A debris monitor apparatus and method using the Faraday Effect to detect accumulation of debris on a magnetic probe is disclosed. The probe (10) contains a magnet (16) and is immersible in the fluid. As debris accumulates on the probe, the magnetic field changes. An optical sensor including magneto-optic material (22) is located within a portion of the changeable magnetic field, preferably near a node of the magnet. Optical fibers (28,30) are provided to bi-directionally address the magneto-optic material (22) with linearly polarized light. Rotation of the light polarization angle is converted into a modulation of the light's intensity. An electro-optic transducer converts the light intensity into electrical signals that indicate the quantity of debris accumulation on the probe, with a differentiated value of these signals with respect to time yielding rate of debris accumulation on the probe. Means are provided to calculate a ratio defined by the difference over sum of two bi-directionally transmitted pulses to provide a self-referenced output dependent only on the change in magnetic field caused by accumulation of debris on the probe. The changed magnetic field provides an inherent real-time memory of the accumulation of debris on the probe, which also permits multiplexed data collection.