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Differential magnetometry based on a diverging laser
beam
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To date, sensitive DC magnetometry applications have
relied on Superconducting Quantum Interference Devices (SQUIDs), which
are large, expensive and require cryogenic cooling. The market for a
magnetometer of comparable sensitivity (~ fT/√Hz), chip-scale size
(< 1cm3), low power (~ mW) and low cost (~ few dollars)
would be vast, with applications ranging from mapping the magnetic fields
of the heart and brain to predicting earthquakes to measuring magnetic
fields in space.
The MEMS technology used to create a chip-scale
atomic clock (CSAC) can be readily adapted to create a chip-scale
atomic magnetometer (CSAM), by measuring magnetically sensitive
rather than insensitive atomic transitions in the S1/2 ground
state of Rb or Cs. Several different CSAM designs have been realized at
NIST [1,2] but all use optical pumping and probing to make a measurement
of the Larmor spin precession frequency, w0
= 2pgB0, where B0
is the field to be measured and g
is the gyromagnetic ratio of the atom.
To achieve a sensitivity comparable to a SQUID, all
noise sources in the CSAM must be minimized. In this paper we present a
new and very simple design for differential detection of the magnetometer
signal, enabling common-mode noise rejection to be achieved in a
chip-scale package.
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A schematic diagram of the
operation of the differential magnetometer. The average direction of
the pump beam defines the z-axis. B0 is the field to be
measured, and lies along z (q=0) for optimal differential signal.
Mosc is the spin component precessing around B0.
The component of Mosc lying along the each probe beam,
modulates the absorption properties of the cell and hence the intensity
reaching each photodiode. The phase of the intensity modulation is
determined by the sign of q - ai, so for
the configuration shown above the oscillating signals on photodiodes 1
and 2 are 180°out of phase.
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The design uses a single uncollimated beam from a
Vertical-Cavity Surface-Emitting diode Laser (VCSEL) to both optically
pump the atoms and to act as multiple probe beams. The simplicity of the
optical design, which doesn’t require beamsplitters, mirrors or
lenses, enables us to demonstrate this device in a physics package volume
of less than 1cm3. Operating the magnetometer in differential
mode gives a sensitivity of 28pT/√Hz, an improvement of 26 over
single channel operation.
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Noise on the magnetometer
signal as a function of frequency. (a) Photodiode 1 only, (b)
Photodiode 2 only, (c) Differential signal (d) Technical and
fundamental noise limit - measured electronic noise (including johnson
noise) with calculated photon shot noise added in quadrature. A
measurement of the noise on each channel around 100 Hz is taken and
displayed on the right hand axis.
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References:
E. Hodby, E.A. Donley and J. Kitching, "Differential
magnetometry based on a diverging
laser beam," submitted.
[1] P. D. D. Schwindt, S. Knappe, V. Shah, L. Hollberg, J. Kitching,
L. Liew and J. Moreland, "Chip-scale
atomic magnetometer," Appl. Phys. Lett. 85, 6409,
2004.
[2] P. D. D. Schwindt, B. Lindseth, S. Knappe,
V. Shah and J. Kitching, "A chip-scale
atomic magnetometer with improved sensitivity using the Mx technique,"
Appl. Phys. Lett.
90, 081102, 2007.
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