In the whole WDM system, the optical wavelength division multiplexer and demultiplexer are key components in WDM technology, and their performance 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 the multi-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 (two-way reversible), that is, as long as the output and input of the demultiplexer are used in reverse, it is a multiplexer. The performance indicators of optical wavelength division multiplexers mainly include access loss and crosstalk. The loss and frequency offset are required to be small, the access loss should be 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 current practical WDM system, there are mainly grating type optical wavelength division multiplexers and dielectric film filter type optical wavelength division multiplexers.
The blazed grating is to carve equal and equidistant grooves on a plane that can be transmitted or reflected, and the grooves have a small step-like shape. When an optical signal with multiple wavelengths is diffracted by the grating, the optical signals with different wavelength components will be emitted at different angles. When the optical signal in the optical fiber is sent to the blazed grating with parallel beams through the lens, due to the diffraction effect of the grating, the optical signals of different wavelengths return to the lens with various parallel lights with slightly different directions, and then after being focused by the lens, the The rules are respectively injected into the output optical fiber, so that the optical signals of different wavelengths are transmitted through different optical fibers to achieve the purpose of demultiplexing. According to the principle of reciprocity, the purpose of multiplexing can be achieved by exchanging the input and output of optical wavelength division multiplexing.
At present, the WDM system works in the 1550nm wavelength range, and uses 8, 16 or more wavelengths to form an optical communication system on a pair of optical fibers (a single optical fiber can also be used). Between each wavelength is 1.6nm, 0.8nm or narrower interval, corresponding to 200GHz, 100GHz or narrower bandwidth.
Five, the main characteristics of WDM technology