Magnetooptic Current Field Sensor with Sensitivity Independent of Verdet Constant and Light Intensity

A new type of ac current sensor utilizing the Faraday effect is proposed. A stabilized dc bias magnetic field H^b is applied to a Faraday element placed between a crossed Nicols pair of polarizers. The amplitudes of the fundamental signal I¿ and second harmonic signal I2¿ induced by the ac field H^x in the intensity of light emerging from the analyzer are phase-detected. The ratio R=I2¿/I¿ gives the output signal. When the Faraday rotation angles a (¿H^x) and ¿ (¿H^b are so small that such higher terms as a² and ¿² can be neglected, we obtain R=H^x/4H^b, which is independent not only of the light intensity, but of the Verdet constant of the Faraday element as well. Experiments performed at 20 to 100°C on FR-5 glass and Bi-substituted YIG Faraday elements corroborated this prediction. The effect due to the nonlinear terms a² and ¿² is calculated, and found to agree with our experimental results.