Alternating current sensor with second-harmonic detection

We describe a novel optical sensor based on the Faraday effect for measuring ac. The sensing element is a piece of material of high Verdet constant placed between polarizers. When the angle between the polarizers differs by 45°, a signal of twice the frequency of the ac is generated-in addition to the usual signal with the same frequency as the current. The quotient of these two signals allows a normalized output to be obtained that is proportional to the amplitude of the current. Experimental results demonstrating the practicality of the procedure are presented.