1. What is single mode and multimode fiber? What is the difference between them?
The concept of single-mode and multi-mode is to classify optical fibers according to the propagation mode—the concept of multi-mode optical fibers and single-mode optical fiber propagation modes. We know that light is an extremely high-frequency (3×1014Hz) electromagnetic wave. When it propagates in an optical fiber, the theory of wave optics, electromagnetic fields, and Maxwell equations solves the following problems:
When the fiber core has a geometric dimension much larger than the wavelength of the light, the light will propagate in the fiber in dozens or even hundreds of propagation modes, such as TMmn mode, TEmn mode, HEmn mode, etc. (where m, n=0, 1, 2, 3, ...).
Among them, the HE11 mode is called the basic mode, and the rest are all called high-order modes.
1) Multimode fiber When the fiber's geometric size (mainly the core diameter d1) is far greater than the wavelength of light (about 1μm), there will be dozens or even hundreds of propagation modes in the fiber. Different propagation modes have different propagation speeds and phases, resulting in delays and widening light pulses after long-distance transmission. This phenomenon is called the modal dispersion of the fiber (also called inter-modal dispersion).
Mode dispersion can narrow the bandwidth of multimode fiber and reduce its transmission capacity. Therefore, multimode fiber is only suitable for smaller-capacity fiber communication.
The refractive index distribution of a multimode fiber is mostly a parabolic distribution, ie, a graded index profile. Its core diameter is about 50μm.
2) Single-mode fiber When the fiber geometry (mainly the core diameter) can be similar to the wavelength of light, such as the core diameter d1 in the range of 5 ~ 10μm, the fiber allows only one mode (base mode HE11) in which to propagate, the rest High-order mode all cut-off, such optical fiber is called single-mode fiber.
Since it only has one mode to propagate and avoids the problem of mode dispersion, single-mode fiber has a very wide bandwidth and is particularly suitable for large-capacity optical fiber communications. Therefore, in order to achieve single-mode transmission, the parameters of the fiber must satisfy certain conditions. Through formulae calculations, for a fiber with NA=0.12, single-mode transmission above λ=1.3 μm, the radius of the fiber core should be ≤ 4.2 μm, ie its core diameter d1 ≤ 8.4 μm.
Because the core diameter of a single mode fiber is very small, more stringent requirements are imposed on its manufacturing process.
2. What are the advantages of using optical fiber?
1) The passband of the fiber is very wide and the theory can reach 30T.
2) The length of non-relay support can reach tens to hundreds of kilometers, and the copper wire is only a few hundred meters.
3) Not affected by electromagnetic fields and electromagnetic radiation.
4) Light weight and small size.
5) Optical fiber communication is not powered, and the use of safety can be used in flammable, prone to storm and other places.
6) The use of a wide range of ambient temperatures.
7) Long service life.
3, how to choose the optical cable?
In addition to selecting the number of optical fiber cores and optical fibers, the optical cable must be selected according to the use environment of the optical cable to select the structure of the optical cable and the outer sheath.
1) When the outdoor optical cable is buried directly, loose-sheathed cable should be used. When overhead, a loose PE cable with a black PE sheath with two or more ribs can be used.
2) Optical fiber cables used in buildings should use tight-fitting optical cables and pay attention to their fire-retardant, toxic and smoke characteristics. The type of flame-retardant but smoke (Plenum) or flammable and non-toxic type (LSZH) can be used in the pipeline or in forced ventilation. The type of flame-retardant, non-toxic and non-smoking (Riser) should be used in the exposed environment.
3) When vertical or horizontal cabling is installed in a building, it can be used when using tight-fitting optical cables, distribution optical cables or branch optical cables that are commonly used in buildings.
4) Select single-mode and multi-mode optical cables based on network applications and optical cable application parameters. Usually, indoor and short-distance applications use multimode optical cables, while outdoor and long-distance applications use single-mode optical cables.
4. In the connection of optical fiber, how to choose different applications of fixed connection and active connection?
The active connection of the fiber is achieved through a fiber optic connector. An active connection point in the optical link is a clear split interface. In the choice of active connections and fixed connections, the advantages of fixed connections are lower costs, less optical loss, but less flexibility, and the active connection is the opposite. When designing the network, it is necessary to flexibly select the use of activities and fixed connections according to the entire link situation, so as to ensure flexibility and stability, so as to give full play to their respective advantages. The active connection interface is an important test, maintenance, and change interface. The active connection is relatively easy to find the fault point in the link than the fixed connection, which increases the convenience of replacement of the faulty device, thereby improving system maintenance and reducing maintenance costs.
