The present invention relates to a laser including a laser amplifier and a laser resonator. The laser amplifier includes a disk-shaped active medium having a reflector. The active medium rests on a cooling means for cooling said active medium and said reflector. The reflector embodies a mirror of said laser resonator. The laser amplifier includes an optical means for refocusing a pumping light configured for repeatedly deviating said pumping light and refocusing said pumping light onto the active medium for causing a multiple passage of said pumping light through said active medium. Such a thin-disk laser is not prone to thermal lensing. According to the invention, the laser resonator is formed as a ring resonator. Two optical waves can travel around within the ring resonator in different directions. An optical selection means is provided for giving preference to propagation of one of the both optical waves traveling around that can be coupled out as an output laser beam. The laser according to the invention is highly stable and can be adjusted and tuned easily. Intra-cavity frequency-multiplication can be achieved easily.