The difference between single-mode fiber and multi-mode fiber and how to choose

1. What are single-mode and multi-mode optical fibers? What are their differences?

The concepts of single-mode and multi-mode are used to classify optical fibers according to their propagation modes: the concepts of multi-mode optical fibers and single-mode optical fibers. We know that light is an electromagnetic wave with a very high frequency (3×1014Hz). When it propagates in an optical fiber, according to theories such as wave optics, electromagnetic fields, and the solution of Maxwell's equations, it is found that:

• When the geometric size of the optical fiber core is much larger than the wavelength of the light wave, light will propagate in the optical 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 fundamental mode, and the rest are called higher-order modes.

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(1) Multimode fiber

When the geometric dimensions of an optical fiber (mainly the core diameter d1) are much larger than the wavelength of light (about 1 μm), 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 light pulse broadening after long-distance transmission. This phenomenon is called optical fiber mode dispersion (also called inter-modal dispersion).

Modal dispersion will narrow the bandwidth of multimode optical fiber and reduce its transmission capacity, so multimode optical fiber is only suitable for smaller capacity optical fiber communications.

The refractive index distribution of multimode optical fiber is mostly parabolic distribution, that is, graded refractive index distribution. Its core diameter is about 50μm.

(2) Single-mode fiber

When the geometric dimensions of an optical fiber (mainly the core diameter) are close to the wavelength of light, such as when the core diameter d1 is in the range of 5 to 10 μm, the optical fiber only allows one mode (fundamental mode HE11) to propagate in it, and all other higher-order modes are cut off. Such an optical fiber is called a single-mode optical fiber.

Since it has only one mode of propagation, it avoids the problem of mode dispersion, so single-mode fiber has a very wide bandwidth and is particularly suitable for large-capacity fiber-optic communications. Therefore, to achieve single-mode transmission, the parameters of the fiber must meet certain conditions. Through the formula calculation, it is found that for NA=0.12 fiber to achieve single-mode transmission above λ=1.3μm, the radius of the 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.

2. What are the advantages of using optical fiber?

• The bandwidth of optical fiber is very wide, theoretically up to 30T.
• The supported length without repeaters can reach tens to hundreds of kilometers, while copper wires can only reach a few hundred meters.
• Not affected by electromagnetic fields and electromagnetic radiation.
• Light weight and small size.
• Optical fiber communication does not carry electricity and is safe to use in flammable and explosive places.
• Wide operating temperature range.
• Long service life.

3. How to choose optical cable?

In addition to the number of optical fiber cores and the type of optical fiber, the selection of optical cable also depends on the structure and outer sheath of the cable according to the environment in which the cable is used.

• When outdoor optical cables are directly buried, loose tube armored optical cables should be used. When overhead, loose tube optical cables with black PE outer sheaths with two or more reinforcing ribs can be used.
• When selecting optical cables for use in buildings, tight-buffered optical cables should be used and attention should be paid to their flame retardant, toxic and smoke characteristics. Generally, flame retardant but smoke-containing types (Plenum) or flammable non-toxic types (LSZH) can be selected in ducts or forced ventilation areas, and flame retardant, non-toxic and smoke-free types (Riser) should be selected in exposed environments.
• When laying cables vertically or horizontally in a building, you can choose tight-buffered optical cables, distribution optical cables or branch optical cables that are commonly used in buildings.
• Single-mode and multi-mode optical cables are selected 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.

4. In the connection of optical fiber, how to choose the different applications of fixed connection and active connection?

The active connection of optical fiber is realized through optical fiber connectors. An active connection point in an optical link is a clear dividing interface. In the choice between active connection and fixed connection, the advantages of fixed connection are low cost and low optical loss, but poor flexibility, while active connection is the opposite. When designing a 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.

5. Fiber is getting closer and closer to user terminals. What is the significance of "fiber to the desktop" and what factors should be considered when designing the system?

In the application of horizontal subsystems, "fiber to the desktop" and copper cables are complementary and indispensable. 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:

• When the transmission distance of information points is greater than 100m, if copper cables are used, repeaters or network equipment and weak current rooms must be added, which increases costs and potential faults. Optical fibers can easily solve this problem.
• In certain working environments (such as factories, hospitals, air-conditioning rooms, power rooms, etc.), there are a large number of electromagnetic interference sources. Optical fiber can be free from electromagnetic interference and operate stably in these environments.
• Optical fiber does not have electromagnetic leakage, 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).
• In 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.

6. Can fiber optic connectors be terminated directly onto 250 μm fiber?

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 fiber, are not strong enough to support the weight of the optical fiber connector, and are very unsafe. To terminate the connector directly on the optical cable, at least a 900 μm tight tube layer is required to wrap the 250 μm optical fiber outside to provide protection for the optical fiber and support the connector.

7. Can an FC connector be directly connected to an SC connector?

Yes, it is just a different method of connection between two different types of connectors.

If you need to connect them, you must choose a mixed adapter. Using the FC/SC adapter, you can connect the FC connector and SC connector at both ends. 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 patch cord and two connection adapters. A hybrid patch cord is one that uses different fiber connector types at both ends. These connectors will connect to where you need them, so you can use a universal adapter in the patch panel to connect to the system, but it will increase the system attenuation budget by one connector pair.

8. The fixed connection of optical fiber includes mechanical optical fiber splicing and thermal fusion splicing. What are the selection principles of mechanical optical fiber splicing and thermal fusion splicing?

Mechanical fiber splicing is commonly known as fiber cold splicing, which refers to a fiber splicing method that does not require a hot melt splicer, but uses simple splicing tools and mechanical connection technology to achieve a single-core or multi-core optical fiber connection. In general, when splicing optical fibers with a small number of cores and dispersed locations, it is advisable to use mechanical splicing instead of hot melt splicing.

Mechanical fiber splicing technology was often used in engineering practices such as line repair and small-scale applications in special occasions in the early days. 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 fiber splicing as an important fiber splicing method.

For fiber-to-the-desktop and fiber-to-the-home applications with large numbers of users and scattered locations, when the user scale reaches a certain level, the complexity of construction and the construction personnel and fusion splicers cannot meet the time requirements for users to open services. Mechanical fiber splicing provides a 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 places in corridors, small 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.

9. How do the requirements for fiber optic splice closures in fiber-to-the-home systems differ from those used by telecom operators in outdoor lines?

First, in the FTTH system, it is necessary to reserve the location for the installation and termination of the optical splitter, and to accommodate and protect the jumpers in and out of the optical splitter in the junction box according to actual needs. Because the actual situation is that the optical splitter may be located in facilities such as the optical cable junction box, optical cable junction box, distribution box, ODF, etc., and the optical cable is terminated and distributed in them.

Secondly, for residential areas, optical cable junction boxes are more often installed underground, so higher requirements are placed on the underground performance of the optical cable junction boxes.

In addition, in fiber-to-the-home projects, you may need to consider the entry and exit of a large number of small-core optical cables.