5.9 Wavelength Stabilization 341 5.9 Wavelength Stabilization Luckily for us, it turns out that the wavelength drift due to temperature variations of some of the key components used in WDM systems is quite small. Typical mul-tiplexers 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 that we studied in Chapter 3 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 . 1 ◦ C, which would stabilize the laser to within ±0 . 01 nm/ ◦ C. The laser comes packaged with a thermistor and a thermo-electric (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
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