A type of diffraction grating comprising grooves ruled into a surface that can be either plane or concave. In a distributed feedback laser (DFB laser), a diffraction grating with a concave surface serves to focus light without affecting the spectra. This approach is much more effective than the mirror technique employed with a Fabry-Perot laser and vertical cavity surface-emitting laser (VCSEL). See also DFB laser, diffraction, diffraction grating, Fabry-Perot laser, and VCSEL.