Role develops for coarse WDM

BY BERNARD R. EICHENBAUM

The International Telecommunication Union (ITU-T) Recommendations G.692 and G.959.1 define frequency grids for DWDM. Channel spacing in these DWDM grids is not wide enough to accommodate the drift of uncooled laser sources. Stabilizing the emission wavelengths from a laser typically requires that its operating temperature be stabilized. A thermo-electric cooler (TEC) with its control circuitry not only adds cost, but also consumes and dissipates power.

Relatively speaking, the cost of terminals in short-haul communications systems such as metro systems and LANs is more critical than in long-haul systems. This trend is especially true for very-short-haul LANs where strategies such as using Fabry-Perot lasers are employed to reduce the terminal cost. For transmission distances not quite so short, eliminating the TECs while retaining a distributed-feedback-device architecture can reduce terminal costs. Because emitted wavelengths could then drift by several nanometers with ambient temperature changes, a range of spectrum must be allocated to a transmission channel to accommodate the drift. To acknowledge the wider-than-DWDM spacing of these channels, the term "coarse WDM" (CWDM) has been proposed.

At its February meeting in Geneva, the ITU-T Question 17/15 experts, chaired by J. Matthews (Corning USA), defined "coarse WDM" as channel spacing exceeding 200 GHz (about 1.6 nm at 1,550 nm) and less than 100 nm, but left these boundaries open to review by the correspondence group that has taken the lead in identifying WDM specifications.

Economics enter into the choice of a frequency plan by considering how the costs of lasers and filters vary with channel spacing. Increasing production yield decreases laser costs as tolerances on 25°C wavelength are relaxed. Wider channel spacing means relaxed 25°C tolerances and reduced thin-film-filter-costs. But the rate at which costs decrease levels off in the range of 20-25-nm channel widths. This effect is separate from the lower packaging costs of CWDM multiplexer/demultiplexer components relative to DWDM.

Economics also enter into the choice of specific center wavelengths for the transmission channels. Various wavelengths such as 1,310, 1,510, and 1,550 nm are already available in uncooled packages. In a grid that uses these, some of the wavelengths would be immediately available, and economies of scale would be realized.

Bernard R. Eichenbaum is a systems engineer representing the Optical Fiber Solutions division of Lucent Technologies in standards work. He can be contacted at tel: 973-386-5483; fax: 973-386-3127; e-mail: eichenbaum@lucent.com.

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