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Definition Of Center Wavelength And Wavelength Spacing In CWDM Applications

Time: 2023-12-07
        CWDM is a low-cost WDM transmission technology for the access layer of metropolitan area networks (MANs). It multiplexes optical signals at different wavelengths into a single fiber using optical multiplexers, maximizing the utilization of fiber resources.
        The ITU-T has clearly specified the center wavelength and wavelength spacing for CWDM applications in Recommendation ITU-T G.694.2.
• Center Wavelength for CWDM Applications
        The first edition of ITU-T G.694.2, published in 2002, defines that a CWDM system can support up to 18 nominal center operating wavelengths over a fiber link, ranging from 1270 nm to 1610 nm.
        Adjacent wavelengths are spaced 20 nm apart, with an allowable center wavelength deviation of ±6.5 nm.
        Uncooled lasers typically have a temperature drift coefficient of 0.08 nm/°C. The nominal center wavelength specified by most laser manufacturers refers to the laser’s output wavelength at 23°C.
        For optical module applications, the standard commercial operating temperature range is 0 °C ~ 70 °C.
        As a result, the output wavelength of uncooled laser-based modules operates in an “asymmetric” state: the actual effective center wavelength of CWDM modules corresponds to the laser output at 35 °C rather than 23 °C.
        Due to temperature drift, the output wavelength shifts toward longer wavelengths by approximately 1 nm, calculated as follows:
(35℃-23℃) × 0.08nm/℃ = 0.96nm ≈ 1nm
        Therefore, during the 2003 ITU-T meeting, it was agreed to shift the center wavelength defined in G.694.2 upward by 1 nm to align with industrial practice, while retaining the symmetrical tolerance for nominal center wavelength deviation.
Figure 1 CWDM Center Wavelength Distribution
• Wavelength Spacing for CWDM Applications
        The principles for selecting center wavelength spacing and the factors determining wavelength variation are described in Appendix I of ITU-T G.694.2: Center Wavelength Spacing and Wavelength Variation.
        Practical CWDM systems using uncooled lasers and wide passband filters require the nominal center wavelength spacing to be no less than 20 nm.
        Wavelength variations within ±6 nm ~ 7 nm are compatible with current optical filtering technologies.
        For the guard band, one-third of the minimum channel spacing is sufficient. To maximize the number of channels, 20 nm wavelength spacing was adopted for CWDM applications.
        Under this specification, CWDM allows a wavelength shift of ±6.5 nm with a guard band width of 7 nm.
        Moduletek provides CWDM optical modules for all rate applications from GE to 25G.
        Moduletek Limited is at your service!
        For further inquiries about the above content, please contact us at: sales@moduletek.com
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