In today's world, optical fiber communication has become the main means of communication. At the same time, optical fiber communication technology is also rapidly developing, making the optical fiber transmission system win the favor of everyone with its numerous advantages.
The optical fiber transmission system has the following significant advantages: large capacity and long transmission distance. The theoretical fiber bandwidth is up to 20000 GHz and the non-relay transmission distance is up to 50-80 km. Made of glass, it resists electromagnetic interference and has good transmission quality. It can be used in strong electromagnetic environments such as power grids and substations. The fiber is lightweight, bendable and easy to lay. It can save precious metals and has strong corrosion resistance. The production of optical fiber is rich in raw materials. With the advancement of the process and the expansion of scale, the cost is further reduced, and the cost of the entire transmission system is also low.
First, the introduction of optical fiber transmission system
Optical fiber transmission is a kind of communication method which uses light wave as carrier frequency and optical fiber as transmission medium, because it has the advantages of transmission frequency bandwidth, low signal loss, strong anti-interference ability and light weight. Optical fiber communications have seen rapid development in the past two decades.
1, the structure of the optical fiber
Fiber bare fiber is generally divided into three layers: the central high-index glass core (core diameter is generally 50 or 62.5μm), the middle is a low refractive index silica glass cladding (diameter is generally 125μm), the outermost is the strengthening of the resin coating Floor.
2, the classification of optical fiber
1) Divide by transmission mode:
Single-mode fiber (Single-mode)
The single mode fiber only transmits the main mode. Since the mode dispersion is completely avoided, the transmission band of the single mode fiber is very wide, and thus it is suitable for a large-capacity, long-distance transmission system. Multi-mode fiber (Multi-mode)
The multi-mode optical fiber has multiple modes to transmit in the optical fiber. Due to dispersion and phase difference, the transmission performance is poor, the frequency band is narrow, the capacity is small, and the distance is short.
2) Divided by refractive index distribution:
Multimode fiber can be divided into step-type (abrupt type) and gradient type (gradient, self-focus type).
Single-mode fiber is stepped.
3, 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.
4, the attenuation of the optical fiber
The main factors that cause fiber attenuation are: intrinsic, bending, extrusion, impurities, non-uniformity, and docking.
Intrinsic: is the inherent loss of optical fiber, including Rayleigh scattering, intrinsic absorption and so on.
Bending: The loss of light in a part of the fiber due to scattering when the fiber bends.
Extrusion: The loss caused by slight bending of an optical fiber when it is squeezed.
Impurities: Losses caused by absorption and scattering of light in the fiber by impurities in the fiber.
Inhomogeneity: The loss due to the non-uniform refractive index of the optical fiber material.
Docking: Loss caused when the optical fiber is docked, such as: different axes (single-mode fiber coaxiality requirements less than 0.8μm), the end surface is not perpendicular to the axis, the end surface is uneven, the mating heart diameter is not matched and the welding quality is poor.
5, principle of optical fiber transmission system
The optical transmission system consists of three parts: a light source (optical transmitter), a transmission medium, and a detector (optical receiver).
Divided by transmission signals, it can be divided into digital transmission systems and analog transmission systems.
In an analog transmission system, the input signal is changed to a continuous change in the amplitude (frequency or phase) of the transmission signal. The analog transmission system of the optical fiber simulates the light intensity, and the modulation power of the light source varies with the amplitude of the modulation signal. However, due to the serious nonlinearity of the light source, the signal-to-noise ratio, transmission distance and transmission frequency are very limited.
The digital transmission system converts the input signal into a pulse signal represented by "1" and "0" and uses it as a transmission signal. At the receiving end, it is restored to the original information. In this way, the nonlinearity of the light source has little effect on the digital code stream. In addition, digital communication can use some coding and error correction methods, and it is easy to implement multiplexing. Therefore, the digital transmission system occupies a great advantage and is obtained in many places. A wide range of applications.
Second, digital video transmission
Currently in the highway, transportation, electronics, surveillance, security, industrial automation, power, customs, water conservancy, banking and other fields video image, audio, data, Ethernet, telephone and other optical transceivers began to use a large number of applications. In the early stage, analog frequency modulation, amplitude modulation, and phase modulation of analog optical transceivers were mainly used, but digital instead of analog was the development trend of optical fiber communication technology. Because digital optical transceivers have high transmission signal quality, there are no shortcomings such as analog FM, phase modulation, and amplitude modulation when the optical transceiver multipath signals are transmitted simultaneously. The interference is serious, the environment is easily affected, the transmission quality is poor, and the long-term working stability is poor. Digital optical transceivers have been widely used in the project.
