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What are the classifications of outdoor optical cables

Divided into single-mode fiber and multi-mode fiber according to the transmission mode of the fiber

Single-mode fiber

This refers to an optical fiber that can transmit only one propagation mode at the working wavelength, and is often referred to as a single-mode fiber (SMF: Single Mode Fiber). At present, it is the most widely used optical fiber in cable television and optical communications. Because the core of the optical fiber is very thin (about 10 μm) and the refractive index is stepwise distributed, when the normalized frequency V parameter is less than 2.4, theoretically, only single-mode transmission can be formed. In addition, SMF does not have multi-mode dispersion. Not only does the transmission band have more mode fibers, but also the material dispersion and structural dispersion of SMF add together to cancel it out. Its synthetic characteristics just form the characteristics of zero dispersion, which makes the transmission band wider. There are many types of SMFs due to differences in dopants and manufacturing methods. Depressed cladding fiber (DePr-essed Clad Fiber), whose cladding forms a double structure, and the cladding adjacent to the core has a lower refractive index than the outer inverted cladding. In addition, there are matching cladding fibers, whose cladding refractive index is uniformly distributed.

Multimode fiber

An optical fiber that has multiple modes according to its working mode and its possible propagation mode is called a multimode fiber (MMF: MUlti Mode Fiber). The core diameter is 50 μm. Since the transmission mode can reach several hundred, the transmission bandwidth is mainly dominated by mode dispersion compared to SMF. Historically used for short-distance transmission of cable television and communication systems. Ever since the emergence of SMF fiber, it seems to be a historical product. But in fact, because MMF has a larger core diameter than SMF and is easy to combine with LED and other light sources, it has more advantages in many LANs. Therefore, MMF is still being re-emphasized in the short-range communication field. When MMF is classified by refractive index distribution, there are two types: gradient (GI) type and step (SI) type. The refractive index of the GI type is highest at the core center, and gradually decreases along the cladding. From the perspective of geometric optics, the light beam advancing in the core appears to propagate in a meandering shape. Because the time required for each path of light is about the same. Therefore, the transmission capacity is larger than that of the SI type.
The refractive index distribution of the SI-type MMF fiber is the same as that of the core index, but the interface with the cladding is stepped. Because the SI type light wave is reflected in the optical fiber during the advancing process, the time difference of each light path is generated, which causes the outgoing light wave to be distorted and the color excitation is large. As a result, the transmission bandwidth is narrowed, and currently SI-type MMF is used less frequently.

Post time: 2020-03-18
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