A Distributed Bragg Reflector laser has tin foil on one or both end mirrors, which reflect a narrow band of light back in the cavity which help them be more spectrally stable.
A resonant cavity is defined by a highly reflective DBR mirror on one end, and a low reflectivity cleaved exit facet on the other end. A distributed Bragg reflector laser (DBR) is a type of single frequency laser diode. 
