Abstract: A rotation rate measuring instrument for measuring the rotation rate of a fiber gyro having a light source, a series of beam splitters, a phase modulator, a coiled optical waveguide and an evaluating circuit provides a first and second light beam which traverse the coiled optical waveguide in opposite directions. At least one of the light beams is modulated at an angular frequency .omega.m in the phase modulator while at least one of the light beams is further modulated at a second frequency .omega.s. The frequencies are distinguishable in the evaluating circuit permitting the suppression of signal components caused by light reflected at the boundaries of the phase modulator.