|<<< | >>>|
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 shift the zero dispersion wavelength into the 1.5-μm region. This is achieved by modifying the refractive index profile of the core. Common index profiles of dispersion-shifted fibers have a triangular, trapezoidal or Gaussian shape.
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 small dispersion in the wavelength range of the data transmission, with the zero dispersion wavelength lying just outside this region. An alternative is to use dispersion-unshifted (i.e., standard) fiber with larger dispersion at 1.5 μm, combined with some kind of dispersion compensation.
There are also dispersion-flattened fibers with a relatively constant group delay dispersion over some wavelength range, i.e., low higher-order dispersion. They can, for example, exhibit near zero dispersion in the telecom C band. Such fibers are important for data transmission with wavelength division multiplexing and for adiabatic soliton compression. They often have a W-shaped profile of the refractive index, although profiles with a graded index and multiple steps have also been developed.
All fibers with tailored dispersion properties can be regarded as specialty fibers.
|||L. G. Cohen et al., “Tailoring zero chromatic dispersion into the 1.5 μm-1.6 μm low-loss spectral region of single-mode fibres”, Electron. Lett. 15 (12), 334 (1979)|
|||M. A. Saifi et al., “Triangular-profile single-mode fiber”, Opt. Lett. 7 (1), 43 (1982)|
|||B. J. Ainslie et al., “Monomode fibre with ultra-low loss and minimum dispersion at 1.55 μm”, Electron. Lett. 18, 842 (1982)|
|||V. A. Bhagavatula and M. S. Spitz, “Dispersion-shifted segmented-core single-mode fibers”, Opt. Lett. 9 (5), 186 (1984)|
|||M. Wandel and P. Kristensen, “Fiber designs for high figure of merit and high slope dispersion compensating fibers”, J. Opt. Fiber Commun. Rep. 3, 25–60 (2005)|
|||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 lightwave communications, fiber optics and waveguides, light pulses
If you like this article, share it with your friends and colleagues, e.g. via social media: