Systems and methods related to optical current and voltage sensors and, more particularly, to filters for use in such sensors. A fiber optic current sensor comprising: a light source; a polarization beam splitter connected to said light source having a reciprocal port and a non-reciprocal port; a Faraday rotator connected to said polarization beam splitter; a first quarter-wave plate connected to said Faraday rotator; a polarization maintaining fiber connected to said first quarter-wave plate; a second quarter-wave plate connected to said polarization maintaining fiber; a sensing fiber connected to said second quarter-wave plate; a detector connected to said polarization beam splitter via said non-reciprocal port and having an output; and an adaptive filter for filtering said output.