c) Coherence Spectroscopy with Pulse Trains
Actively
mode-locked
lasers
or
pulsed
semiconductor
lasers,
which
are
controlled
by
a
frequency
synthesizer,
are
generating
ultrashort
light
pulses
with
well
defined
and
highly
stable
pulse
repeti
tion rates in the MHz up to the GHz range.
Each
individual
pulse
can
create
a
coherent
superposition
of
nearly
degenerate
atomic
states
or
sub-states, when the laser wavelength is tuned to an optical transition of the atoms or molecules
and
the
pulse
is
short
enough
to
excite
the
involved
atomic
states
coherently,
i.e.,
the
Fourier
trans
form spectrum of the pulse covers the respective sub-level splitting.
Under
such
conditions
a
new
pulse
already
interacts
with
the
sample,
while
a
substantial
coherent
contribution
created
by
previous
pulses
is
still
existing.
If
the
individual
pulses
are
sufficiently
weak
and
are
not
erasing
the
already
existing
coherences,
they
can
be
assumed
to
generate
a
small
but
constant
contribution
to
the
atomic
coherences,
each
oscillating
with
the
splitting frequencies of the involved sub-states and damped at a rate gamma.
Constructive
interference
of
these
different
contributions
give
rise
to
very
sharp
resonances
in
the
resulting
macroscopic
coherence
amplitude,
whenever
the
excitation
rate
or
a
higher
har
mo
-
nic co
incides with the atomic splitting frequency.
This
coherence
phenomenon
is
quite
similar
to
multiple
beam
interference
of
optical
rays
with
damped amplitudes as known from Fabry-Perot interferometers.
Polarization Selective Detection
Such
an
enhanced
coherent
superposi
-
tion
can
sensitively
be
measured
by
a
si
-
milar
experimen
tal
set-up
as
used
for
quantum
beats
in
forward
scattering,
but
now
tuning
the
laser
repetition
rate
and
monitoring
the
average
intensity
of
probe
pulses
be
hind
a
crossed
polarizer.
There
-
fore,
this
periodic
pulse
excita
tion
meth
-
od
is
directly
measuring
level
splittings
in
the frequency domain.
Since
ground
state
coherent
transients
are
characterized
by
extremely
low
relaxation
rates,
very
sharp
resonances
of
ajacent
level
splittings
can
be
expected.
But
under
usual
experimental
con
-
ditions
the
observed
resonance
width
is
limited
by
transient
time-of-flight
broadening
of
the
atoms through the probe beam.
From
optical
pumping
experiments
in
alkaline
atoms
it
is
well
known
that
an
additional
buffer
gas
in
the
vapor
cell
considerably
reduces
diffusion
of
the
atoms
out
of
the
interaction
region
without suf
ficiently destroying the ground state coherence.
Under
such
conditions
periodic
pulse
excitation
allows
ultrahigh
resolution
spectroscopy.
So,
the
133
Cs
hyperfine
ground
state
splitting
of
9.2
GHz,
e.g.,
can
be
measured
with
a
resonance
width
of
on
ly
30
Hz,
which
corresponds
to
a
Q-factor
(9.2
GHz
to
30
Hz)
of
3*10
8
.
In
this
case
the
laser
operates
with
a
pulse
repetition
rate
of
84
MHz
and
scans
through
the
atomic
resonance
with
the 120th har
monic of the pulse rate, which is only tuned over 0.25 Hz.
Fluorescence Detection
In
an
appropriate
basis
of
atomic
eigenstates
an
atomic
coherence
can
be
described
as
an
oscill
-
ating
population
difference
between
adjacent
sub-states.
This
oscillation
occurs
with
the
respec
-
tive splitting frequency of the sub-levels.
Pulses
with
the
right
polarization
only
act
on
special
sub-states
and
can
change
the
population
den
sity
of
these
states
with
a
definite
phase.
When
the
pulse
repetition
rate
or
some
higher
harmonic
coincides
with
the
oscillation
frequency
between
two
sub-states,
the
pulse
train
burns
a
‘hole’
into
the
population
density
with
the
consequence
that
succeeding
pulses
interact
with
the
atoms
at
times
when
the
population
is
low.
Therefore,
less
atoms
are
excited
on
the
optical
transition and reduced fluorescence radiation is observed.
Such
signal
results
from
coherent
trapping
of
atoms
in
the
ground
state
and
can
be
described
as
a non-absorbing resonance.
