a wavelength selector for use in a laser cavity comprising a diffraction grating mounted at an angle near grazing incidence with respect to the beam travelling away from the excited medium and a reflector which reflects the beam diffracted by the grating back along its incidence path . wavelength tuning is accomplished by rorating this reflector , while the grating remains fixed . rotation of the grating provides linewidth variation . field of the invention the present invention relates generally to a new improvement in mounting a diffraction grating in optical systems , and more particularly to the applicability of this improvement for use in a laser cavity . the invention is envisaged as being applicable to dye lasers , high - pressure gas lasers , semiconductor diode lasers , and in general any type of laser which may be frequency - tuned by an external cavity . it is also applicable , mutatis mutandis , for use in spectrometers and parametric devices .