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Improvement of the Spatial Amplitude Isotropy of a 4He Magnetometer Using a Modulated Pumping Beam
B. Chéron, H. Gilles, J. Hamel, O. Moreau, E. Noël
To cite this version:
B. Chéron, H. Gilles, J. Hamel, O. Moreau, E. Noël. Improvement of the Spatial Amplitude Isotropy of a 4He Magnetometer Using a Modulated Pumping Beam. Journal de Physique III, EDP Sciences, 1997, 7 (8), pp.1735-1740. �10.1051/jp3:1997215�. �jpa-00249677�
Improvement of the Spatial Amplitude Isotropy of a ~He Magnetometer Using a Modulated Pumping Beam
B. Chbron (~,~), H. Gilles (~'*), J. Hamel (~), O. Moreau (~) and E. NoAl (~)
(~) Laboratoire de Spectroscopie Atomique, ISMRA, 14050 Caen cedex, France (~) Universitd de Caen, UFR de Sciences, 14032 Caen cedex, France
(Received 24 January1997, revised 5 May 1997, accepted 7 May 1997)
PACS 32 80.Bx Level crossing and optical pumping PACS 67.65.+z Spin-polar12ed hydrogen and helium
PACS 07 55.Ge Magnetometers for magnetic field measurements
Abstract. Optically pumped magnetometers are scalar magnetometers. Contrary to vecto-
riel magnetometers, they measure the total magnetic field whatever the direction of the sensor
However, for some orientations of the magnetometer with respect to the magnetic field direc- tion, the resonant signal vanishes and the measurement is impossible. In this paper we present a
simple solution to reduce the amplitude spatial anisotropy and apply it to a ~He magnetometer developed in our Laboratory.
R4sum4. Les magndtomAtres h pompage optique sont des magndtomAtres scalaires Contrai~
rement aux magn4tomAtres vectoriels, its mesurent le module du champ magndtique quelle que
soit l'orientation du capteur dans l'espace Cependant, pour certames orientations du magndto~
mAtre par rapport h la direction du champ h mesurer, l'amplitude du signal de rdsonance s'annule
et la mesure devient impossible. Dans cet article, nous prdsentons une solution simple pour rd~
duire l'anisotropie spatiale d'amphtude et nous l'apphquons h un magn4tomAtre h h41ium~4 ddveloppd dans notre Laboratoire.
Optically pumped magnetometers are known from the 60s Iii. They are based on the Zeeman effect of fundamental or excited state of atoms such as rubidium, caasium or helium. They
measure the total magnetic field whatever the orientation of the sensor, while most magne-
tometers (for example SQUID) measure only a component of the magnetic field along one axis.
This point is essential for magnetic anomalies detection and ocean observation. However, if the optically pumped magnetometer is not properly oriented with respect to the magnetic field
direction, the resonance signal can vanish and the measurement is impossible. This represents
a serious limitation to the use of such a sensor for mobile application (in particular for airborne
application).
In this paper we present a very simple solution to reduce the amplitude spatial anisotropy and apply it to a ~He optically pumped magnetometer developed in our Laboratory at ISMRA
Caen.
(* Author for correspondence (e-mail: gillestispalp255.ismra.fr)
@ Les iditions de Physique 1997
1736 JOURNAL DE PHYSIQUE III N°8
Fig 1 Geometrical parameters (a, @) definition for the spatial amplitude dependence. E is the
electric field of the linearly polarized beam.
Let us first recall briefly the principle of optically pumped magnetometers (especially ~He magnetometer) and discuss their amplitude spatial anisotropy ~He atoms at a pressure of a few Torrs are submitted to a weak H F. discharge to populate the metastable 2~Si state. Optical pumping between 2~Si state and 2~Po state at 1
= 1.083 /Jm modifies the repartition of the atoms within the three sublevels of2~Si and creates an electronic orientation or alignment of the metastable state. A driven spin precession is then induced either by applying a radiofrequency magnetic field (it is the traditional method [2j or by modulating the pumping light (intensity [3,4j, polarisation [5j or frequency modulation [6j). The resonance is obtained when the RF field frequency or the modulation frequency F is equal to or twice the Larmor frequency FL of helium atoms in a given static magnetic field The resonance appears as a modification of either the average intensity of the pumping beam transmitted through the cell containing
helium atoms or the modulation of the transmitted light intensity at frequency F and other harmonics 2F...
