Dense Wavelength Division Multiplexing (DWDM) systems are an important part of modern long-haul backbone networks and metropolitan networks. In long-haul transmission applications, fiber attenuation and dispersion are the main factors affecting optical signal transmission, so active devices such as Erbium Doped Fiber Amplifier (EDFA) and Dispersion Compensator (DCM) are commonly used in Dense Wavelength Division Multiplexing (DWDM) systems to ensure the quality of fiber transmission.

Fiber Attenuation & Dispersion

Optical signal in the transmission process will be affected by a variety of factors, including the type of fiber, specific applications, etc., of which, fiber attenuation and dispersion is an important factor that directly affects the quality of optical signal transmission.

Optical fiber attenuation is the process of optical signal transmission in the fiber, its power with the increase in transmission distance in the form of exponential decay. The main factors that cause fiber attenuation are: intrinsic, bending, extrusion, impurities, inhomogeneity, docking, etc. Intrinsic is the inherent loss of the fiber, including Rayleigh scattering, inherent absorption, etc.; bending is the fiber bending part of the light will be scattered due to loss; extrusion is the fiber is squeezed by the loss caused by micro-bending; impurities is the fiber impurities in the absorption and scattering of optical signals caused by loss; inhomogeneity is the fiber material refractive index of the loss caused by uneven; docking is the loss of fiber docking. In DWDM system, the fiber attenuation largely determines the transmission distance of the system.

The optical signal transmitted in optical fiber is composed of different frequency components and different mode components, which have different propagation speeds, and these will cause changes in the shape of the pulse waveform, which is dispersion, fiber dispersion generally includes mode dispersion, material dispersion and waveguide dispersion. Mode dispersion is the dispersion caused by the different tangential components of the transmission constants and different group velocities of each mode of the fiber at the same wavelength during multimode transmission; material dispersion is caused by the own characteristics of the fiber material; waveguide dispersion is caused by the optical waveguide in the fiber. For multimode fiber, the total dispersion is equal to the sum of the three; for single-mode fiber, because 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, and to minimize these effects, erbium-doped fiber amplifiers (EDFAs) and dispersion compensators (DCMs) are often used to compensate for fiber loss and dispersion.

Erbium-doped fiber amplifier (EDFA) is an optical device that directly amplifies optical signals in fiber optic communication systems, with the advantages of high gain, wide bandwidth, low noise, and high saturation output power, etc. However, in DWDM systems, the wavelength gain characteristics of erbium-doped fiber amplifier (EDFA) are not flat, which will cause the power (optical signal-to-noise ratio) of each optical channel of the system to be seriously unequal, and channels that deviate from the gain peak will The channels that deviate from the gain peak will be detected incorrectly, resulting in a serious impact on the system performance. Therefore, in long-haul DWDM networks, erbium-doped fiber amplifiers (EDFAs) are usually installed in fiber lines at certain interval distances, and the number of EDFAs used in a certain area is usually limited by the fiber type, number of channels, data transmission distance, and the allowed optical signal-to-noise ratio value.

The widespread use of erbium-doped fiber amplifier (EDFA) has largely solved the transmission loss problem of high-speed fiber optic communication systems, but fiber dispersion is also one of the main factors limiting the relay distance and communication capacity of fiber optic transmission systems, so DWDM networks also use dispersion compensator (DCM), dispersion compensator (DCM) is mainly used to compensate for material dispersion and waveguide dispersion in fiber optic lines. dispersion. It should be noted that the dispersion compensator (DCM) in DWDM networks also causes some fiber attenuation in the fiber line, and must be used in conjunction with an erbium-doped fiber amplifier (EDFA).