For increasing the laser damage threshold (LDT) of diffraction gratings, particularly of multi-layer dielectric diffraction (MLD) gratings, the invention proposes a method for treating a diffraction grating to be used in a high energy laser apparatus having a first laser with the steps of providing the diffraction grating, providing a second treatment laser and irradiating the diffraction grating with laser light from the second treatment laser until the laser damage threshold of the diffraction grating has increased.Furthermore, the invention proposes a diffraction grating treated according to the method, as well as a laser system, which comprises such a diffraction grating.