Novel nulling/balancing technique for fiber optic magnetic gradiometer

A technique for balancing a fiber-optic magnetic gradiometer configured as a Mach-Zehnder interferometer is presented. The method uses different dither frequencies for nulling the DC field experienced by each transducer and a third frequency common to both transducers for detecting the gradient. The method overcomes the hysteresis when each transducer is separately nulled, and the balance is performable in real time. Noise spectra are shown in the near DC and a gradient sensitivity of 0.14 gamma /ft- square root Hz RMS at 0.22 Hz is demonstrated. This is believed to be the most sensitive direct measurement of a magnetic field gradient by a fiber-optic sensor that has been reported.<>