The invention is directed to an apparatus and a method for reducing noise, in particular RIN noise, in optical sensors, in particular fiber-optic sensors. The optical sensor produces a sensor output signal representing a physical quantity to be measured and also contains a first noise component, which is detected by a first detector. A separate detector detects an input light noise component of the input light source of the optical sensor and produces a second noise component. The second noise component is electronically processed and time-delayed or phase shifted by a time delay which substantially correspond to the time delay experienced by the sensor input light traversing the optical sensor. The time-delayed processed signal is then subtracted from the sensor output signal to produce a reduced noise optical sensor signal corresponding to the measured physical quantity.