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IMPACT OF LASERS ON PRIMARY FREQUENCY
STANDARDS AND PRECISION SPECTROSCOPY
L. Lewis, M. Feldman, J. Bergquist
To cite this version:
L. Lewis, M. Feldman, J. Bergquist. IMPACT OF LASERS ON PRIMARY FREQUENCY STAN-
DARDS AND PRECISION SPECTROSCOPY. Journal de Physique Colloques, 1981, 42 (C8), pp.C8-
271-C8-281. �10.1051/jphyscol:1981834�. �jpa-00221729�
JOURNAL
DE
PHYSIQUE
Colloque C8, suppZ6ment au n
'1
2,
Tome
42,
d6cembre
2981
page
C8-271
IMPACT OF LASERS ON PRIMARY FREQUENCY STANDARDS AND PRECISION
SPECTROSCOPY
L.L.
Lewis,
M.
Feldman and
J.C.
Bergquist
Frequency and Time Standads Group, Time and Frequency Division, National
Bureau of Standards,
BouMer, Colorado,
U.
S.
A.
Abstract.
-
Lasers available at new wavelengths, powers, linewidths,
and stabilities have made possible advances in precision atomic fre-
quency standards and spectroscopy. Laser spectrometers with resolving
power exceeding
1011
have been constructed qnd used to measure the pho-
ton recoil structure of spectral lines and in new tests of relativity.
Recent progress
in
frequency stabilation methods and in laser-cooled
ions indicates the possibility of an optical frequency standard with an
accuracy and stability of less than
10-Is. Diode lasers may enable the
construction of an optically-pumped cesium standard with an order-of-
magnitude improvement in accuracy over existing primary standards.
I. CESIUM PRIMARY STANDARDS RESEARCH
Introduction.
-
The
U.S.
primary frequency standard,
NBS-6,
is a cesium atomic-
beam apparatus with an interaction length of
3.74
meters. The experimental
uncertainty that limits its accuracy is the microwave cavity phase shift
[l].
This shift is cancelled to some degree by averaging the clock rates obtained from
the two directions of atomic beam propagation in the standard. However, the
degree of cancellation is limited by the faithfulness of the atomic beam retrace,
which is affected by factors of beam axial symmetry.
Since the frequency shift produced by a given cavity phase shift is propor-
tional to the resonance linewidth, one way to improve this standard would be
to
increase the microwave
Q.
This factor can be increased in a cesium clock only by
lengthening the interaction time with the atoms. It is unlikely that a new
primary standard could be made physically longer than
NBS-6,
because magnetic
shielding becomes difficult to achieve for such large dimensions. In addition, a
suitable means of cooling neutral atoms in an atomic beam has not yet been de-
veloped. Therefore, the microwave linewidth in future cesium clocks will probably
not decrease, and any improvement in accuracy must come from a direct assault on
the sources of error.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981834
C8-272
JOURNAL
DE PHYSIQUE
In recent years, the development of reliable single-mode diode lasers has
made possible new methods that may significantly reduce the errors found in a
cesium clock of given length. While much admirable work has been performed in
the past using laboratory-quality diode lasers
[Z],
the advent of low-cost,
long-life, commercial lasers with excellent quality now makes long-term operation
of an optically-pumped laboratory standard feasible.
Cesium Optical Pumping.
-
The atomic-state-preparation technique of most interest
at the National Bureau of Standards (NBS) is one which promises to change the
state of nearly every cesium atom within the atomic beam to a single magnetic
sublevel
[3,4]. Two lasers can be used for this purpose, one tuned to the (2S4,
F
=
3
-,
2P3,2,
F
=
4) transition, and the other adjusted to the (2S+, F
=
4
-,
2P3,2,
F
=
4)
line, both at
852
nm wavelength. The latter laser is plane polar-
ized, with the vector of the electric field parallel to a small magnetic field,
which establishes a quantization axis. With both lasers crossing the atomic
beam, atoms move from each energy level into every other level, with the excep-
tion of the
(2S+,
F
=
4,
M
=
0) ground-state sublevel.
In this case, selection
rules prohibit excitation of the atom, and eventually nearly all atoms should
fall into this substate. A similar arrangement of lasers will allow the pumping
of atoms into the F
=
3,
M
=
0 state.
A number of advantages result from this process.
The most obvious is that
the signal should increase considerably, since, instead of being rejected by
state selection magnets, atoms are converted to the desired state by optical
pumping. In the case of NBS-6, which uses both the
(F
=
3,
M
=
0) and (F
=
4,
M
=
0) substates, the increase in signal is a factor of eight, and the expected
increase in signal-to-noise ratio is consequent1 y
a.
A more significant benefit of optical pumping is the generation of a highly
symmetrical cesium beam, since optical pumping is largely independent of velocity
and position. This feature, combined with a new Cs oven design, should permit a
much better retrace of the atomic beam upon reversal. Improved microwave cavity
design with smaller cavity windows may help reduce the actual size of phase
shifts which vary across cavity windows [5] and lower the total uncertainty in
frequency due to cavity phase shift by an order of magnitude.
Another uncertainty of unknown magnitude in present Cs standards is that
introduced by Majorana transitions as atoms pass from the large
(1
T) fields of
the state selection magnets to the small
(0.1
T)
field of the C-field region
161.
Since the C-field can now be extended to include the optical pumping region, such
Majorana transitions should not occur. In addition, pumping of the atoms into a
single magnetic sublevel will permit study of Majorana transitions, which should
appear as changes in Rabi resonances adjacent to the central clock transition.
