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What is Optical Fiber? What is the Difference Between Single mode Fiber and Multimode Fiber?

What is Optical Fiber? What is the Difference Between Single mode Fiber and Multimode Fiber?

Do You Know What is Optic Fiber?


The optical fiber is generally divided into three layers: fiber core, cladding and coating. The fiber core and cladding are composed of glass with different refractive indexes. The fiber core is a glass (germanium-doped silica) with high refractive index , and the fiber cladding is a silica glass (pure silica) with low refractive index. Light enters the fiber at a specific angle of incidence, and full emission occurs between the fiber and the cladding (because the refractive index of the cladding is slightly lower than the core), so that it can propagate in the optical fiber.

The main function of the coating is to protect the optical fiber from external damage, while increasing the flexibility of the optical fiber. As mentioned earlier, the core and cladding are made of glass and cannot be bent and fragile. The use of the coating layer protects and prolongs the life of the optical fiber.

A layer of outer sheath is added to the non-bare fiber. In addition to protecting it, the outer sheath of different colors can also be used to distinguish various optical fibers. 

Optical fiber can be categorized as singlemode fiber and multimode fiber according to the transmission mode. Light enters the optical fiber at a specific incident angle, and full emission occurs between the optical fiber and the cladding. When the diameter is small, only one direction of light is allowed to pass through, which is a singlemode fiber. When the fiber diameter is large, it can allow light to enter and propagate at multiple angles of incidence. At this time, it is called a multimode fiber.


What is Characteristics of Optical fiber


Optical fiber has two main transmission characteristics: loss and dispersion. The loss of an optical fiber refers to the attenuation per unit length of the optical fiber, which unit is dB/km. The level of optical fiber loss directly affects the transmission distance of optical fiber communication system or the distance between relay stations. Fiber dispersion refers to the fact that the signal transmitted by the fiber is carried by different frequency components and different mode components, and the transmission speeds of different frequency components and different mode components are different, which leads to signal distortion.

Fiber dispersion is divided into material dispersion, waveguide dispersion and modal dispersion. The first two kinds of dispersion are caused by the signal not being a single frequency, and the latter kind of dispersion is caused by the signal being not a single mode. The signal is not a single mode will cause mode dispersion. Singlemode fiber has only one fundamental mode, so there is only material dispersion and waveguide dispersion, and no modal dispersion. The multimode fiber has inter-mode dispersion. The dispersion of the optical fiber not only affects the transmission capacity of the optical fiber, but also limits the relay distance of the optical fiber communication system.

What is Single mode Optical Fiber


Light enters the optical fiber at a specific incident angle, and full emission occurs between the optical fiber and the cladding. When the diameter is small, only one direction of light is allowed to pass through, which is a singlemode optical fiber. The central glass core of a singlemode fiber is very thin, the core diameter is generally 8.5 or 9.5 μm, and it works at wavelengths of 1310 and 1550 nm.

What is Multimode Optical Fiber

Multimode fiber is a fiber that allows multiple guided mode transmission. The core diameter of a multimode fiber is generally 50μm/62.5μm. Because the core diameter of a multimode fiber is relatively large, it can allow different modes of light to be transmitted on one fiber. The standard wavelengths of multimode are 850nm and 1300nm respectively. There is also a new multimode fiber standard called WBMMF (Wideband Multimode Fiber), which uses wavelengths between 850nm and 953nm.Both single-mode fiber and multi-mode fiber have a cladding diameter of 125μm.

What is the Difference Between Singlemode Fiber and Multimode Fiber? 


Transmission Distance of Optical Fiber

The smaller diameter of the singlemode fiber makes the reflection tighter, allowing only one mode of light to propagate, so that the optical signal can propagate farther. As the light passes through the core, the amount of light reflection is reduced, reducing attenuation and causing further signal propagation. Because it has no inter-mode dispersion or small inter-mode dispersion, singlemode fiber can transmit 40 kilometers or more without affecting the signal. Therefore, singlemode fiber is generally used for long-distance data transmission ,telecommunications companies, cable TV providers and universities, etc. 

Multimode fiber has a larger diameter core and can transmit light in multiple modes. In multi-mode transmission, due to the larger core size, the inter-mode dispersion is larger, that is, the optical signal "spreads" faster. The signal quality will be reduced during long-distance transmission, so multimode fiber is usually used for short-distance, audio/video applications and local area networks (LANs). And OM3/OM4/OM5 multimode fiber can support high-speed data transmission.

Bandwidth and Capacity of Optical Fiber

Bandwidth is defined as the ability to carry information. The main factors affecting the width of the optical fiber transmission band are various dispersions, of which the modal dispersion is the most important. The dispersion of the singlemode fiber is small, so it can transmit light in a wide frequency band for a long distance. Because multimode fiber will produce interference and other complex problems, it is inferior to singlemode fiber in bandwidth and capacity. The bandwidth of the latest generation of multi-mode fiber OM5 is set to 28000MHz/km, while the bandwidth of single-mode fiber is much larger.

Cost of Optical Fiber

If singlemode fiber has higher bandwidth and longer transmission distance, why do we need multimode fiber? Cost may be the key to this problem. Because the core diameter of the singlemode fiber is too small, it is difficult to control the optical transmission, so a laser is required as the light source body. Since the optic transmitter and receiver is very expensive, the cost of using a singlemode fiber will be higher than that of a multimode fiber. This fact has prompted most data centers to use multimode fiber to save costs.

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