An isotropic earth field scalar magnetometer using optically pumped helium 4

Optical pumping was originally suggested by Kastler [I] in 1950 and since the early 1960's numerous helium magnetometers based upon that technique have been developed. Since then the main evolution has consisted in the advent in 1986 of tunable lasers suitable for helium optical pumping. They led to a renewed interest in helium magnetometry essentially because the magnetometer resolution is expected to significantly increase with the optical power. For airborne applications however, these improvements must be associated with a reduction of the anisotropy inherent to optically pumped helium sensors in order to be useful. Our group has therefore recently designed a new helium magnetometer structure which suppresses the sensor orientation effects. To achieve this we perform both the laser optical pumping and the RF resonance excitation in a fixed configuration whatever the sensor orientation with respect to the magnetic field to be measured is.This is performed by rotating simultaneously the laser linear polarization and the RF field direction to maintain them perpendicular to the static field. In this paper, we explain the principles of operation of such a magnetometer and describe this new sensor which drastically reduces the amplitude and frequency anisotropies of standard helium pumped resonance magnetometers. Finally, we present the preliminary sensor isotropy test results which show a perfect amplitude isotropy and frequency effects smaller than 150 pT.