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How does WDM system work?

Date: 2020-11-30 10:02:05 From: C-light Onclick:

WDM carries multiple wavelength (channel) systems on one optical fiber, converts one optical fiber into several "virtual" fibers. Of course, each virtual fiber works independently at different wavelengths, which greatly improves the transmission capacity of optical fibers. Because of the economy and effectiveness of WDM system technology, it has become the main means to expand the capacity of optical fiber communication network.

Why using WDM?

In the past, a single optical fiber was used to transmit signals of only one wavelength at a time, but now it multiplexes signals of multiple wavelengths (such as 40 or 50) through WDM (in this case, wavelength division multiplexer), transmits them through optical fibers, and separates the light of each wavelength with a demultiplexer at the receiving end. For example, optical fibers can transmit wavelength signals from 1310 to 1550 nm. If you only transmit 1310, it is possible, but the bandwidth resources are wasted. This intermediate bandwidth is 240 nm. If the wavelength spacing is 0.8 nm (equivalent to 10 GHz), 300 signal wavelengths can be transmitted simultaneously (theoretically).

How it work
Generally speaking, the WDM system is mainly composed of the following five parts: optical transmitter, optical relay amplification, optical receiver, optical monitoring channel and network management system,as shown in the following figure:
Optical wavelength division multiplexer(mux) is a device to realize different combination and division of optical signals. Optical wavelength division multiplexer(mux) can be divided into two types: transmitter and receiver. The combined signal is also called multiplexer, whose function is to synthesize multiple single-path optical signals satisfying g.692 specification into a single-path combined signal, which is then coupled to the same optical fiber for transmission. The splitter, also known as demultiplexer, is used to restore the combined signal transmitted by an optical fiber to a single wavelength signal at the receiving end, and then coupled into different optical fibers. Optical wavelength division multiplexer(mux) is a key device in WDM system.
Previous article: Steps before you use your Wavelength Division Multiplexer
Wherein the optical transmitter is a WDM core of the system, according to ITU-T recommendations and standards, in addition to the WDM central wavelength of the emitted laser systems have special requirements, but also need to WDM different application systems (mainly transmission type fiber The distance from the radio relay transmission is selected to select a light signal with a certain chromatic dispersion tolerance . The optical signal output by the terminal device (such as an SDH terminal) is first transmitted by an Optical Transponder Unit ( OTU ). An optical signal of a non-specific wavelength conforming to ITU-T G.957 is converted into an optical signal having a stable specific wavelength, a multi-channel optical signal is synthesized by a combiner, and an optical signal of the output path is amplified by an optical power amplifier ( BA ).
After long-distance optical fiber transmission ( 80 to 120 km ), optical relay amplification of the optical signal is required. Most of the optical amplifiers currently in use are erbium doped fiber amplifiers ( EDFAs ). In the WDM system, the gain flattening technique must be adopted to make the EDFA have the same amplification gain for optical signals of different wavelengths. At the same time, it is necessary to consider various situations in which different numbers of optical channels work simultaneously, and the gain competition of the optical channel can be ensured. Does not affect the transmission performance. When applied, the EDFA can be used as “ line ( LA ) ” , “ amplifier ( BA ) ” and “ preamplifier ( PA ) ” depending on the situation .
At the receiving end, the optical placement amplifier ( PA ) amplifies the transmitted and attenuated main channel optical signal, and uses a demultiplexer to separate the optical channel of a specific wavelength from the main channel optical signal.
The main function of the optical monitoring channel is to monitor the transmission of each channel in the system.
The network management system transmits the overhead bytes to other nodes through the physical layer of the optical monitoring channel, or receives the overhead bytes from other nodes to manage the WDM system, and implements configuration management, fault management, performance management, security management, etc., and the upper layer. Management systems (such as TMN ) are connected.
The component modules mainly include:
( 1 ) Optical repeater / optical wavelength converter ( OTU ).
( 2 ) The passive part of the wavelength division multiplexing device ( demultiplexer / multiplexer ).
( 3 ) Optical amplifiers (optical post amplifiers ( OBA ), optical line amplifiers ( OLA ), optical preamplifiers ( OPA ).
( 4 ) Optical Monitoring Channel / Path ( OSC ).
Connection between mux and transceivers  
As fiber resources become more and more tight, wavelength division multiplexers with fiber expansion capabilities are used more and more. The wavelength division multiplexer must be used in conjunction with the wave splitting module. Today, Fiber Yitong will share with you how the wavelength division multiplexer and the wavelength division optical module should be connected.
Double fiber wave division connection
Wavelength division multiplexers are divided into single fiber and double fiber, and the connection methods are different. The following takes a dual-fiber coarse wavelength division multiplexer as an example. Fiber Yitong introduces the connection method of the dual-fiber wavelength division multiplexer and the wavelength division optical module.
As shown above, this is a common point-to-point optical transmission scheme. Both the Aand B points use a wavelength division multiplexer and a wavelength division optical module.See the figure below for details of the connection:
As shown in the figure above, the wave splitting module is inserted on the switch, each optical module emits light of a specific wavelength, and the light emitted by the optical module is connected to the combined wave of the wavelength corresponding to the wavelength division multiplexer (the upper row of the Mux) channel port, and the light The module light collection port is also connected to the demodulation (lower row of Demux) channel ports of the wavelength division multiplexer. Each optical module is connected by such a method. After the link is connected, the light from each channel's optical module is multiplexed into a beam of light, which comes out of the M-COM , and the light from the opposite end enters the D-COM .
Single fiber wavelength division connection
Since the single fiber wavelength division multiplexer is on one fiber because of the light collection and illumination, half of the channels must be connected to the light port of the optical module in one wavelength division multiplexer, and the other half is connected to the light module. mouth. The following is a description of connecting the optical module with a dense wavelength division multiplexer.
As shown in the above figure, the user needs to transmit 80G service with single fiber wavelength division , using 40 channel single fiber wavelength division.

As shown above, a dense optical module with a wavelength of C21-C28 is used at point A , and a dense optical module with a wavelength of C31-38 is used at point B. Then the connection method is as shown above. The C21 channel of the C21 module at point A is connected to the C21 channel of the wave division , and the light receiving port is connected to the C31 channel of the wave division ; the connection method of C22-C28 is similar. B point C31module wave emission port access points C31 passage, the light receiving port is connected to wavelength division C21 channels; C32-C38 similar connection methods. By this connection, the received light is multiplexed onto a single core fiber for transmission.
C-light products including WDM series
The above is the connection method of the wavelength division multiplexer and the wavelength division optical module, focusing on the fiber billion to obtain more knowledge of optical communication.