Abstract: An optical sensing apparatus for use in a ring laser interferometer measuring a physical quantity of an object includes a ring laser cavity formed by several internal mirrors, of which at least two internal mirrors are partly light-transmitting. The ring laser cavity generates two laser beams of an identical wavelength, the two laser beams propagating in counter direction to each other in the ring laser cavity. External mirrors are disposed at a distance from the ring laser cavity, whereby portions of the two laser beams, which pass through the internal mirrors and travel on optical paths toward the external mirrors, are reflected back into the ring laser cavity. The apparatus further includes at least one platform for mounting the external mirrors.