Speed collection! The most comprehensive introduction to fiber optic cables in history
Optical fiber, the full name is optical fiber, and the English name is OPTIC FIBER.
It is a fiber made of glass or plastic that can be used as a light transmission tool.
The main purpose of optical fiber is communication. At present, the optical fiber used in communication is basically a silica optical fiber, and its main component is high-purity silica glass, namely silicon dioxide (SiO2).
Optical fiber communication systems use optical fibers to transmit light waves that carry information to achieve the purpose of communication.
▎The development history of optical fiber communication
In 1880, Alexander Graham Bell invented the "optical telephone".
In 1887, British scientist Charles Vernon Boys pulled out the first optical fiber in the laboratory.
In 1938, the American Owens Illinois Glass Company and Japan's Nitto Textiles Company began to produce long glass fibers.
In 1951, the optical physicist Brian O’Brian proposed the concept of cladding.
In 1956, a student at the University of Michigan made the first glass-clad fiber. He used a glass tube with a low refractive index to melt it onto a glass rod with a high refractive index.
In 1960, Theodore Maiman showed people the first laser. This has ignited people's interest in optical communication. Laser seems to be a very promising communication method that can solve the problem of transmission bandwidth. Many laboratories have begun experiments.
In 1966, the British-Chinese scholar Gao Kun pointed out the possibility and technical approach of using optical fiber for information transmission, which laid the foundation for modern optical communication-optical fiber communication.
In 1970, Corning in the United States successfully developed a quartz optical fiber with a loss of 20dB/km.
In 1973, Bell Laboratories of the United States achieved greater results, reducing the optical fiber loss to 2.5dB/km.
In 1976, Nippon Telegraph and Telephone (NTT) reduced the optical fiber loss to 0.47 dB/km (wavelength 1.2μm).
▎The characteristics of optical fiber communication
Huge communication capacity
In theory, an optical fiber can transmit 10 billion voice channels at the same time. At present, the test of 500,000 voice channels at the same time has been successful, which is thousands or even hundreds of thousands times higher than traditional coaxial cables and microwaves.
Long relay distance
Optical fiber has a very low attenuation coefficient, coupled with appropriate optical transmission, optical receiving equipment, optical amplifier, forward error correction and RZ code modulation technology, etc., can make the relay distance of more than thousands of kilometers, while the traditional cable can only Transmission 1.5km, microwave 50km, can not compare with it at all.
Good confidentiality
Adaptable
It has the advantages of not afraid of interference from strong external electromagnetic fields, corrosion resistance, etc.
Small size and light weight
Rich sources of raw materials and low prices
▎Fiber structure
The typical structure of an optical fiber is a multi-layer coaxial cylinder, which is mainly composed of a core, a cladding, and a coating layer.
Fiber core
Located in the center of the optical fiber, the composition is high-purity silica with a very small amount of dopant. The refractive index of the core is slightly higher than that of the cladding, and the loss is lower than that of the cladding. The light energy is mainly transmitted in the core.
layers
Located around the core, its composition is also high-purity silica containing a very small amount of dopants. The cladding layer provides a reflective surface and light isolation for light transmission, and plays a certain mechanical protection role.
Coating
The outermost layer of the optical fiber is composed of acrylate, silicone rubber and nylon. The coating protects the optical fiber from water vapor erosion and mechanical abrasion.
▎The working principle of optical fiber
Principle of Total Reflection
If the light beam is directed from the optically dense medium to the optically thin medium, the angle of refraction is greater than the angle of incidence, as shown in the figure.
If θ0 is continuously increased, the refraction angle θ1 can reach 90°, at this time θ1 is called the critical angle.
When the light radiates from the optically dense medium to the optically thin medium, and the incident angle is greater than the critical angle, the phenomenon of total reflection will occur.
Optical fiber uses this total reflection to transmit optical signals.
▎Dispersion of optical fiber
Causes of fiber dispersion
In the optical fiber, the optical signal is composed of many different components. Because the propagation speed of each frequency component or each mode component of the signal is different, after a certain distance through the optical fiber, there will be a time delay difference between the different components, causing the transmission signal waveform Distortion, pulse broadening, this phenomenon is called fiber dispersion.
Influence of fiber dispersion
The existence of fiber dispersion distorts and broadens the transmitted signal pulse, thereby causing inter-symbol interference. In order to ensure communication quality, the inter-symbol interval must be increased, that is, the transmission rate of the signal must be reduced, which limits the communication capacity and transmission distance of the optical fiber system.
Classification of fiber dispersion
According to the causes of dispersion, fiber dispersion can be divided into modal dispersion, material dispersion, waveguide dispersion and polarization dispersion.
▎The electromagnetic spectrum of optical fiber
▎The loss of optical fiber
The loss of optical fiber refers to the reduction of optical power due to absorption, scattering and other reasons after the optical signal is transmitted through the optical fiber.
Classification of fiber loss
Schematic diagram of the attenuation of ordinary single-mode fiber with wavelength
▎The classification of optical fiber
Step fiber
In the core and cladding regions, the refractive index distributions are uniform, n1 and n2, respectively. At the boundary between the core and the cladding, the refractive index changes stepwise (n2<n1).
Graded fiber
The refractive index at the fiber axis is the largest (n1), but it gradually decreases with the increase in the radial direction of the cross-section. At the boundary between the core and the cladding, it just drops to the refractive index n2 with the cladding region.
Multimode fiber (MMF, multimode fiber)
Can transmit multiple modes of light. However, the inter-mode dispersion is relatively large, which limits the frequency of the transmission of digital signals, and it becomes more serious with the increase of distance.
