Differential Measurement With the Balanced Response of Optically Pumped Atomic Magnetometers

Highly sensitive magnetic sensors for biomagnetic measurements are often used in gradiometers to suppress the magnetic noise from the environment. We propose a differential measurement method in optically pumped atomic magnetometers, where the frequency responses from the two magnetometers are balanced separately and the signals are subtracted. A magnetometer system that has two pairs of pump and probe beams was constructed to confirm the noise reduction experimentally. A general form for the transfer function of the magnetometer was derived and the parameters were experimentally fitted for the two magnetometers. The frequency responses were balanced with the inverse of the transfer function in the frequency domain, and the differential measurement improved the signal-to-noise ratio of the magnetometer system, even when the resonance frequency at the two sensing volumes differed by several hertz. This method considerably relaxes the requirement for the uniformity of the magnetic bias field at the two sensing volumes, which previously needed to be identical.