Briefly introduce the basic principles, structural composition, functional configuration, key technical components and technical characteristics of the optical wavelength division multiplexing system, and explain that the optical wavelength division multiplexing WDM system is the future development direction of optical communications.
1. Optical Wavelength Division Multiplexing (WDM) technology Optical Wavelength Division Multiplexing (Wavelength Division MulTIplexing, WDM) technology is an optical carrier signal of multiple wavelengths in a fiber at the same time, and each optical carrier can be through FDM or TDM. Each carries multiple analog or multiple digital signals. The basic principle is to combine (multiplex) optical signals of different wavelengths at the sending end and couple them to the same optical fiber on the optical cable line for transmission, and separate these combined signals of different wavelengths at the receiving end ( Demultiplexing), and further processing, restore the original signal and send it to different terminals. Therefore, this technology is called optical wavelength division multiplexing, referred to as optical wavelength division multiplexing technology.
WDM technology is of great significance for network expansion and upgrading, development of broadband services, mining of fiber bandwidth capabilities, and realization of ultra-high-speed communication. Especially, WDM with erbium-doped fiber amplifier (EDFA) is more powerful for modern information networks. Attractive.
Second, the basic structure of the WDM system The basic structure of the WDM system is mainly divided into two modes: dual fiber unidirectional transmission and single fiber bidirectional transmission. Unidirectional WDM refers to that all optical channels are simultaneously transmitted in the same direction on one optical fiber. At the sending end, the modulated light signals with different wavelengths carrying various information are combined through an optical extender, and are combined in one Unidirectional transmission in optical fiber, because the signals are carried by light of different wavelengths, they will not be confused with each other. At the receiving end, optical signals of different wavelengths are separated by optical multiplexers to complete the transmission of multiple optical signals. The opposite direction is transmitted through another fiber. Bidirectional WDM means that the optical path is simultaneously transmitted in two different directions on a main fiber, and the wavelengths used are separated from each other to achieve full-duplex communication between the two parties. At present, unidirectional WDM systems are more extensive in development and application, while bidirectional WDM is affected by factors such as interference of various channels, light reflection, isolation between two-way channels and crosstalk during design and application. less.
III. Composition of dual-fiber unidirectional WDM system Take dual-fiber unidirectional WDM system as an example. Generally speaking, WDM system is mainly composed of the following 5 parts: optical transmitter, optical relay amplifier, optical receiver, optical monitoring channel and Network management system.
1. Optical transmitter The optical transmitter is the core of the WDM system. In addition to the special requirements for the center wavelength of the emitting laser in the WDM system, it should also be based on different applications of the WDM system (mainly the type of transmission fiber and the transmission distance). Select a transmitter with a certain chromatic dispersion capacity. At the sending end, the optical signal output from the terminal device is first converted into an optical signal of a non-specific wavelength into a signal with a specific wavelength by using an optical repeater, and then a multiplexer is used to synthesize a multi-channel optical signal through an optical power amplifier (BA ) Amplify the output.
2. After long-distance (80 ~ 120km) optical fiber transmission, the optical relay amplifier needs to perform optical relay amplification on the optical signal. Most of the optical amplifiers currently used are erbium-doped fiber optical amplifiers (EDFA). Gain flattening technology must be used in WDM systems to enable EDFA to have the same amplification gain for optical signals of different wavelengths, and to ensure that gain competition for optical channels does not affect transmission performance.
3. At the receiving end of the optical receiver, the optical preamplifier (PA) amplifies the main channel signal that has been attenuated by transmission, and uses a demultiplexer to separate the optical channel of a specific wavelength from the main channel optical signal. The requirements of optical signal sensitivity, overload power and other parameters must also be able to withstand a certain amount of optical noise and have sufficient electrical bandwidth performance.
4. Optical monitoring channel The main function of optical monitoring channel is to monitor the transmission of each channel in the system. Insert the optical monitoring signal with a wavelength of λs (1550nm) generated by this node at the sending end, and combine with the optical signal of the main channel to output. At the receiving end, the received optical signal is demultiplexed, and the optical monitoring signal of the wavelength λs (1550 nm) and the optical signal of the service channel are output respectively. Frame synchronization bytes, official bytes and overhead bytes used by the network management are all transmitted through the optical monitoring channel.
5. Network management system The network management system transmits overhead bytes to other nodes through the optical monitoring channel or receives overhead bytes from other nodes to manage the WDM system, and implement functions such as configuration management, fault management, performance management, and security management.
4. Optical wavelength division multiplexer and demultiplexer In the entire WDM system, optical wavelength division multiplexer and demultiplexer are key components in WDM technology, and the performance of the system has a decisive effect on the transmission quality of the system. A device that combines signals of different light source wavelengths and outputs them through a transmission fiber is called a multiplexer; conversely, a device that decomposes multiple wavelength signals sent by the same transmission fiber into individual wavelengths and outputs them separately is called a demultiplexer. In principle, the device is reciprocal (bidirectional reversible), that is, as long as the output and input of the demultiplexer are used in reverse, it is a multiplexer. The performance indexes of optical wavelength division multiplexer mainly include access loss and crosstalk. The loss and frequency deviation are required to be small. The access loss is less than 1.0 ~ 2.5db. The crosstalk between channels is small, the isolation is large, and the influence between signals of different wavelengths is small. In the currently applied WDM system, there are mainly grating type optical wavelength division multiplexer and dielectric film filter type optical wavelength division multiplexer.
