Optical fiber 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 carrying 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, the British scientist Charles Vernon Boys pulled out the first optical fiber in the laboratory.
In 1938, Owens Illinois Glass Company of the United States and Nittobo Co., Ltd. of Japan began to produce long glass fibers.
In 1951, 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, laying 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 made greater achievements, 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, a single fiber can transmit 10 billion voice channels at the same time. The current 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 It can transmit 1.5km and microwave 50km, which is simply incomparable.
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
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.
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.
Located around the core, its composition is also high-purity silica containing a very small amount of dopants. The cladding provides a reflective surface and light isolation for light transmission, and plays a certain mechanical protection role.
▎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 refraction angle is greater than the incident angle, 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, total reflection will occur.
Optical fiber uses this total reflection to transmit optical signals.
▎The dispersion of 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 signal transmission rate 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 refers to the decrease of optical power due to absorption, scattering and other reasons after optical signal is transmitted through optical fiber.
▎The classification of optical 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).
The refractive index at the axis of the fiber 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 the inter-mode dispersion is small, which is suitable for remote communication.
▎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
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 dual fiber transceiver integrated
Common representation methods, such as “FC/PC”, “SC/PC”, and “SC/APC” What do they 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 telecom operators’ equipment.
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.
The 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 itself.
▎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)
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 of observing the core wire of the optical fiber under a microscope, positioning it through image processing, making the central axis of the core wire consistent, and then discharging. The fusion splicing machine equipped with two-way observation cameras is used for positioning from two directions.
Fixed V-groove alignment method
This is a fusion method that uses high-precision V-groove arrangement of optical fibers, and uses the aligning effect produced by the surface tension of the melted fiber to perform 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: 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
Optical cable is made of one or more optical fibers or optical fiber bundles in accordance with 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.
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 stable optical fiber transmission performance. When the optical cable is subjected to certain external forces such as tension and lateral pressure, the optical fiber should not be affected by external forces.
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 reinforcing elements in the cable core, and the cable core needs to be filled with grease, which has reliable moisture resistance and prevents moisture from spreading in the cable core.
As long as the protective layer 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 protective layer 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 generally adopts polyethylene or polyvinyl chloride, etc. The outer protective layer can be determined according to the laying conditions, using aluminum tape and polyethylene LAP outer jacket and steel wire armor.
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 strengthening elements 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.
▎The classification of optical cable
Outdoor optical cable
Mainly used for direct burial, pipeline and overhead construction of trunk lines and metropolitan area networks.
Mainly used for the construction of metro backbone networks with high core count and high density.
Figure 8 Optical Cable
The optical cable integrates the cable core part and the steel wire suspension wire into a “8”-shaped PE sheath to form a self-supporting structure. There is no need to erect suspension wires and hooks during the laying process, and the construction efficiency is high and the construction cost is low. It is very simple to realize the overhead laying between electric poles, between electric poles and buildings, and between buildings and buildings.
Indoor optical cable
Mainly used for building LAN construction and vertical wiring inside the building.
▎The model of the optical cable
According to the relevant recommendations of ITU-T, the current optical cable model is composed of two parts: the type code of the optical cable and the specification code of the optical fiber, separated by a short horizontal line in the middle.
The type code of the fiber optic cable is composed of five parts: classification, strengthening member, derivative features, sheath and outer layer.
The code and significance of optical cable classification
GY: Communication room (field) outdoor optical cable
GM: Mobile Optical Cable for Communication
GJ: Optical cable in communication room (office)
GS: Optical cable in communication equipment
GH: Submarine optical cable for communication
GT: Special optical cable for communication
The code name of the strengthening member and its significance
Unsigned: Metal reinforcement member
F: Non-metallic reinforcing member
The code names of derived features and their meanings
The structural characteristics of the optical cable should be able to show the main type of cable core and the derived structure of the optical cable. When the optical cable type has several structural features that need to be noted, it can be indicated by the combination code, and the combination code is arranged in the order of the following corresponding codes from top to bottom.
D: Optical fiber ribbon structure
Unsigned: Fiber loose tube coating structure
J: Optical fiber tight sleeve coating structure
Unsigned: Stranded structure
G: Skeleton groove structure
X: Central beam tube structure
T: ointment filled structure
Z: Self-supporting structure
B: Flat shape
Z: Flame retardant
The code name of the sheath and its significance
Y: Polyethylene sheath
V: PVC sheath
U: Polyurethane sheath
A: Aluminum-polyethylene bonded sheath (A sheath)
S: Steel-polyethylene bonded sheath (S sheath)
W: Steel-polyethylene bonded sheath with parallel steel wires (W sheath)
L: Aluminum sheath
G: Steel sheath
Q: Lead sheath
The code name of the outer protective layer and its significance
The outer sheath refers to the armor layer and the outer layer outside the armor layer.