As shown below, the spatial amplitude isotropy depends upon the technique used to induce
resonance. while the sensitivity is nearly independent (about 1 pTllfi).
The spatial amplitude isotropy has already been studied in the past (see for example Ref. [7j).
Among the great variety of magnetic resonance signals, we restrict our study to a few cases presenting some interest m magnetometry. In particular we examine only the case of optical pumping with linearly polarized light (resonances obtained with a circularly polarized light are
very sensitive to light shifts [8j which induce resonance frequency shifts) When the polarization
of the pumping beam is linear, the angle fl between the polarization vector of the beam and the magnetic field direction is sufficient to describe the spatial amplitude dependence. Let us define fl from Figure 1. The orientation of the magnetic probe with respect to the magnetic field direction is defined from the angles a and fl. a is the angle between the probe axis (which
coincides with the pumping beam axis u) and the magnetic field B. u, ui and u2 are mutually orthogonal unitary vectors with ui lying in the plane defined by B and the probe axis. From
Figure 1, we deduce: cos 9
= sin a cos fl.
The study of resonance signals from the well known equations of optical pumping [9j shows that the signal amplitude presents one or several extinctions whativer the magnetic
resonance
technique. For example Figure 2 displays the spatial isotropy depejdence for a signal obtained with the double resonance technique It represents the evolution of the signal amplitude,
excited at the Larmor frequency FL and detected at 2FL, in polar ~coordinates with respect to
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~o 0
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~ J
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270
Fig. 2. Amplitude in polar coordinates of a resonance signal obtained with the double magnetic
resonance technique versus the angle fl defined in Figure 1. o
= 90°
a = 45°
a = 0°. The angle (BRF, B) is equal to 90°.
the angle fl when a
= 0°, 45° and 90°. The signal isotropy is described by the geometrical
factor 11 3sin~
o cos~fl) ii sin~ a cos~fl). This signal presents several extinctions For these orientations of the probe, the magnetic field value can not be measured. In the case of the double magnetic resonance technique, it is important to note that another parameter has to be adjusted: the resonance signal amplitude vanishes when (BRF) is parallel to the static field
(B) and is maximum when (BRF) is perpendicular to (B) (Fig. 2 is drawn assuming BRF
perpendicular to B).
Figure 3,shows the evolution for a signal obtained with the frequency modulated pumping beam technique when the modulation frequency is equal to the Larmor frequency and the
detection is made at null frequency. The signal amplitude can be written as follows:
ii sm~ a cos~fl)~. II)
It vanishes when a
=
90° and fl = 0°.
To our knowledge. two methods have already been tested to reduce the spatial amplitude anisotropy The first one uses a magnetometer with three cells disposed orthogonally [10j.
In fact three independent magnetometers are employed and at least one cell gives a resonant
signal whatever the orientation is. In the second one the resonant signal obtained with the double magnetic resonance technique presents the spatial anisotropy of Figure 2 In order to avoid signal extinctions, the angle fl between the polarization vector of the pumping beam and the magnetic field direction is maintained at 90° by adjusting and controlling the laser polarization with an amagnetic piezoelectric motor rotating a linear polarizer Ill] The same motor rotates the RF coils in order to maintain their axis parallel to the polarizer axis and to
adjust the angle (BRF, B) to 90°.
