Classification of outdoor optical cables
Aug 13, 2021
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 fiber that can only transmit one propagation mode in the working wavelength, usually referred to as single-mode fiber for short
(SMF: Single ModeFiber). In cable television and optical communications, it is the most widely used optical fiber.
Because the core of the optical fiber is very thin (about 10μm) and the refractive index has a step-like distribution, when the normalized frequency V parameter
When the number is less than 2.4, in theory, only single-mode transmission can be formed. In addition, SMF has no multi-mode dispersion, not only the transmission frequency band
More mode fibers are wider, plus the addition and cancellation of SMF material dispersion and structural dispersion, its synthesis characteristics are exactly the shape
The characteristic of zero dispersion makes the transmission frequency band wider.
In SMF, there are many types due to differences in dopants and manufacturing methods. Depressed cladding fiber (DePr-
essed Clad Fiber), its cladding forms a double structure, the cladding adjacent to the core is more refraction
The rate is still low. In addition, there are matched cladding fibers whose cladding refractive index is uniformly distributed.
Multimode fiber
Multimode fiber (MMF) is called multimode fiber (MMF:
MUlti ModeFiber). The fiber core diameter is 50μm, because the transmission mode can reach several hundred, compared with SMF transmission
The bandwidth is dominated by mode dispersion. Historically, it has been used for short-distance transmission in cable television and communication systems. since
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 different from LED
It is easy to combine other light sources, which is more advantageous in many LANs. Therefore, in the field of short-distance communication, MMF is still renewing
Valued.
When MMF is classified according to the refractive index distribution, there are two types: gradient (GI) type and step (SI) type. GI type
The index of refraction is the highest in the center of the core, and gradually decreases along the cladding. From a geometrical optics point of view, in the core
The advancing beam appears to travel in a serpentine manner. Because the time required for each path of light is roughly the same. So, pass
The transmission capacity is larger than the SI type.
The refractive index distribution of the SI type MMF fiber, the core refractive index distribution is 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 optical path is generated, resulting in
The emitted light wave is distorted and the color shock is large. As a result, the transmission bandwidth becomes narrower and there are fewer applications of SI type MMF.
