Alternating current sensor with second-harmonic detection

We describe a novel optical sensor based on the Faraday effect for measurin g ac. The sensing element is a piece of material of high Verdet constant pl aced between polarizers. When the angle between the polarizers differs by 4 5 degrees, a signal of twice the frequency of the ac is generated-in additi on 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 pro portional to the amplitude of the current. Experimental results demonstrati ng the practicality of the procedure are presented. (C) 2000 Optical Societ y of America OCIS codes: 040.2840, 060.2370, 280.0280, 230.2240.