Erbium doped fiber amplifier (EDFA) and dispersion compensator (DCM) in active DWDM systems
Fiber attenuation&dispersion
Optical signals are influenced by various factors during transmission, including the type and specific application of optical fibers. Among them, fiber attenuation and dispersion are important factors that directly affect the quality of optical signal transmission.
Fiber optic attenuation refers to the process in which the power of an optical signal decreases exponentially with the increase of transmission distance when transmitted in a fiber optic. The main factors causing fiber optic attenuation include intrinsic, bending, squeezing, impurities, unevenness, docking, etc. Intrinsic is the inherent loss of optical fibers, including Rayleigh scattering, inherent absorption, etc; Bending refers to the loss of some light caused by scattering during the bending of optical fibers; Squeezing is the loss caused by slight bending of optical fibers due to compression; Impurities refer to the losses caused by the absorption and scattering of light signals by impurities in optical fibers; Uneven refractive index refers to the loss caused by the uneven refractive index of optical fiber materials; Docking is the loss generated during fiber optic docking. In DWDM systems, fiber attenuation largely determines the transmission distance of the system.
The optical signals transmitted in optical fibers are composed of different frequency components and mode components, which have different propagation speeds. These will cause changes in the shape of the pulse waveform, which is called dispersion. Fiber dispersion generally includes mode dispersion, material dispersion, and waveguide dispersion. Mode dispersion refers to the dispersion caused by different tangent components of the transmission constant and group velocities of different modes in a fiber optic cable at the same wavelength during multimode transmission; Material dispersion is caused by the inherent characteristics of optical fiber materials; Waveguide dispersion is caused by optical waveguides in optical fibers. For multimode fibers, the total dispersion is equal to the sum of the three; For single-mode optical fibers, since there is only one transmission mode, the total dispersion is the sum of material dispersion and waveguide dispersion.
As mentioned above, fiber attenuation and dispersion have a significant impact on the performance of DWDM systems. To minimize these effects, erbium-doped fiber amplifiers (EDFAs) and dispersion compensators (DCMs) are often used to compensate for fiber loss and dispersion.
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Erbium doped fiber amplifier (EDFA) is an optical device that directly amplifies optical signals in fiber optic communication systems. It has advantages such as high gain, wide bandwidth, low noise, and high saturation output power. However, in DWDM systems, the wavelength gain characteristics of EDFA are not flat, which can cause severe power imbalance in each optical channel (optical signal-to-noise ratio) of the system. Channels deviating from the gain peak will result in detection errors, leading to serious performance degradation of the system. Therefore, in long-distance DWDM networks, erbium-doped fiber amplifiers (EDFAs) are usually installed in fiber optic lines at certain intervals. In a certain area, the number of EDFAs used is usually limited by factors such as fiber type, channel number, data transmission distance, and allowable optical signal-to-noise ratio.
The widespread application of erbium-doped fiber amplifiers (EDFAs) has largely solved the transmission loss problem of high-speed fiber optic communication systems. However, fiber dispersion is also one of the main factors restricting the relay distance and communication capacity of fiber optic transmission systems. Therefore, DWDM networks also need to use dispersion compensators (DCMs), which are mainly used to compensate for material dispersion and waveguide dispersion in fiber optic lines. It should be noted that the dispersion compensator (DCM) in DWDM networks can also cause certain fiber attenuation to the fiber optic line, and must be used in conjunction with erbium-doped fiber amplifiers (EDFAs).
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