The spacing of the lines in the grating determines the wavelength at which the feedback is strongest, and light amplification by stimulated emission concentrates emission at that wavelength. For a grating with line spacing D in a material with refractive index n, the peak wavelength l is
where m is the order of the grating, usually 1 or 2. That means that for a first-order grating in InGaAsP, with n=3.4, a grating period of 228 nm is needed to generate 1550 nm light.
Distributed feedback is used mostly in semiconductor diode lasers, where parallel lines etched in the active layer form a conventional diffraction grating (see figure). In fiber lasers, distributed feedback is produced by fabricating fiber Bragg gratings -- alternating regions of high and low refractive index perpendicular to the fiber axis -- in the optically pumped fiber. Note that distributed Bragg reflector (DBR) lasers are not considered DFB lasers because the feedback comes from gratings outside the gain region; DBR cavities are used for both diode and fiber lasers.
FIGURE. Distributed feedback laser. (From Jeff Hecht, Understanding Fiber Optics, 5th edition)
The big advantage of distributed feedback is its ability to stabilize lasers so they emit in a fixed-narrow range of frequencies, and DFB diode lasers were crucial for high-speed fiber-optic systems. A typical Fabry-Perot diode oscillates on multiple longitudinal modes spread across a few nanometers, but a temperature-stabilized DFB laser can prevent mode-hopping and limit oscillation to a single longitudinal mode with megahertz linewidth. DFB diode lasers provide the narrow linewidth essential for dense wavelength-division multiplexing (DWDM) and high-speed transmission. DFB lasers also can be tuned over limited ranges, and provide narrow-line emission for sensing and other demanding applications. can be used in other applications such as sensing.