1. Multimode Fiber
When the geometric size of the optical fiber (mainly the core diameter d1) is much larger than the wavelength of the light wave (about 1µm), there will be dozens or even hundreds of propagation modes in the optical fiber. Different propagation modes have different propagation speeds and phases, resulting in time delays and broadening of light pulses after long-distance transmission. This phenomenon is called mode dispersion of optical fiber (also called inter-mode dispersion). Mode dispersion will narrow the bandwidth of multimode optical fiber and reduce its transmission capacity. Therefore, multimode optical fiber is only suitable for optical fiber communications with smaller capacity. The refractive index distribution of multimode optical fiber is mostly parabolic distribution, that is, gradient refractive index distribution. Its core diameter is about 50µm.
2. Single-mode fiber
When the geometric size of the optical fiber (mainly the core diameter) can be close to the wavelength of light, such as the core diameter d1 is in the range of 5 to 10µm, the optical fiber only allows one mode (fundamental mode HE11) to propagate in it, and all other higher-order modes are cut off. Such optical fiber is called single-mode optical fiber. Since it has only one mode of propagation, it avoids the problem of mode dispersion, so single-mode optical fiber has an extremely wide bandwidth and is particularly suitable for large-capacity optical fiber communication. Therefore, to achieve single-mode transmission, the parameters of the optical fiber must meet certain conditions. It is calculated by formula that for optical fiber with NA=0.12, when single-mode transmission is achieved above λ=1.3µm, the radius of the optical fiber core should be ≤4.2µm, that is, its core diameter d1≤8.4µm.Since the core diameter of single-mode optical fiber is very small, more stringent requirements are placed on its manufacturing process.
3. What are the advantages of using optical fiber?
1) The bandwidth of optical fiber is very wide, theoretically up to 30T.
2) The length of non-relay support can reach tens to hundreds of kilometers, while copper wire is only a few hundred meters.
3) It is not affected by electromagnetic fields and electromagnetic radiation.
4) It is light in weight and small in size.
5) Optical fiber communication is not electrified, so it is safe to use and can be used in flammable and explosive places.
6) The operating environment temperature range is wide.
7) It has a long service life.
4. How to choose optical cable?
In addition to the number of optical fiber cores and the type of optical fiber, the structure and outer sheath of the optical cable should also be selected according to the use environment of the optical cable.
1) When directly buried, loose-tube armored optical cable should be selected for outdoor optical cable. When overhead, loose-tube optical cable with black PE outer sheath with two or more reinforcing ribs can be selected.
2) When selecting optical cables for use in buildings, tight-tube optical cables should be selected and attention should be paid to their flame retardant, toxic and smoke properties. Generally, flame retardant but smoke-containing types (Plenum) or flammable and non-toxic types (LSZH) can be selected in pipelines or forced ventilation, and flame retardant, non-toxic and smoke-free types (Riser) should be selected in exposed environments.
3) When laying cables vertically or horizontally in a building, tight-tube optical cables, distribution optical cables or branch optical cables commonly used in buildings can be selected.
4) Select single-mode and multi-mode optical cables according to network applications and optical cable application parameters. Generally, multi-mode optical cables are used for indoor and short-distance applications, while single-mode optical cables are used for outdoor and long-distance applications.
5. In the connection of optical fiber, how to choose different applications of fixed connection and active connection?
The active connection of optical fiber is realized through optical fiber connector. An active connection point in the optical link is a clear segmentation interface. In the choice of active connection and fixed connection, the advantages of fixed connection are low cost and small optical loss, but poor flexibility, while active connection is the opposite. When designing the network, it is necessary to flexibly choose the use of active and fixed connections according to the situation of the entire link to ensure both flexibility and stability, so as to give full play to their respective advantages. The active connection interface is an important interface for testing, maintenance, and change. Active connection is relatively easier to find the fault point in the link than fixed connection, which increases the convenience of replacing faulty components, thereby improving system maintainability and reducing maintenance costs.
