What is fiber optic cable

What is fiber optic cable

Definition of fiber optic cable

Optical fiber cable is manufactured to meet optical, mechanical or environmental performance specifications, it is a communication using one or more optical fibers placed in a sheath as the transmission medium and can be used individually or in groups cable assembly.

Optical cables are mainly composed of optical fibers (glass filaments as thin as hair), plastic protective sleeves and plastic outer skins. There are no metals such as gold, silver, copper and aluminum in the optical cables, and generally have no recycling value.

Optical cable is a kind of communication line in which a certain number of optical fibers form a cable core according to a certain method, which is covered with a sheath, and some are also covered with an outer sheath to realize optical signal transmission. That is: a cable formed by an optical fiber (optical transmission carrier) through a certain process. The basic structure of optical cable is generally composed of cable core, reinforcing steel wire, filler and sheath, etc. In addition, there are waterproof layer, buffer layer, insulated metal wire and other components as needed.

The history of fiber optic cables

In 1976, the Bell Institute of the United States built the first optical fiber communication experimental system in Atlanta, using the optical cable containing 144 optical fibers manufactured by Western Electric Company. In 1980, commercial optical cables made of multimode optical fibers began to be used on inter-city inter-office trunks and a few long-distance lines. Commercial fiber optic cables made of single-mode fiber began to be used on long-distance lines in 1983. In 1988, the first trans-Atlantic submarine cable connecting the United States and Britain and France was successfully laid, and soon the first trans-Pacific submarine cable was built. China developed its own communication optical cable in 1978, using multimode optical fiber, and the cable core structure is layered. Field tests have been carried out in Shanghai, Beijing, Wuhan and other places successively. Soon after, it was used as an inter-office trunk in the local telephone network. After 1984, it was gradually used for long-distance lines, and single-mode fiber was used. Communication optical cables have larger transmission capacity than copper cables, long relay distance, small size, light weight, and no electromagnetic interference. Since 1976, they have developed into long-distance trunk lines, urban relays, offshore and transoceanic submarine communications , as well as the backbone of wired transmission lines for local area networks, private networks, etc., and began to develop into the field of user loop distribution networks in the city, providing transmission lines for fiber-to-the-home and broadband integrated service digital networks.

Optical cable is the main transmission tool of various information networks in today's information society. If the "Internet" is called the "information superhighway", then the optical cable network is the cornerstone of the information superhighway---the optical cable network is the physical route of the Internet. Once a certain optical cable is damaged and blocked, the "information superhighway" in that direction will be destroyed. In addition to the usual telephones, telegrams, and faxes, the information transmitted through the optical cable is now transmitted in large quantities, including television signals, bank remittances, stock market quotations, and other information that cannot be interrupted for a moment.

At present, the transmission mode of long-distance communication optical cable has been developed from PDH to SDH, and the transmission rate has developed from the original 140MB/S to 2.5GB/S, 4×2.5GB/S, 16×2.5GB/S or even higher, that is It is said that a pair of fiber cores can open 30,000, 120,000, 480,000 or even more channels. With such a large transmission capacity, once the optical cable is blocked, it will not only cause huge losses to the telecommunications sector, but also cause many inconveniences to the general public due to poor communication, such as computer users being unable to access the Internet, not knowing the stock market, bank remittances, and remote deposits. Take it into a bubble, and all kinds of information cannot be transmitted. In remote mountainous areas, once the optical cable is interrupted, the entire county and even several counties along the optical cable will be isolated from the world in communication and become isolated islands.

Optical cable structure

Overview

Optical cable is composed of cable core, reinforcing steel wire, filler and sheath, etc. In addition, there are waterproof layer, buffer layer, insulated metal wire and other components as needed.

optical fiber

The most important part of the optical fiber cable core is the optical fiber. Let's take a look at the optical fiber theory and optical fiber structure.

Light and its properties

1. Light is an electromagnetic wave

The wavelength range of visible light is: 390~760nm (nanometer). The part larger than 760nm is infrared light, and the part smaller than 390nm is ultraviolet light. At present, the most commonly used optical fibers are: 850, 1310, and 1550.

2. Refraction, reflection and total reflection of light.

Because light travels at different speeds in different substances, when light is emitted from one substance to another, refraction and reflection occur at the interface between the two substances. Also, the angle of the refracted light varies with the angle of the incident light. When the angle of the incident light reaches or exceeds a certain angle, the refracted light will disappear, and all the incident light will be reflected back, which is the total reflection of light. Different substances have different refraction angles to light of the same wavelength (ie, different substances have different refractive indices of light), and the same substance has different refraction angles to light of different wavelengths. Optical fiber communication is formed based on the above principles.

Optical fiber structure and type:

1. Optical fiber structure:

The bare fiber is generally divided into three layers: the central high-refractive-index glass core (the core diameter is generally 50 or 62.5μm), the middle is the low-refractive-index silica glass cladding (the diameter is generally 125μm), and the outermost is the resin coating for reinforcement. Floor.

