1995/1996 Technical Highlights
- Rabi Pedestal Shifts as a Diagnostic Tool in
Primary Frequency Standards. Jon Shirley,
David Lee, and Bob Drullinger of the Time and
Frequency Division in Boulder, in collaboration
with Daniel Rovera of the Laboratoire Primaire du
Temps et des Fréquences in Paris, have developed
a new method for evaluating certain systematic
frequency shifts in primary cesium frequency
standards. They use a digital servo system to
measure the frequency offset between the Rabi
pedestal and the Ramsey fringe for all seven
components of the hyperfine transition observed
in a cesium-beam standard. Measurement of the
dependence of each of these shifts on microwave
power enables them to separate three distinct
causes: Rabi pulling, cavity pulling, and magnetic field inhomogeneity.
The method was used to evaluate these three shifts for NIST-7, NIST's new
optically pumped cesium-beam frequency standard. The method
indicates a shift due to magnetic-field inhomogeneity
of 2 × 10-15, a shift due to cavity pulling of
6 × 10-15, and a shift due to Rabi pulling of less
than 1 × 10-16. One advantage of the method is that
it requires frequency measurements no better
than 1 × 10-11 to evaluate these exceedingly small
frequency biases. This work is part of a continuing effort
to find additional independent methods for measuring each of the systematic
errors in NIST-7. The shifts measured in this manner are consistent with
previous measurements using other methods, and add confidence to the NIST-7
uncertainty statement which is now 5 × 10-15.
This and other new evaluation methods hold the potential for improving
accuracy, perhaps by a factor of five. This means that the original goal of an
improvement of a factor of ten could be extended to a factor of one hundred,
producing the largest performance advance for any frequency standard
constructed at NIST. These advances give NIST a more comfortable margin over
commercial frequency standards which are now approaching uncertainties of
1 × 10-13. (R. Drullinger)
- Electronic Control Systems for Primary Frequency Standards.
David Lee and Craig Nelson are developing completely new electronic control
systems for NIST-7. While their first system will
be specific to NIST-7, their design philosophy
provides for an evolution that can support future
standards such as cesium-fountain and linear-ion-trap
frequency standards. The design brings together four work stations
interconnected with a local optical-fiber network. The fiber connections
dramatically reduce problems associated with
electrical ground loops bringing substantially more order to
the overall grounding of the different components of the standard.
The hardware changes are the most visible
aspect of this advance, but perhaps the most
important progress in the new system is the
introduction of object-oriented programming
techniques in the software. Using these techniques,
software objects such as microwave frequency synthesizers, digital c-field
supplies, and cesium-oven controllers are developed to
perform the various functions needed to run the
standard. These software objects are not specifically
designed around a given piece of hardware,
but rather define the very general functionality of
the object. The software objects interact with the
hardware components through device drivers.
Thus, new or different hardware components can
be introduced without major rewriting of software.
This will greatly simplify development work on
future frequency standards. Furthermore, the
objects can communicate with each other through
the network, so that interactions among the
various components can be easily reconfigured.
One benefit of this work is that new concepts for
evaluating errors or modifying the mode of operation
of the standard can be tested quickly, since
little effort is needed to implement such changes. (D. Lee)
- Development of a Cesium-Fountain Frequency
Standard. A cesium-fountain frequency standard
has been designed and is now under construction
in a collaborative effort involving staff of the Time
and Frequency Division and the Atomic Physics
Division as well as staff of the Politecnico di
Torino and the Istituto Elettrotecnico Nacionale
Galileo Ferraris (also in Torino). An agreement
between NIST and the two Italian institutions
formally establishes the international collaboration.
The end point of this work will have two
fountain standards at NIST and one at each of the
two Italian Institutions. Bob Drullinger, Fred
Walls, Tom Parker, Steve Jefferts, David Lee, Jon
Shirley, Leo Hollberg, Don Jennings, and Dawn
Meekhof of the Time and Frequency Division are
now assembling the first of these devices in
Boulder. This should be in preliminary operation
by early summer. Steve Ralston and Bill Phillips
of the Atomic Physics Division are studying a new
method for transverse cooling. If successful, this
cooling method might then be incorporated in the
Boulder standard allowing for operation at a higher flux without serious
collisional effects. Andrea DeMarchi of the Politecnico di Torino has been
involved with the design, particularly that of the microwave cavity.
This new standard operates by launching
laser-cooled cesium atoms vertically through a
microwave cavity. Then, under the influence of
gravity, they fall back through the same cavity
allowing Ramsey-type interrogation without the
usual end-to-end cavity phase shift. The atoms
move more slowly than those in an atomic beam
standard, so the Doppler shifts are much lower.
