An optical resonator is provided made of low-outgassing materials, including at least one chamber, a non-linear crystal arranged in the chamber, and an array of mirrors arranged in the chamber and including a plurality of mirrors for deflecting a light beam. To specify such a resonator which is low-outgassing and which ensures fine adjustment of the optical elements at the same time, the present invention proposes that the non-linear crystal and at least one mirror of the array of mirrors is arranged on one movable carrier each, wherein the said carrier is fabricated from a low-outgassing material and seals the chamber. Furthermore, a sealing system is provided including a housing, an optical element and a sealing element of indium or indium alloy, which is arranged between the housing and the optical element, wherein the optical element has a lateral surface and the sealing element is arranged on the lateral surface.