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Application of the Optogalvanic Detection Technique to the 4He Magnetometer
B. Chéron, H. Gilles, J. Hamel, O. Moreau, E. Noël
To cite this version:
B. Chéron, H. Gilles, J. Hamel, O. Moreau, E. Noël. Application of the Optogalvanic Detection Technique to the 4He Magnetometer. Journal de Physique III, EDP Sciences, 1995, 5 (4), pp.459-465.
�10.1051/jp3:1995131�. �jpa-00249323�
Classification Physics Abstracts
07.55 32.808 42.60
Application of the Optogalvanic Detection Technique to the ~He Magnetometer
B. Chdron (~,~), H. Gilles (~), J. Hamel (~), O. Moreau (~) and E. No@1 (~) (~) Laboratoire de Spectrocopie Atomique, ISMRA 14050 Caen Cedex, France (~) Universit4 de Caen, UFR de Sciences, 14032 Caen Cedex, France
(Received 7 October 1994, accepted 23 December 1994)
Abstract. The optogalvanic detection of the magnetic resonance on the Helium (2~Si)
metastable state is investigated in view of its application to ~He magnetometers. Metastable
atoms are optically pumped with an intensity modulated beam and resonance signals are mea-
sured versus geometrical parameters such as light polarization and magnetic field orientations.
Introduction
Optical pumping and electronic magnetic resonance are very traditional techniques in atomic
physics. An application of these methods is the realization of high sensitivity magnetometers [1-3].
Radio Frequency optical double resonance has been intensively studied in the past. How- ever, another solution using an intensity of polarization modulated pumping beam has also been proposed [4,5]. The development in our laboratory of an infra-red diode pumped laser using a LMA crystal [6] allows for the study of a new laser pumped ~He magnetometer with
interesting features. In view of the previous results [7], we have undertaken the development
of a magnetometer prototype based on the intensity modulated pumping beam technique. The unknown magnetic field is determined by monitoring the pumping beam absorption through
the ~He cell.
A very beautiful variant of the previous experiment consists in replacing the optical detection
by an optogalvanic detection. This technique has been for the first time associated with ~He optical pumping by Sevast'yanov and Zhitnikov [8]. Recently, Schearer and Tin [9] proposed
for the same atoms the optogalvanic detection with an intensity modulated pumping beam and
suggested an application to magnetometry.
In this paper, in addition to the application of a new method for detecting the optogalvanic effect, we present the results of our measurements of the spatial and polarization dependence
of the resonance signals. Finally, in a view of a magnetometry application, the performances
are compared with those obtained using an optical detection.
© Les Editions de Physique 1995
460 JOURNAL DE PHYSIQUE III N°4
1. Theory
The optogalvanic detection of an optical or magnetic resonance is based on impedance changes
in a discharge plasma produced by an induced modification of the excited states populations [10]. It is currently used in the determination of spectroscopic constants of atoms and molecules
and plasma properties ill].
In a weak helium discharge, electron density is governed by collisional processes involving
excited atoms, ions and electrons. In these processes, an important contribution to the electron density is the collisional quenching between two helium metastable (2~Si) atoms (Penning collisions):
He (2~Si) + He (2~Si) - He (l~so) + He (l~S)~~) + e~ (I)
The efficiency of this reaction depends in a predictable manner on the magnetic spin states
population of the reactant He (2~Si) metastable atoms. In particular, the Penning process
between two He (2~Si it is forbidden since it does not conserve the spin angular momentum.
Hill et al. [ll] have shown that the rate coefficient for the ionizing collision (I) is proportional
to:
y~2 y~2 y~2
fl ~
+ (~)
~~
where n = no + n+ + n- is the stationary metastable density and no, n+, n- are respectively
the populations densities of the mj
= 1, 0, -1 Zeeman sublevels.
When the modulation frequency Q of an intensity modulated pumping beam matches the Larmor frequency, a resonance occurs leading to a rapid change of the electron density and hence the cell impedance.
The rate coefficient fl may be calculated using the well known equations of optical pumping.
