The disclosure relates to an amplifying optical cavity of the Fabry-Perot type that can be used in combination with a high-rate picosecond pumped laser for generating monochromatic X-rays. The disclosure relates to an amplifying optical cavity of the Fabry-Perot type that can be used for obtaining a strongly focused pumped laser beam having a high stability at the average power PMOY. The disclosure more particularly relates to an amplifying optical cavity of the Fabry-Perot type for generating monochromatic X-rays by the Compton reaction between a high-rate picosecond pumped laser beam and a synchronised electron beam, the cavity including a closed enclosure that can be placed under a vacuum and through which extends an electron beam tube, the enclosure including a laser beam input means, a means for maintaining and positioning two planar optical reflectors, and a means for maintaining and positioning two spherical optical reflectors capable of focusing the laser beam at an interaction point with the electron beam. The means for maintaining and positioning the optical reflectors are arranged so that said optical reflectors substantially define the vertexes of a tetrahedron.