Spectral resolution enhancement without increasing the number of grooves in grating-based spectrometers

A simple scheme of an optical setup to enhance the spectral resolution of diffraction gratings without increasing the number of grooves is presented. A plane-parallel mirror pair, which consists of a 100% reflectivity mirror and a 50% reflectivity mirror, is placed upon a grating. For certain values of the mirror gap and the angle between the grating and the mirrors, the effective size of the grating doubles, as does the resolution. Using a grating of 830 grooves/mm and a narrow-band diode laser that is tunable around 773 nm, we experimentally demonstrated an enhancement of the resolution by a factor of 2.