Zero Dispersion Wavelength
is zero. For standard telecom fibers, this wavelength is ≈ 1.3 μm, but by employing designs with modified waveguide dispersion it is possible to shift the zero dispersion wavelength to the 1.5-μm region (→ dispersion-shifted fibers). The dispersion is anomalous for wavelengths longer than the zero dispersion wavelength, and normal for shorter wavelengths.
For photonic crystal fibers with small mode areas, which can exhibit particularly strong waveguide dispersion, the zero dispersion wavelength can be shifted e.g. into the visible spectral region, so that anomalous dispersion is obtained in the visible wavelength region, allowing for, e.g., soliton transmission. Photonic crystal fibers as well as some other fiber designs can exhibit two or even three different zero dispersion wavelengths.
Effects of Vanishing Dispersion
When ultrashort pulses of light propagate in a medium with zero chromatic dispersion, dispersive pulse broadening is avoided. Similarly, operation of a telecom system around the zero dispersion wavelength greatly reduces dispersive broadening of optical signals.
At the same time, however, the signals become relatively sensitive to optical nonlinearities of the fiber, such as four-wave mixing, which can be phase matched under these conditions. It is therefore not always advantageous to operate in that regime; an improved approach is dispersion management in the form of alternatively using fibers with different signs of group velocity dispersion.
In other situations, phase matching of nonlinearities near the zero dispersion wavelength can be useful for nonlinear devices, such as optical parametric oscillators based on the χ(3) nonlinearity of optical fibers. Also, supercontinuum generation can lead to particularly broad optical spectra when the pump light has a wavelength near the zero dispersion wavelength.
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