The major purpose of optical fibers complying with ITU-T G.656 recommendation is to reduce the growth of non-linear effects. Non-linear effects such as Self Phase Modulation (SPM), Cross Phase Modulation (XPM), Four wave mixing (FWM), Stimulated Raman Scattering (SRS), and Stimulated Brillouin Scattering (SBS) limits the capacity of optical transmission.

Non-linear effects are deleterious in DWDM systems, in which the channel spacing becomes narrow. Non-linear effects are stronger in an optical fiber that has zero dispersion at its operating wavelength. A zero value for Chromatic dispersion can lead to unwanted troubles in your fiber network. Dispersion could be as low as zero, but not zero.

ITU-T has classified optical fibers into seven major groups. Under these major groups, there may be subcategories based on specific characteristics specified in tables. Such major groups are ITU-T G.651 (Multimode fibers), ITU-T G.652 (Conventional Single-mode fibers), ITU-T G.653 (Dispersion Shifted Single-mode fibers), ITU-T G.654 (Cut-off shifted Single-mode fibers), ITU-T G.655 (Non-zero dispersion-shifted single-mode fibers), ITU-T G.656 (Non-zero dispersion for Wideband Optical transport), and ITU-T G.657 (Bend Insensitive Single-mode fiber).

ITU-T Recommendation G.656 describes a single-mode fiber with chromatic dispersion that is greater than some non-zero value throughout the wavelength range of 1460-1625 nm. This dispersion ensures a reduction of the growth of non-linear effects that can be particularly deleterious in dense wavelength division multiplexing (DWDM) systems.

ITU-T G.656 compliant optical fibers use non-zero dispersion characteristics to reduce four-wave mixing and cross-phase modulation over a wider wavelength range than the fiber described in ITU-T G.655 (Non-zero dispersion-shifted single-mode fibers).

ITU-T G.656 compliant optical fiber can be utilized for CWDM and DWDM systems throughout the extended wavelength transmission region between 1460 and 1625 nm. ITU-T says that the extension of the operating wavelength is possible in the future to add wavelengths beyond the 1460-1625 nm region.

For an ITU-T G.656, compliant non-zero dispersion-shifted fiber for wideband operation, the geometrical, optical, transmission, and mechanical parameters are described below in three categories such as those fiber attributes that are retained throughout cabling and installation, the cable attributes that are recommended for cables as they are delivered, and the link attributes that are characteristics of concatenated cables, describing estimation methods of system interface parameters based on measurements, modeling, or other considerations.

There is only one type of G.656 fiber, the properties of which are listed in Table 1 by the ITU-T. The range of nominal values for mode field diameter at 1550nm could be from 7 to 11 micrometers. The tolerance to the Mode field diameter could be 0.7micrometers to the plus and minus side.

Chromatic dispersion coefficients have specific requirements on different wavelengths as shown below;

– Minimum dispersion at any wavelength (λ) between 1460 and 1550nm: 0.0289 x (λ – 1460) + 1 (For example, the minimum dispersion at 1460nm is 1ps/nm.km and that at 1461nm is 1.0289ps/nm.km

– Maximum dispersion at any wavelength (λ) between 1460 and 1550nm: 0.052 x (λ – 1460) + 4.6 (For example, the maximum dispersion at 1460nm is 4.6ps/nm.km and that at 1461nm is 4.652ps/nm.km

– Minimum dispersion at any wavelength (λ) between 1550 and 1625nm: 0.01307 x (λ – 1550) + 3.6 (For example, the minimum dispersion at 1550nm is 3.6ps/nm.km and that at 1551nm is 3.61307ps/nm.km

– Maximum dispersion at any wavelength (λ) between 1550 and 1625nm: 0.0629 x (λ – 1550) + 9.28 (For example, the maximum dispersion at 1550nm is 9.28ps/nm.km and that at 1551nm is 9.3429ps/nm.km

(Approximate numbers in terms of the digits after the decimal).

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