2. Numerical aperture:

The light incident on the end face of the fiber cannot be all transmitted by the fiber, only the incident light within a certain angle range can. This angle is called the numerical aperture of the fiber. A larger numerical aperture of the optical fiber is advantageous for the butt-joining of the optical fiber. Optical fibers produced by different manufacturers have different numerical apertures.

3. Types of optical fibers:

A. According to the transmission mode of light in the fiber, it can be divided into: single-mode fiber and multi-mode fiber.

Multimode fiber: The central glass core is thicker (50 or 62.5μm) and can transmit light in multiple modes. But its intermodal dispersion is large, which limits the frequency of transmitting digital signals, and it will be more serious with the increase of distance. For example: 600MB/KM fiber has only 300MB bandwidth at 2KM. Therefore, the distance of multimode fiber transmission is relatively short, generally only a few kilometers.
Single-mode fiber: The central glass core is relatively thin (the core diameter is generally 9 or 10 μm), and only one mode of light can be transmitted. Therefore, its intermodal dispersion is very small, which is suitable for long-distance communication, but its chromatic dispersion plays a major role, so the single-mode fiber has higher requirements on the spectral width and stability of the light source, that is, the spectral width is narrower and the stability is better. .

B. According to the optimal transmission frequency window: conventional single-mode fiber and dispersion-shifted single-mode fiber.

Conventional type: The optical fiber manufacturer optimizes the optical fiber transmission frequency on a single wavelength of light, such as 1310nm.

Dispersion-shifted type: Optical fiber manufacturers optimize the optical fiber transmission frequency at two wavelengths of light, such as: 1310nm and 1550nm.

C. According to the distribution of refractive index, it is divided into: abrupt and graded fibers.

Abrupt type: The refractive index from the central core of the fiber to the glass cladding is abrupt. It has low cost and high intermodal dispersion. It is suitable for short-distance low-speed communication, such as industrial control. However, due to the small intermodal dispersion of single-mode fiber, the single-mode fiber adopts abrupt type.

Graded fiber: The refractive index from the center core of the fiber to the glass cladding is gradually reduced, so that the high-mode light can propagate in a sinusoidal form, which can reduce the dispersion between modes, improve the bandwidth of the fiber, and increase the transmission distance, but the cost is high. Mode fibers are mostly graded fibers.

4. Common fiber specifications:

Single mode: 8/125μm, 9/125μm, 10/125μm
Multimode: 50/125μm, European standard
62.5/125μm, American standard
Industrial, Medical and Low Speed ​​Networks: 100/140μm, 200/230μm
Plastic: 98/1000μm for automotive control

Optical fiber manufacturing and attenuation:

1. Optical fiber manufacturing:

At present, optical fiber manufacturing methods mainly include: in-tube CVD (chemical vapor deposition) method, in-rod CVD method, PCVD (plasma chemical vapor deposition) method and VAD (axial vapor deposition) method.

2. Optical fiber attenuation:

The main factors that cause fiber attenuation are: intrinsic, bending, extrusion, impurities, non-uniformity and butt joint.

Intrinsic: It is the inherent loss of the fiber, including: Rayleigh scattering, intrinsic absorption, etc.
Bending: When the fiber is bent, part of the light in the fiber will be lost due to scattering, resulting in loss.
Squeeze: Loss caused by tiny bends in an optical fiber when it is squeezed.
Impurities: Losses caused by impurities in the fiber absorbing and scattering light propagating in the fiber.
Non-uniformity: Loss caused by non-uniform refractive index of the fiber material.
Docking: The loss generated when the optical fiber is docked, such as: different axes (the coaxiality of single-mode fiber is required to be less than 0.8μm), the end face is not perpendicular to the axis, the end face is not flat, the butt core diameter does not match and the welding quality is poor.

Advantages of optical fiber:

1. The passband of the optical fiber is very wide. Theoretically, it can reach 3 billion megahertz.
2. No hop length. Dozens to more than 100 kilometers, only a few hundred meters of copper wire.
3. Not affected by electromagnetic fields and electromagnetic radiation.
4. Light weight and small size. For example: 900 pairs of twisted-pair cables with 21,000 voice channels, the diameter is 3 inches, and the weight is 8 tons/KM. The optical fiber cable with ten times the communication capacity has a diameter of 0.5 inches and a weight of 450P/KM.
5. Optical fiber communication is not electrified, and can be used safely in flammable and violent places.
6. Use a wide range of ambient temperature.
7. Chemical corrosion, long service life.

Optical cable classification

1. According to the laying method, there are: self-supporting overhead optical cable, pipeline optical cable, armored buried optical cable and submarine optical cable.
2. According to the optical cable structure, it is divided into: bundled optical cable, layered optical cable, tightly hugged optical cable, ribbon optical cable, non-metallic optical cable and branchable optical cable.
3. According to the use, there are: optical cable for long-distance communication, short-distance outdoor optical cable, hybrid optical cable and optical cable for building.

