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Fiber optic cables are designed to meet certain optical, mechanical, and environmental requirements. It is a communication cable assembly that uses one or more optical fibers placed in a sheath as a transmission medium and can be used individually or in groups. It has the advantages of large capacity, long relay distance, good confidentiality, immunity to electromagnetic interference, and saving copper. Optical fiber communication is one of the important ways of modern information transmission.
fiber optic cable
Optical fiber is a kind of fiber composed of two layers of transparent media in concentric circles. Quartz glass is the most extensively used dielectric substance (SiO2). The inner medium is known as the core, and it has a greater refractive index than the outer media (called the cladding). Doping impurities in the quartz glass, such as germanium, phosphorus, fluorine, boron, and others, change the refractive index of the core or cladding. The wavelength of light transmitted by optical fiber for communication is primarily near-infrared light with a wavelength of 0.8 to 1.7 microns. The core diameter of the optical fiber varies with the type, usually a few microns to 100 microns, and the outer diameter is mostly about 125 microns.
A multi-layer coaxial cylinder, consisting primarily of a core, cladding, and coating layer, is the usual structure of an optical fiber.
Typical structure of the optical fiber
The fiber core is at the center of the optical fiber. The material is made up mostly of high-purity silica with a tiny quantity of dopant. The core has a little higher refractive index than the cladding, and it has a smaller loss than the cladding. The core is where the majority of the light energy is conveyed.
The cladding is located around the core. It also contains a minor amount of dopants and is made out of high-purity silica. The cladding acts as a reflector and light isolator for light transmission, as well as providing some mechanical protection.
The optical fiber's outermost layer is made up of acrylate, silicone rubber, and nylon. The coating protects the optical fiber from water vapor erosion and mechanical abrasion.
Optical fiber transmission is based on the principle of total reflection of available light at the interface of two media. n1 is the refractive index of the core medium. n2 is the refractive index of the cladding medium. n1 is greater than n2. The light entering the core has an incident angle when it reaches the interface between the core and the cladding. When the incident angle is greater than the critical angle of total reflection θc, total reflection can occur without light energy passing through the core. And the incident light can be transmitted forward through countless total reflections at the interface.
When the optical fiber is bent, the normal line of the interface turns, and the incident angle is small. Therefore, the incident angle of a part of the light becomes smaller than θc and cannot be totally reflected. However, those rays with a larger incident angle can still be totally reflected, so the light can still be transmitted when the optical fiber is bent, but it will cause energy loss. Generally, when the bending radius is greater than 50 to 100 mm, the loss is negligible. Small bending will cause serious "micro bending loss".
Total reflection path
People often use electromagnetic wave theory to further study the mechanism of optical fiber transmission. They the boundary conditions of the optical fiber dielectric waveguide to solve the wave equation. The light propagating in the optical fiber contains many modes, and each mode represents an electromagnetic field distribution and corresponds to a certain light described in geometric optics. The conduction mode existing in the fiber depends on the normalized frequency ν value of the fiber.
Where NA is the numerical aperture, which is related to the refractive index of the core and cladding medium. ɑ is the core radius, and λ is the wavelength of the transmitted light. When the fiber is bent, mode coupling occurs. A part of the energy is transferred from the conduction mode to the radiating mode and is lost outside the core of the fiber.
Fiber optic cable and network cable look similar in appearance, and they are both used in network communication. What are the specific differences?
1. Different materials: Most optical fibers are made of glass fiber, which is used as a light transmission tool. The transmission principle is "total reflection of light". The network cable is a copper core. There are mainly three types of lines: coaxial cable, optical cable, and twisted pair. The difference in materials also led to the difference in technical indicators between the two.
2. Transmission distance: Theoretically, the transmission distance of the network cable is within 100 meters. The single-mode fiber optic cable can realize the transmission of 10-20km, and the multi-mode fiber optic cable can realize the information transmission of 2km and 3km, which is much higher than that of the network cable.
3. Role: The optical fiber is to accelerate the transmission speed of network information, and exists as an information transmission tool. The network cable is used to connect the equipment to ensure intercommunication between the equipment.
4. Application scenarios: The network cable is used for communication transmission, which can provide users with interactive multimedia information of communication, data, and views, such as network interactive games, remote video monitoring, work conference videos, etc. The network cable is to connect the equipment, engineering wiring, network connection, equipment port connection, etc...
5. Product nature: Network cable is the carrier of information transmission and a kind of physical medium. The optical fiber is an abstract concept, which is the transmission rate of dynamic information.
6. Existence mode: Optical fiber exists in a virtual manner, and optical fiber cable is the carrier of its existence. The network cable is a real cable with concrete objects.
Modern people's lives are inseparable from the Internet. It can even be said to be a very important part of life. Recently, a lecturer from University College London (UCL) has researched the fastest data transmission speed in the world. Using 16.8 THz bandwidth reached 178 Tbps, which is 178,000,000 Mbps, setting a world record. The speed of data transmission is really unimaginable. If you take a practical example, 178 Tbps can download all Netflix videos in less than 1 second.
Fiber optic cables are divided into single-mode and multi-mode. Single-mode refers to an optical fiber that can only transmit one mode, and multi-mode refers to an optical fiber that can transmit multiple modes. How to see whether the fiber optic cable is single-mode or multi-mode:
1. Look at the color, usually orange is multi-mode, and yellow is single-mode.
2. Look at the model. The single-mode fiber optic cables are usually GYTS and GYXTW, while the multi-mode models are GYFTZY and GYFTY.
3. It is distinguished by basic identification, SM is single-mode, and MM is multi-mode.
4. It can also be distinguished according to the transmission mode of light in the optical fiber.
Common types of optical cables: G.651, G.652, G.653, G.654, G.655, and G.656.
(1) The G.651 category is a multimode fiber. IEC and GB/T are further subdivided into four subcategories A1a, A1b, A1c, and A1d according to their core diameter, cladding diameter, and numerical aperture parameters.
(2) G.652 category is a conventional single-mode fiber, currently divided into four sub-categories G.652A, G.652B, G.652C, and G.652D, IEC and GB/T named G.652C as B1.3. And the rest are named B1.1.
(3) G.653 fiber is a dispersion-shifted single-mode fiber. IEC and GB/T classify G.653 fiber as type B2 fiber. G.653 fiber is suitable for point-to-point long-distance, high-speed single-channel systems.
(4) G.654 fiber is a cut-off wavelength shifted single-mode fiber, also known as 1550nm best performance fiber. IEC and GB/T classify G.654 fiber as type B1.2 fiber. It is mainly used for non-relay submarine optical cable communication systems that have long transmission distances and cannot be inserted into active devices that require particularly high attenuation.
(5) G.655 fiber is a non-zero dispersion shift single-mode fiber, currently divided into three sub-categories G.655A, G.655B, and G.655C. IEC and GB/T classify G.655 fiber as B4 Class fiber.
(6) Non-zero dispersion single-mode fiber for wideband optical transmission of G.656 fiber, currently not named by IEC and GB/T. G.656 fiber can be used in the wide band of S+C+L (1460～1625nm).