6. Fiber is getting closer and closer to user terminals. What are the significance of "fiber to the desktop" and what factors should be paid attention to when designing the system?
"Fiber to the desktop" is complementary and indispensable to copper cables in the application of horizontal subsystems. Fiber has its own unique advantages, such as long transmission distance, stable transmission, no influence of electromagnetic interference, high bandwidth support, and no electromagnetic leakage. These characteristics make fiber play an irreplaceable role for copper cables in some specific environments:
1) When the transmission distance of the information point is greater than 100m, if you choose to use copper cables. Repeaters must be added or network equipment and weak current rooms must be added, thereby increasing costs and potential faults. The use of fiber can easily solve this problem.
2) In specific working environments (such as factories, hospitals, air-conditioning rooms, power rooms, etc.), there are a large number of electromagnetic interference sources. Fiber can be free from electromagnetic interference and operate stably in these environments.
3) There is no electromagnetic leakage in optical fiber, and it is very difficult to detect the signal transmitted in the optical fiber. It is a good choice in places with high confidentiality requirements (such as military, R&D, auditing, government and other industries).
4) For environments with high bandwidth requirements, reaching more than 1G, optical fiber is a good choice.
The application of optical fiber is gradually extending from the backbone or computer room to desktop and residential users, which means that more and more users who do not understand the characteristics of optical fiber are beginning to come into contact with optical fiber systems. 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, using compatible systems and products, facilitating maintenance and management as much as possible, and adapting to the ever-changing actual on-site conditions and user installation requirements.
5) Can fiber optic connectors be directly terminated on 250µm optical fibers?
No. Loose-tube optical cables contain bare optical fibers with an outer diameter of 250 µm, which are very small and fragile. They cannot fix the optical fibers, are not strong enough to support the weight of the optical fiber connectors, and are very unsafe. To terminate the connector directly on the optical cable, at least a 900 µm tight-tube layer must be used to wrap the 250 µm optical fiber outside to provide protection for the optical fiber and support for the connector.
6) Can FC connectors be directly connected to SC connectors?
Yes, this is just a different way to connect two different types of connectors.
If you need to connect them, you must choose a mixed transfer adapter. Using the FC/SC adapter, you can connect the FC connector and SC connector at both ends respectively. This method requires that the connectors should be flat-polished. If you must connect an angled (APC) connector, you must use the second method to prevent damage.
The second method is to use a hybrid jumper and two connection adapters. A hybrid jumper means that different fiber optic connector types are used at both ends. These connectors will be connected to where you need to connect, so that you can use a universal adapter in the patch panel to connect to the system, but it will increase the amount of connector pairs to the system attenuation budget.
7) The fixed connection of optical fiber includes mechanical optical fiber splicing and hot fusion splicing. What are the selection principles of mechanical optical fiber splicing and hot fusion splicing?
Mechanical optical fiber splicing is commonly known as optical fiber cold splicing. It refers to the optical fiber splicing method that does not require a hot fusion splicer, but uses simple splicing tools and mechanical connection technology to achieve permanent connection of single-core or multi-core optical fibers. In general, when splicing optical fibers with a small number of cores and scattered locations, it is advisable to use mechanical splicing instead of hot fusion splicing.
In the early days, mechanical optical fiber splicing technology was often used in engineering practices such as line repair 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), people have realized the significance of mechanical optical fiber splicing as an important means of optical fiber splicing.
For fiber to the desktop and fiber to the home applications with large number of users and scattered locations, when the user scale reaches a certain level, the complexity of construction and the construction personnel and fusion splicing machines cannot meet the time requirements for users to open services. Mechanical fiber splicing provides the most cost-effective fiber splicing solution for large-scale fiber deployment due to its simple operation, short personnel training cycle, and small equipment investment. For example, in high-rise corridors, narrow spaces, insufficient lighting, and inconvenient on-site power supply, mechanical fiber splicing provides a convenient, practical, fast, and high-performance fiber splicing method for design, construction, and maintenance personnel.