5, the optical fiber is closer and closer to the user terminal, "fiber to the desktop" significance and system design need to pay attention to what factors?
"Fiber-to-the-desktop" in the application of the horizontal subsystem, and the relationship between copper and copper cable are complementary to each other. Optical fiber has its unique advantages, such as long transmission distance, stable transmission, free from electromagnetic interference, high support bandwidth, and no electromagnetic leakage. These characteristics make the optical fiber play an irreplaceable role in some specific environments:
1) If the information point transmission distance is greater than 100m, if you choose to use copper cable. Replicators must be added or network equipment and weak rooms must be added to increase costs and hidden troubles. Using fiber can easily solve this problem.
2) In a specific working environment (such as factories, hospitals, air-conditioning rooms, power equipment rooms, etc.), there are a large number of sources of electromagnetic interference, and the optical fibers can be operated stably without electromagnetic interference in these environments.
3) There is no electromagnetic leakage in the optical fiber. It is very difficult to detect the signal transmitted in the optical fiber. It is a good choice for places where security requirements are high (such as military, R&D, auditing, government, etc.).
4) The environment with high demand for bandwidth has reached 1G or more, and optical fiber is a good choice.
The application of fiber optics is gradually extending from the trunk or the computer room to the desktop and residential users, which means that more and more users who do not understand the characteristics of the fiber are beginning to contact the fiber optic system. Therefore, when designing optical fiber link systems and selecting products, full consideration should be given to the current and future application requirements of the system, use of compatible systems and products, the greatest possible ease of maintenance and management, and adaptation to the ever-changing field conditions and user installation requirements.
6. Can a fiber optic connector be terminated directly on a 250μm fiber?
No. Loose-sheathed fiber optic cable contains bare fiber with an outer diameter of 250 μm, which is very small and fragile. It cannot fix the fiber and is not enough to support the weight of the fiber optic connector and is very insecure. The connector is terminated directly on the fiber optic cable. At a minimum, a 900 μm tight jacket is required to wrap around the outside of the 250 μm fiber to protect the fiber and support the connector.
7, FC connector can be directly connected with the SC connector?
Yes, this is just a different connection method for two different types of connectors.
If you need to connect them, you must select a mixed adapter and use the FC/SC adapter to connect the FC connector and the SC connector at both ends. This method requires that the connectors should all be flat ground. If you absolutely need to connect APC connectors, you must use a second method to prevent damage.
The second method is to use a hybrid jumper and two connection adapters. Hybrid patch cords use different types of fiber connectors at both ends. These connectors will connect to the place where you need to connect. In this way, you can use a universal adapter to connect the system in the patch panel, but bring the system budget to budget. The increase in the number of connector pairs.
8. The fixed connection of optical fiber includes mechanical fiber connection and thermal fusion. What are the selection principles for mechanical fiber connection and thermal fusion?
Mechanical fiber optic connection, commonly known as fiber optic cold connection, refers to an optical fiber connection method in which a single or multi-fiber optical fiber is permanently connected through a simple connection tool and a mechanical connection technology without the need of a thermal fusion bonding machine. In general, mechanical splices should be used instead of thermal fusion when splices are made at small and multi-location dispersed optical fibers.
Mechanical fiber optic connection technology is often used in engineering practices such as line repairs and small-scale applications in special occasions. In recent years, with the large-scale deployment of fiber-to-the-desktop and fiber-to-the-home (FTTH), it has been recognized that mechanical fiber optic connection is an important means of fiber optic connection.
For fiber-to-the-desktop and fiber-to-the-home applications with a large number of users and geographically dispersed features, when the scale of the users reaches a certain level, the construction complexity and construction personnel and fusion splicer cannot meet the time requirements for users to open services. Because of the simple operation, short training cycle, and low equipment investment, the mechanical fiber connection method provides the most cost-effective solution for optical fiber connection for large-scale deployment of optical fibers. For example, in the high corridors, narrow spaces, insufficient lighting, inconvenient on-site power and other occasions, mechanical fiber optic connection provides a convenient, practical, fast and high-performance optical fiber continuation method for design, construction and maintenance personnel.
