Page 233 - Optical Communications Essentials

Constructing the WDM Network Puzzle

Constructing the WDM Network Puzzle 223

■ Since a thermoelectric cooler is not needed, nominal power requirements are
0.5W compared to 4W for temperature-controlled DWDM lasers.
■ Whereas typical wavelength tolerances for DWDM lasers are on the order of
0.1nm, the manufacturing tolerances for CWDM lasers are around 3nm.
This significantly reduces yield costs.
■ The elimination of thermoelectric cooler requirements and the higher manu-
facturing yields reduce both the cost and the size of CWDM transmitters by a
factor of 4 or 5 compared to DWDM devices.
■ Wideband thin-film-filter technology can be used for wavelength multiplexing
at about one-half the cost of DWDM components. Figure 13.5 shows that a
TFF with a 3-dB passband of typically 13nm can accommodate the wave-
length drift of 0.09nm/°C of a typical DFB laser between the temperature
ranges of 0 to 50°C.

13.2.3. Wavelength multiplexing devices
As described in Chap. 12, four of the basic device types for wavelength multi-
plexing are thin-film filters, fiber Bragg gratings, arrayed waveguide gratings,
and bulk diffraction gratings. The performance demands on these components
are increasing with the desire to support higher channel counts and longer dis-
tances between terminals.
Whereas prior to 2000 the standard wavelength spacing was 100GHz for
2.5-Gbps DWDM links, subsequently service providers started deploying
10-Gbps ultradense systems operating with channels that are spaced 25 or
12.5GHz apart. An even further squeezing of the channels is seen in the hyper-
fine WDM products that have separations down to 3.125GHz. For 40-Gbps
systems the channels nominally are spaced 50 or 100GHz apart because of
the greater impact from nonlinear dispersion effects at these higher data rates.
The expansion of WDM channels beyond the C-band into the S- and L-bands
has demonstrated the possibility of sending at least 320 wavelengths spaced
25GHz apart with 10-Gbps transmission rates per channel in the combined

CWDM filter passband
Relative amplitude 0°C 25°C 50°C
3-dB passband = 13 nm

1547.75 1550 1552.25
Figure 13.5. A CWDM thin-film filter with a 3-dB pass-
band of 13nm allows a wavelength drift of 0.09nm/°C in
the temperature range of 0 to 50°C.

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