This is done under the following conditions:
the pumping beam intensity is modulated at frequency Q and we suppose that the power density is low enough in order to avoid absorption saturation;
the polarization of the pumping beam is linear or circular;
resonances can occur if nQ
= pUJL (n and p are integers, UJL is the Larmor frequency of the 2~Si state in a magnetic field ED)i we restrict to the cases Q
= UJL and Q
= 2uJL.
This study provides the amplitude angular dependence A:
A(Q = UJL, circ.)
= 1.(sin~9).(I + cos~9) A(Q = 2uJL> circ)
= 1/2.sin~9
where 9 is the angle between the propagation direction of the circularly polarized beam and the magnetic field ED
A(Q = UJL, lin=
= I. sin~(2b) A(Q
= 2uJL, [in. = 2.sin~(b)
where b is the angle between the electric field of the linearly polarized beam and the magnetic
field.
It is worth noting that the amplitudes of both optogalvanic and optical detection signals
exhibit the same spatial dependence.
2. Experiment
Several methods can be used to detect the electron density variations in a weak discharge.
For example, the cell impedance can be monitored by measuring the time-averaged discharge
current intensity or by detecting the resonance frequency shifts of a microwave cavity containing the cell.
The method used in the present investigation consists in measuring the reflected power at the end of the discharge circuits. To our knowledge, it has never been reported before.
The experimental setup is shown in Figure I: the measure probe (SWR) is placed between
Gi
L AO
~ s
G2
A SWR
~ o
Fig. I. Experimental setup. L-LMA laser; AOM-acoustooptic modulator; H-Helmholtz coils;
C-helium Lell; S-magnetic shield; Gl-DC current generator; G2-low frequency sinusoidal generator;
A-lock-in amplifier; SWR-stationary wave rate probe; R-chart recorder; O-HF oscillator.
the 25 MHz oscillator and the discharge circuit and its output voltage is proportional to the reflected wave amplitude. The tuning of the discharge circuit is experimentally adjusted in order to optimize the sensitivity: the maximum value is obtained when the reflexion rate is close to 15%.
The helium cell is placed inside a magnetic shield in order to avoid the ambiant magnetic field fluctuations. A pair of Helmholtz coils [Hi provides the static magnetic field (Bo " 50 ~tT).
The resonance signal is detected as follows: a small low frequency (300 Hz) magnetic field
is superimposed to the static magnetic field; the resulting modulation is then synchronously
detected and provides a signal which goes to zero at resonance.
Figure 2 represents such a resonance signal obtained when the modulation frequency is scanned over the resonance frequency (Q
= UJL). As predicted, the resonance width is close to that obtained with an optical detection (width
= 9 kHz). We check that the amplitudes have
462 JOURNAL DE PHYSIQUE III N°4
14 KHz
Fig. 2. Amplitude of the resonant signal when the modulation frequency (Q) is scanned over the Larmor frequency (Q = wL). The propagation direction of the circularly polarized beam is perpendic-
ular to the magnetic field direction.
the same dependence versus optical power density in optogalvanic or optical detections.
The evolution of the amplitudes observed at Q
= ~JL, are represented in polar coordinates in Figure 3 (circular polarization) and in Figure 4 (linear polarization). A good agreement is
found with theoretical predictions (full lines).
The comparison between the amplitude of the different signals is shown in Table I. For each
signal, we choose the geometrical configuration corresponding to the maximum amplitude and the values are normalized on A(Q
= 2uJL,lin.).
Table 1.
A(a w Ala 2w Ala
w Ala 2w
circ l~
circ l~ i>n 1~
(in 1~
theory I 5 2
experiment 4 O 48 2 2
The A(Q
= 2uJL, [in. signal is expected to have the highest amplitude. However, experiment clearly indicates that the maximum amplitude is observed for the A(Q
= UJL circ. signal. This
discrepancy is quite independent of the optical power density and remains at vanishing power.
It is unexplained at the present time but it is interesting to note that the starting formula
(2) derived by Hill et al., is valid in the afterglow when the electron density goes to zero.
In a continuous discharge, superelastic collisions between metastable Helium atoms and spin oriented electrons may affect the population density of the Helium magnetic sublevels.
