The information transmission in the digital era is so efficient that part of it benefits from the rapid development of optical fiber communication technology. Optical fiber was originally born as a medical endoscope. Because of the limited technology at the time, the transmission attenuation of light in the optical fiber was very large, so researchers at that time did not favor the application of optical fiber in communication. Later, after the British Chinese scholar Takashi Takashi's diligent research, and made bold assumptions, it rekindled the researchers' enthusiasm for optical communications.
First, the technical background Information transmission in the digital era is so efficient, in part due to the rapid development of optical fiber communication technology. Optical fiber was originally born as a medical endoscope. Because of the limited technology at the time, the transmission attenuation of light in the optical fiber was very large, so researchers at that time did not favor the application of optical fiber in communication. Later, after the British Chinese scholar Takashi Takashi's diligent research, and made bold assumptions, it rekindled the researchers' enthusiasm for optical communications. After several years of research, there have been major breakthroughs in lasers and low-loss fibers, which have made optical communications possible.
Optical fiber was first widely used in the broadcasting and TV industry, while the security industry in the same period is still using coaxial cable as the main transmission medium. However, with the digital and high-definition development of the security industry, the traditional transmission media can no longer meet the needs of the security industry for transmission bandwidth, transmission rate, transmission distance, and confidentiality. Optical fiber communication technology has been introduced into the security industry. And it quickly developed. At present, optical transmission is the core technology of video surveillance long-distance transmission solutions. As the core of the entire transmission solution, high-performance optical transceiver equipment has also become the focus of research and development of security vendors such as Hikvision, and it occupies a pivotal position in the entire security monitoring system.
Second, the classification of optical transceivers According to the different input signal processing methods, optical transceivers can be divided into analog optical transceivers and digital optical transceivers.
(1) Analogue Optical Transmitter The analog optical transceiver performs the PFM modulation of the analog signal and then performs electro-optical conversion. The optical-electrical conversion is performed at the receiving end, and then the video signal is restored through PFM demodulation. This method has a higher signal-to-noise ratio and less nonlinear distortion, so that the signal can easily transmit tens of kilometers, but the number of signal paths transmitted by a single optical fiber is limited.
(II) Digital optical transceivers The digital optical transceivers perform analog-to-digital conversion of the input analog signals. Therefore, digital signals are transmitted in the optical fiber. The digital signals directly control the strength of the optical signal transmitted from the optical head (that is, the signal strength is represented by the number 1 and the signal is weakly used. 0 indicates that a number of analog conversions are performed at the receiver to restore the original video and audio signals. The digital signal has a lower bit error rate than the analog signal, which can ensure the distortion-free reduction of the video signal, and multiple signals can be transmitted without interference in one optical fiber, so that the multiplexing efficiency of one optical fiber is greatly improved. Can achieve tens of roads and even hundreds of roads of optical fiber transmission. Just as digital replaces analog in the field of cameras, digital optical transceivers in the world of optical transceivers will certainly replace analog optical transceivers. This article focuses on digital optical transceivers. The digital optical transceiver is further divided into a digital video optical transceiver and an Ethernet optical transceiver that is a network optical transceiver.
1. Digital video transmitter and receiver Digital video transmitter and receiver can transmit uncompressed video signals, such as HD-SDI signal, to solve the problem of transmission bandwidth under the traditional transmission mode, making real-time transmission of uncompressed high-definition video signals possible, greatly improving real-time monitoring The quality of the picture. The specific digital video transmitter can transmit 485 control signals, audio signals, etc. while transmitting video signals. According to different application scenarios, digital video optical transceivers can be classified into peer-to-peer optical transceivers, node optical transceivers, convergent optical transceivers, coarse wavelength division multiplexing optical transceivers, and integrated service video optical transceivers.
(1) Point-to-point Optical Multiplexer A fiber can only have one sending end and one receiving end, and the sending end and receiving end pair up (see Figure 1). Therefore, there are several transmitters that need several optical fibers and can only form a star network. Each monitoring point uses its own optical fiber lines for independent transmission. If the number of input terminals is too large, it is likely to cause waste of optical fibers, which is not conducive to laying cables on the project. Cable and control of costs.
Figure 1 Point-to-point optical transceiver connection diagram
(2) Node-type optical transceivers Each node-type optical transceiver is transmitted to the center receiver through the serial connection. The main use of signal multiplexing technology is to reuse multiple optical signals for each node, and multiplex the signals of the node. The last signal is passed to the next node (Figure 2). Therefore, there may be multiple nodes on one optical fiber line, which greatly saves the cost of laying optical fibers. This type of optical transceiver is widely used in highway projects.
