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Laser frequency stabilization using Zeeman effect
B. Chéron, H. Gilles, J. Hamel, O. Moreau, H. Sorel
To cite this version:
B. Chéron, H. Gilles, J. Hamel, O. Moreau, H. Sorel. Laser frequency stabilization using Zeeman effect.
Journal de Physique III, EDP Sciences, 1994, 4 (2), pp.401-406. �10.1051/jp3:1994136�. �jpa-00249111�
Classification
Physics Abstracts 42.60
Laser frequency stabilization using Zeeman effect
B. Chdron11- 2), H. Gilles ('), J. Hamel (I), O. Moreau (I) and H. Sorel (') ii) Laboratoire de Spectroscopie Atomique (URA 19), ISMRA, 14050 Caen Cedex, France (2) Universit6 de Caen, UFR de Sciences, 14032 Caen Cedex, France
(Received 5 July 1993, revised 18 October 1993, accepted 4 November 1993)
Rksumd. Nous ddcrivons une nouvelle mdthode, facile h mettre en ceuvre pour asservir la longueur d'onde d'un laser sur une raie atomique. Cette mdthode basde sur l'effet Zeeman, met en
jeu le dichroisme circulaire prdsentd par une vapeur d'atomes soumise h un champ magndtique.
Elle est appliqude h la stabilisation de la frdquence d'un laser LNA monomode sur la transition (2 ~Si-23Po) de l'hdlium.
Abstract. We describe a
new and easy to handle method to stabilize the laser frequency on an
atomic transition. This method, based on Zeeman effect, involves the circular dichroism of an atomic vapour submitted to a magnetic field. It is applied to the frequency stabilization of a single frequency LNA laser on (2 (2 ~Si-2~Po) helium transition.
Several papers describe solid state lasers [1-6], end pumped by a high power diode laser
array in order to excite the helium resonance transition 2 3Si-2 3P (A
= 1.083 ~Lm). These
tunable lasers can be used for such applications as atomic trapping experiments [7] or
realization of high sensitivity He optically pumped magnetometers [8, 9]. Since 1986, our
group has been working on the last application using a LNA laser as a pumping source [10-13].
Magnetic probes using optical fibers have been designed and built for outdoor magnetometry.
As compared to a laboratory application, it requires a more compact, reliable and easy to handle stabilized laser.
The various frequency stabilization methods for He optical pumping experiments use
absorption or fluorescence signal detection resulting from interaction between the laser beam and helium atoms. Such a signal exhibits an extremum when the wavelength is changing over
the atomic transition wavelength. Generally, the laser frequency is modulated in order to obtain an error signal. A simple method consists in modulating the laser cavity length as a
result the optical pumping beam is frequency modulated and this can be an handicap for
magnetometer in which other low frequency modulations are used for signal processing.
Moreover, small intensity modulation is superimposed to the frequency modulation. In order to avoid these effects, an elegant but expensive method makes use of an external modulation of the laser beam with an electro-optic modulator device. This modulation is applied to a small
part of the laser beam and does not affect the stabilized pumping beam [7, 14].
402 JOURNAL DE PHYSIQUE III N° 2
The precision of the frequency stabilization is limited by the very large absorption or
fluorescence Doppler width (1.7GHz). More complicated alternative methods based on
saturated absorption give narrower error signal and then better performances [7, 14-17], but outdoor application is not realistic.
After a brief description of our laser, we present an original method using helium magnetic circular dichroism and allowing a laser frequency stabilization without modulation. This method is suitable for the Do(2 3Si_2 3Po) transition.
Our laser is a LNA diode pumped laser. The output of the pumping laser diode (SDL 2432) is collimated, shaped and focused with standard Melles Griott laser diode optics. The LNA
crystal, provided by Union Carbide and Co, is coated on one face to form the end of the laser
cavity. It has a high transmission (~90 %) at the pump wavelength (800 nm) and a high reflectivity (~ 99 fb at 1.083 ~Lm. The output end of the laser cavity is a spherical concave
mirror with a curvature radius equal to 5 cm.
The LNA material has two principal laser bands the stronger one at A
=
1.054 ~Lm and the
weaker at A =1.083 ~Lm. In order to impose the laser oscillation at A =1.083 ~Lm, the
multidielectric coating of the output mirror has a reflectivity coefficient equal to 99 % at this
wavelength and lower than 50 % at 1.054 ~Lm. To enhance the longitudinal modes separation
and then to make easier single mode operation, the laser cavity is made as short as possible (= 2 cm ). The laser frequency is adjusted by a solid intracavity glass (talon (200 ~Lm thick,
F-S.R.
=
500 GHz) with a reflectivity coefficient R
=
20 fb on each face. Fine tunning of the
cavity mode is controlled by mounting the output mirror on to a piezoelectric translator. The laser cavity is placed inside a thermally stabilizated box to avoid detuning due to temperature
fluctuations.
It is well-known that LNA laser performances are much better for c-axis pumped crystals. In this case, the laser output is theoretically unpolarized and the pumping light absorption by the
crystal is independent of the crystal orientation with respect to the pumping light polarization.
On the other hand, for a LNA crystal pumped along the a-axis, the pumping light absorption
is strongly dependent of crystal orientation with respect to the pumping source polarization
direction. The laser output is then linearly polarized and laser optical efficiency is reduced as
compared to c-axis pumped crystals.
