1.What is singlemode and multimode 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 extremely high-frequency (3×1014Hz) electromagnetic wave. When it propagates in an optical fiber, it is found from theories of wave optics, electromagnetic fields, and Maxwell equations.
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.
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.
When the fiber’s geometry (mainly the core diameter) can be similar to the wavelength of light, if the core diameter d1 is in the range of 5~10μm, the fiber only allows one mode (base mode HE11) to propagate in it, and all other high-order modes are all cut off. This kind of 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 singlemode 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 is up to 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, volatile 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) Optical cable for outdoor use When loosely buried, it is better to use loose-sheathed cable. 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-fit optical cable, distribution optical cable or branch optical cable that are common in the building.
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 fibers, 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 connection and fixed connection, the advantages of fixed connection are reflected in lower cost, light 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 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.Fibers are getting closer to user terminals. What do you need to pay attention to when it comes to the meaning of “fiber to the desktop” and system design?
“Fiber-to-the-desktop” in the application of the horizontal subsystem, and the relationship between copper and copper cable is complementary and indispensable. 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) There are a large number of sources of electromagnetic interference in specific work environments (such as factories, hospitals, air-conditioning rooms, power equipment rooms, etc.), and optical fibers can be operated stably without electromagnetic interference in these environments.
3) There is no electromagnetic leakage in the fiber. It is very difficult to detect the signal transmitted in the fiber. It is a good choice for places where the security level is relatively high (such as military, R&D, auditing, government, etc.).
4) The environment with high demand for bandwidth has reached more than 1G. Optical fiber is a good choice.
There are many differences between single-mode fiber and multi-mode fiber, and the selection method is not the same. Let’s talk about it today. For more details, please keep an eye on Singlemode fiber and multimode fiber different and selection method(2).