8) In the fiber-to-the-home system, what are the differences between the requirements for the optical cable junction box and the optical cable junction box used in the outdoor lines of telecom operators?
First, in the fiber-to-the-home system, it is necessary to reserve the installation and termination of the splitter in the junction box, and to accommodate and protect the jumpers entering and exiting the splitter according to actual needs. Because the actual situation is that the splitter may be located in facilities such as the optical cable junction box, the optical cable junction box, the distribution box, and the ODF, and the optical cable is terminated and distributed in them.
Secondly, for residential areas, the optical cable junction box is more often installed in the form of buried, so the buried performance of the optical cable junction box is required to be higher.
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-core optical cables.
The core diameter of multimode optical fiber is 50~62.5μm, the outer diameter of the cladding is 125μm, and the core diameter of single-mode optical fiber is 8.3μm, and the outer diameter of the cladding is 125μm. The working wavelengths of optical fiber include short wavelength 0.85μm, long wavelength 1.31μm and 1.55μm. The optical fiber loss generally decreases with the increase of wavelength. The loss of 0.85μm is 2.5dB/km, the loss of 1.31μm is 0.35dB/km, and the loss of 1.55μm is 0.20dB/km, which is the lowest loss of optical fiber. The loss of wavelengths above 1.65μm tends to increase. Due to the absorption of OHˉ, there are loss peaks in the range of 0.90~1.30μm and 1.34~1.52μm, and these two ranges have not been fully utilized. Since the 1980s, there has been a tendency to use more single-mode optical fibers, and the long wavelength 1.31μm has been used first.
Multimode Fiber
Multimode Fiber: The central glass core is thicker (50 or 62.5μm), and can transmit multiple modes of light. However, its inter-mode dispersion is large, which limits the frequency of digital signal transmission, and it becomes more serious with the increase of distance. For example, a 600MB/KM optical fiber only has a bandwidth of 300MB at 2KM. Therefore, the transmission distance of multimode optical fiber is relatively short, generally only a few kilometers.
Single-mode fiber
Single-mode fiber: The central glass core is very thin (the core diameter is generally 9 or 10μm), and can only transmit one mode of light. Therefore, its inter-mode dispersion is very small, which is suitable for long-distance communication, but there are still material dispersion and waveguide dispersion. In this way, single-mode fiber has 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. Later, it was discovered that at a wavelength of 1.31μm, the material dispersion and waveguide dispersion of single-mode fiber are positive and negative, and the sizes are exactly equal. This means that at a wavelength of 1.31μm, the total dispersion of single-mode fiber is zero. From the loss characteristics of optical fiber, 1.31μm is exactly a low-loss window for optical fiber. In this way, the 1.31μm wavelength region has become an ideal working window for optical fiber communication, and it is also the main working band of practical optical fiber communication systems. The main parameters of 1.31μm conventional single-mode fiber are determined by the International Telecommunication Union ITU-T in the G652 recommendation, so this fiber is also called G652 fiber.
7. What is the difference between multimode fiber optic transceivers and single-mode fiber optic transceivers?
Price: Multimode is cheap, single-mode is expensive
Distance: Multimode is less than 2KM, single-mode can transmit about 100KM
Wavelength: Multimode 850/1310NM, single-mode 1310/1550NM, others are similar
Multimode transceivers receive multiple transmission modes and have a shorter transmission distance.
Single-mode transceivers only receive a single mode. The transmission distance is longer.
As for which one is more widely used, it is hard to say. Although multimode is being phased out, it is still widely used in monitoring and short-distance transmission because of its lower price. I personally recommend single-mode.
Single-mode cables have two cores, one for receiving and one for transmitting; there are also single-fiber bidirectional cables that use one core and use WDM technology to achieve bidirectional transmission on the same core. Currently, most of the cables on the market use single-mode single-fiber.
Multimode cables have two cores, not single-core, because multimode optical cables cannot do WDM.
