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Recently, optically pumped atomic magnetometers (OPAMs) replacing superconductive quantum interference device-based magnetometers have attracted a considerable amount of attention. To apply the OPAMs to biomagnetic field measurements, simultaneous multilocation measurements are required. To fulfill the specific requirement, we have developed a hybrid cell, which contains potassium and rubidium atoms. The OPAM with a hybrid cell can enhance the spatial homogeneity due to the suppression of pump-beam absorption. Thus, we can have multiple measurement locations inside a hybrid cell with multiple pump and probe lasers. In this paper, we proposed a novel method to accomplish simultaneous multilocation measurements based on pump-beam modulation. Using two optical choppers, we operated a two-channel OPAM with two modulated pump beams. We investigated the signal strength of the OPAM as a function of modulation frequency and found that the signal strength decreased as the modulation frequency increased. Subsequently, we measured the magnetic field distributions generated from a loop coil to validate the proposed method and could confirm that they showed good agreements with the theoretical values. These results demonstrate the feasibility of the proposed method based on pump-beam modulation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

We measured the spatial homogeneity of spin polarization, which is directly related to the sensitivity and magnetic linewidth of an optically pumped atomic magnetometer with a hybrid cell of K and Rb atoms. This was done by changing the position of the probe beam. Furthermore, for comparison, we also measured the sensitivity and magnetic linewidth of atomic magnetometers with single K and single Rb cells, and found that optically thick atoms were spin polarized homogeneously with the hybrid cell. Optically thin alkali metal vapor can be spin polarized homogeneously, and the spin polarization transfers to an optically thick alkali metal vapor. An atomic magnetometer with the hybrid cell was found to be more effective in realizing simultaneous signal measurements at different locations with the uniform sensitivity and magnetic linewidth. [ABSTRACT FROM AUTHOR]

An optically pumped atomic magnetometer using a hybrid cell of potassium and rubidium atoms was demonstrated to yield high sensitivity to magnetic fields. We operated the magnetometer with the four possible combinations of optically pumped and optically probed atoms and found that the combination of optically pumped potassium and optically probed rubidium showed the highest sensitivity among the four combinations because the rubidium atoms were denser than those of potassium. Furthermore, we investigated the dependence of the sensitivity on the power densities of the pump and probe beams and the wavelength of the probe beam. The magnetometer using the hybrid cell required higher pump-beam power and had narrower magnetic linewidth than those of the single alkali-metal cell. However, the magnetic linewidth was larger than the theoretical value, ignoring the spin relaxation caused by the spin-exchange collisions. By adjusting the laser conditions, the highest sensitivity approached 30 fTrms/\rm Hz^1/2. [ABSTRACT FROM AUTHOR]

Presents information on a study that proposed an extended Learning Vector Quantization (LVQ) neural networks (NN) to land cover mapping. Description of the LVQ NN; Experimental results of the study; Conclusions.

We have developed a K-Rb hybrid optically pumped atomic magnetometer (OPAM) for biomagnetic measurements. With this hybrid OPAM, we used a linear photodiode array and a charge-coupled device (CCD) sensor to instantaneously measure 1-D and 2-D magnetic field distributions generated by a test coil, respectively. The measured distributions were compared with those calculated using the Biot–Savart law and showed good agreement for the linear photodiode array; however, in the case of the CCD sensor in the area away from the test coil, the intensity of the detected magnetic field was slightly different from the calculated one, possibly because of the diffusion of the spin-polarized atoms as well as smear and blooming in the CCD sensor. The sensitivity of the OPAM was 5–6 pT/Hz ^1/2 using the linear photodiode array and $\sim 10$ pT/Hz ^1/2 using the CCD sensor. In future experiments, we plan to fabricate a sensor cell with a high density of probe atoms and reduce the noise generated in the electronic circuitry of the detectors in order to increase sensitivity. [ABSTRACT FROM AUTHOR]