\"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 ..