Abstract: An unstable optical cavity for laser beams, having at least 3 deflection means for making a beam created in the cavity undergo at least three changes in direction so to form an optical ring path, and an optical magnification set disposed among the deflecting means. The at least 3 deflection means are prisms, each of which has an air-glass reflecting interface that provides one of the changes in direction by total reflection. A first of the at least 3 prims is positioned at an output of the cavity and has an air-glass reflecting interface with an insufficient size for a total reflection of the beam in the cavity, and remaining ones of the prisms have an air-glass reflecting interface with a sufficient size for a total reflection of the beam in the cavity.