Page 107 - Optical Communications Essentials

Light Sources and Transmitters

Light Sources and Transmitters 97

wavelengths compared to other wavelengths, and thus defines the lasing modes
of the FP laser. This effect produces a broad output spectrum that has a width
of several nanometers at its half-maximum point. Although this broad spectral
output does not make the FP laser feasible for high-speed, long-haul trans-
missions, it is popular for short- and intermediate-distance (up to 15 km)
applications running at data rates of up to 622 Mbps. These applications use the
small-form-factor (SFF) transceiver described in Sec. 6.4.4.

6.3.2. Distributed-feedback laser
In a distributed-feedback (DFB) laser, a series of closely spaced reflectors pro-
vide light feedback in a distributed fashion throughout the cavity, as shown in
Fig. 6.11. Through a suitable design of these reflectors, which normally are
some type of grating, the device can be made to oscillate in only a single mode
with a very narrow linewidth. This means that it emits at a fairly well-defined
wavelength. The particular operating wavelength can be selected at the time of
device fabrication by an appropriate choice of the reflector spacing. Single-mode
DFB lasers are used extensively in high-speed transmission systems. Table 6.2
lists some typical performance parameter values of commercially available DFB
lasers used for data transmission. Note that a 2-MHz linewidth is equivalent to
10 5 pm at 1550nm. As described in Chap. 12, uncooled lasers are cost-effective
sources for coarse WDM networks, whereas the more expensive cooled lasers
are needed for dense WDM applications.

Figure 6.11. In a distributed-feedback (DFB) laser, a
series of closely spaced reflectors provides light feed-
back for lasing.

TABLE 6.2. Typical Performance Parameter Values of DFB Lasers
Center Peak optical
wavelength, Transmitter Threshold Wavelength output
nm type current, mA Linewidth drift, pm/°C power, mW
1310 Uncooled 15 1nm 100 0.5–1
1550 Cooled 12–25 2 MHz (10 5 pm) 1 2–5

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