A Distributed Feedback Laser (DFB) is an edge emitting laser( EEL). It is characterized by its extremely high spectral purity, which is reflected in a very narrow spectral width of the laser light. While spectral widths of up to 1 nm are achieved with normal laser diodes, DFB lasers achieve spectral widths of only 0.1 nm.
This high spectral purity is reflected in a lower chromatic dispersion within the monomode fiber, which in turn results in a significant increase in the bridgeable distances. For example, distances of 100 km can be bridged with DFB lasers using direct modulation, at data rates of 2.5 Gbit/s. At transmission rates of 10 Gbit/s, the distance is reduced to a few kilometers. With external modulation, in which the DFB laser operates with constant output light and the modulation is performed via a modulation unit connected between the DFB laser and the optical fiber, the speed values increase by a factor of about 10, since the external modulation improves the conditions for chromatic dispersion. The output power of DFB lasers ranges from 1 mW (0 dBm) to 10 mW.
DFB lasers are used in optical networks with CWDM and DWDM, but also in Gigabit Ethernet( GbE), 10 Gigabit Ethernet and 100 Gigabit Ethernet with bridgeable distances of up to 40 km, as in 100GBase-ER4.