Editing Dispersion (optics) (section)

== Dispersion control ==
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The result of GVD, whether negative or positive, is ultimately temporal spreading of the pulse. This makes dispersion management extremely important in optical communications systems based on optical fiber, since if dispersion is too high, a group of pulses representing a bit-stream will spread in time and merge, rendering the bit-stream unintelligible. This limits the length of fiber that a signal can be sent down without regeneration. One possible answer to this problem is to send signals down the optical fibre at a wavelength where the GVD is zero (e.g., around 1.3–1.5&nbsp;μm in [[silica]] [[fibres]]), so pulses at this wavelength suffer minimal spreading from dispersion. In practice, however, this approach causes more problems than it solves because zero GVD unacceptably amplifies other nonlinear effects (such as [[four-wave mixing]]). Another possible option is to use [[soliton (optics)|soliton]] pulses in the regime of negative dispersion, a form of optical pulse which uses a [[nonlinear optics|nonlinear optical]] effect to self-maintain its shape. Solitons have the practical problem, however, that they require a certain power level to be maintained in the pulse for the nonlinear effect to be of the correct strength. Instead, the solution that is currently used in practice is to perform dispersion compensation, typically by matching the fiber with another fiber of opposite-sign dispersion so that the dispersion effects cancel; such compensation is ultimately limited by nonlinear effects such as [[self-phase modulation]], which interact with dispersion to make it very difficult to undo.

Dispersion control is also important in [[laser]]s that produce [[ultrashort pulse|short pulses]]. The overall dispersion of the [[laser construction|optical resonator]] is a major factor in determining the duration of the pulses emitted by the laser. A pair of [[Prism (optics)|prisms]] can be arranged to produce net negative dispersion, which can be used to balance the usually positive dispersion of the laser medium. [[Diffraction grating]]s can also be used to produce dispersive effects; these are often used in high-power laser amplifier systems. Recently, an alternative to prisms and gratings has been developed: [[chirped mirror]]s. These dielectric mirrors are coated so that different wavelengths have different penetration lengths, and therefore different group delays. The coating layers can be tailored to achieve a net negative dispersion.