Source: Dongguan HX Fiber Technology Co., Ltd
1. Multimode Fiber
When the geometric size of the optical fiber (mainly the core diameter d1) is much larger than the wavelength of the light wave (about 1µm), there will be dozens or even hundreds of propagation modes in the optical fiber. Different propagation modes have different propagation speeds and phases, resulting in time delays and broadening of light pulses after long-distance transmission. This phenomenon is called mode dispersion of optical fiber (also called inter-mode dispersion). Mode dispersion will narrow the bandwidth of multimode optical fiber and reduce its transmission capacity. Therefore, multimode optical fiber is only suitable for optical fiber communications with smaller capacity. The refractive index distribution of multimode optical fiber is mostly parabolic distribution, that is, gradient refractive index distribution. Its core diameter is about 50µm.
2. Single-mode fiber
When the geometric size of the optical fiber (mainly the core diameter) can be close to the wavelength of light, such as the core diameter d1 is in the range of 5 to 10µm, the optical fiber only allows one mode (fundamental mode HE11) to propagate in it, and all other higher-order modes are cut off. Such optical fiber is called single-mode optical fiber. Since it has only one mode of propagation, it avoids the problem of mode dispersion, so single-mode optical fiber has an extremely wide bandwidth and is particularly suitable for large-capacity optical fiber communication. Therefore, to achieve single-mode transmission, the parameters of the optical fiber must meet certain conditions. It is calculated by formula that for optical fiber with NA=0.12, when single-mode transmission is achieved above λ=1.3µm, the radius of the optical fiber core should be ≤4.2µm, that is, its core diameter d1≤8.4µm.Since the core diameter of single-mode optical fiber is very small, more stringent requirements are placed on its manufacturing process.
3. What are the advantages of using optical fiber?
1) The bandwidth of optical fiber is very wide, theoretically up to 30T.
2) The length of non-relay support can reach tens to hundreds of kilometers, while copper wire is only a few hundred meters.
3) It is not affected by electromagnetic fields and electromagnetic radiation.
4) It is light in weight and small in size.
5) Optical fiber communication is not electrified, so it is safe to use and can be used in flammable and explosive places.
6) The operating environment temperature range is wide.
7) It has a long service life.
4. How to choose optical cable?
In addition to the number of optical fiber cores and the type of optical fiber, the structure and outer sheath of the optical cable should also be selected according to the use environment of the optical cable.
1) When directly buried, loose-tube armored optical cable should be selected for outdoor optical cable. When overhead, loose-tube optical cable with black PE outer sheath with two or more reinforcing ribs can be selected.
2) When selecting optical cables for use in buildings, tight-tube optical cables should be selected and attention should be paid to their flame retardant, toxic and smoke properties. Generally, flame retardant but smoke-containing types (Plenum) or flammable and non-toxic types (LSZH) can be selected in pipelines or forced ventilation, and flame retardant, non-toxic and smoke-free types (Riser) should be selected in exposed environments.
3) When laying cables vertically or horizontally in a building, tight-tube optical cables, distribution optical cables or branch optical cables commonly used in buildings can be selected.
4) Select single-mode and multi-mode optical cables according to network applications and optical cable application parameters. Generally, multi-mode optical cables are used for indoor and short-distance applications, while single-mode optical cables are used for outdoor and long-distance applications.
5. In the connection of optical fiber, how to choose different applications of fixed connection and active connection?
The active connection of optical fiber is realized through optical fiber connector. An active connection point in the optical link is a clear segmentation interface. In the choice of active connection and fixed connection, the advantages of fixed connection are low cost and small optical loss, but poor flexibility, while active connection is the opposite. When designing the network, it is necessary to flexibly choose the use of active and fixed connections according to the situation of the entire link to ensure both flexibility and stability, so as to give full play to their respective advantages. The active connection interface is an important interface for testing, maintenance, and change. Active connection is relatively easier to find the fault point in the link than fixed connection, which increases the convenience of replacing faulty components, thereby improving system maintainability and reducing maintenance costs.
