Basic knowledge and system design of optical fiber and cable wiring

As a data transmission medium with high bandwidth and high security, optical fiber is widely used in various large and medium-sized networks. Due to the high cost of cables and equipment, most optical fibers are only used in the network backbone, that is, they are used in the system wiring of vertical backbone subsystem and building group subsystem to realize the connection between buildings and floors. At present, they are also used in the horizontal wiring subsystem with high requirements for transmission rate and security.

Basic knowledge and system design of optical fiber and cable wiring

1、 Optical fiber

1. Light and its characteristics:

1) Light is an electromagnetic wave

The wavelength range of visible light is 390 ~ 760nm (nm). The part greater than 760nm is infrared light, and the part less than 390nm is ultraviolet light. There are three kinds of optical fibers: 85013001550.

2) Refraction, reflection and total reflection of light.

Because the propagation speed of light in different substances is different, when light is emitted from one substance to another, refraction and reflection will occur at the interface between the two substances. Moreover, the angle of refracted light will change with the angle of 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 of light with the same wavelength (that is, different substances have different light refractive indexes), and the same substances have different refraction angles of light with different wavelengths. Optical fiber communication is based on the above principles.

2. Optical fiber structure and type:

1) Optical fiber structure:

The bare fiber of optical fiber is generally divided into three layers: the central high refractive index glass core (the core diameter is generally 50 or 62.5) μ m) , with low refractive index silicon glass cladding in the middle (generally 125 in diameter) μ m) , the outermost is the resin coating for reinforcement.

2) Numerical aperture:

The light incident on the end face of the optical fiber cannot be transmitted by the optical fiber, only the incident light within a certain angle range. This angle is called the numerical aperture of the optical fiber. The larger numerical aperture of optical fiber is beneficial to the docking of optical fiber. The numerical aperture of optical fibers produced by different manufacturers is different (at & T? Corning).

3) Type of optical fiber:

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

Multimode fiber: the central glass core is thick (50 or 62.5 μ m) , it can transmit multiple modes of light. However, its inter mode dispersion is large, which limits the frequency of digital signal transmission, and will become more serious with the increase of distance. For example, 600MB / km optical fiber has only 300MB bandwidth at 2km. Therefore, the transmission distance of multimode optical fiber is relatively short, generally only a few kilometers.

Single mode optical fiber: the central glass core is thin (the core diameter is generally 9 or 10) μ m) , only one mode of light can be transmitted. Therefore, its inter mode dispersion is very small, which is suitable for remote communication, but its chromatic dispersion plays a major role. In this way, the single-mode fiber has high requirements for the spectral width and stability of the light source, that is, the spectral width should be narrow and the stability should be good.

B. According to the optimal transmission frequency window, it is divided into 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, such as 1300nm.

Dispersion shift type: the optical fiber manufacturer optimizes the optical fiber transmission frequency at two wavelengths, such as 1300nm and 1550nm.

C. According to the refractive index distribution, it can be divided into abrupt and gradual optical fibers.

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

Graded optical fiber: the refractive index from the central core of the optical fiber to the glass cladding is gradually reduced, which can make the high-mode light propagate in a sinusoidal form, which can reduce the inter mode dispersion, improve the optical fiber bandwidth and increase the transmission distance, but the cost is high. Most multimode optical fibers are graded optical fibers.

4) Common optical fiber specifications:

Single mode: 8 / 125 μ m，9/125 μ m，10/125 μ m

Multimode: 50/125 μ m. European standard 62.5/125 μ m. American Standard

Industrial, medical and low speed networks: 100 / 140 μ m，200/230 μ m

Plastic: 98 / 1000 μ m. For vehicle control

3. Optical fiber manufacturing and attenuation:

1) Optical fiber manufacturing:

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

2) Attenuation of optical fiber:

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

Intrinsic: it refers to the inherent loss of optical fiber, including Rayleigh scattering, inherent absorption, etc.

Bending: when the optical fiber is bent, the light in some optical fibers will be lost due to scattering, resulting in loss.

Extrusion: the loss caused by slight bending when the optical fiber is extruded.

Impurity: the loss caused by the absorption and scattering of light propagating in the optical fiber by impurities in the optical fiber.

Nonuniformity: loss caused by uneven refractive index of optical fiber material.

Butt joint: the loss caused by optical fiber butt joint, such as the coaxiality of different axes (single-mode optical fiber) is required to be less than 0.8 μ m) , the end face is not perpendicular to the axis, the end face is uneven, the butt center diameter does not match, and the welding quality is poor.

4. Advantages of optical fiber:

1) The passband of optical fiber is very wide The theory can reach 3 billion MHz.

2) No trunk length Tens to more than 100 kilometers, copper wire is only a few hundred meters.

3) Not affected by electromagnetic field and electromagnetic radiation.

4) Light weight and small volume. For example: 900 pairs of twisted pair with 21000 channels, with a diameter of 3 inches and a weight of 8 tons / km. The optical cable with ten times the communication volume has a diameter of 0.5 inches and a weight of 450p / km.