People have long known that digital communication has many advantages and it is best suited for long-distance communication. Since digital signals can be continuously regenerated by shaping and decision-making during transmission, it can realize high-quality long-distance transmission without noise accumulation and without nonlinear distortion. The extremely wide transmission bandwidth and minimal transmission loss of optical fibers make it possible to use digital communications widely. Compared with traditional analog optical transmission, digital video optical transmission has the following remarkable features:
1) It can be cascaded. As the distance increases, the SNR does not decrease.
2) Because it is a digital transmission method, it adopts digital coding error correction method, which has high stability and high reliability.
3) When multiplex signals are transmitted simultaneously, digital time division multiplexing (TMD) is used, and no intermodulation distortion is generated during analog transmission.
4) It has good stability and high adaptability to environment. It is easier to maintain and adjust than analog transmission system.
5) It is easy to realize large-capacity transmission with high cost performance.
6) Using no compression coding, the image signal quality is high and reaches the broadcast level.
The digital video transmitter and receiver adopts internationally advanced 10-bit digital video coding technology, and the image signal can reach broadcast quality. The video format is compatible with PAL, NTSC, and SECAM formats. Can use a stand-alone, card type (with card chassis) installation.
Digital Video Multiplexers can use one fiber to transmit up to eight video streams simultaneously (up to 64 channels using CWDM) as well as multiple bidirectional audio and multiplexed bidirectional data. The data formats are: RS-232, RS-422, RS-485, Manchester code. Its front panel has power indication and working status indication. This series of digital video optical transceivers has excellent performance and is a long-distance optical transmission system with high reliability and cost performance.
Third, application examples
The digital video transmitter and receiver have complete models and flexible configuration, which can be widely applied to all walks of life in the society. Its main application industries are: Intelligent Transportation Monitoring System (ITS), Security System, Intelligent Building/Smart Community, Campus Network, Industrial Monitoring (Power, Chemical, Steel, Railway, etc.), Military Monitoring (Border, Guard, etc.) and Stadium (live video transmission).
1. Example of urban traffic monitoring application
The traffic monitoring systems used in many cities today use optical transceivers to transmit video images. Each monitoring point is equipped with a CCD camera, a PTZ, and an optical transmitter. Traffic monitoring center to each monitoring point to lay a single-core or dual-core optical cable. One end of the optical cable is connected to the transmitter of the optical end, and the other end is connected to the optical receiver of the monitoring center. The video image signal is output from the on-site camera to the optical transmitter. The optical transmitter converts the electrical signal into an optical signal and sends it to the optical cable. The optical signal passes through the optical cable to the optical receiver of the monitoring center. The optical receiver restores the receiver to a video signal for processing in an image display control system. PTZ control signals are also transmitted through the optical transceiver. The PTZ control signal of the traffic command center is input through the serial interface of the optical transceiver. The optical signal of the surveillance site is converted into an electrical signal and sent to the decoder to output the pan/tilt and lens control signals. At the same time, the electronic * takes all the way to the same RS-232 data; the traffic signal indicator occupies all the way to reverse RS-232 data; the intercom system transmits through the two-way audio channel of the optical transceiver.
2, highway monitoring application example
Because each toll station of the highway is far away from the monitoring center, and there are also more images to be uploaded by the whole-line monitoring system, the cost performance of image transmission using point-to-point optical transceivers is not ideal, so multi-channel video transmission is adopted at highway toll stations. Nodes are cascaded for image transmission, that is, N toll stations switch out 2 (or 3, 4) video images through the video matrix and transmit them to the monitoring center through a 1-core optical fiber cascade through video node optical transceivers. At the same time, the monitoring of each toll station (pan/tilt, lens, matrix switch, etc.) reverse control data transmission provides a data channel from each toll communication station.
3, security system monitoring application example
A few channels of video monitored in the same cell are transmitted to the monitoring center through the optical transceiver, and then displayed on the monitor screen after being switched by the video matrix. At the same time, the audio signals collected by the microphones in the monitoring positions are also transmitted to the monitoring center through the optical transceiver. The intercom system of the monitoring center is also transmitted through the two-way audio channels of the optical transceivers. Control and data transmission are also accomplished by optical transceiver bidirectional data channels. Figure 3 shows the block diagram of the security monitoring system.
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