Basic composition of WDM system
The basic composition of the WDM system is mainly divided into two modes: dual-fiber one-way transmission and single-fiber two-way transmission. One-way WDM means that all optical channels are transmitted in the same direction on one optical fiber at the same time. At the sending end, the modulated optical signals with different wavelengths carrying various information are combined through an optical extension device, and transmitted in one optical fiber. One-way transmission in the optical fiber, because each signal is carried by light of different wavelengths, so they will not be confused with each other. At the receiving end, the optical multiplexer separates the optical signals of different wavelengths to complete the transmission of multiple optical signals. The opposite direction is transmitted through another optical fiber. Bi-directional WDM means that the optical path is transmitted in two different directions simultaneously on a main optical fiber, and the wavelengths used are separated from each other to achieve full-duplex communication between the two parties. At present, the unidirectional WDM system is widely used in development and application, while the design and application of bidirectional WDM are affected by factors such as interference of each channel, influence of light reflection, isolation and crosstalk between two-way channels, and the current practical application is relatively limited. few.
Composition of dual-fiber unidirectional WDM system
Taking the dual-fiber unidirectional WDM system as an example, generally speaking, the WDM system is mainly composed of the following five parts: optical transmitter, optical relay amplifier, optical receiver, optical monitoring channel and network management system.
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 selected according to the different applications of the WDM system (mainly the type of transmission fiber and the transmission distance). Dispersion capacity transmitter. At the sending end, the optical signal output from the terminal equipment is first converted into a stable signal with a specific wavelength by using an optical transponder to convert the optical signal output from the terminal device into a signal with a stable specific wavelength, and then the multi-channel optical signal is synthesized by a multiplexer, and passed through an optical power amplifier (BA ) to amplify the output.
After long-distance (80~120km) optical fiber transmission, optical relay amplification is required for the optical signal. Most of the optical amplifiers currently used are erbium-doped fiber optical amplifiers (EDFA). Gain flattening technology must be adopted in the WDM system, so that EDFA has the same amplification gain for optical signals of different wavelengths, and ensures that the gain competition of optical channels does not affect transmission performance.
At the receiving end, the optical preamplifier (PA) amplifies the main channel signal attenuated by transmission, and uses a demultiplexer to separate the optical channel of a specific wavelength from the main channel optical signal. The receiver must not only meet the requirements for optical signal sensitivity, overload Requirements for parameters such as power, but also a signal that can withstand a certain amount of optical noise, and sufficient electrical bandwidth performance.