With
Rb
and
additional
buffer
gas
in
the
vapor
cell
the
atoms
are
excited
by
a
semiconductor
laser
on
the
D
2
resonance
line
at
780
nm.
The
diode
laser
is
directly
triggered
by
a
step
reco
-
very
diode
(comb
generator)
and
produces
pulses
as
short
as
40
ps
at
a
repetition
rate
of
488
MHz.
To
ensure
single-mode
operation,
the
laser
is
additionally
coupled
to
an
external
re
-
sonator.
The
hyperfine
ground
state
splitting
of
Rb-87
with
6.835
GHz
is
measured
with
the
14th
harmonic
of
the
pulse
rate
by
monitoring
the
fluorescence
radiation
on
the
D
2
line
as
a
function
of
the
pulse re
petition rate.
To
discriminate
laser
straylight
from
the
fluorescence
light,
the
atoms
are
subjected
to
a
slightly
mo
dulated
magnetic
field,
which
also
slightly
and
pe
riodically
shifts
the
atomic
resonance
by
some
tenth
Hz.
This
allows
to
use
lock-in
detection
tech
nique,
which
suppresses
a
lar
ger
back
-
ground
and
in
this
case
measures
the
deriva
-
tive
of
the
reso
nance
signal
as
a
disper
sion-
like curve.
In
this
way
the
ground
state
splitting
can
be
de
termined
with
an
uncertainty
of
less
than
1
Hz,
and
with
a
stabilization
of
the
pulse
gene
-
rator
to
the
center
frequency
of
the
hyperfine
splitting
such
a
set-up
has
the
potential
to
operate as a secondary frequency standard.
Doctoral Theses
H. Burggraf
Hochauflösende Spektroskopie mit ultrakurzen Laserpulsen am Natrium
School of Electrical Engineering, Helmut-Schmidt-Uiversity, Hamburg 1985
H. Lehmitz
Kohärenzspektroskopie mit ultrakurzen Lichtimpulsen – Zeit- und frequenzaufgelöste Messungen am Beispiel von
Cäsium sowie Voruntersuchungen zu Messungen an gespeicherten Ytterbiumionen
School of Electrical Engineering, Helmut-Schmidt-Uiversity, Hamburg 1989
W. Kattau
Frequenzaufgelöste Kohärenzspektroskopie am Rubidium 87 mit kurzen Lichtimpulsen eines Halbleiterlasers
School of Electrical Engineering, Helmut-Schmidt-Uiversity, Hamburg 1990
Refereed Publications in Journals and Conference Digests
J. Mlynek, K.H. Drake, G. Kersten, W. Lange, H. Burggraf, H. Harde
Modulated Pumping Transmission Spectroscopy
Eleventh International Quantum Electronics Conference, Bosten, 1980
H. Harde, H. Burggraf, J. Mlynek, W. Lange
High-Resolution Level Splitting Measurements Using Pulse Trains
Europhysics Conference Abstracts 5A, part II of European Conference on Atomic Physics (European Physical
Society, Heidelberg, 1981), p. 1060 (1981)
J. Mlynek, W. Lange, H. Harde, H. Burggraf
High-Resolution Coherence Spectroscopy Using Pulse Trains
Physical Review A 24, 1099 (1981)
H. Harde, H. Burggraf
High-Resolution Coherence Spectroscopy by Means of Periodic Excitation with Picosecond Pulses
Digest of Fifth International Conference on Laser Spectroscopy (Canadian Physical Society, Jasper Park, 1981), in
"Laser Spectroscopy V", ed. by B. Stoicheff (Springer Series in Optical Sciences, 1981)
H. Harde, H. Burggraf
Ultrahigh-Resolution Coherence Spectroscopy by Means of Periodic Excitation with Picosecond Pulses
Optics Communications 40, 441 (1982)
H. Harde, H. Burggraf
High-Resolution Coherence Spectroscopy with Ultrashort Light Pulses
Digest of 12th International Quantum Electronics Conference Munich,
Applied Physics B 28, p. 246 (1982),
H. Harde, H. Burggraf
Periodic Pumping Spectroscopy with Picosecond Light Pulses
Technical Digest of Eighth International Conference on Atomic Physics, Calmers University Gothenburg, Sweden,
p. B 48 (1982)
H Harde, H. Burggraf
High Precision Level Splitting Measurements with Picosecond Light Pulses
Technical Digest of Fifth Rochester Conference on Coherence and Quantum Optics (University of Rochester,
Rochester), p. 335 (1983)