8 = 90 °
+
0 =
0~
Fig. 3. Amplitude in polar coordinates of the signal observed at Q
= wL versus angle 6 between
the propagation direction of the circularly polarized beam and the magnetic field direction.
a 90°
+
+
+
Fig. 4. Amplitude in polar coordinates of the signal observed at Q
= wL versus angle 6 between
the electric field of the linearly polarized beam and the magnetic field direction.
3. Application to Magnetometry
Optical and optogalvanic detections can be studied simoultaneously with experimental setup.
For the comparison, we choose the A(Q
= UJL, circ.) signal. Figure 5 represents a time variation
464 JOURNAL DE PFIYSIQUE III N°4
OPTICAL DETECTION
~ m~
OPTOGALVANIC DETECTION
Fig. 5. Record of the time variation of the resonance signal when field steps of I nT are applied to helium atoms (optical detection and optogalvanic detection).
of the resonance signal when the frequency modulation is close to the resonance frequency.
A field step of I nT is applied periodically. The vertical shift between the two curves is
arbitrary and due to the setting of the chart recorder. However, the precise measurement of the resonance frequency clearly indicates a shift of the order of 5 nT between optical and
optogalvanic detection techniques. This discrepancy is likely to be the result of magnetic field
inhomogeneities inside the magnetic shield (of the order of 50 nT over the Helium cell) because the tested cell volume is lightly different for both techniques. For each signal, the output time
constant is set at 0.3 s. We can see in Figure 5 that high frequency f > I Hz) noise is higher for optogalvanic detection than for optical detection (this result is confirmed by a spectral noise
density analysis). At low frequency, noise levels are similar for both detection. It is interesting
to note that the good correlation between the fluctuations of the two signals implies a common origin: pumping beam fluctuations and (or) magnetic field fluctuations.
It appears that sensibilities are similar for both detection. Moreover, optogalvanic detection is an easier to handle method than optical detection. But the conditions of the previous
test (circular polarization) are not optimal for a magnetometry application. In particular the
resonance frequency can be shifted due to the interaction of the metastable atoms with the
laser light, leading to an error in the determination of the magnetic field intensity. On the
other hand, this light shift is negligible when the polarization of the pumping beam is linear:
in that case, the optical detection is more sensitive than optogalvanic detection [7].
However, several parameters of the optogalvanic detection must be optimized (cell dimen- sions, Helium pressure, pumping beam section...) before drawing definitive conclusions and we
may then hope to reach similar sensitivities for both techniques.
References
iii Colegrove F-D-, Schearer L-D- and Walters G-K-, Polarization of ~He
gas by optical pumping, Phys. Rev. 132 (1963) 2561.
[2] Slocum R-E- and Reilly F-N-, Low field helium magnetometer for space application, IEEE Trans.
Nucl. Sci. lo (1965) 165.
[3] Cassimi A., Cheron B. and Hamel J., ~He optical pumping with intensity modulated laser light,
J. Phys. II France1 (1991) 1213.
[4] Gilles H.~ Cheron B. and Hamel J., ~He optical pumping with polarization modulated light, Opt. Comm. 81 (1991) 369.
[5] Cheron B., Gilles H., Hamel J., Moreau O. and Sorel H., Laser frequency stabilization using
Zeeman effect, J. Phys. III France 4 (1994) 401.
[6] Gilles H., Cheron B. and Hamel J., Magn4tomktre h ~He pomp4 par laser. Isotropie spatiale des
signaux de r4sonance en r4sonance magn4tique et en modulation de lumikre, J. Phys. II France 2 (1992) 781.
[7] Sevast'yanov B.N. and Zhitnikov R-A-, Effect of optical orientation of ~He atoms in the 2~Si state
on the electron density and radiation of helium atoms in plasma, Sov. Phys. JETP 29 (1969) 809.
[8] Schearer L-D- and Tin P., Spin dependent, optogalvanic effect of laser-pumped He(2~Si) atoms, J. Opt. Sac. Am. B 6 (1989) 1771.
[9] Penning F-M-, Physica 8 (1928) 137.
[lo] Julien L. and Pinard M., Optogalvanic detection of optical pumping, J. Phys. B 15 (1992) 2881.
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