1. Grating type optical wavelength division multiplexer blazed grating is to scribe equal and equidistant groove marks on a plane that can be transmitted or reflected, and its groove has a small step-like shape. When optical signals containing multiple wavelengths are diffracted by the grating, optical signals of different wavelength components will be emitted at different angles. When the optical signal in the optical fiber strikes the blazed grating with a parallel beam through the lens, due to the diffraction effect of the grating, the optical signals of different wavelengths are transmitted back to the lens with various parallel lights with slightly different directions, and then focused by the lens Regularly inject the output fiber separately, so that the optical signals of different wavelengths are transmitted through different fibers, respectively, to achieve the purpose of demultiplexing. According to the principle of reciprocity, the input and output of optical wavelength division multiplexing can be interchanged to achieve the purpose of multiplexing.
2. Dielectric film filter type optical wavelength division multiplexer The current WDM system works in the 1550nm wavelength band, with 8, 16 or more wavelengths, on a pair of optical fibers (a single optical fiber can also be used) to form an optical communication system. The relationship between its wavelength and fiber loss is shown in Figure 4. The interval between each wavelength is 1.6nm, 0.8nm or narrower, corresponding to the bandwidth of 200GHz, 100GHz or narrower.
Fifth, the main features of WDM technology 1. Make full use of the huge bandwidth resources of optical fiber, so that the transmission capacity of an optical fiber is increased several times to tens of times than single wavelength transmission, thereby increasing the transmission capacity of optical fiber, reducing costs, and has a great Application value and economic value.
2. Because the wavelengths used in the WDM technology are independent of each other, they can transmit signals with completely different characteristics, complete the synthesis and separation of various signals, and achieve mixed transmission of multimedia signals.
3. Since many communications use full-duplex mode, the use of WDM technology can save a lot of line investment.
4. According to needs, WDM technology can have many application forms, such as long-distance trunk network, broadcast distribution network, multi-channel multi-local area network, etc., so it is very important for network applications.
5. As the transmission rate continues to increase, the response speed of many optoelectronic devices is obviously insufficient. The use of WDM technology can reduce the extremely high performance requirements of some devices, while achieving large-capacity transmission.
6. Use WDM technology to select routes to achieve network exchange and recovery.
1. Optical Wavelength Division Multiplexing (WDM) technology Optical Wavelength Division Multiplexing (Wavelength Division MulTIplexing, WDM) technology is an optical carrier signal of multiple wavelengths in a fiber at the same time, and each optical carrier can be through FDM or TDM. Each carries multiple analog or multiple digital signals. The basic principle is to combine (multiplex) optical signals of different wavelengths at the sending end and couple them to the same optical fiber on the optical cable line for transmission, and separate these combined signals of different wavelengths at the receiving end ( Demultiplexing), and further processing, restore the original signal and send it to different terminals. Therefore, this technology is called optical wavelength division multiplexing, referred to as optical wavelength division multiplexing technology.
WDM technology is of great significance for network expansion and upgrading, development of broadband services, mining of fiber bandwidth capabilities, and realization of ultra-high-speed communication. Especially, WDM with erbium-doped fiber amplifier (EDFA) is more powerful for modern information networks. Attractive.
Second, the basic structure of the WDM system The basic structure of the WDM system is mainly divided into two modes: dual fiber unidirectional transmission and single fiber bidirectional transmission. Unidirectional WDM refers to that all optical channels are simultaneously transmitted in the same direction on one optical fiber. At the sending end, the modulated light signals with different wavelengths carrying various information are combined through an optical extender, and are combined in one Unidirectional transmission in optical fiber, because the signals are carried by light of different wavelengths, they will not be confused with each other. At the receiving end, optical signals of different wavelengths are separated by optical multiplexers to complete the transmission of multiple optical signals. The opposite direction is transmitted through another fiber. Bidirectional WDM means that the optical path is simultaneously transmitted in two different directions on a main fiber, and the wavelengths used are separated from each other to achieve full-duplex communication between the two parties. At present, unidirectional WDM systems are more extensive in development and application, while bidirectional WDM is affected by factors such as interference of various channels, light reflection, isolation between two-way channels and crosstalk during design and application. less.
III. Composition of dual-fiber unidirectional WDM system Take dual-fiber unidirectional WDM system as an example. Generally speaking, WDM system is mainly composed of the following 5 parts: optical transmitter, optical relay amplifier, optical receiver, optical monitoring channel and Network management system.
