Optical Networks - _3_5 Transmitters_39

They provide the flexibility to choose the transmit

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They provide the flexibility to choose the transmit wavelength at the source of a lightpath. For instance, if we wanted to have a total of, say, four lightpaths starting at a node, we would equip that node with four tunable lasers. This would allow us to choose the four transmit wavelengths in an arbitrary manner. In contrast, if we were to use fixed-wavelength lasers, either we would have to preequip the node with a large number of lasers to cover all the possible wavelengths, or we would have to manually equip the appropriate wavelength as needed. We will see more of this application in Chapter 7. The tuning time required for such applications is on the order of milliseconds because the wavelength selection happens only at the times where the lightpath is set up, or when it needs to be rerouted in the event of a failure. Another application for tunable lasers is in optical packet-switched networks, where data needs to be transmitted on different wavelengths on a packet-by-packet basis. These networks are primarily in their early stages of research today, but sup- porting such an application would require tuning times on the order of nanoseconds to microseconds, depending on the bit rate and packet size used. Finally, tunable lasers are a staple in most WDM laboratories and test environ- ments, where they are widely used for characterizing and testing various types of
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3.5 Transmitters 185 optical equipment. These lasers are typically tabletop-type devices and are not suit- able for use in telecom applications, which call for compact, low-cost semiconductor lasers. The InGaAsP/InP material used for most long-wavelength lasers is enhanced by the use of quantum well structures and has an overall gain bandwidth of about 250 nm at 1.55 μ m, large enough for the needs of current WDM systems. However, the tuning mechanisms available potentially limit the tuning range to a small fraction of this number. The following tuning mechanisms are typically used: Injecting current into a semiconductor laser causes a change in the refractive index of the material, which in turn changes the lasing wavelength. This effect is fairly small—about a 0.5–2% change in the refractive index (and the wavelength) is possible. This effect can be used to effect a tuning range of approximately 10– 15 nm in the 1.55 μ m wavelength window. Temperature tuning is another possibility. The wavelength sensitivity of a semi- conductor laser to temperature is approximately 0.1 nm/ C. In practice, the al- lowed range for temperature tuning is about 1 nm, corresponding to a 10 C temperature variation. Operating the laser at significantly higher temperatures than room temperature causes it to age rapidly, degrading its lifetime. Mechanical tuning can be used to provide a wide tunable range in lasers that use a separate external cavity mechanism. Many of these lasers tend to be bulky.
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