9. What is the difference between fiber optic cable splice box requirements and fiber optic cable splice boxes used in telecom operators' outdoor lines in fiber-to-the-home systems?
First of all, in the fiber-to-the-home system, the position of the optical splitter is required to be installed and terminated, accommodated, and protected in the splitter box in accordance with actual needs. Because the actual situation is that the optical splitter may be located in the cable joint box, optical cable transfer box, wiring box, ODF and other facilities, and in which the optical cable termination and distribution.
Secondly, for residential quarters, the optical fiber cable splice box is installed in a buried manner. Therefore, the optical cable splice box has higher requirements for buried performance.
In addition, in the fiber-to-the-home project, it may be necessary to consider the entry and exit of a large number of small number of optical fiber cables.
10. What should be noticed for the construction of ordinary layer stranded cable?
1) The construction of optical cables must be strictly in accordance with the specifications of the construction;
2) When the cable is turning, its turning radius should be 15-20 times larger than the diameter of the cable itself. If the overhead cable is in the upper and lower towers, the bending angle should be minimized, and the cable tray should be assisted to reduce the line tension of the cable.
3) Before laying fiber optic cables, the construction and related personnel shall be given appropriate training on matters that should be taken care of during construction, such as the method of releasing the lines and safety, and ensuring that the construction personnel obey the command;
4) Relevant personnel should be arranged at the place where the cable is placed on the cable tray, crossing the obstacle point, turning the terrain, etc. in order to find the problem in time, troubleshoot, control the speed in the pay-off line, and reduce the tension of the pay-off plate;
5) If any obstacles are encountered in the deployment process of optical fiber cables, drag and drop them and remove them in time. Do not use force, or it will cause damage to the cable;
6) When the cable is put on line, the tension should be stable, and it must not exceed the required tension of the cable standard.
7) When the optical cable is subjected to large tension and passes through a pulley with a small bending radius or a hard surface with angular edges at a small angle, the optical cable is locally subjected to a side pressure much greater than the rated value, causing damage to the internal structure of the optical cable, causing serious disconnection. Fiber.
8) The construction unit of optical fiber cables should constantly sum up experience and strive to improve construction quality and prevent similar construction accidents.
The concept of single-mode and multi-mode is to classify optical fibers according to the propagation mode—the concept of multi-mode optical fibers and single-mode optical fiber propagation modes. We know that light is an extremely high-frequency (3×1014Hz) electromagnetic wave. When it propagates in an optical fiber, the theory of wave optics, electromagnetic fields, and Maxwell equations solves the following problems:
When the fiber core has a geometric dimension much larger than the wavelength of the light, the light will propagate in the fiber in dozens or even hundreds of propagation modes, such as TMmn mode, TEmn mode, HEmn mode, etc. (where m, n=0, 1, 2, 3, ...).
Among them, the HE11 mode is called the basic mode, and the rest are all called high-order modes.
1) Multimode fiber When the fiber's geometric size (mainly the core diameter d1) is far greater than the wavelength of light (about 1μm), there will be dozens or even hundreds of propagation modes in the fiber. Different propagation modes have different propagation speeds and phases, resulting in delays and widening light pulses after long-distance transmission. This phenomenon is called the modal dispersion of the fiber (also called inter-modal dispersion).
Mode dispersion can narrow the bandwidth of multimode fiber and reduce its transmission capacity. Therefore, multimode fiber is only suitable for smaller-capacity fiber communication.
The refractive index distribution of a multimode fiber is mostly a parabolic distribution, ie, a graded index profile. Its core diameter is about 50μm.
2) Single-mode fiber When the fiber geometry (mainly the core diameter) can be similar to the wavelength of light, such as the core diameter d1 in the range of 5 ~ 10μm, the fiber allows only one mode (base mode HE11) in which to propagate, the rest High-order mode all cut-off, such optical fiber is called single-mode fiber.
Since it only has one mode to propagate and avoids the problem of mode dispersion, single-mode fiber has a very wide bandwidth and is particularly suitable for large-capacity optical fiber communications. Therefore, in order to achieve single-mode transmission, the parameters of the fiber must satisfy certain conditions. Through formulae calculations, for a fiber with NA=0.12, single-mode transmission above λ=1.3 μm, the radius of the fiber core should be ≤ 4.2 μm, ie its core diameter d1 ≤ 8.4 μm.
