In a fibre-optic Sagnac interferometer, in particular for determining rotational speeds (fibre gyro), having an optical directional coupler (3) which is located in the light path from a light source (1) and whose at least one output is connected via a spatial filter (6) and a polarization filter (7) to a beam splitter (8) which, on the one hand, splits the light transmitted by the polarization filter (7) into two component light beams which are irradiated into the two ends of an optical fibre (9), it being the case, on the other hand, that the component light beams returning after traversing the optical fibre (9) are reunited in an interfering fashion and applied to a light-sensitive receiver (5), it is provided for the purpose of reducing the amplitude error, that is to say, the polarization ratio of the light upstream of the polarization filter (7), to produce the spatial filter (6) from monomode fibre. It is possible, in particular, as a result to save substantial costs in the case of the optical directional coupler (3), which can now be fabricated from simple monomode fibre.