6. Fiber is getting closer and closer to user terminals. What are the significance of "fiber to the desktop" and what factors should be paid attention to when designing the system?
"Fiber to the desktop" is complementary and indispensable to copper cables in the application of horizontal subsystems. Fiber has its own unique advantages, such as long transmission distance, stable transmission, no influence of electromagnetic interference, high bandwidth support, and no electromagnetic leakage. These characteristics make fiber play an irreplaceable role for copper cables in some specific environments:
1) When the transmission distance of the information point is greater than 100m, if you choose to use copper cables. Repeaters must be added or network equipment and weak current rooms must be added, thereby increasing costs and potential faults. The use of fiber can easily solve this problem.
2) In specific working environments (such as factories, hospitals, air-conditioning rooms, power rooms, etc.), there are a large number of electromagnetic interference sources. Fiber can be free from electromagnetic interference and operate stably in these environments.
3) There is no electromagnetic leakage in optical fiber, and it is very difficult to detect the signal transmitted in the optical fiber. It is a good choice in places with high confidentiality requirements (such as military, R&D, auditing, government and other industries).
4) For environments with high bandwidth requirements, reaching more than 1G, optical fiber is a good choice.
The application of optical fiber is gradually extending from the backbone or computer room to desktop and residential users, which means that more and more users who do not understand the characteristics of optical fiber are beginning to come into contact with optical fiber systems. 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, using compatible systems and products, facilitating maintenance and management as much as possible, and adapting to the ever-changing actual on-site conditions and user installation requirements.
5) Can fiber optic connectors be directly terminated on 250µm optical fibers?
No. Loose-tube optical cables contain bare optical fibers with an outer diameter of 250 µm, which are very small and fragile. They cannot fix the optical fibers, are not strong enough to support the weight of the optical fiber connectors, and are very unsafe. To terminate the connector directly on the optical cable, at least a 900 µm tight-tube layer must be used to wrap the 250 µm optical fiber outside to provide protection for the optical fiber and support for the connector.
6) Can FC connectors be directly connected to SC connectors?
Yes, this is just a different way to connect two different types of connectors.
If you need to connect them, you must choose a mixed transfer adapter. Using the FC/SC adapter, you can connect the FC connector and SC connector at both ends respectively. This method requires that the connectors should be flat-polished. If you must connect an angled (APC) connector, you must use the second method to prevent damage.
The second method is to use a hybrid jumper and two connection adapters. A hybrid jumper means that different fiber optic connector types are used at both ends. These connectors will be connected to where you need to connect, so that you can use a universal adapter in the patch panel to connect to the system, but it will increase the amount of connector pairs to the system attenuation budget.
7) The fixed connection of optical fiber includes mechanical optical fiber splicing and hot fusion splicing. What are the selection principles of mechanical optical fiber splicing and hot fusion splicing?
Mechanical optical fiber splicing is commonly known as optical fiber cold splicing. It refers to the optical fiber splicing method that does not require a hot fusion splicer, but uses simple splicing tools and mechanical connection technology to achieve permanent connection of single-core or multi-core optical fibers. In general, when splicing optical fibers with a small number of cores and scattered locations, it is advisable to use mechanical splicing instead of hot fusion splicing.
In the early days, mechanical optical fiber splicing technology was often used in engineering practices such as line repair 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), people have realized the significance of mechanical optical fiber splicing as an important means of optical fiber splicing.
For fiber to the desktop and fiber to the home applications with large number of users and scattered locations, when the user scale reaches a certain level, the complexity of construction and the construction personnel and fusion splicing machines cannot meet the time requirements for users to open services. Mechanical fiber splicing provides the most cost-effective fiber splicing solution for large-scale fiber deployment due to its simple operation, short personnel training cycle, and small equipment investment. For example, in high-rise corridors, narrow spaces, insufficient lighting, and inconvenient on-site power supply, mechanical fiber splicing provides a convenient, practical, fast, and high-performance fiber splicing method for design, construction, and maintenance personnel.