H. Harde, H. Burggraf
High Precision Measurements of Atomic Level Splittings by Means of Periodic Pumping with Picosecond Light
Pulses
in ''Laser Spectroscopy VI'', Springer Series of Optical Science 40, ed. by H. P. Weber and W. Lüthy (Springer,
Heidelberg, 1983), p. 117 (1983)
H. Harde, H. Burggraf
Hochauflösende Spektroskopie mit ultrakurzen Laserimpulsen
Laser und Optoelektronik 3, 235 (1983)
H. Harde, H. Burggraf
High Precision Level Splitting Measurements with Picosecond Light Pulses from an Injection Laser
in ''Coherence and Quantum Optics V'', ed. by L. Mandel and E. Wolf (Plenum Publishing Corporation, New York,
1984), p. 993 (1984)
H. Burggraf, H. Harde
Measurement of the Pressure and Temperature Shift in the Na Hyperfine Frequency by a Train of Ultrashort Light
Pulses
Conference Abstracts of Ninth International Conference on Atomic Physics, ed. by R. S. Van Dyck Jr. and
E. N. Fortson (University of Washington, Seattle, 1984), p. B 41 (1984)
H. Burggraf, W. Kattau, M. Kuckartz, H. Harde
Applications of Ultrashort Light Pulses for High-Precision Measurements in Atoms
Proceedings of Optics in Engineering Measurement, SPIE 599, 350 (1985)
H. Lehmitz, W. Kattau, H. Harde
Modulated Pumping in Cs with Picosecond Pulse Trains
in ''Methods of Laser Spectroscopy'', ed. by Y. Prior et al., Plenum Press, New York, p. 97 (1986)
W. Kattau, H. Harde
Optical Pulse Train Interference Spectroscopy with an Injection Laser
Technical Digest of International Quantum Electronics Conference (Optical Society of America, Washington, D.C.,
1986), p. 202 (1986)
Contributions on National Conferences and Meetings
H. Burggraf, H. Harde
Untersuchungen zum Betrieb eines synchron gepumpten modengekoppelten Farbstofflasers mit veränderlicher
Pulsrepetitionsrate
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Fachausschuss Kurzzeitphysik, Würzburg, 1. März
1982, Verhandl. DPG (VI) 17, 474 (1982)
H. Burggraf, H. Harde
Untersuchungen zur Druckverschiebung in der Na-Hyperfeinstrukturaufspaltung mit Hilfe der Periodischen
Pulsanregung durch Pikosekunden Laserimpulse
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Sektion Quantenoptik, Regensburg, 17. März 1983,
Verhandl. DPG (VI) 18, 453 (1983)
H. Burggraf, B. Schröder, H. Harde
Messung der Druck- und Temperatureinflüsse von Edelgasen auf die Hyperfeinaufspaltung des Na-
Grundzustandes durch Kohärenzspektroskopie mit ultrakurzen Laserimpulsen
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Sektion Quantenoptik, Gießen, 20. März 1984,
Verhandl. DPG (VI) 19, 821 (1984)
H. Hoidis, H. Lehmitz, H. Harde
Periodische Pulsanregung am Cs
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Sektion Quantenoptik, Bayreuth, 27. März 1985,
Verhandl. DPG (VI) 20, 1054 (1985)
W. Kattau, C. Peine, H. Harde
Hochauflösende Kohärenzspektroskopie mit Halbleiterlasern
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Sektion Quantenoptik, Bayreuth, 27.März 1985,
Verhandl. DPG (VI) 20, 1055 (1985)
H. Lehmitz, H. Harde
Moduliertes Pumpen mit Pikosekunden Lichtimpulsen am Cs
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Sektion Quantenoptik, Heidelberg, 17. März 1986,
Verhandl. DPG (VI) 21, 729 (1986)
W. Kattau, H. Harde
Hochauflösende Kohärenzspektroskopie am Rb mit einem Halbleiterlaser
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Sektion Quantenoptik, Heidelberg, 17. März 1986,
Verhandl. DPG (VI) 21, 724 (1986)
W. Kattau, H. Harde
Periodische Pulsanregung zum Betrieb eines
87
Rb-Sekundärnormals
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Arbeitsgemeinschaft Quantenoptik, Essen, 6. März
1989, Verhandl. DPG (VI) 24, Q 5.4 (1989)
Physics & Climate