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Recently developed ultrasensitive optically pumped atomic magnetometers are promising for biomagnetic measurements such as magnetoencephalograms and magnetocardiograms. The magnetometer's bandwidth and amplitude of signal response are important factors for biomagnetic measurements. These factors depend on various operating parameters such as power density and wavelengths of laser beams. By focusing on the transverse spin relaxation time, we theoretically and experimentally studied the change in the bandwidth and amplitude of the signal response. By considering the effect of the attenuation of a pump beam inside a cell, we showed good agreement between theoretical and experimental results. Furthermore, the magnetometer's integrated performances for the factors were evaluated by changing the power density of the pump beam. Measured data indicated that the bandwidth of signal response depended on the power density of the pump beam and that the bandwidth could be tuned to a desirable frequency range.

Optically pumped alkali-metal atomic magnetometers are expected to be used not only for biomagnetic field measurements but also for magnetic resonance imaging because of their potential ultrahigh sensitivity. Here, we studied magnetic field mapping and biaxial vector operation using atomic magnetometers. A potassium atomic magnetometer was used in these measurements. First, we obtained sensor output signals by solving the Bloch equation. Next, we measured magnetic field distributions generated by a current dipole electrode that was placed in a spherical phantom, which simulated a group of simultaneously activated neurons in the human brain. We obtained vector contour maps of the magnetic field distributions from the dipoles oriented parallel and orthogonal to the pump laser beam and have found good agreement with theoretical magnetic field distributions. These results demonstrate practical applications of magnetic field mapping and biaxial vector operation using optically pumped atomic magnetometers.