In order to obtain a well polarized single mode emission, we have tested three cavity
configurations (Fig. I) i
I. ii : c-axis pumped LNA crystal with its back face at Brewster angle and without coating.
(1.2) : c-axis pumped LNA crystal with its back end nearly perpendicular to the c-axis. The
polarization of the laser beam is forced by an uncoated intracavity single glass plate at Brewster angle.
(1.3) a-axis pumped LNA crystal with nearly parallel faces.
In cases (1.2) and (1.3), the angle between the two faces of the crystal is about 3° in order to
avoid the « dtalon effect » between these faces. The back face of the crystal is A-R- coated at
1.083 ~Lm.
The performances obtained with the three configurations are quite similar. A fine
comparison is not significant because crystals and coatings have not the same origin. With a 500 mW pumping beam at 800 nm, we obtain a 20 mW CW single frequency emission at 1.083 ~Lm when the intracavity dtalon is inserted. It is interesting to note that the third
configuration is very easy to use and permits the shortest length cavity.
Our performances are quite similar with that obtained by [3-5]. However, recently, a
Konstanz group [6] has developed an elegant twisted cavity laser giving 150mW single
frequency with 2 W pumping beam.
our method used to stabilize the laser frequency on the Do(2 3Si-2 3Po) transition consists
di
<.z
c axis
Brewstef plate
I .3
fiQ ~w
a axis
Fig. I. Cavity configurations. (I.I) c-axis pumped LNA crystal with its back face at Brewster angle and without coating. (1.2) c-axis pumped LNA crystal with its back end nearly perpendicular to the c- axis. The polarization is forced by a plate at Brewster angle. (1.3) a-axis pumped LNA crystal with nearly parallel faces.
in measuring the difference between absorption coefficients for left and right circular polarized components («~ and «~) by helium atoms placed in a constant magnetic field. We call it
« Zeeman locking
».
Figure 2 shows the experimental set-up a helium cell is submitted to a weak HF discharge
in order to populate the (23Si) metastable state, and to a magnetic field created by a
permanent magnet. A part of the laser beam (4 %), parallel to the magnetic field direction and linearly polarized, is absorbed by the cell. The (« ~ ) and («~ ) components of the transmitted beam are analysed using a quaterwave plate and a polarizing cubic beam splitter. The magnetic
field amplitude (0.03 T) is adjusted in order to separate the components by a quantity equal to
404 JOURNAL DE PHYSIQUE III N° 2
abs%t"on
~ ~
+
C [@
S
~
~
,
~
, ,,
l' A
' l
L
PzT
Fig. 2. Experimental setup for Zeeman locking (L), LNA laser (PZT), piezoelectnc translator IA), permanent magnet (C), helium cell IA /4), quaterwave plate (S), beam-splitter (D), differential
amplifier ; (I), integrator.
the Doppler width (1.7 GHz) of the Do transition (Fig. 2) : the two absorption curves cross at
their inflexion point. The difference between the (« ~ and («~ absorption signals provides
the error signal which is, after integration, applied to the piezoelectric translator. The laser
stabilization at the center of the atomic frequency needs a carefull balance between the
(«~ ) and («~ ) channels : I % relative difference involves a shift of 15 MHz.
In order to test the short and long term laser frequency stability, a conventional light beat
experiment involving two identical LNA lasers was undertaken. Each laser is locked to the 1.083 ~Lm He line using the Zeeman locking technique described above. The beat between the two laser beams propagating coaxially is detected with a high speed photodiode I ns rise time with a 50 n load resistance) and the output voltage is analysed with a HP 8560 A Spectrum Analyser. Figure 3 shows the beat spectrum scanned over 20 MHz and with a 500 ms sweep time the beat width is about 2 MHz. Due to the small bandwidth of the locking loop (a few Hz), the short term frequency fluctuations are not reduced by the Zeeman locking loop (same
fluctuations are observed with a free running laser).
Short term frequency fluctuations are greatly enhanced by surrounding acoustic noise.
However, without such noise, the main contribution comes from intensity and spatial fluctuations of the laser diode due to the optical feedback noise. All optical surfaces are A.R.
coated but are not perfect. The main back reflexion contribution comes from the crystal end.
Due to the focusing of the pumping beam, it cannot be remove by simply tilting the beam with respect to the cavity axis.
With the Zeeman locking technique, the long term frequency stability, evaluated by looking
at the fluctuation of the mean beat frequency, is of the order of lo MHz during one hour. These
performances are fully sufficient for magnetometry application.
It is interesting to note that the controlled signal applied to the piezoelectric translator
provides an upper limit value for the short term frequency stability of the laser. From the
BEAT SPECTRUM
~~~t~4 aO. QQMe~
~w~ %Qomm
Fig. 3. Beat spectrum the frequency is scanned over 20 MHz with a 500 ms sweep time.
analysis of this signal, we deduce a frequency noise equal to I MHz in a 10 Hz bandwidth.
This result is in good agreement with the previous light beat experiment measurement.
The Zeeman locking technique is compared with the traditional arrangement described in [9]
using fluorescence signal detection emitted by (23Po)4He atoms. In this case, the laser
frequency is modulated at 300 Hz with a modulation depth equal to 200 MHz. The fluctuations of the laser mean frequency are estimated to 30 MHz.
It appears that the Zeeman locking technique provides a good and easy to handle method for stabilizing a laser without frequency modulation and can be easily extended to other types of laser as tunable laser diodes.
References
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406 JOURNAL DE PHYSIQUE III N° 2
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