ITU standard tackles chromatic dispersion

May 13, 2004
May 13 2004 Geneva -- The ITU has set a global standard for a new optical fiber that will make it easier for network operators to deploy bandwidth to maximize technology in their core networks. Recommendation G.656 permits easier deployment of Coarse WDM (CWDM) in metropolitan areas and increases the capacity of fiber in DWDM systems.

May 13 2004 Geneva -- The ITU has set a global standard for a new optical fiber that will make it easier for network operators to deploy bandwidth to maximize technology in their core networks. Recommendation G.656 permits easier deployment of Coarse WDM (CWDM) in metropolitan areas and increases the capacity of fiber in DWDM systems.

"G.656 is another significant step in the evolution of optical networks, because it allows a more economical deployment of optical transport networks," explains says Peter Wery, chairman of ITU-T Study Group 15, which is responsible for the Recommendation.

G.656 allows operators using CWDM to deploy systems without the need to compensate for chromatic dispersion, a phenomenon that at low levels counteracts distortion but at high-levels can make a signal unusable. The management of chromatic dispersion becomes increasingly more important as the number of wavelengths used in WDM systems increase.

G.656 also specifies the addition of at least 40 more channels to DWDM systems. In this case, chromatic dispersion is used to control harmful interference over the increased range of the optical spectrum.

According to the IEEE, the most important new feature in Recommendation G.656 fiber is the chromatic dispersion coefficient. In G.656, this coefficient has an allowed range of 2 to 14 ps/nm*km in the 1460- to 1625-nm band, compared to 1 to 10 ps/nm*km for G.655.B and G.655.C, which is only related to the 1530- to 1565-nm band. This low value of the chromatic dispersion coefficient in the S, C, and L bands is the real novelty of G.656 because it allows the utilization of a larger wavelength band.

The other characteristics are very similar to previous Recommendations. The range of mode field diameter permitted in G.656 of 7 to 11 μ m compares to 8 to 11 μ m in the G.655 non-zero dispersion-shifted fiber. G.656 fiber has a maximum PMD link design value of 0.20 ps/sqrtkm, which is the lowest value recommended by ITU-T (the same value that ITU-T recently adopted for G.655.C). G.656 has the same cable cut-off wavelength and cable attenuation coefficients in the C and L bands as G.655.

ITU-T G.656 (Characteristics of a fiber and cable with Non-Zero Dispersion for Wideband Optical Transport) is the most recent in the G-series, which specifies the geometrical, physical, mechanical, and transmission characteristics of the optical fibers. Other Recommendations in this series include:
• ITU-T G.652 - Characteristics of a singlemode optical fiber and cable
• ITU-T G.653 - Characteristics of a dispersion-shifted singlemode optical fiber and cable
• ITU-T G.654 - Characteristics of a cut-off shifted singlemode optical fiber cable
• ITU-T G.655 - Characteristics of a non-zero dispersion-shifted singlemode optical fiber and cable

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