Because the core diameter of a single mode fiber is very small, more stringent requirements are imposed on its manufacturing process.
2. What are the advantages of using optical fiber?
1) The passband of the fiber is very wide and the theory can reach 30T.
2) The length of non-relay support can reach tens to hundreds of kilometers, and the copper wire is only a few hundred meters.
3) Not affected by electromagnetic fields and electromagnetic radiation.
4) Light weight and small size.
5) Optical fiber communication is not powered, and the use of safety can be used in flammable, prone to storm and other places.
6) The use of a wide range of ambient temperatures.
7) Long service life.
3, how to choose the optical cable?
In addition to selecting the number of optical fiber cores and optical fibers, the optical cable must be selected according to the use environment of the optical cable to select the structure of the optical cable and the outer sheath.
1) When the outdoor optical cable is buried directly, loose-sheathed cable should be used. When overhead, a loose PE cable with a black PE sheath with two or more ribs can be used.
2) Optical fiber cables used in buildings should use tight-fitting optical cables and pay attention to their fire-retardant, toxic and smoke characteristics. The type of flame-retardant but smoke (Plenum) or flammable and non-toxic type (LSZH) can be used in the pipeline or in forced ventilation. The type of flame-retardant, non-toxic and non-smoking (Riser) should be used in the exposed environment.
3) When vertical or horizontal cabling is installed in a building, it can be used when using tight-fitting optical cables, distribution optical cables or branch optical cables that are commonly used in buildings.
4) Select single-mode and multi-mode optical cables based on network applications and optical cable application parameters. Usually, indoor and short-distance applications use multimode optical cables, while outdoor and long-distance applications use single-mode optical cables.
4. In the connection of optical fiber, how to choose different applications of fixed connection and active connection?
The active connection of the fiber is achieved through a fiber optic connector. An active connection point in the optical link is a clear split interface. In the choice of active connections and fixed connections, the advantages of fixed connections are lower costs, less optical loss, but less flexibility, and the active connection is the opposite. When designing the network, it is necessary to flexibly select the use of activities and fixed connections according to the entire link situation, so as to ensure flexibility and stability, so as to give full play to their respective advantages. The active connection interface is an important test, maintenance, and change interface. The active connection is relatively easy to find the fault point in the link than the fixed connection, which increases the convenience of replacement of the faulty device, thereby improving system maintenance and reducing maintenance costs.
5, the optical fiber is closer and closer to the user terminal, "fiber to the desktop" significance and system design need to pay attention to what factors?
"Fiber-to-the-desktop" in the application of the horizontal subsystem, and the relationship between copper and copper cable are complementary to each other. Optical fiber has its unique advantages, such as long transmission distance, stable transmission, free from electromagnetic interference, high support bandwidth, and no electromagnetic leakage. These characteristics make the optical fiber play an irreplaceable role in some specific environments:
1) If the information point transmission distance is greater than 100m, if you choose to use copper cable. Replicators must be added or network equipment and weak rooms must be added to increase costs and hidden troubles. Using fiber can easily solve this problem.
2) In a specific working environment (such as factories, hospitals, air-conditioning rooms, power equipment rooms, etc.), there are a large number of sources of electromagnetic interference, and the optical fibers can be operated stably without electromagnetic interference in these environments.
3) There is no electromagnetic leakage in the optical fiber. It is very difficult to detect the signal transmitted in the optical fiber. It is a good choice for places where security requirements are high (such as military, R&D, auditing, government, etc.).
4) The environment with high demand for bandwidth has reached 1G or more, and optical fiber is a good choice.
The application of fiber optics is gradually extending from the trunk or the computer room to the desktop and residential users, which means that more and more users who do not understand the characteristics of the fiber are beginning to contact the fiber optic system. Therefore, when designing optical fiber link systems and selecting products, full consideration should be given to the current and future application requirements of the system, use of compatible systems and products, the greatest possible ease of maintenance and management, and adaptation to the ever-changing field conditions and user installation requirements.
6. Can a fiber optic connector be terminated directly on a 250μm fiber?
No. Loose-sheathed fiber optic cable contains bare fiber with an outer diameter of 250 μm, which is very small and fragile. It cannot fix the fiber and is not enough to support the weight of the fiber optic connector and is very insecure. The connector is terminated directly on the fiber optic cable. At a minimum, a 900 μm tight jacket is required to wrap around the outside of the 250 μm fiber to protect the fiber and support the connector.
