We demonstrate an optical magnetometer based on a microfabricated 87Rb vapor cell in a micromachined silicon sensor head. The alkali atom density in the vapor cell is increased by heating the cell with light brought to the sensor through an optical fiber, and absorbed by colored filters attached to the cell windows. A second fiber-optically coupled beam optically pumps and interrogates the atoms. The magnetometer operates on 140 mW of heating power and achieves a sensitivity below 20 fT/√Hz throughout most of the frequency band from 15 Hz to 100 Hz. Such a sensor can measure magnetic fields from the human heart and brain.
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The brain field noise spectrum shown in Figure 3 B is an average of the magnetic field measured with 102 SQUID magnetometers in a helmet-shaped sensor array placed 18 mm from the scalp. Environmental interference in this data as suppressed with signal-space projection (SSP) prior to computing the spectra. We assume the signal originated from a current dipole field source situated in the range 2 cm to 5 cm below the scalp and calculate the expected increase in the measured signal for a sensor placed 2.5 mm above the scalp. This leads to an improvement by a factor of 2.2 to 4.8 in the signal strength at the closer location. The resulting enhanced brain signal is shown by the red hatched region A in Figure 3.