Abstract: The cavity of the invention has a pumping source, a ring cavity, a planar convex lens, an optical gain medium, and/or a nonlinear medium. The laser cavity is formed by two mirrors with the same curvature. The curving surfaces of the two mirrors are coupled by a manner of face to face, whereby a space is defined as the ring cavity. The planar-convex lens is located between the pump beam and the ring cavity to work together with the two mirrors of the ring cavity to focus the pump beam. The pumping light enters the ring cavity at a desired distance of d mm from the mirror axis. The laser light also enters the ring cavity in a propagating direction nearly parallel to the mirror axis. Due to the two curving surfaces of the two mirrors, the light beam is reflected back and forth to form the 3-D figure-“8” light path. The optical gain medium and the nonlinear medium are located between the two mirrors on the light path.