Optical Networks - _3_5 Transmitters_39

Lasers using this approach have been demonstrated and

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Lasers using this approach have been demonstrated and used in system experiments [Zah92, You95]. Another approach is to use Fabry-Perot–type laser arrays and use an external mechanism for selecting the lasing wavelength. Several structures have been proposed [Soo92, ZJ94], one using an external waveguide grating and the other using an external arrayed waveguide grating. With these structures, the wavelength accuracy is determined by the external grating. The long cavity length results in potentially a large number of cavity modes within the grating wavelength selection window, which could cause the laser to hop between cavity modes during operation. 3.5.4 Direct and External Modulation The process of imposing data on the light stream is called modulation. The simplest and most widely used modulation scheme is called on-off keying (OOK), where the light stream is turned on or off, depending on whether the data bit is a 1 or 0. We will study this in more detail in Chapter 4. OOK modulated signals are usually realized in one of two ways: (1) by direct modulation of a semiconductor laser or an LED, or (2) by using an external modu- lator. The direct modulation scheme is illustrated in Figure 3.56. The drive current into the semiconductor laser is set well above threshold for a 1 bit and below (or slightly above) threshold for a 0 bit. The ratio of the output powers for the 1 and 0 bits is called the extinction ratio . Direct modulation is simple and inexpensive since no other components are required for modulation other than the light source
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3.5 Transmitters 193 Figure 3.56 Direct modulation of a semiconductor laser. (laser/LED) itself. In fact, a major advantage of semiconductor lasers is that they can be directly modulated. In contrast, many other lasers are continuous wave sources and cannot be modulated directly at all. These lasers require an external modulator. For example, because of the long lifetime of the erbium atoms at the E 2 level in Figure 3.35, erbium lasers cannot be directly modulated even at speeds of a few kilobits per second. The disadvantage of direct modulation is that the resulting pulses are consider- ably chirped. Chirp is a phenomenon wherein the carrier frequency of the transmitted pulse varies with time, and it causes a broadening of the transmitted spectrum. As we saw in Section 2.4, chirped pulses have much poorer dispersion limits than unchirped pulses. The amount of chirping can be reduced by increasing the power of a 0 bit so that the laser is always kept well above its threshold; the disadvantage is that this reduces the extinction ratio, which in turn, degrades the system performance, as we will see in Section 5.3. In practice, we can realize an extinction ratio of around 7 dB while maintaining reasonable chirp performance. This enhanced pulse broadening of chirped pulses is significant enough to warrant the use of external modulators in high-speed, dispersion-limited communication systems.
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