ITU revises Recommendation G.655 for non-zero dispersion-shifted fibre

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Earlier this year ITU-T SG15/Q5 consented to a modified version of Recommendation G.655, “Characteristics of a Non-zero Dispersion-shifted Single-mode Optical Fibre (NZDSF) and Cable”. Among other things, two new tables were added with tightened chromatic dispersion specifications.

Previously, chromatic dispersion was described as a constant minimum and maximum value over the 1,530 to 1,565 nm C-band range. In reality, the chromatic dispersion of such fibres usually increases with wavelength. System experts requested more detailed information on this parameter for use in CWDM and DWDM recommendations.

To address this request, SG15/Q5 surveyed commercially available NZDSF and agreed on the following modifications:

  • 1. Most commercially available NZDSF fall into two new categories: G.655.D representing low-dispersion fibres (with dispersion at 1,550 nm ranging from 2.80 to 6.2 ps/nm·km), and G.655.E representing medium-dispersion fibres (with dispersion at 1,550 nm ranging from 6.06 to 9.31 ps/nm·km).
  • 2. Rather than specify constant minimum and maximum values over a wavelength range, the chromatic dispersion is now defined by two limiting curves, Dmin(λ) and Dmax(λ), autonomously specified for the two new fibre categories (D and E). Consequently, the limiting dispersion values vary with wavelength. Furthermore, the wavelength range has been extended to include not only the C-band (1,530 to 1,565 nm), but also the S- and L-bands to cover 1,460 to 1,625 nm.

The two new categories do not contain fibres with negative dispersion. These can still be referred to Table C of Rec. G.655 (type G.655.C). Although the older Tables A and B have not been included in the latest edition of G.655, they are still available in the 2003 edition.

The new edition of Recommendation G.655 (03/2006) was recently published and is available at the ITU-T web site ( To harmonise its standards with ITU, IEC SC86A is in the process of updating its singlemode-fibre product specification (IEC 60793-2-50, fibre type B.4).

Gerard Kuyt is product line manager at Draka Comteq Optical Fibre in The Netherlands, a delegate to ITU-T SG15/Q5, and convenor of IEC SC86A/WG1. He can be reached at

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