Page 232 - Optical Communications Essentials

Constructing the WDM Network Puzzle

222 Chapter Thirteen

Figure 13.4. Construction of a laser transmitter that has a thermoelectric cooler and a monitoring photo-
diode.
■ Continuous adjustment of the laser bias current (e.g., to within 7mV).
■ Stable optical output-power monitoring in order to have a reference for output-
level control. The monitoring is done by built-in low-profile photodiodes.
■ Constant optical-output control (e.g., within 0.2dB) provided by a transmitter-
power controller that works in conjunction with a variable optical attenuator.
■ Monitoring and alarm notification of abnormal operating conditions such as
instability in temperature, output power, or drive current.

13.2.2. CWDM optical transmitters
The concept of coarse WDM arose from the production of full-spectrum fibers
and the desire to have low-cost optical links operating in metro- and local-area
networks. In 2002 the ITU-T released Recommendation G.694.2, entitled
Coarse Wavelength Division Multiplexing (CWDM). The CWDM grid is made
up of 18 wavelengths defined within the range of 1270 to 1610nm (O- through
L-bands) spaced by 20nm with wavelength drift tolerances of 2nm.
The wider wavelength spacing in CWDM applications results in the following
operational characteristics:
■ Distributed-feedback (DFB) lasers or vertical cavity surface emitting lasers,
popularly known as VCSELs (see Chap. 6), which are not temperature-
controlled. These can be used since it is not necessary to maintain the lasers
at a precise wavelength.

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