The solution proposed m this paper is suitable only for the ~He magnetometer based on modulated pumping beam techniques. The synoptic diagram of this solution is illustrated in
Figure 4 (in this figure the angle a equals 90°). The pumping light comes out of a laser diode tuned on the (2~Si 2~Po) ~He transition. The current intensity of the diode is modulated at
frequency F (F
= 1.3MHz) in order to modulate (at the same frequency F) the frequency of the emitted light. The pumping beam is separated into two parallel pumping beams orthogonally
1738 JOURNAL DE PHYSIQUE III N°8
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," "'
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W 0
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/
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270
Fig. 3. Amplitude in polar coordinates of a resonance signal obtained with
a frequency or intensity modulated pumping beam technique versus the angle fl a
= 90°; a
=
45°; a =
0°.
~~
LD ~
~
BS i L
-j P
(~ ~~~
~
~'~He
G
D
Fig. 4. Synoptic diagram for an isotropic magnetometer- LD laser diode; G: sinus generator mod-
ulating the diode current; P. photodiode; D. Electronic detection, L: lens; BS polarizing beamsplitter;
Ptri total reflexion prism Ei is the electric field of the beam n°1. k2
is the electric field of the beam n°2
polarized. The two beams of1 cm diameter and same optical power are distant from 1.2 cm
and do not cross each other. They propagate through a Pyrex cell containing ~He atoms and
placed in a static magnetic field (B
= 46 /JT). After the cell, they are focused with the same lens on the same photodiode. Each beam gives its own resonance signal respectively noted Si
and 52. In focusing the two transmitted beams on the same photodiode, we obtain the sum S = Si + 52. An electronic detection analyses the resonance signal S when F is swept around the Larmor frequency. We note fl the angle between the polarization vector of the beam n°I and the unitary vector ui (Fig. 1). The total resonant signal amplitude is given by the half sum of expression (1) taken with fl for beam n°1 and the same expression (1) taken with fl + 90°
90
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,' ',
, '
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'
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, I
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I j
I j
I j
' /
I
' /
' '
' /
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' '
' '
' '
"
'
27Q
Fig. 5 Amplitude in polar coordinates of the resonance signal versus the angle fl between the
electric field of the linearly polarized beam ii°I and the magnetic field B. o
= 90°;
a = 45° a
= 0° (m). experiment.
for beam n°2:
Ala, fl) = 0.5[(1 sin~ a cos~fl)~ + II sin~
a cos~(fl + 90))~j. (2)
Both beams are well separated in the cell in order to avoid interferences effects between them.
We also checked that the effect of diffusion of atoms from one beam to the other is negligible.
The following complementary experiment has been done under the geometrical conditions:
a = 90° and fl
= 45°. If one of the beam (for example beam n°1) is masked in front of or behind the cell, no modification of the resonant signal amplitude is observed on the other beam
(beam n°2). This result shows that the resonant signal is the sum Si + 52 when the two beams act simultaneously on the cell.
The comparison between calculation (Eq (2)) and experiment is presented m Figure 5.
The experimental observations are in good agreement with the predictions. Whatever the orientation of the sensor with respect to the magnetic field direction, one can make the magnetic field measurement with a very good signal to noise ratio. The sensitivity varies from 1 pT lift
when a
= 0° to 4 pTllfi in the worst condition (a
= 90°, fl
= 45°). It is worth noting that the technique used in reference Ill] provides a constant sensitivity (1 pTllfi) regardless of the probe orientation However, like other techniques using several beams, our solution may
exhibit a spatial frequency anisotropy if the magnetic field to be measured is not uniform.
Without artificial perturbation, the mean natural magnetic field gradient value is close to 200
pT/m. A separation between the two beams of1.2 cm should induce a spatial frequency anisotropy of 2.4 PT. Of course, in the case of an airborn application, this value may be much higher due to the magnetism of the vehicle.
This paper shows that the realization of an isotropic scalar ~He magnetometer becomes simpler when a modulated pumping beam technique is used The solution proposed could be applied to the double magnetic resonance technique but in this case, it is necessary to adjust
the angle between the RF field direction and the static magnetic field direction.
Our solution seems to be similar to the one using several cells. But contrary to [10j only one electronic detection line is employed and the signal processing is then simpler Moreover this solution is less sensitive to the spatial frequency anisotropy because both beams are very close to each other.
1740 JOURNAL DE PHYSIQUE III N°8
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