A method of measuring an electromagnetic field in a formation can include installing an electromagnetic sensor with improved sensitivity, the sensor including multiple optical waveguides and respective multiple materials, and in response to exposure to the electromagnetic field, the materials changing shape in opposite directions. A well system can include an optical electromagnetic sensor which measures an electromagnetic field in a formation, and wherein optical path lengths or phases in optical waveguides of the sensor change both positively and negatively in response to exposure to the electromagnetic field. A method of monitoring a formation can include installing an optical electromagnetic sensor in a wellbore which penetrates the formation, and an optical path length/phase in an optical waveguide of the sensor increasing in response to exposure to the electromagnetic field, and an optical path length/phase in another optical waveguide of the sensor decreasing in response to exposure to the electromagnetic field.