 1dB/point = 2dB
Fusion of two points results in: 2 points × 0.07dB/point = 0.14dB
Attenuation margin: -16dBm - (-29.5dBm) - 5.25dB - 0.14dB - 2dB = 6.11 (dB)
After the above calculation, it can be seen that the system capacity is greater than 4dB, and the above selection can meet the requirements.