8) In the fiber-to-the-home system, what are the differences between the requirements for the optical cable junction box and the optical cable junction box used in the outdoor lines of telecom operators?
First, in the fiber-to-the-home system, it is necessary to reserve the installation and termination of the splitter in the junction box, and to accommodate and protect the jumpers entering and exiting the splitter according to actual needs. Because the actual situation is that the splitter may be located in facilities such as the optical cable junction box, the optical cable junction box, the distribution box, and the ODF, and the optical cable is terminated and distributed in them.
Secondly, for residential areas, the optical cable junction box is more often installed in the form of buried, so the buried performance of the optical cable junction box is required to be higher.
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-core optical cables.
The core diameter of multimode optical fiber is 50~62.5μm, the outer diameter of the cladding is 125μm, and the core diameter of single-mode optical fiber is 8.3μm, and the outer diameter of the cladding is 125μm. The working wavelengths of optical fiber include short wavelength 0.85μm, long wavelength 1.31μm and 1.55μm. The optical fiber loss generally decreases with the increase of wavelength. The loss of 0.85μm is 2.5dB/km, the loss of 1.31μm is 0.35dB/km, and the loss of 1.55μm is 0.20dB/km, which is the lowest loss of optical fiber. The loss of wavelengths above 1.65μm tends to increase. Due to the absorption of OHˉ, there are loss peaks in the range of 0.90~1.30μm and 1.34~1.52μm, and these two ranges have not been fully utilized. Since the 1980s, there has been a tendency to use more single-mode optical fibers, and the long wavelength 1.31μm has been used first.
Multimode Fiber
Multimode Fiber: The central glass core is thicker (50 or 62.5μm), and can transmit multiple modes of light. However, its inter-mode dispersion is large, which limits the frequency of digital signal transmission, and it becomes more serious with the increase of distance. For example, a 600MB/KM optical fiber only has a bandwidth of 300MB at 2KM. Therefore, the transmission distance of multimode optical fiber is relatively short, generally only a few kilometers.
Single-mode fiber
Single-mode fiber: The central glass core is very thin (the core diameter is generally 9 or 10μm), and can only transmit one mode of light. Therefore, its inter-mode dispersion is very small, which is suitable for long-distance communication, but there are still material dispersion and waveguide dispersion. In this way, single-mode fiber has 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. Later, it was discovered that at a wavelength of 1.31μm, the material dispersion and waveguide dispersion of single-mode fiber are positive and negative, and the sizes are exactly equal. This means that at a wavelength of 1.31μm, the total dispersion of single-mode fiber is zero. From the loss characteristics of optical fiber, 1.31μm is exactly a low-loss window for optical fiber. In this way, the 1.31μm wavelength region has become an ideal working window for optical fiber communication, and it is also the main working band of practical optical fiber communication systems. The main parameters of 1.31μm conventional single-mode fiber are determined by the International Telecommunication Union ITU-T in the G652 recommendation, so this fiber is also called G652 fiber.
7. What is the difference between multimode fiber optic transceivers and single-mode fiber optic transceivers?
Price: Multimode is cheap, single-mode is expensive
Distance: Multimode is less than 2KM, single-mode can transmit about 100KM
Wavelength: Multimode 850/1310NM, single-mode 1310/1550NM, others are similar
Multimode transceivers receive multiple transmission modes and have a shorter transmission distance.
Single-mode transceivers only receive a single mode. The transmission distance is longer.
As for which one is more widely used, it is hard to say. Although multimode is being phased out, it is still widely used in monitoring and short-distance transmission because of its lower price. I personally recommend single-mode.
Single-mode cables have two cores, one for receiving and one for transmitting; there are also single-fiber bidirectional cables that use one core and use WDM technology to achieve bidirectional transmission on the same core. Currently, most of the cables on the market use single-mode single-fiber.
Multimode cables have two cores, not single-core, because multimode optical cables cannot do WDM.
Source: Dongguan HX Fiber Technology Co., Ltd
