• The wavelength drift due to temperature variations of some of the key components used in WDM systems is quite small. Typical multiplexers and demultiplexers made of silica/silicon have temperature coefficients of 0.01 nm/◦C, whereas DFB lasers have a temperature coefficient of 0.1 nm/◦C. Some of the other devices have even lower temperature coefficients. The DFB laser source used in most systems is a key element that must be kept wavelength stabilized.
• In practice, it may be sufficient to maintain the temperature of the laser fairly constant to within ±0.1ºC, which would stabilize the laser to within ±0.01 nm/ºC.
• The laser comes packaged with a thermistor and a thermoelectric (TE) cooler. The temperature can be sensed by monitoring the resistance of the thermistor and can be kept constant by adjusting the drive current of the TE cooler.
• However, the laser wavelength can also change because of aging effects over a long period. Laser manufacturers usually specify this parameter, typically around ±0.1mnm. If this presents a problem, an external feedback loop may be required to stabilize the laser. A small portion of the laser output can be tapped off and sent to a wavelength discriminating element, such as an optical filter, called a wavelength locker.
• The output of the wavelength locker can be monitored to establish the laser wavelength, which can then be controlled by adjusting the laser temperature.
• Depending on the temperature range needed (typically −10 to 60ºC for equipment in telco centraloffices), it may be necessary to temperature-control the multiplexer/demultiplexer as well.
• One problem with temperature control is that it reduces the reliability of the overall component because the TE cooler is often the least reliable component. An additional factor to be considered is the dependence of laser wavelength on its drive current, typically between 100 MHz/mA and 1 GHz/mA.
• A laser is typically operated in one of two modes, constant output power or constant drive current, and the drive circuitry incorporates feedback to maintain these parameters at constant values. Keeping the drive current constant ensures that the laser wavelength does not shift because of current changes.
• However, as the laser ages, it will require more drive current to produce the same output power, so the output power may decrease with time.
• On the other hand, keeping the power constant may require the drive current to be increased as the laser ages, inducing a small wavelength shift.
• With typical channel spacing’s of 100 GHz or thereabouts, this is not a problem, but with tighter channel spacing’s, it may be desirable to operate the laser in constant current mode and tolerate the penalty (if any) due to the reduced output power.
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