July 24, 2019
With the rapid growth of IP data services on the Internet, the demand for transmission bandwidth is increasing. Although DWDM (Dense Wavelength Division Multiplexing) technology is the most effective method to solve bandwidth expansion, but CWDM (Coarse Wavelength Division Multiplexing) technology has more advantages in system cost and maintainability compared to DWDM.
Both CWDM and DWDM belong to wavelength division multiplexing technology, which can couple different wavelengths of fiber into a single-core fiber and transmit them together.
The latest ITU standard for CWDM is G.695, which specifies 18 wavelength channels with a spacing of 20 nm from 1271 nm to 1611 nm. Considering the water peak effect of ordinary G.652 fiber, 16 channels are generally used. Because the channel spacing is large, both the multiplexer and the laser are cheaper than the DWDM device.
The channel spacing of DWDM is 0.4 nm, 0.8 nm, 1.6 nm, etc., and the interval is small, requiring additional wavelength control devices. Therefore, devices based on DWDM technology are more expensive than devices based on CWDM technology.
The PIN photodiode is between a P-type and N-type semiconductor with a high doping concentration, and a lightly doped N-type material called an I (Intrinsic) layer. Due to the light doping, the electron concentration is very low, and after diffusion, a wide depletion layer is formed, which can improve the response speed and conversion efficiency.
APD avalanche photodiode, which not only has optical/electrical conversion, but also has internal amplification, and its amplification is achieved by the avalanche multiplication effect inside the tube. APD is a photodiode with gain. In the case where the sensitivity of the optical receiver is high, the use of APD is beneficial to extend the transmission distance of the system.
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