Optical cable manufacturing

The manufacturing process of optical cable is generally divided into the following processes:

1. Screening of optical fibers: select optical fibers with excellent transmission characteristics and qualified tension.
2. Optical fiber dyeing: use standard full chromatogram to identify, requiring no fading and no migration at high temperature.
3. Secondary extrusion: Use plastic with high elastic modulus and low linear expansion coefficient to extrude into a tube of a certain size, incorporate the optical fiber and fill it with moisture-proof and waterproof gel, and store it for a few days (not less than two days) .
4. Optical fiber cable twisting: twist several extruded optical fibers and strengthening units together.
5. Squeeze the outer sheath of the optical cable: add a layer of sheath to the stranded optical cable.

Optical cable installation

Over the years, human society has developed a set of mature methods and experiences for optical cable construction. For example, outdoor construction of optical cables: the most important thing for laying long-distance optical cables is to choose a suitable path. The shortest path is not necessarily the best, but also pay attention to the right to use the land, the possibility of erection or burial, etc.

There must be very complete design and construction drawings for convenient and reliable construction and future inspections. During construction, be careful not to put the optical cable under heavy pressure or be punctured by hard objects.

When the optical cable turns, its turning radius is 20 times larger than the diameter of the optical cable itself.

Construction of outdoor aerial optical cable:

A. The overhead method of hanging wire brackets is simple and cheap, and is the most widely used in my country, but it is time-consuming to add hooks and arrange them.
B. Hanging wire winding overhead method, this method is more stable and less maintenance work. But a special wrapping machine is required.
C. The self-supporting overhead method has high requirements on the trunk line, difficult construction and maintenance, and high cost. It is rarely used in China at present.
D. When overhead, a guiding device must be installed at the place where the optical cable leads to the trunk, and the optical cable should not be dragged to the ground. Pay attention to reducing friction when pulling the optical cable. A length of fiber optic cable should be left on each trunk for expansion and contraction.
E. Pay attention to the reliable grounding of metal objects in the optical cable. Especially in mountainous areas, high-voltage power grid areas and many areas, there are generally 3 grounding points per kilometer, and even non-metallic optical cables are used.

Construction of outdoor pipeline optical cable:

A. Before construction, the occupancy of the pipeline should be checked, the plastic sub-pipes should be cleaned and placed, and the traction line should be placed at the same time.
B. Calculate the deployment length, and there must be enough reserved length.
C. The length of one laying should not be too long (generally 2KM). When wiring, it should be pulled from the middle to both sides.
D. The traction force of the cable is generally not more than 120kg, and the reinforcing core part of the optical cable should be pulled, and the waterproof and strengthening treatment of the head of the optical cable should be done.
E. The lead-in and lead-out of the optical cable must be equipped with a follow-up device, and the floor cannot be directly mopped.
F. The pipeline optical cable should also pay attention to reliable grounding.

Laying of directly buried optical cables:

A. The depth of direct buried optical cable trench should be excavated according to the standard. The standard is shown in the following table:
B. Where trenches cannot be dug, the pipelines can be laid overhead or drilled.
C. The bottom of the ditch should be flat and firm, and a part of sand, cement or support can be pre-filled if necessary.
D. Manual or mechanical traction can be used when laying, but attention should be paid to guidance and lubrication.
E. After the laying is completed, the soil should be covered and compacted as soon as possible.

Laying of optical cables in buildings:

A. When laying vertically, special attention should be paid to the load-bearing problem of the optical cable. Generally, the optical cable should be fixed once every two layers.
B. When the optical cable passes through the wall or floor, a protective plastic pipe with a mouth guard should be added, and the pipe should be filled with flame retardant filler.
C. A certain amount of plastic pipes can also be laid in advance in the building, and when the optical cable is to be laid in the future, the optical cable can be laid by traction or vacuum method.

Optical cable selection

The selection of optical cables is not only based on the number of optical fibers and the type of optical fibers, but also the outer sheath of the optical cable according to the use environment of the optical cable.

1. When the outdoor optical cable is directly buried, the armored optical cable should be selected. When overhead, an optical cable with a black plastic outer sheath with two or more reinforcing ribs can be used.
2. When selecting optical cables used in buildings, attention should be paid to their flame retardant, toxic and smoke characteristics. Generally, the flame retardant but smoke-free type (Plenum) can be used in the pipeline or forced ventilation, and the flame-retardant, non-toxic and smoke-free type (Riser) should be used in the exposed environment.
3. When cabling vertically in the building, distribution cables can be used; when wiring horizontally, breakout cables can be used.
4. If the transmission distance is less than 2km, you can choose multi-mode optical cable. If it exceeds 2km, you can use repeater or choose single-mode optical cable.