5) Optical fiber communication is not charged and safe to use. It can be used in flammable and explosive places.

6) Wide ambient temperature range.

7) Chemical corrosion, long service life.

2、 Optical cable

1. Optical cable manufacturing:

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

1) Screening of optical fiber: select the optical fiber with excellent transmission characteristics and qualified tension.

2) Dyeing of optical fiber: it shall be marked by standard full chromatography, and the color shall not fade or migrate at high temperature.

3) two extrusion: use high elastic modulus and low expansion coefficient plastic to be extruded into a certain size pipe, and put the optical fiber into the waterproof and waterproof gel, and store it for several days (no less than two days).

4) Optical cable stranding: stranding several extruded optical fibers with reinforcing units.

5) Extruded optical cable outer sheath: a layer of sheath is added to the stranded optical cable.

2. Type of optical cable:

1) According to the laying method, it is divided into: 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: bundle tube optical cable, layer twisted optical cable, cling optical cable, belt optical cable, non-metallic optical cable and branchable optical cable.

3) There are optical cables for long-distance communication, short-distance outdoor optical cables, hybrid optical cables and optical cables for use in buildings.

3. Construction of optical cable:

Over the years, doing optical cable construction has made us have a set of mature methods and experience.

Outdoor construction of optical cable:

The most important thing for long-distance optical cable laying is to choose a suitable path. Here, the shortest path is not necessarily the best, but also pay attention to the land use right, the possibility of erection or burial, etc.

There must be complete design and construction drawings for convenient and reliable construction and future inspection. During construction, pay attention not to make the optical cable under heavy pressure or hurt by hard objects.

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

1) Outdoor overhead optical cable construction:

A. Hanging wire hanging overhead mode, which is simple and cheap, is the most widely used in China, but it is time-consuming to hang and tidy the hook.

B. Hanging wire winding overhead mode, which is more stable and less maintenance work. But a special binding machine is needed.

C. The self-supporting overhead mode has high requirements for line trunk, great difficulty in construction and maintenance and high cost, which is rarely used in China at present.

D. When overhead, a guiding device shall be added at the dry part of the optical cable lead-in line, and the optical cable shall not be dragged to the ground. Pay attention to reducing friction when pulling optical cables. A section of optical cable for expansion shall be left on each trunk.

E. Pay attention to the reliable grounding of metal objects in optical cables. Especially in mountainous areas, high-voltage power grid areas and many areas, there are generally three grounding points per kilometer, and even non-metallic optical cables are selected.

2) Outdoor pipeline and optical cable construction:

A. Before construction, check the occupation of the pipeline, clean and place the plastic sub pipe, and put in the traction line at the same time.

B. After calculating the laying length, there must be enough reserved length.

C. The laying length at one time shall not be too long (generally 2km), and the wiring shall be pulled from the middle to both sides.

D. The traction force of cable laying is generally not more than 120kg, and the reinforcing core of optical cable shall be pulled, and the waterproof reinforcement treatment of optical cable head shall be done well.

E. The lead-in device must be added at the leading in and leading out of the optical cable, and it is not allowed to drag the ground directly.

F. Pay attention to reliable grounding of pipeline and optical cable.

3) Laying of directly buried optical cables:

A. The depth of directly buried optical cable trench shall be excavated according to the standard, and the standard is shown in the following table:

B. Where ditches cannot be dug, pipes can be laid overhead or drilled and embedded.

C. The trench bottom shall be smooth and firm, and a part of sand, cement or support can be pre filled when necessary

D. Manual or mechanical traction can be used during laying, but pay attention to guidance and lubrication.

E. After the laying is completed, the soil shall be covered and compacted as soon as possible.

4) Laying of optical cables in buildings:

A. When laying vertically, special attention shall be paid to the bearing of optical cables. Generally, the optical cables shall be fixed once every two layers.

B. When the optical cable passes through the wall or floor, the protective plastic pipe with protective mouth shall be added, and the pipe shall be filled with flame-retardant filler.

C. A certain amount of plastic pipes can also be laid in the building in advance. When the optical cable is to be laid in the future, the optical cable can be laid by traction or vacuum method.

4. Selection of optical cables:

The selection of optical cable not only depends on the number of optical fiber cores and types of optical fibers, but also the outer sheath of optical cable according to the service environment of optical cable.

1) When outdoor optical cables are directly buried, armored optical cables should be selected. When overhead, the optical cable with black plastic outer sheath with two or more stiffeners can be selected.

2) When selecting optical cables used in buildings, attention should be paid to their flame retardant, toxic and smoke characteristics. Generally, flame-retardant but smoke-free type (plenum) can be selected in the pipeline or forced ventilation, and flame-retardant, non-toxic and smoke-free type (riser) should be selected in the exposed environment.

3) When laying cables vertically in the building, distribution cables can be selected; Breakout cables can be selected for horizontal wiring.

