Reverse sagnac effect in ring lasers

We report here the possibility of controlling the position of the standing-wave pattern in a ring-laser cavity at rest thanks to diffraction. Modulating the position of the aperture along the beam axis induces a beat frequency between the two counterpropagating waves in the motionless ring laser, the so-called reverse Sagnac effect. This effect permits us to observe rotation rates like the Earth’s one, several orders of magnitude smaller than the width of the locking region, by purely internal measurements.