Single-mode fiber (SMF, single-mode fiber)
Only one mode of light can be transmitted, so its inter-mode dispersion is very small, which is suitable for long-distance communication.
Comparison of multimode fiber and single mode fiber
Application of multimode fiber and single mode fiber
▎Fiber optic interface
There are the following types of optical fiber interfaces:
FC round with thread (most used on patch panels)
ST snap-on round type
SC card-connected square type (most used on router switches)
The LC connector is similar in shape to the SC connector, but smaller than the SC connector
MT-RJ square type, one head dual fiber transceiver integrated
MPO/MTP type
BFOC type
DIN type
FDDI type
MU type
What do the common representation methods, such as "FC/PC", "SC/PC", and "SC/APC" mean?
The part in front of "/" indicates the connector type of the pigtail, FC and SC are as mentioned above, omitted;
The part after "/" indicates the cross-section process of the optical fiber connector, that is, the grinding method.
"PC: Physical Contact":
Its joint cross-section is flat, in fact it is micro-spherical grinding and polishing, which is the most widely used in the equipment of telecom operators.
"APC":
It has an 8 degree angle and is polished and polished by a microsphere. It is the model that is mostly used in radio and television and early CATV. Its pigtail head adopts an angled end face, which can improve the quality of the TV signal. The main reason is that the TV signal is analog. Light modulation, when the coupling surface of the joint is vertical, the reflected light returns along the original path.
"UPC":
Attenuation is smaller than that of "PC", and it is generally used for equipment with special needs. Some foreign manufacturers use FC/UPC for internal fiber jumpers in ODF racks, mainly to improve the ODF equipment's own indicators.
▎Fiber optic module
Optical transceiver, the full name is optical transceiver, is an important component in optical fiber communication system.
Generally, the following types of network equipment are included:
SFP (Small Form-factor Pluggable transceiver):
Small package pluggable transceiver (LC interface), SFP supports speeds of 100M, 155M, 622M, 1000M, 1250M, 2500M.
GBIC (GigaBit Interface Converter):
Gigabit Ethernet interface converter (SC interface)
XFP (10-Gigabit small Form-factor Pluggable transceiver ):
10 Gigabit Ethernet interface small package pluggable transceiver (LC interface)
XENPAK (10 Gigabit EtherNet Transceiver PAcKage):
10 Gigabit Ethernet interface transceiver package (SC interface)
▎Fiber splicing
Fusion splicing is a wiring technology that uses the heat generated by the discharge between the electrode rods to melt the optical fiber into a whole. It is divided into the following two categories:
Fiber core alignment method
This is a fusion method in which the core wire of the optical fiber is observed under a microscope, and is positioned through image processing to make the center axis of the core wire consistent, and then discharges. The fusion splicing machine equipped with a two-way observation camera is used for positioning from two directions.
Fiber Fusion Splicer
Fixed V-groove alignment method
This is a fusion method in which the fibers are arranged in a high-precision V-groove, and the aligning effect produced by the surface tension of the melted fiber is used for outer diameter alignment. Recently, due to the development of manufacturing technology, the dimensional accuracy of the position of the optical fiber core has been improved, and therefore, low-loss wiring can be realized.
This method is mainly used for multi-core one-time wiring.
Optical cable
Optical cable: Use appropriate materials and cable structure to house and protect the communication optical fiber, so that the optical fiber is protected from mechanical and environmental influence and damage, and is suitable for use in different occasions.
▎The structure of the optical cable
An optical cable is made of one or more optical fibers or fiber bundles in a structure that conforms to chemical, mechanical, and environmental characteristics. Regardless of the structure of the optical cable, it is basically composed of three parts: the cable core, the strengthening element and the sheath.
Cable core
The cable core structure should meet the following basic requirements:
① Keep the optical fiber in the best position and state in the cable to ensure the stable transmission performance of the optical fiber. When the optical cable is subjected to certain external forces such as pulling, lateral pressure, etc., the optical fiber should not be affected by the external force.
② The reinforcing elements in the cable core should be able to withstand the allowable tension.
③ The cross-section of the cable core should be as small as possible to reduce costs. There are optical fibers, sleeves or skeletons and strengthening elements in the cable core, and the cable core needs to be filled with grease, which has a reliable moisture-proof performance and prevents moisture from spreading in the cable core.
Protective layer
As long as the sheath of the optical cable protects the fiber core of the cable, it can avoid external mechanical force and environmental damage, so that the optical fiber can be adapted to various laying occasions. Therefore, the sheath is required to have pressure resistance, moisture resistance, and good temperature characteristics. Light weight, chemical resistance and flame retardant characteristics.
The sheath of the optical cable can be divided into an inner sheath and an outer sheath. The inner protective layer is generally made of polyethylene or polyvinyl chloride, and the outer protective layer can be determined according to the laying conditions, using aluminum tape and polyethylene LAP outer jacket and steel wire armor.
Strengthening element
The reinforcing element is mainly to bear the external force applied during laying and installation. The configuration of fiber optic cable strengthening elements is generally divided into a "central strengthening element" method and a "peripheral strengthening element" method.
Generally, the strengthening elements of stranded and skeleton optical cables are located in the center of the cable core and belong to the "central strengthening element" (strengthening core); the strengthening element of the central tube type optical cable moves from the cable core to the sheath and belongs to the "peripheral strengthening element".
Reinforcing elements generally include metal steel wire and non-metallic glass fiber reinforced plastic (FRP). Non-metallic fiber optic cables using non-metallic strength components can effectively reverse lightning strikes.
▎Typical structure of optical cable
Commonly used fiber optic cable structures have four types: stranded, skeleton, central beam tube, and ribbon.