4) If the transmission distance is within 2km, multi-mode optical cable can be selected. If it is more than 2km, relay or single-mode optical cable can be used.

3、 Connection and detection

1. Connection of optical cable:

The main methods are permanent connection, emergency connection and active connection.

1) Permanent optical fiber connection (also known as hot melt):

This kind of connection is to melt and connect the connection points of connecting optical fibers together by means of discharge. It is generally used for long-distance connection, permanent or semi permanent fixed connection. Its main feature is that the connection attenuation is the lowest among all connection methods, and the typical value is 0.01 ~ 0.03db/point. However, during connection, special equipment (welding machine) and professional personnel are required to operate, and the connection point also needs to be protected by special containers.

2) Emergency connection (also known as cold melting):

The emergency connection mainly uses mechanical and chemical methods to fix and bond the two optical fibers together. The main feature of this method is that the connection is fast and reliable, and the typical attenuation of the connection is 0.1 ~ 0.3db/point. However, the long-term use of the connection point will be unstable and the attenuation will increase significantly, so it can only be used urgently in a short time.

3) Active connection:

Mobile connection is a method of connecting stations or stations with optical cables by using various optical fiber connection devices (plugs and sockets). This method is flexible, simple, convenient and reliable. It is often used in computer network wiring in buildings. Its typical attenuation is 1dB / connector.

2. Optical fiber detection:

The main purpose of optical fiber detection is to ensure the quality of system connection, reduce fault factors and find out the fault point of optical fiber in case of fault. There are many detection methods, mainly divided into manual simple measurement and precision instrument measurement.

1) Simple manual measurement:

This method is generally used to quickly detect the on-off of optical fiber and distinguish the optical fiber during construction. It uses a simple light source to inject visible light from one end of the optical fiber and observe which one emits light from the other end. Although this method is simple, it can not quantitatively measure the attenuation of optical fiber and the breaking point of optical fiber.

2) Precision instrument measurement:

Using optical power meter or optical time domain reflection grapher (OTDR) to quantitatively measure the optical fiber, the attenuation of optical fiber and joint can be measured, and even the breakpoint position of optical fiber can be measured. This measurement can be used to quantitatively analyze the causes of optical fiber network failures and evaluate optical fiber network products.

4、 Application and system design of optical fiber

1. Application of optical fiber:

Human society has now developed into an information society, and the exchange of information such as sound, image and data is very large. The previous means of communication can no longer meet the current requirements, and optical fiber communication is widely used for its advantages of large information capacity, good confidentiality, light weight, small volume, long distance without relay section and so on. Its application fields cover communication, transportation, industry, medical treatment, education, aerospace, computer and other industries, and are developing to a broader and deeper level. The application of light and optical fiber is bringing profound influence and change to human life.

2. Optical fiber network system design:

The design of optical fiber system generally follows the following steps:

1) First of all, find out what kind of network to design, its current situation and why to use optical fiber.

2) Select appropriate optical fiber network equipment, optical cables, jumpers and other items for connection according to the actual situation. Selection shall be based on availability, and then determined according to performance, price, service, place of origin and brand.

3) Determine the route of the line according to the customer's requirements and network type, and draw the wiring diagram.

4) When the route is long, the attenuation allowance of the system needs to be calculated according to the following formula:

Attenuation margin = transmitted optical power - Acceptance sensitivity - line attenuation - connection attenuation (DB), where line attenuation = optical cable length × Unit attenuation;

The unit attenuation is closely related to the quality of optical fiber, and the single-mode is generally 0.4 ~ 0.5db/km; Multimode is 2 ~ 4dB / km.

Connection attenuation includes fusion attenuation and joint attenuation. Fusion attenuation is related to fusion means and personnel quality. Generally, hot fusion is 0.01 ~ 0.3db/point; Cold melting 0.1 ~ 0.3db/point; The attenuation of the joint is closely related to the quality of the joint, which is generally 1dB / point. The system attenuation margin is generally not less than 4dB.

5) If the calculation is unqualified, the design shall be modified according to the situation, and then the calculation shall be carried out. This situation may sometimes be repeated several times.

3. Design example:

Transformation of a campus network:

According to the situation, a three port repeater (twisted pair optical fiber thin cable) is used on one side of the existing thin cable network, and a twisted pair hub with optical fiber backbone is used on the other side. The bundle tube type 4-core outdoor multimode optical cable with overhead or ground burial can be used in the middle, and then welded into an indoor jumper with ST head (because the optical fiber interface of the equipment is st type).

Attenuation Accounting: (generally, multi-mode equipment does not need accounting within 2km. Here is only an example)

Transmitting power: - 16dbm

Receiving sensitivity: - 29.5dbm

Line attenuation: 1.5km × 3.5dB/km=5.25dB

Connection attenuation: 2 joints attenuation: 2 points × 1dB / point = 2dB

The two welding points are: 2 points × 0.07d