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According to the transmission mode of the fiber, it is divided into single-mode fiber and multi-mode fiber.
Single mode fiber
This refers to the optical fiber that can only transmit one propagation mode in the working wavelength, usually referred to as Single Mode Fiber (SMF: Single Mode Fiber). In cable television and optical communications, it is the most widely used optical fiber. Because the core of the fiber is very thin (about 10μm) and the refractive index is in a step-like distribution, when the normalized frequency V parameter is less than 2.4, in theory, only single-mode transmission can be formed. In addition, SMF does not have multi-mode dispersion. Not only does the transmission frequency band be wider than the fiber with more modes, plus the addition and cancellation of SMF material dispersion and structural dispersion, its synthesis characteristics just form the characteristic of zero dispersion, which makes the transmission frequency band wider.
In SMF, there are many types due to differences in dopants and manufacturing methods. DePr-essed Clad Fiber (DePr-essed Clad Fiber), its 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 matched cladding fibers whose cladding refractive index is uniformly distributed.
Multimode fiber (MMF: MUlti ModeFiber) is a fiber in which the possible propagation modes of the fiber are multi-mode according to the working length. The core diameter is 50μm, and because the transmission mode can reach several hundred, compared with SMF, the transmission bandwidth is mainly dominated by modal dispersion. Historically, it has been used for short-distance transmission in cable television and communication systems. Since the emergence of SMF fiber, it seems to have formed a historical product. But in fact, because MMF has a larger core diameter than SMF and is easier to combine with light sources such as LEDs, it has more advantages in many LANs. Therefore, MMF is still receiving attention again in the field of short-range communication.
When MMF is classified according to the refractive index distribution, there are two types: gradient (GI) type and step (SI) type. The refractive index of the GI type is the highest in the center of the core, and gradually decreases along the cladding. From a geometrical optics point of view, the light beam traveling in the core appears to travel in a serpentine shape. Because the time required for each path of light is roughly the same. Therefore, the transmission capacity is larger than that of the SI type.
The refractive index distribution of the SI-type MMF fiber and the core refractive index distribution are the same, but the interface with the cladding is stepped. As the SI type light wave is reflected in the optical fiber, the time difference of each light path is generated, which causes the emitted light wave to be distorted and the color shock is large. As a result, the transmission bandwidth is narrowed, and there are fewer applications of SI-type MMF.