7, FC connector can be directly connected with the SC connector?
Yes, this is just a different connection method for two different types of connectors.
If you need to connect them, you must select a mixed adapter and use the FC/SC adapter to connect the FC connector and the SC connector at both ends. This method requires that the connectors should all be flat ground. If you absolutely need to connect APC connectors, you must use a second method to prevent damage.
The second method is to use a hybrid jumper and two connection adapters. Hybrid patch cords use different types of fiber connectors at both ends. These connectors will connect to the place where you need to connect. In this way, you can use a universal adapter to connect the system in the patch panel, but bring the system budget to budget. The increase in the number of connector pairs.
8. The fixed connection of optical fiber includes mechanical fiber connection and thermal fusion. What are the selection principles for mechanical fiber connection and thermal fusion?
Mechanical fiber optic connection, commonly known as fiber optic cold connection, refers to an optical fiber connection method in which a single or multi-fiber optical fiber is permanently connected through a simple connection tool and a mechanical connection technology without the need of a thermal fusion bonding machine. In general, mechanical splices should be used instead of thermal fusion when splices are made at small and multi-location dispersed optical fibers.
Mechanical fiber optic connection technology is often used in engineering practices such as line repairs and small-scale applications in special occasions. In recent years, with the large-scale deployment of fiber-to-the-desktop and fiber-to-the-home (FTTH), it has been recognized that mechanical fiber optic connection is an important means of fiber optic connection.
For fiber-to-the-desktop and fiber-to-the-home applications with a large number of users and geographically dispersed features, when the scale of the users reaches a certain level, the construction complexity and construction personnel and fusion splicer cannot meet the time requirements for users to open services. Because of the simple operation, short training cycle, and low equipment investment, the mechanical fiber connection method provides the most cost-effective solution for optical fiber connection for large-scale deployment of optical fibers. For example, in the high corridors, narrow spaces, insufficient lighting, inconvenient on-site power and other occasions, mechanical fiber optic connection provides a convenient, practical, fast and high-performance optical fiber continuation method for design, construction and maintenance personnel.
9. What is the difference between fiber optic cable splice box requirements and fiber optic cable splice boxes used in telecom operators' outdoor lines in fiber-to-the-home systems?
First of all, in the fiber-to-the-home system, the position of the optical splitter is required to be installed and terminated, accommodated, and protected in the splitter box in accordance with actual needs. Because the actual situation is that the optical splitter may be located in the cable joint box, optical cable transfer box, wiring box, ODF and other facilities, and in which the optical cable termination and distribution.
Secondly, for residential quarters, the optical fiber cable splice box is installed in a buried manner. Therefore, the optical cable splice box has higher requirements for buried performance.
In addition, in the fiber-to-the-home project, it may be necessary to consider the entry and exit of a large number of small number of optical fiber cables.
10. What should be noticed for the construction of ordinary layer stranded cable?
1) The construction of optical cables must be strictly in accordance with the specifications of the construction;
2) When the cable is turning, its turning radius should be 15-20 times larger than the diameter of the cable itself. If the overhead cable is in the upper and lower towers, the bending angle should be minimized, and the cable tray should be assisted to reduce the line tension of the cable.
3) Before laying fiber optic cables, the construction and related personnel shall be given appropriate training on matters that should be taken care of during construction, such as the method of releasing the lines and safety, and ensuring that the construction personnel obey the command;
4) Relevant personnel should be arranged at the place where the cable is placed on the cable tray, crossing the obstacle point, turning the terrain, etc. in order to find the problem in time, troubleshoot, control the speed in the pay-off line, and reduce the tension of the pay-off plate;
5) If any obstacles are encountered in the deployment process of optical fiber cables, drag and drop them and remove them in time. Do not use force, or it will cause damage to the cable;
6) When the cable is put on line, the tension should be stable, and it must not exceed the required tension of the cable standard.
7) When the optical cable is subjected to large tension and passes through a pulley with a small bending radius or a hard surface with angular edges at a small angle, the optical cable is locally subjected to a side pressure much greater than the rated value, causing damage to the internal structure of the optical cable, causing serious disconnection. Fiber.
8) The construction unit of optical fiber cables should constantly sum up experience and strive to improve construction quality and prevent similar construction accidents.
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