Optical Dispersion
Two Types of Optical Dispersion
Chromatic Dispersion (CD) and Polarization Mode Dispersion (PMD) are both important linear distortions that affect the performance of optical systems.
Polarization Mode Dispersion (PMD)
PMD is a consequence of certain physical properties of optical fiber that result in distortion of optical pulses. These distortions result in dispersion of the optical pulse over time, as well as a reduction in peak power. While the outcome of PMD-induced distortion is similar to Chromatic Dispersion, it is not accumulated linearly, but stochastically. This means that its value cannot be predicted from one instant to the next, but its distribution (occurrence within a predictable range) can be.
Chromatic Dispersion (CD)
Chromatic dispersion is a variation in the velocity of light (group velocity) according to wavelength. This variation in velocity causes the pulses of a modulated laser source to broaden when traveling through the fiber; up to a point where pulses overlap and bit error rate increases. As this increase in bit error rate interferes with both the quality and speed of the signal, chromatic dispersion (CD) is a major limiting factor in high-speed transmission. Therefore, to ensure adequate quality of service, it is extremely important that carriers compensate for this type of signal distortion. The widespread use of DWDM systems covering the C and L bands (1530 nm to 1625 nm) will certainly create a need for accurate wideband CD compensators. However, ensuring proper CD compensation starts with proper testing.
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