Furthermore, the long observation time results in
a narrower linewidth. This standard should
operate with an uncertainty of less than 1 × 10-15,
and has the advantage of using the same time-defining transition as is used in
present cesium-beam standards. (R. Drullinger)
- Frequency Synthesizers for Primary Frequency Standards.
Over the last several years Fred Walls of the Division has developed a
state-of-the-art synthesizer design that should prove useful for
the next several generations of primary atomic
frequency standards. These new generations of
standards demand exceptionally low-phase-noise
sources for interrogation of the clock transition.
His synthesizer exhibits a fractional frequency
stability of 1 × 10-16 at
20 min and 1 × 10-17 at
1 day. This should be more than adequate for all
frequency standards now under development. The
synthesizer is being used on NIST-7 and the
linear ion standard, and it will be used for
cesium-fountain projects in both the Time and
Frequency Division and the Atomic Physics
Division. Copies of the synthesizer have also been
delivered to the Naval Research Laboratory and to
a standards laboratory in Brazil, and four more
synthesizers are being constructed for standards
projects in other countries. (F. Walls)
- A 100 MHz Timing Distribution Amplifier. Fred
Walls, in collaboration with Marco Siccardi,
Stephania Römisch, and Andrea De Marchi of the
Politecnico de Torino in Italy, have developed a
100 MHz frequency distribution amplifier that can
support advanced timing measurements, even at
the performance levels of the next generations of
atomic frequency standards. Typical timing
distribution systems use 5 MHz or 10 MHz signals,
but, due to the sensitivity of currently available
phase detectors to both temperature and rf amplitude,
much better timing performance can be achieved at 100 MHz.
The amplifier provides five outputs for one input and exhibits signal
isolation of better than 100 dB. The 1/f noise added by the amplifier
stages is extremely low, -163 dBc/Hz at 10 Hz. In addition, a very
good match to 50 %Omega;
at both the input and output reduces the voltage-standing-wave ratio, thus
minimizing the phase errors that accompany signal reflection. (F. Walls)
- Demonstration of a Cryogenic Linear-Ion-Trap Frequency Standard.
John Miller, Dana Berkeland, Jim Bergquist, Wayne Itano, and Dave
Wineland of the Time and Frequency Division
have developed a cryogenic linear-ion-trap system
that can be used to investigate both microwave
and optical frequency standards. Presently, the
ion used for both standards is 199Hg+. Trapping of
ions eliminates the first-order Doppler shift and
laser cooling reduces the second-order Doppler
shift to a very low value. Since observation times
can be extremely long, linewidths can be very
narrow leading to high stability, even for a small
number of ions. For this system, fundamental
systematic uncertainties are known to nearly one
part in 1018 on the optical clock transition. Of
course, other technical problems might limit performance short of this goal.
I
n a preliminary test of the microwave standard, the group has demonstrated
a frequency stability of 3 × 10-15 at 1,000 s,
but further work should improve this result. The dominant systematic effect
appears to be caused by a second-order magnetic field shift due to the
presence of currents flowing in the trap electrodes at the trap drive frequency.
Efforts are currently underway to reduce this shift and establish a
calibration procedure to remove its effects. (J. Bergquist)
- Crystalline Nonneutral Plasmas. An atomic
frequency standard with good signal-to-noise
performance can be constructed using large
numbers (>105) of ions contained in a Penning
trap (which achieves trapping using static magnetic
and electric fields). However, to date it has
been difficult to precisely characterize and control
the Doppler shifts associated with the magnetron
rotation of such a stored ion plasma. Recently,
Joseph Tan, John Bollinger, Pei Huang, Wayne
Itano, Brana Jelenkovic, and Dave Wineland of
the Division have cooled such a plasma to form a
rigid solid, and have developed a method for
controlling the rotation rate of this rotating solid.
Their methods bring promise for the development
of a frequency standard of high accuracy and
excellent short-term stability. In addition, they
now have strong indications that their plasmas
are sufficiently large that they exhibit bulk behavior
and could be used to study infinite, strongly
coupled one-component plasmas. This is significant
because such plasmas are models of dense
astrophysical matter and this is the first laboratory
system with the potential of generating them in the strongly coupled
regime.
The group had previously observed Bragg
scattering of laser light from crystallized plasmas,
but the rotation of the plasma converted the
usual Laue dot pattern to one of concentric rings.
This did not allow identification of the lattice type.
In recent experiments they have gated (gate time
small compared to the rotation period) an imaging
system synchronously with the plasma rotation.
