How to choose single mode fiber and multimode fiber?


1. What is single-mode and multi-mode fiber? What is the difference between them?

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


When the geometric size of the fiber core is much larger than the wavelength of the light wave, 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, ...).


The HE11 mode is called the fundamental mode, and the rest are called higher-order modes.


1) Multimode fiber

When the geometric size of the 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 fiber. Different propagation modes have different propagation velocities and phases, resulting in time delay and widening of optical pulses after long-distance transmission. This phenomenon is called the modal dispersion of the fiber (also called intermodal dispersion).

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Modal dispersion will narrow the bandwidth of multimode fiber and reduce its transmission capacity, so multimode fiber is only suitable for small-capacity fiber communication.


Most of the refractive index distributions of multimode fibers are parabolic distributions, ie graded refractive index distributions. Its core diameter is about 50µm.


2) Single mode fiber

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


Since it has only one mode of propagation, the problem of modal dispersion is avoided, so the single-mode fiber has a very wide bandwidth, which is especially suitable for large-capacity optical fiber communication. Therefore, in order to achieve single-mode transmission, the parameters of the fiber must meet certain conditions. It is calculated by the formula that for a fiber with NA=0.12 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 the single-mode fiber is very small, more stringent requirements are placed on its manufacturing process.


2. What are the advantages of using optical fiber?

1) The passband of the optical fiber is very wide, and the theoretical value can reach 30T.


2) The length of support without repeater can reach dozens to hundreds of kilometers, and the copper wire is only a few hundred meters.


3) Unaffected by electromagnetic fields and electromagnetic radiation.


4) Light weight and small size.


5) Optical fiber communication is not electrified, and can be used in flammable and violent places for safe use.


6) Wide range of ambient temperature for use.


7) Long service life.


3. How to choose the optical cable?

The selection of optical cables is not only based on the number of optical fibers and the type of optical fibers, but also the structure and outer sheath of the optical cable according to the use environment of the optical cable.


1) When the outdoor optical cable is directly buried, loose armored optical cable should be used. For overhead use, a loose-sheathed fiber optic cable with black PE outer sheath with two or more reinforcing ribs can be used.


2) When selecting the optical cable used in the building, the tight sleeve optical cable should be selected and attention should be paid to its flame retardant, toxic and smoke characteristics. Generally, the flame retardant but smoke-free type (Plenum) or the flammable and non-toxic type (LSZH) can be used in the pipeline or forced ventilation, and the flame-retardant, non-toxic and smoke-free type (Riser) should be used in the exposed environment.


3) When cabling vertically or horizontally in the building, you can choose the tight-sleeve optical cable, distribution optical cable or branch optical cable that is common in the building.


4) Select single-mode and multi-mode optical cables according to network application and optical cable application parameters. Usually indoor and short-distance applications are dominated by multi-mode optical cables, and outdoor and long-distance applications are dominated by 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 optical fibers is realized through optical fiber connectors. An active connection point in an optical link is a clear dividing interface. In the choice of active and fixed connections, the advantages of fixed connections are lower cost, less optical loss, but less flexible, while active connections are the opposite. During network design, it is necessary to flexibly select the use of active and fixed connections according to the conditions 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. Compared with the fixed connection, the active connection is relatively easier to find the fault point in the link, which increases the convenience for the replacement of faulty components, thereby improving system maintainability and reducing maintenance costs.


5 Optical fibers are getting closer and closer to user terminals. What factors should be paid attention to in the meaning of "fiber to the desktop" and system design?

In the application of "fiber to the desktop" in the horizontal subsystem, the relationship with copper cables is complementary and indispensable. Optical fiber has its unique advantages, such as long transmission distance, stable transmission, not affected by electromagnetic interference, high support bandwidth, and no electromagnetic leakage. These characteristics make optical fiber play an irreplaceable role in copper cables in some specific environments:


1) When the information point transmission distance is greater than 100m, if choose to use copper cable. It is necessary to add repeaters or add network equipment and weak current rooms, thereby increasing costs and hidden troubles, which can be easily solved by using optical fibers.


2) There are a large number of electromagnetic interference sources in specific working environments (such as factories, hospitals, air-conditioning rooms, power rooms, etc.), and optical fibers can be immune to electromagnetic interference and operate stably in these environments.


3) There is no electromagnetic leakage in the 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) In the environment with high demand for bandwidth, 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 contact the optical fiber system. Therefore, when designing an optical fiber link system and selecting products, the current and future application requirements of the system should be fully considered, and compatible systems and products should be used to facilitate maintenance and management to the greatest extent possible, and to adapt to the ever-changing on-site actual conditions and user installation needs.


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


Loose-tube fiber optic cable contains bare fiber with an outer diameter of 250 µm, which is very small in size and is very fragile. It cannot be fixed to the fiber, insufficient to support the weight of the fiber optic connector, and is very unsafe. Terminate the connector directly on the fiber optic cable. , at least a 900 µm tight jacket is required to wrap around the 250 µm fiber to protect the fiber and support the connector.


7. Can the FC connector be directly connected with the SC connector?

Yes, it's just a different connection method for two different types of connectors.


If you need to connect them, you must choose a hybrid adapter, use the FC/SC adapter to connect the FC connector and the SC connector on both ends respectively. This method requires that the connectors should be flat ground, and if you absolutely need to connect angled (APC) connectors, the second method of preventing damage must be used.


The second method is to use a hybrid patch cord and two connection adapters. Hybrid patch cords are those with different fiber optic connector types on both ends that will go where you need to connect, allowing you to use a generic adapter in the patch panel to connect to the system, but at the cost of the system attenuation budget. The increment of one connector pair.


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

Mechanical optical fiber splicing, commonly known as optical fiber cold splicing, refers to a fiber splicing method that does not require a thermal fusion splicer, and uses simple splice tools and mechanical connection technology to achieve permanent connection of single-core or multi-core fibers. In general, mechanical splicing should be used instead of thermal fusion splicing when splicing optical fibers with a small number of cores scattered in multiple locations.


Mechanical fiber splicing technology was 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-desk and fiber-to-the-home (FTTH), people realize the significance of mechanical fiber splicing as an important fiber splicing method.


For fiber-to-desktop and fiber-to-the-home applications with a large number of users and scattered locations, when the number of users reaches a certain level, the complexity of construction and construction personnel and fusion splicers cannot meet the time requirements for users to open services. The mechanical optical fiber splicing method provides the most cost-effective optical fiber splicing solution for large-scale deployment of optical fibers due to its simple operation, short personnel training period, and small equipment investment. For example, in high corridors, small spaces, insufficient lighting, inconvenient on-site power access, etc., mechanical optical fiber splicing provides a convenient, practical, fast and high-performance optical fiber splicing method for designers, construction and maintenance personnel.


9. What is the difference between the requirements for fiber optic cable splice boxes in fiber-to-the-home systems and those used in telecom operators' outdoor lines?

First of all, in the fiber-to-the-home system, it is necessary to reserve positions for the installation of the optical splitter and the termination, accommodation and protection of the jumper wires entering and leaving the optical splitter in the splice box according to actual needs. Because the actual situation is that the optical splitter may be located in the optical cable splice box, optical cable junction box, distribution box, ODF and other facilities, and the termination and distribution of the optical cable are performed in it.


Secondly, for residential quarters, the optical cable splice box is mostly installed in a buried way, so the buried performance of the optical cable joint box is higher.


In addition, in a fiber-to-the-home project, the entry and exit of a large number of small-core-count cables may need to be considered.



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