Optical Networks - _3_5 Transmitters_39

Optical Networks - _3_5 Transmitters_39 - 172 Components...

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172 Components 3.5 Transmitters We will study many different types of light sources in this section. The most im- portant one is the laser, of which there are many different types. Lasers are used as transmitters as well as to pump both erbium-doped and Raman amplifiers. When using a laser as a light source for WDM systems, we need to consider the following important characteristics: 1. Lasers need to produce a reasonably high output power. For WDM systems, the typical laser output powers are in the 0–10 dBm range. Related parameters are the threshold current and slope efficiency. Both of these govern the efficiency of converting electrical power into optical power. The threshold current is the drive current at which the laser starts to emit optical power, and the slope efficiency is the ratio of output optical power to drive current. 2. The laser needs to have a narrow spectral width at a specified operating wave- length so that the signal can pass through intermediate filters and multiple chan- nels can be placed close together. The side-mode suppression ratio is a related parameter, which we will discuss later. In the case of a tunable laser, the operating wavelength can be varied. 3. Wavelength stability is an important criterion. When maintained at constant temperature, the wavelength drift over the life of the laser needs to be small relative to the wavelength spacing between adjacent channels. 4. For lasers that are modulated, chromatic dispersion can be an important limiting factor that affects the link length. We will see in Chapter 5 that the dispersion limit can be stated in terms of a penalty as a function of the total accumulated dispersion along the link. Pump lasers are required to produce much higher power levels than lasers used as WDM sources. Pump lasers used in erbium-doped fiber amplifiers put out 100– 200 mW of power, and pump lasers for Raman amplifiers may go up to a few watts. 3.5.1 Lasers A laser is essentially an optical amplifier enclosed within a reflective cavity that causes it to oscillate via positive feedback. Semiconductor lasers use semiconductors as the gain medium, whereas fiber lasers typically use erbium-doped fiber as the gain medium. Semiconductor lasers are by far the most popular light sources for optical communication systems. They are compact, usually only a few hundred micrometers
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3.5 Transmitters 173 Figure 3.42 Reflection and transmission at the facets of a Fabry-Perot cavity. in size. Since they are essentially pn -junctions, they can be fabricated in large volumes using highly advanced integrated semiconductor technology. The lack of any need for optical pumping, unlike fiber lasers, is another advantage. In fact, a fiber laser typically uses a semiconductor laser as a pump! Semiconductor lasers are also highly efficient in converting input electrical (pump) energy into output optical energy.
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Optical Networks - _3_5 Transmitters_39 - 172 Components...

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