![]() ![]() In optical fibers, there is usually some slight difference in the propagation characteristics of light waves with different polarization states. Polarization mode dispersion (PMD) represents the polarization dependence of the propagation characteristics of light waves in optical fibers. For example, the G.652 fibers are designed in this way. These varying speeds cause pulses to either spread out or compress as they travel down the fiber, making it possible to customize the index of refraction profile to produce fibers for different applications. Light travels at various speeds at different wavelengths or materials. What’s more, chromatic dispersion isn’t always a bad thing. At some particular frequency, these two effects can cancel each other out giving a wavelength with approximately 0 chromatic dispersion. Waveguide dispersion occurs due to dependence of the mode propagation constant on the fiber parameters (core radius, and difference between refractive indexes in fiber core and fiber cladding) and signal wavelength. Material dispersion is caused by the wavelength dependence of the refractive index on the fiber core material. For example, the high-order modes (light entering at sharp angles) have more model dispersion than low-order modes (light entering at smaller angles). The longer the path is, the higher the model dispersion will be. ![]() Whenever there is a bounce off, modal dispersion (or intermodal dispersion) happens. Some of these light rays will travel straight through the center of the fiber (axial mode) while others will repeatedly bounce off the cladding/core boundary to zigzag their way along the waveguide, as illustrated below with a step-index multimode fiber. As we know, light rays entering the fiber at different angles of incidence will go through different paths/modes. Modal dispersion is a distortion mechanism occurring in multimode fibers and other waveguides, in which the signal is spread in time because of different propagation velocity for all modes. Normally, dispersion in fiber optic cable includes modal dispersion, chromatic dispersion and polarization mode dispersion. Optical fiber dispersion describes the process of how an input signal broadens/spreads out as it propagates/travels down the fiber. ![]()
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