All-Optical Nonzero-Field Vector Magnetic Sensor for Magnetoencephalography

We present the concept and the results of an investigation of an all-optical vector-magnetic-field sensor scheme developed for biological applications such as nonzero-field magnetoencephalography and magnetocardiography. The scheme differs from the classical two-beam Bell-Bloom scheme in that the detecting laser beam is split into two beams, which are introduced into the cell in orthogonal directions, and the ratio of the amplitudes of the magnetic resonance signals in these beams and their phase difference are measured; strong optical pumping from the lower hyperfine level of the ground state ensures the resonance line narrowing, and detection in two beams is carried out in balanced schemes by measuring the beam-polarization rotation. The proposed sensor is compact, resistant to variations of parameters of laser radiation and highly sensitive to the angle of deflection of the magnetic field vector-with an estimated scalar sensitivity of the order of 16 fT/Hz1/2 in the 8 x 8 x 8 mm3 cell, an angular sensitivity of 4 x 10-7 rad, or 0.08", was demonstrated.