The present strength of the C-field is mandated by the presence of asymmetry
in the size of microwave resonances near the clock transition. AS atoms are
pumped into the
(F
=
4, M
=
0) level, these auxiliary features will fade, permit-
ting the reduction of the C-field to a smaller, more stable value. Also, the
sign of the C-field is irrelevant in this scheme; therefore, the field can be
reversed with no anticipated shifts
in clock frequency. This check on systematics
is not possible with the current system, since improper sign of the C-field would
result in
Majorana transitions of unknown effect.
Fluorescence Detection.
-
Complete removal of state-selection magnets is possible
if
laser methods are employed to detect atoms that have made the microwave transi-
tion. Perhaps the simplest way to accomplish this is with fluorescence detection.
In the case where atoms are pumped to the
F
=
3,
M
=
0 level, a "flop-in" inter-
rogation is possible with a laser tuned to the
F
=
4
+
F
=
5
transition. The
advantage of using of this particular transition is that atoms excited to the
2S,,,,
F
=
5
level must decay to the
F
=
4
level, where they are repeatedly ex-
cited, contributing many photons per atom to the fluorescence signal.
It
is
possible in this manner to obtain 100% total detection efficiency even when the
collection efficiency is considerably less than one. By appropriate adjustment
of laser frequency and angle of crossing of the atomic beam, different atomic
velocity distributions
a1 so may be selected using such fluorescence detection.
A device constructed with laser-induced detection and optical-pumping regions
could, perhaps, be operated simultaneously with counter-propagati ng atomic beams.
With appropriate arrangement of pumping and detection regions, only atoms which
have made a microwave transition will
contribute to the fluorescence signal
;
this
allows distinction between the two beam directions. One direction could be used
for locking the microwave frequency, and the other direction for locking the
cavity phase shift value. This may even permit the use of separate cavities for
the two ends of the Ramsey resonance region. Even
if
difficulties with background
levels and collisions prevent implementation of this technique, switching of the
two directions should be much faster with the laser methods, since the cesium
ovens can be operated continuously.
There are certain disadvantages of the new methods.
One, the light shift of
the cesium atoms as they pass through the microwave region, due
to the fluor-
escence light coming from the two laser regions, may be as large as
Av
-
10-14.
The magnitude of this effect was first estimated by A. Brillet (Universit6 Paris-
Sud).
E.
Smith
(NBS)
has also calculated the shift using a tensorial analysis,
which accounts for differences in fluorescence polarization. Care must be taken
to reduce the intensity of this fluorescence radiation through the use of aper-
tures and by properly spacing the laser regions from the microwave regions.
A second problem is photon-recoil beam deflection
[7]. Since slower atoms
are deflected more than faster ones, a velocity-dependent spatial distribution
may be formed. The value of frequency shifts due to this effect can be reduced
below
-
10-l5 by symmetric laser illumination of the beam and proper cavity
design
[4].
C8-274
JOURNAL
DE
PHYSIQUE
Laser Requirements.
-
The characteristics of diode lasers appropriate for this
task include properties essential to attainment of good signal-to-noise ratio and
complete optical pumping, as
we1 1 as properties important for operating consider-
ations. In the latter category, low cost and ready availability are desirable
from the standpoint of construction and repair, and lifetime is important when
considering maintenance of a continuously operating standard. Diode lasers are
now easily available with projected lifetimes in excess of ten years and costs of
about $100 each. Since manufacturers specify lifetime in terms of power output,
however, and not spectral properties, independent measurements are in progress at
NBS to measure the length of time diode lasers may be locked to an atomic beam
resonance line. To be useful, this lifetime will have to be longer than one
year.
In order to completely optically pump each atom, the laser spectral density
should be greater than about
mW/cm2-Hz for an atomic beam path length of
about a millimeter. With output powers of
3
to
10
mW and laser linewidths of
10 to 100 MHz, this requirement is easily met with a properly collimated beam.
It is also necessary, both for pumping and detection, that the laser fre-
quency fluctuations be small. Measured standard deviations per unit frequency
interval for typical
commercially available devices are less than 10
k~z/@ for
frequencies below 10 kHz
[8].
At higher frequencies, the frequency noise de-
creases, and should not introduce appreciable noise through the pumping process.
Long-term
(lo2
5
T
5
lo4 s) diode laser frequency stability of about 10 kHz
has been demonstrated at
NBS
[3],
where one laser was simply locked to a rubidium
vapor absorption cell, and light shift of the rubidium ground-state hyperfine
splitting was interpreted in terms of laser frequency drift. Although it is
probably not needed, greater stability is expected if the lock is made to an
atomic beam or a saturated absorption cell.
Finally, laser intensity f
1
uctuations also appear as undesirable noise,
especially when a cycling transition is used for detection. If we assume a
cesium beam magnitude of
1010 atoms/s, fractional laser intensity noise, AI/I
,
per unit frequency interval should be less than lO-'/Kz. Measurements at
NBS
indicate that some of the diode lasers tested have intensity noise less than
10-6/fi
[3].
Other authors have measured noise as small as
-
10-'/m
[9].
Newly developed diode lasers appear to have the characteristics needed for
use in laboratory cesium standards. Implementation of these new light sources,
with various other modifications of the traditional cesium beam clock, may enable
the construction of a cesium primary standard with an order of magnitude improve-
ment in accuracy.