1. Optical transmitter The optical transmitter is the core of the WDM system. In addition to the special requirements for the center wavelength of the emitting laser in the WDM system, it should also be based on different applications of the WDM system (mainly the type of transmission fiber and the transmission distance). Select a transmitter with a certain chromatic dispersion capacity. At the sending end, the optical signal output from the terminal device is first converted into an optical signal of a non-specific wavelength into a signal with a specific wavelength by using an optical repeater, and then a multiplexer is used to synthesize a multi-channel optical signal through an optical power amplifier (BA ) Amplify the output.
2. After long-distance (80 ~ 120km) optical fiber transmission, the optical relay amplifier needs to perform optical relay amplification on the optical signal. Most of the optical amplifiers currently used are erbium-doped fiber optical amplifiers (EDFA). Gain flattening technology must be used in WDM systems to enable EDFA to have the same amplification gain for optical signals of different wavelengths, and to ensure that gain competition for optical channels does not affect transmission performance.
3. At the receiving end of the optical receiver, the optical preamplifier (PA) amplifies the main channel signal that has been attenuated by transmission, and uses a demultiplexer to separate the optical channel of a specific wavelength from the main channel optical signal. The requirements of optical signal sensitivity, overload power and other parameters must also be able to withstand a certain amount of optical noise and have sufficient electrical bandwidth performance.
4. Optical monitoring channel The main function of optical monitoring channel is to monitor the transmission of each channel in the system. Insert the optical monitoring signal with a wavelength of λs (1550nm) generated by this node at the sending end, and combine with the optical signal of the main channel to output. At the receiving end, the received optical signal is demultiplexed, and the optical monitoring signal of the wavelength λs (1550 nm) and the optical signal of the service channel are output respectively. Frame synchronization bytes, official bytes and overhead bytes used by the network management are all transmitted through the optical monitoring channel.
5. Network management system The network management system transmits overhead bytes to other nodes through the optical monitoring channel or receives overhead bytes from other nodes to manage the WDM system, and implement functions such as configuration management, fault management, performance management, and security management.
4. Optical wavelength division multiplexer and demultiplexer In the entire WDM system, optical wavelength division multiplexer and demultiplexer are key components in WDM technology, and the performance of the system has a decisive effect on the transmission quality of the system. A device that combines signals of different light source wavelengths and outputs them through a transmission fiber is called a multiplexer; conversely, a device that decomposes multiple wavelength signals sent by the same transmission fiber into individual wavelengths and outputs them separately is called a demultiplexer. In principle, the device is reciprocal (bidirectional reversible), that is, as long as the output and input of the demultiplexer are used in reverse, it is a multiplexer. The performance indexes of optical wavelength division multiplexer mainly include access loss and crosstalk. The loss and frequency deviation are required to be small. The access loss is less than 1.0 ~ 2.5db. The crosstalk between channels is small, the isolation is large, and the influence between signals of different wavelengths is small. In the currently applied WDM system, there are mainly grating type optical wavelength division multiplexer and dielectric film filter type optical wavelength division multiplexer.
1. Grating type optical wavelength division multiplexer blazed grating is to scribe equal and equidistant groove marks on a plane that can be transmitted or reflected, and its groove has a small step-like shape. When optical signals containing multiple wavelengths are diffracted by the grating, optical signals of different wavelength components will be emitted at different angles. When the optical signal in the optical fiber strikes the blazed grating with a parallel beam through the lens, due to the diffraction effect of the grating, the optical signals of different wavelengths are transmitted back to the lens with various parallel lights with slightly different directions, and then focused by the lens Regularly inject the output fiber separately, so that the optical signals of different wavelengths are transmitted through different fibers, respectively, to achieve the purpose of demultiplexing. According to the principle of reciprocity, the input and output of optical wavelength division multiplexing can be interchanged to achieve the purpose of multiplexing.
2. Dielectric film filter type optical wavelength division multiplexer The current WDM system works in the 1550nm wavelength band, with 8, 16 or more wavelengths, on a pair of optical fibers (a single optical fiber can also be used) to form an optical communication system. The relationship between its wavelength and fiber loss is shown in Figure 4. The interval between each wavelength is 1.6nm, 0.8nm or narrower, corresponding to the bandwidth of 200GHz, 100GHz or narrower.
Fifth, the main features of WDM technology 1. Make full use of the huge bandwidth resources of optical fiber, so that the transmission capacity of an optical fiber is increased several times to tens of times than single wavelength transmission, thereby increasing the transmission capacity of optical fiber, reducing costs, and has a great Application value and economic value.
2. Because the wavelengths used in the WDM technology are independent of each other, they can transmit signals with completely different characteristics, complete the synthesis and separation of various signals, and achieve mixed transmission of multimedia signals.
3. Since many communications use full-duplex mode, the use of WDM technology can save a lot of line investment.
4. According to needs, WDM technology can have many application forms, such as long-distance trunk network, broadcast distribution network, multi-channel multi-local area network, etc., so it is very important for network applications.
5. As the transmission rate continues to increase, the response speed of many optoelectronic devices is obviously insufficient. The use of WDM technology can reduce the extremely high performance requirements of some devices, while achieving large-capacity transmission.
6. Use WDM technology to select routes to achieve network exchange and recovery.
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