The acronym DFB stands for distributed feedback laser. Their key features relative to other semiconductor lasers are their single longitudinal mode (single frequency) emission profile, their high stability and their tunability. These single frequency laser diodes are used in applications such as fiber optic telecommunications, spectroscopy, metrology and atomic physics. This sub-category of semiconductor lasers employs Bragg diffraction gratings in the active gain region of the semiconductor to form a waveguide. These diffraction gratings are designed to deliver periodic changes in the refractive index which cause a reflection back into the cavity of the laser. Unlike Fabry-Perot laser diodes, which utilize mirrors at both ends of the laser cavity, the reflection of the laser light in a DFB Laser Diode is along the entire cavity. DFB semiconductor materials include indium-phosphide and gallium-indium-arsenide-antimony.