\"DFB\" here means distributed feedback.

DFB laser diode is an improved version of more conventional FP laser diode. The difference of a DFB laser is it has a periodically structured diffractive grating written in its active layer.

The Bragg grating is written within the laser cavity. Hence it forms a feedback \"mirror\" of the cavity. Unlike FP laser cavities, where a number of longitudinal modes are supported by the mirrors, grating only reflects one single longitudinal mode back into the cavity. So, DFB laser diode has single wavelength laser output.

Temperature is used to tune the wavelength of DFB lasers, because the Bragg wavelength of the grating changes with temperature. Driving current can also alter the center wavelength because of the temperature effects and index changes associated with change of current.

The typical linewidth of a DFB laser diode is several megahertz. Introduction of phase shift (1/4 phase shift at the center of the grating) can greatly reduce the linewidth to kilohertz level. Very narrow linewidth laser diodes have applications in coherent optical fiber communication systems and coherent optical fiber sensor systems.

The linewidth of the laser diode can be measured using self-heterodyne detection technique.