Optical Networks - _3_5 Transmitters_39

Longitudinal modes for laser oscillation to occur at

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Longitudinal Modes For laser oscillation to occur at a particular wavelength, two conditions must be satisfied. First, the wavelength must be within the bandwidth of the gain medium that is used. Thus, if a laser is made from erbium-doped fiber, the wavelength must lie in the range 1525–1560 nm. The second condition is that the length of the cavity must be an integral multiple of half the wavelength in the cavity. For a given laser, all the wavelengths that satisfy this second condition are called the longitudinal modes of that laser. The adjective “longitudinal” is used to distinguish these from the waveguide modes (which should strictly be called spatial modes) that we studied in Section 2.2. The laser described earlier is called a Fabry-Perot laser (FP laser) and will usu- ally oscillate simultaneously in several longitudinal modes. Such a laser is termed a multiple-longitudinal mode (MLM) laser. MLM lasers have large spectral widths, typically around 10 nm. A typical spectrum of the output of an MLM laser is shown in Figure 3.43(a). We saw in Section 2.4 that for high-speed optical communication systems, the spectral width of the source must be as narrow as possible to minimize the effects of chromatic dispersion. Similarly, a narrow spectral width is also needed to minimize crosstalk in WDM systems (see Section 3.3). Thus it is desirable to de- sign a laser that oscillates in a single-longitudinal mode (SLM) only. The spectrum of the output of an SLM laser is shown in Figure 3.43(b). Single-longitudinal mode oscillation can be achieved by using a filtering mechanism in the laser that selects the desired wavelength and provides loss at the other wavelengths. An important
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3.5 Transmitters 175 (a) (b) c 2nl / ~ 100-200 GHz f f A few nanometers Figure 3.43 The spectrum of the output of (a) an MLM laser and (b) an SLM laser. The laser cavity length is denoted by l , and its refractive index by n . The frequency spacing between the modes of an MLM laser is then c/ 2 nl . attribute of such a laser is its side-mode suppression ratio, which determines the level to which the other longitudinal modes are suppressed, compared to the main mode. This ratio is typically more than 30 dB for practical SLM lasers. We will now consider some mechanisms that are commonly employed for realizing SLM lasers. Distributed-Feedback Lasers In the Fabry-Perot laser described earlier, the feedback of the light occurs from the reflecting facets at the ends of the cavity. Thus the feedback can be said to be localized at the facets. Light feedback can also be provided in a distributed manner by a series of closely spaced reflectors. The most common means of achieving this is to provide a periodic variation in the width of the cavity, as shown in Figure 3.44(a) and (b). In the corrugated section of the cavity, the incident wave undergoes a series of reflections. The contributions of each of these reflected waves to the resulting transmitted wave from the cavity add in phase if the period of the corrugation is an integral multiple of half the wavelength in the cavity. The reasoning for this
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