Sunao Ichihara

Development of a Highly Sensitive Optically Pumped Atomic Magnetometer for Biomagnetic Field Measurements: A Phantom Study

We developed a highly sensitive optically pumped atomic magnetometer for measuring biomagnetic fields from the human body noninvasively. The sensor head was a cubic Pyrex glass cell containing potassium metal and buffer gases. A pump laser beam for spin-polarizing potassium atoms and a probe laser beam for detecting magneto-optical rotation crossed at right angles in the cell, which was heated in an oven to vaporize potassium atoms. The sensitivity of the magnetometer reached to 10-100 fT/Hz1/2 at frequencies below several hundred hertz. To test system performance, we made a phantom which models the human brain, taking into account the contribution of distributed electric currents. First, we tested the phantom by a 306-channel whole-head MEG system and confirmed good agreement of the measured field distributions with theoretical calculations. Subsequently, we measured magnetic field distribution with the phantom scanning two-dimensionally above the oven. The signal source location was estimated by least squares fitting to the measured distribution. The goodness of fit value between the measured and the theoretical distributions was 97.9%.

Noise reduction and signal-to-noise ratio improvement of atomic magnetometers with optical gradiometer configurations.

In the field of biomagnetic measurement, optically-pumped atomic magnetometers (OPAMs) have attracted significant attention. With the improvement of signal response and the reduction of sensor noise, the sensitivity of OPAMs is limited mainly by environmental magnetic noise. To reduce this magnetic noise, we developed the optical gradiometer, in which the differential output of two distinct measurement areas inside a glass cell was obtained directly via the magneto-optical rotation of one probe beam. When operating in appropriate conditions, the sensitivity was improved by the differential measurement of the optical gradiometer. In addition, measurements of the pseudo-magnetic noise and signal showed the improvement of the signal-to-noise ratio. These results demonstrate the feasibility of our optical gradiometer as an efficient method for reducing the magnetic noise.

Development of high-sensitivity portable optically pumped atomic magnetometer with orthogonal pump and probe laser beams

Optically pumped atomic magnetometers (OPAMs) have been demonstrated to show sensitivities better than the order of a few femtoteslas. In this study, we developed a portable potassium OPAM module using an electrically heated oven and orthogonal pump and probe beams coupled from polarization-maintaining optical fibers. The OPAM module shared a volume as small as 700 cm3, and its footprint was as small as 60 × 60 mm2; it operated with a single-channel sensitivity of 14 fTrms/Hz1/2 at 100 Hz when the 2-cm cubic potassium cell was heated to 180 °C. In our module, the optical beam path was folded along the magnetic field to be measured. The development of a magnetic sensor device comprising an array of OPAM modules will be an important step toward realizing a biomagnetic imaging system based on OPAMs. The orthogonal two-beam OPAMs will be beneficial for realizing a flexible placement of the array.

A plateau in the sensitivity of a compact optically pumped atomic magnetometer

In a compact optically pumped atomic magnetometer (OPAM), there is a plateau in the sensitivity where the dependence of the sensitivity on pumping power is small compared with that predicted by the uniform polarization model. The mechanism that generates this plateau was explained by numerical analysis. The distribution of spin polarization in the alkali metal cell of an OPAM was modeled using the Bloch equation incorporating a diffusion term and an equation for the attenuation of the pump beam. The model was well-fitted to the experimental results for a module with a cubic cell with 20 mm sides and pump and probe beams with 8 mm diameter. On the plateau, strong magnetic response was generated at the regions that were not illuminated directly by the intense pump beam, while at the same time spin polarization as large as 0.5 was maintained due to diffusion of the spin-polarized atoms. Thus, the sensitivity of the magnetometer monitored with a probe beam decreases only slightly with increasing pump beam int...

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.