Abstract
A new type of magnetic field sensor which measures ac electric current utilizing Faraday rotation is proposed. On a Faraday element a stabilized dc bias magnetic field Hb is applied. Output signal is given by the ratio R = I2ω/Iω, where Iω and I2ω are fundamental and second harmonic signals generated by the field Hx which the ac current induces. Since R is calculated as R = Hx/4Hb, we can measure Hx independent not only of light intensity but also of the Verdet constant (and therefore its temperature dependence) of the Faraday element. This was supported by experiments performed at 20–100 °C for HeNe laser (λ=0.633 and 1.15 μm) light on Faraday elements of FR-5 paramagnetic glass and Bi-substituted YIG crystal. The bias field Hb was generated by a stabilized dc current in a coil and using a SmGdCo plastic magnet with very weak temperature dependence of remanent magnetization (<1% at T=20–120 °C). According to theory the equation holds even when broad spectrum polychromatic light is used. This was confirmed by experiments performed with a halogen-lamp white light transmitted through an optical fiber. Calculated signal-to-noise ratio of the sensor with a polychromatic light agreed with experiments.
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