Standard telecom fibers exhibit zero chromatic dispersion in the 1.3-μm wavelength region. This was convenient for early optical fiber communications systems, which often operated around 1310 nm. However, the 1.5-μm region later became more important, because the fiber losses are lower there, and erbium-doped fiber amplifiers (EDFAs) are available for this region (whereas 1.3-μm amplifiers do not reach comparable performance). In this wavelength region, however, standard single-mode fibers (now sometimes called dispersion-unshifted fibers) exhibit significant anomalous dispersion. For linear transmission, this can be a problem, because it leads to significant dispersive pulse broadening, limiting the achievable transmission rates or distances. Therefore, so-called dispersion-shifted fibers  have been developed, which have modified waveguide dispersion so as to optimize the dispersion properties.
Dispersion-shifted Fibers with Substantial Dispersion Slope
An early approach was to develop specialty fibers where the refractive index profile is modified such that the zero dispersion wavelength is shifted into the 1.5-μm region, i.e., close to the used signal wavelengths. This is achieved by modifying the refractive index profile of the core. Common refractive index profiles of dispersion-shifted fibers have a triangular, trapezoidal or Gaussian shape.
However, it turned out that zero chromatic dispersion is not necessarily ideal for data transmission. Particularly for the transmission of multiple channels (→ wavelength division multiplexing), four-wave mixing effects can be phase-matched and thus introduce significant distortions, if the dispersion is too weak. Therefore, it can be advantageous to use non-zero dispersion-shifted fibers , which are designed to have a moderate amount of group velocity dispersion (GVD) in the wavelength range of the data transmission, with the zero dispersion wavelength lying somewhat outside this region. There are different versions:
- NZD+ fibers have their zero dispersion wavelength at 1510 nm and anomalous dispersion at the typical signal wavelengths (longer than 1510 nm).
- NZD− fibers have a zero dispersion wavelength of 1580 nm, resulting in normal dispersion at the signal wavelengths.
Such fibers still typically have a substantial positive dispersion slope (specified with units of ps/(nm2 km)), i.e., their group velocity dispersion decreases with increasing wavelength, or increases with increasing optical frequency. That implies positive third-order dispersion (TOD).
Further, there are dispersion-flattened fibers with a much reduced dispersion slope, i.e., relatively constant group velocity dispersion over some wavelength range. In other words, they exhibit low higher-order dispersion. They can, for example, exhibit near zero dispersion or some moderate relatively constant amount of GVD in the telecom C band. Such fibers are important e.g. for adiabatic soliton compression, but also for signal processing like using four-wave mixing for wavelength channel translation, and for the generation of broadband frequency combs. Dispersion-flattened fibers often have a W-shaped profile of the refractive index, although profiles with a graded index and multiple steps have also been developed.
Unfortunately, polarization mode dispersion is often stronger in dispersion-shifted fibers, because they have a relatively high numerical aperture, which causes an increased sensitivity to a slight ellipticity of the fiber core.
An alternative solution can be to use dispersion-unshifted (i.e., standard) fiber with larger dispersion at 1.5 μm, combined with some kind of dispersion compensation. Note, however, that in many cases it is not that only the total chromatic dispersion of a fiber span is what matters. For example, if substantial signal distortions due to four-wave mixing occur in some initial length of fiber, that problem may remain even if FWM is suppressed in a later piece of the fiber. On the other hand, dispersion compensation can be effective where the problem is only dispersive broadening of pulses or signals.
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|||ITU standard G.653 (07/10), “Characteristics of a dispersion-shifted single-mode optical fibre and cable”, International Telecommunication Union (2007)|
|||ITU standard G.655 (11/09), “Characteristics of a non-zero dispersion-shifted single-mode optical fibre and cable”, International Telecommunication Union (2011)|
See also: chromatic dispersion, telecom fibers, fibers, wavelength division multiplexing, specialty fibers
and other articles in the categories fiber optics and waveguides, lightwave communications, light pulses