EGAS41 - Swansea University

41 st EGAS CP 88 Gdańsk 2009
Noise reduction in optically pumped magnetometer assemblies
V. Schultze ∗ , R. IJsselsteijn, H.-G. Meyer
Institute of Photonic Technology, Dept. of Quantum Detection, Albert-Einstein-Str. 9,
D-07745 Jena, Germany
∗ Corresponding author: volkmar.schultze@ipht-jena.de
The ultimate sensitivity of an optically pumped magnetometer is limited by the shot
noise of the photo receiver. In real measurement setups this shot noise limitation can not
be reached, however. Noise contributions of the light source, the magnetic field and the
signal electronics are superimposed. Whereas the latter should easily be cancelled, the
other ones degrade the noise limited magnetic field resolution drastically in practice.
We look for methods to decrease the extra noise sources in order to approach the shot
noise limit. The basic concept is to use gradiometer configurations. The subtraction of
the signals of two adjacent magnetometers is not enough, however. Because of frequency
mixing at the Larmor frequencies only direct noise contributions of the homogeneous
magnetic field can be cancelled this way. Therefore the fluctuations themselves need
to be reduced wherever possible. The pump laser frequency fluctuations, converted to
amplitude fluctuations by the alkali vapour absorption, indeed could be reduced drastically
by a subtraction of the photo diode signals of the two magnetometer channels, prior to
the signal processing with the lock-in amplifiers. So, each of the lock-ins has to select
its own Larmor frequency from the now common signal. This demands a magnetic field
gradient between the two magnetometer cells, which has to be assured somehow.
In our investigations with two Cs cells from the University of Fribourg, Switzerland,
we use an M x measurement configuration. Homogeneous and gradient magnetic fields are
generated in a large shielding barrel in order to get rid of environmental disturbances.
Inserting the magnetometer cells into the laser beams enhances the photo current noise
by one order of magnitude as compared to the direct illumination without Cs vapour
(where it was on shot noise level). In the result of the above sketched photo current
subtraction this excess noise could be reduced again by a factor ample of about four.
This directly reflects in the white noise levels of the magnetic field signals (measured
in a field of 5 µT). The noise of the single magnetometers (∼700 fT/ √ Hz) reduces to
about 200 fT/ √ Hz. The remaining difference to the intrinsic noise of 60 fT/ √ Hz will at
least partially be due to the noise of the magnetic field sources. The common gradient
measurement, realized by the subtraction of the two magnetometer signals, additionally
results in a drastic reduction of the low frequency magnetic field noise.
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