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Definition: fibers with a non-standard zero dispersion wavelength
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 [6] 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.
Figure 1: Approximate chromatic dispersion for different types of telecom fibers.
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 [7], 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.
Bibliography
| [1] | 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) |
| [2] | M. A. Saifi et al., “Triangular-profile single-mode fiber”, Opt. Lett. 7 (1), 43 (1982) |
| [3] | B. J. Ainslie et al., “Monomode fibre with ultra-low loss and minimum dispersion at 1.55 μm”, Electron. Lett. 18, 842 (1982) |
| [4] | V. A. Bhagavatula and M. S. Spitz, “Dispersion-shifted segmented-core single-mode fibers”, Opt. Lett. 9 (5), 186 (1984) |
| [5] | 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) |
| [6] | ITU standard G.653 (12/06), “Characteristics of a dispersion-shifted single-mode optical fibre and cable”, International Telecommunication Union (2006) |
| [7] | ITU standard G.655 (03/06), “Characteristics of a non-zero dispersion-shifted single-mode optical fibre and cable”, International Telecommunication Union (2006) |
See also: chromatic dispersion, telecom fibers, fibers, wavelength division multiplexing
Categories: communications, fibers and other waveguides, pulses
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