Figure 2 node optical transceiver connection diagram
(3) The multiplex optical fiber signals at the front end of the converged optical transceiver are transmitted to a convergence node through the respective transmission optical transceivers. The optical multiplexer at the convergence node combines the multiple optical fiber signals into one optical signal to transmit to the central convergence receiving optical transceiver (Figure 3). Such a transmission method can convert the traditional star topology into a tree topology, which saves the cost of fiber laying to a great extent and increases the flexibility of the entire network.
Figure 3 Converged optical transceiver connection diagram
(4) Coarse Wavelength Division Multiplexing Optical Multiplexing Optical Multiplexers Optical Multiplexers use optical multiplexers to multiplex optical signals of different wavelengths to a single optical fiber for transmission. At the receiving end of the link, optical fiber demultiplexers are used to transmit optical signals. The mixed signal is decomposed into signals of different wavelengths and connected to corresponding receiving devices (see Figure 4). The interval between each wavelength in the coarse wavelength division multiplexing system is relatively wide, so the requirement for the accuracy of the laser transmitter is low, and the cost of the entire transmission system can be saved.
Figure 4 Coarse wavelength division multiplexing optical transceiver working principle
(6) Integrated Services Optical Multiplexer Optical Multiplexer Combines digital non-compression technology with TDM and WDM technologies to transmit video signals, Ethernet signals, telephone signals, voice intercom, two-way data, and digital signals simultaneously using a single optical fiber (eg, Figure 5). Because in large-scale projects and complex applications, the mere transmission of video signals can no longer meet the growing security monitoring needs. For example, if the front end is a simulated dome camera, if only the video signal is transmitted and the dome camera cannot be controlled, the video signal must be transmitted along with the 485 control signal to achieve the low latency control of the dome camera. Under such a demand, an integrated service optical transceiver has emerged to meet the needs of large-scale networking of the entire system.
Figure 5 Topology diagram of an integrated service optical transceiver
2. Network Optical Network Optical transceiver can convert the electrical signals in the twisted pair into optical signals to achieve long-distance transmission, and can also be used through optical fiber cascades and group rings. Mostly used for the construction of networks in factories, transportation, railways, mines, and power stations.
Third, the important parameters of optical transceivers and product selection (a) data types The signal transmitted in a large-scale comprehensive project is very diverse, first in the purchase of optical transceiver first clear what type of front-end input signal source, according to different types of input source, such as Analog signals, network signals, high-definition raw data, etc., select different types of optical transceivers for matching.
(b) Optical fiber type optical transceivers are classified into single-fiber transmission and dual-fiber transmission. When using single-fiber transmission, the transmitted and received signals are transmitted using different wavelengths in one optical fiber and do not interfere with each other. When using dual-fiber transmission, the receiving and transmitting both use an independent optical fiber transmission, and the wavelengths of both can be the same. When single-fiber transmission is used, it is necessary to multiplex two-wavelength signals in a single light, so that one pair of wavelength division multiplexers is transmitted in each optical transceiver than dual-fiber transmission, and dual-fiber transmission is used more than single-fiber transmission. One fiber. Optical fibers can be classified into single-mode optical fibers and multi-mode optical fibers according to different transmission modes. Single-mode optical fibers have a longer transmission distance but are also more expensive than multi-mode optical fibers. Before designing the plan, it is necessary to fully consider the actual requirements and select the appropriate optical transceiver and optical fiber.
(C) Wavelength The transmit wavelength of the optical transceiver needs to match the receive wavelength of the receive end. For example, Tx1310nm/Rx1550 versus Tx1550nm/Rx1310.
(4) Transmission rate The transmission rate of the transmitter of the optical transceiver needs to match the rate of the receiver.
(5) Transmission distances The optical transceivers may differ in price depending on the transmission distance. In practical projects, please select suitable optical transceiver products according to the transmission distance. Common transmission distances are 10 km, 20 km, and 40 km.
Conclusion: With the implementation of large-scale security monitoring projects and the emergence of ultra-high-definition video surveillance requirements, optical fiber transmission will become a video surveillance transmission network due to its good confidentiality, light weight, strong anti-interference ability, long distance, and high data bandwidth. The core technology of construction has been widely used in telecommunications, electric power, *, *, highway traffic, railways, customs, large factories, mines, government agencies and other projects.
