What is Optical Fiber Dispersion? Dispersion is the spreading out of a light pulse in time as it propagates down the fiber. Dispersion in optical fiber includes model dispersion, material dispersion and waveguide dispersion. Each type is discussed in detail below.
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What is Optical Fiber Dispersion? Dispersion is the spreading out of a light pulse in time as it propagates down the fiber. Dispersion in optical fiber includes model dispersion, material dispersion and waveguide dispersion. Each type is discussed in detail below. Model Dispersion in Multimode Fibers Multimode fibers can guide many different light modes since they have much larger core size. This is shown as the 1st illustration in the picture above.
Each mode enters the fiber at a different angle and thus travels at different paths in the fiber. Since each mode ray travels a different distance as it propagates, the ray arrive at different times at the fiber output. So the light pulse spreads out in time which can cause signal overlapping so seriously that you cannot distinguish them any more. Model dispersion is not a problem in single mode fibers since there is only one mode that can travel in the fiber.
Material Dispersion Material dispersion is the result of the finite linewidth of the light source and the dependence of refractive index of the material on wavelength. It is shown as the 2nd illustration in the first picture. Material dispersion is a type of chromatic dispersion. Chromatic dispersion is the pulse spreading that arises because the velocity of light through a fiber depends on its wavelength.
The following picture shows the refractive index versus wavelength for a typical fused silica glass. Waveguide Dispersion Waveguide dispersion is only important in single mode fibers. It is caused by the fact that some light travels in the fiber cladding compared to most light travels in the fiber core. It is shown as the 3rd illustration in the first picture.
Since fiber cladding has lower refractive index than fiber core, light ray that travels in the cladding travels faster than that in the core. Waveguide dispersion is also a type of chromatic dispersion.
It is a function of fiber core size, V-number, wavelength and light source linewidth. While the difference in refractive indices of single mode fiber core and cladding are minuscule, they can still become a factor over greater distances.
It can also combine with material dispersion to create a nightmare in single mode chromatic dispersion. Various tweaks in the design of single mode fiber can be used to overcome waveguide dispersion, and manufacturers are constantly refining their processes to reduce its effects. Recent Articles.
This broadening of light pulses is known as dispersion. This Increase in width of the pulses makes it very difficult to distinguish them at the receiving end. Now observe the same diagram carefully. Due to this dispersion effect broadening of light pulses the digital bit pattern at the input side is not indistinguishable at the output side as the same bit pattern. As we know that optical sources emit a band of frequencies so do not emit just a single frequency. Therefore different spectral components present in the optical source take different propagation delay while travelling through the optical fiber. This phenomena results in the broadening of each transmitted mode and is responsible for the intramodal dispersion.
What is Optical Fiber Dispersion?
Material dispersion 2. Waveguide dispersion Intramodal Dispersion Chromatic or intramodal dispersion may occur in all types of optical fiber and results from the finite spectral linewidth of the optical source. Since optical sources do not emit just a single frequency but a band of frequencies in the case of the injection laser corresponding to only a fraction of a percent of the center frequency, whereas for the LED it is likely to be a significant percentage , then there may be propagation delay differences between the different spectral components of the transmitted signal. This causes broadening of each transmitted mode and hence intramodal dispersion. The delay differences may be caused by the dispersive properties of the waveguide material material dispersion and also guidance effects within the fiber structure waveguide dispersion. Material dispersion Pulse broadening due to material dispersion results from the different group velocities of the various spectral components launched into the fiber from the optical source. It occurs when the phase velocity of a plane wave propagating in the dielectric medium varies nonlinearly with wavelength, and a material is said to exhibit material dispersion when the second differential of the refractive index with respect to wavelength is not zero.