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Modal dispersion occurs even with an ideal, monochromatic light source. A special case of modal dispersion is polarization mode dispersion (PMD), a fiber dispersion phenomenon usually associated with single-mode fibers. PMD results when two modes that normally travel at the same speed due to fiber core geometric and stress symmetry (for example ...
This dispersion limits both the bandwidth and the distance over which the information can be transmitted. This is why for long communications links, it is desirable to use a laser with a very narrow linewidth. Distributed Feedback Lasers (DFB) are popular for communications because they have a single longitudinal mode with a very narrow line width.
Dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency. [1] Sometimes the term chromatic dispersion is used to refer to optics specifically, as opposed to wave propagation in general. A medium having this common property may be termed a dispersive medium.
VCSEL power profiles, along with variations in fiber uniformity, can cause modal dispersion which is measured by differential modal delay (DMD). Modal dispersion is caused by the different speeds of the individual modes in a light pulse. The net effect causes the light pulse to spread over distance, introducing intersymbol interference. The ...
Polarization mode dispersion (PMD) is a form of modal dispersion where two different polarizations of light in a waveguide, which normally travel at the same speed, travel at different speeds due to random imperfections and asymmetries, causing random spreading of optical pulses. Unless it is compensated, which is difficult, this ultimately ...
The name "dispersion relation" originally comes from optics. It is possible to make the effective speed of light dependent on wavelength by making light pass through a material which has a non-constant index of refraction , or by using light in a non-uniform medium such as a waveguide .
In an optical fiber, the material dispersion coefficient, M(λ), characterizes the amount of pulse broadening by material dispersion per unit length of fiber and per unit of spectral width. It is usually expressed in picoseconds per ( nanometre · kilometre ).
Other dispersion models that can be used to derive n and k, such as the Tauc–Lorentz model, can be found in the literature. [19] [20] Two well-known models—Cauchy and Sellmeier—provide empirical expressions for n valid over a limited measurement range, and are only useful for non-absorbing films where k=0. Consequently, the Forouhi ...