The invention relates to an optical device for focusing a laser beam, comprising a source of emission of a laser beam, for example a substantially elliptic and astigmatic laser beam, a focusing lens of the laser beam and first aperture adapted to select a central portion of the laser beam. The first aperture is directly applied on the focusing lens, and comprises a coating made of a substantially opaque material which are applied (for example, by spraying, sputtering, evaporation, printing, painting) on a peripheral portion of a front surface of the focusing lens so as to allow the propagation of the central portion of the laser beam and obstruct the propagation of a surrounding portion of beam. The device of the invention is extremely simple from the constructive point of view, and is inexpensive and small-sized.