The scientific objectives for this proposal were: (1) development of a rugged laser cooled source of atoms using DBR/DFB laser technology; (2) to use this source and these lasers to demonstrate coherent atom manipulation techniques; and (3) to incorporate these techniques into atom interferometer inertial force sensors. Our fulfillment of these objectives is detailed below

Kasevich, Mark

English

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Mark Kasevich
Stanford University
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FINAL REPORT
NASA Grant NAG8-1088
"Atom Interferometry in a Microgravity Environment"
June 1997
The scientific objectives for this proposal were: (i) development of a rugged laser cooled
source of atoms using DBR/DFB laser technology; (ii) to use this source and these lasers
to demonstrate coherent atom manipulation techniques; and (iii) to incorporate these
techniques into atom interferometer inertial force sensors. Our fulfillment of these
objectives is detailed below.
/ ,
Gyroscope
We have demonstrated a Sagnac effect atom interferometer gyroscope that uses
stimulated Raman transitions to coherently manipulate atomic wavepackets. We have
used this gyroscope to measure the Earth's rotation rate, and have measured a short-term
stability for rotations of 2x 10.8 rad/sec/(Hz) la [ 1].
Our approach has been to use stimulated Raman transitions to coherently
manipulate a transversely laser cooled atomic beam, as shown in Fig. 1. In this geometry
approximately 107 atoms/sec contribute to the interference signal. We initially prepare
atoms into the Cs F=3, mf = 0 level, and subsequently subject the atoms to a three pulse
sequence to divide, redirect and finally recombine atomic wavepackets. Interference is
manifested through the number of atoms in the F=4, mr=0 level after the three pulse
sequence. The resulting Sagnac loop has an area of 25 mm z. This corresponds to a
fringe shill of-8 tad for the Earth's rotation rate fl, = 7x104 rad/sec. Our signal to noise
was good enough to split the fringe by 1 part in 500 atler 1 second of data collection time.
Our apparatus consists ofa - 2 m long UHV vacuum chamber (base pressure
-5x10 _° ton') mounted just above a floating optical table. Care was taken to minimize
vibrational coupling between the table and the chamber, The optical table contained the
optics and laser system used for the wavepacket manipulation as well as for the laser
cooling. In order to observe interference fringes the entire optical table was made to
rotate by driving horizontal table rotational modes. This was done with a PZT, which was
attached at one end to the floating table, and on the other end to an independent platform
just next to the optical table. The induced rotation rate was independently measured with
a seismometer located on the optical table. Fig. 2 shows the number of detected atoms
in an 800 msec interval vs. the induced rotation rate. Note that the center of the fringe is
https://ntrs.nasa.gov/search.jsp?R=19980007953 2020-06-16T00:23:11+00:00Z
Fig. 1. Schematic illustration of the gyroscope apparatus.
offset from the zero rotation rate inferred from the seismometer. This offset arises
because the seismometer is not sensitive to low frequency excitations such as those arising
from the rotation of the Earth. The gyroscope, on the other hand, is. Accurate
measurement of this offset allows for determination of the Earth's rotation rate.
We developed a new laser system to drive the stimulated Raman transitions needed
for wavepacket manipulation [2]. This laser system was developed with the aim of
creating a system which is robust and reliable enough so as not to limit the long-term
stability of the gyroscope. Recall that the stimulated Raman method requires two high
power laser beams which differ in frequency by 9.2 GHz, the cesium clock frequency. Our
approach has been to derive these two beams from a master diode laser and a high
frequency acousto-optic modulator, and then to amplify these two beams using optical
injection locking techniques. A key issue is whether the amplification process introduces
low frequency phase noise. By driving the Cs clock transition in the classic Ramsey
configuration, we characterized the phase noise in the 1 - 100 Hz band, and demonstrated
that these sources can be Used for precision measurements.
Gravity Gradiometer
We have just recently demonstrated a proof-of-principle of the gravity gradiometer
configuration discussed above. We have simultaneously measured the gravitational
acceleration of two ensembles of laser cooled atoms separated vertically by approximately
1 m using a common set of Raman beams. Our near term goal is to demonstrate a
sensitivity to gradients at the 10 E/Hz la level.
Approximatelyl0 s atoms are initially captured from a dilute Cs vapor into a
magneto-optic trap in each of the two UHV (base pressure - lxl0 -9 torr) vacuum
chambers. The vacuum chambers are manufactured from non-magnetic AI blocks, with
high optical quality, anti-reflection coated, indium seal windows. Laser light for trapping
and cooling is delivered to the chamber with optical fibers. After loading the traps, the
quadrupole trapping fields are switched off, and the atoms are further cooled in an optical
molasses. Following this cooling interval, the laser cooling light is switched off, and the
atoms fall freely under the influence of gravity. A sequence of pulses is used to prepare
the Cs atoms in the F=3, mr = 0 groundstate. Atoms in this state are then subjected to the
three-pulse interferometer sequence. The fraction of atoms driven to the F=4, mr=0 state
are detected using a normalizing resonance fluorescence technique.
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Fig. 2. Interference fringes vs. rotation rate for the atom interferometer
gyroscope. The shift in the contrast envelope provides a measurement
of the Earth's rotation rate.
We are using an all diode laser system similar to that developed for the gyroscope
(described above). Unlike the gyroscope experiment, we need to dynamically control the
relative frequencies of the Raman laser pulses. Using state-of-the-art direct digital
synthesis techniques, we are able to rapidly, and phase continuously, switch the frequency
and phase of the Raman laser pulses. Fig. 3 shows a scan of interference fringes recorded
from both chambers simultaneously. These fringes were recorded with a low resolution
T=10 msec time between pulses. In the near future we hope to extend this interrogation
time to T= 50 msec, and are working on improvements which should increase the signal to
noise by more than two orders of magnitude. If we are successful, our instrument
sensitivity should be -I0 E/(Hz) la.
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Fig. 3. Interference fringes obtained from the proof-of-principle gravity
gradiometer experiment. The fringes were obtained simultaneously from
two interferometers separated vertically by -1 m. In this low-resolution
mode, we do not expect a visible phase shift between the two traces.
References
1. Precision rotation measurements with an atom interferometer gyroscope, T. Gustavson,
P. Bouyer, and M. Kasevich, Phys. Rev. Lett. 78, 2046 (1997).
2. Microwave signal generation with optical injection locking, P. Bouyer, T. Gustavson, K.
Haritos and M. Kasevich, Optics Letters 21, 1502 (1996).
4


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