With the rapid growth of Internet IP data services, the demand for transmission line bandwidth is increasing. Although DWDM (Dense Wavelength Division Multiplexing) technology is the most effective way to solve the problem of line bandwidth expansion, CWDM (Rough Wavelength Division Multiplexing) technology has advantages over DWDM in system cost and maintainability.
Both CWDM and DWDM belong to wavelength division multiplexing technology. They can couple different wavelengths into single core optical fibers and transmit them together.
The latest ITU standard for CWDM is G.695, which specifies 18 channels with a 20-nm interval from 1271 to 1611 nm. Considering the peak effect of G.652 optical fibers, 16 channels are generally used. Because of the large channel spacing, the combined-division devices and lasers are cheaper than the DWDM devices.
The channel spacing of DWDM is 0.4nm, 0.8nm, 1.6nm and so on. The channel spacing of DWDM is smaller and requires additional wavelength control devices. Therefore, the equipment based on DWDM technology is more expensive than that based on CWDM technology.
PIN photodiode is a layer of lightly doped N-type material between P-type and N-type semiconductors with high doping concentration, which is called I (intrinsic) layer. Because of light doping and low electron concentration, a wide depletion layer is formed after diffusion, which can improve the response speed and conversion efficiency.
APD avalanche photodiode not only has the function of light/electricity conversion, but also has the function of internal amplification, which is accomplished by avalanche multiplication effect inside the tube. APD is a photodiode with gain. In the case of high sensitivity of optical receiver, the use of APD is conducive to extending the transmission distance of the system.
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