This has allowed them to recover the Laue dot
pattern and helped identify the favored lattice
type as body centered cubic (bcc). This is the
predicted lattice for an infinite, strongly coupled
one-component plasma. The group has also used
a "rotating wall" to precisely control the magnetron
rotation of the crystalline plasma. The rotating wall
is a rotating electric field generated by six
electrodes arranged around the equator of the
trap. Bragg scattering studies show that the
crystalline plasma orientation can be phase
locked to the rotating electric field. This allows
precise reproduction of the same rotation rate
from experiment to experiment, an important step
in controlling the time dilation shift due to the
plasma rotation. Other conditions that need to be
controlled to produce a constant time dilation
shift are the number of trapped ions and the
strengths of the trapping fields. (J. Bollinger)
Figure 1. Synchronized Bragg-scattering pattern for a cooled ion
plasma. The crystallized plasma rotates at a frequency of 140 kHz, so
the imaging system required for these observations must be gated in proper
phase with the rotation to obtain this pattern. The rectangular outline near
the center is a stop inserted to block the direct laser beam. A regular grid
can be drawn through these dots providing evidence that this is a bcc lattice.
- Quantum Limits to Measurement and a Quantum Computer.
In earlier experiments, the Ion Storage Group observed what can be called
"quantum projection noise." In spectroscopy, this
source of noise is caused by the statistical fluctuations
in the measured number of absorbers (atomic ions in this case) that are
observed to undergo the transition of interest. This was the
first observation of this noise source in spectroscopy,
and the Group developed and confirmed the theory that describes it.
Having reached this noise floor, John
Bollinger, Wayne Itano, and Dave Wineland of the
Group then proposed a method for using
quantum-mechanically correlated states to reduce
the noise below this level. The promise of this
method was sufficiently high that a program
aimed at realizing the concept was initiated. Their
approach involved the development of a linear ion
trap capable of storing one-dimensional "strings"
of ions along the trap axis. As this work progressed,
I. Cirac and P. Zoller from the University
of Innsbruck proposed using an identical configuration
to produce a quantum computer. The
synergism of these two apparently disparate
projects, an atomic frequency standard and a
quantum computer, is extremely high. In fact, the
work that needs to be done to prove the quantum-computer
idea will lead quite naturally to the
reduced-noise frequency standard, so work on
these two projects is now being pursued concurrently.
In the first phase of the computer project,
Chris Monroe, Dawn Meekhof, Brian King, Wayne
Itano, and Dave Wineland of the Time and Frequency
Division have demonstrated the operation
of a two-bit "controlled-NOT" quantum logic gate,
a fundamental building block of a quantum
computer. The two quantum bits are stored in the
internal and external degrees of freedom of a
single trapped ion which is first laser-cooled to
the zero-point energy. This gate is a simplified
version of the Cirac/Zoller scheme. Although this
minimal system is not useful for computation, it
illustrates the basic operations and the problems
associated with constructing a large scale quantum computer.
The interest in quantum computation stems
from the fact that certain problems can be more
efficiently solved on a quantum computer than on
a classical computer. In particular, a quantum
computer should be able to factorize large numbers very
efficiently. This is of interest, because
the security of many data encryption schemes
relies on the inability of classical computers to
factorize large numbers. (D. Wineland)
- Observation of a Schrödinger Cat State. In
recent experiments in the Division, Chris Monroe,
Dawn Meekhof, Brian King, and Dave Wineland
generated a "Schrödinger cat-like" state of matter
at the single atom level. In 1935 Schrödinger
developed a thought experiment where a cat is
placed in a quantum superposition of being dead
and alive (correlated with a single radioactive
atom that has and has not decayed, respectively).
The state of the system is represented by an
entangled quantum mechanical wavefunction
involving a superposition of two different states.
This situation of course defies our sense of reality,
where we only observe live or dead cats and we
expect that there exist only live and dead cats
independent of our observation. This is a classic
illustration of the conflict between the existence of
quantum superpositions and our real world
experience of observation and measurement.
Although superposition states such as
Schrödinger cat states appear to be absent from
the macroscopic world, there is great interest in
creating mesoscopic systems (systems having
both microscopic and macroscopic features and
hence bridging the gap between the quantum and
classical worlds). These types of experiments may
provide an interesting proving ground in the
controversial theory of quantum measurement.
In their experiments, a single laser-cooled and
trapped 9Be+ ion is prepared in a quantum
superposition of two separate localized positions
correlated with different internal states of the ion.
This state is prepared by applying several pulses
of laser radiation, which "entangle" the internal
(electronic) and external (classical-like motional)
states of the ion. They verify the superposition by
detecting the quantum mechanical interference
between the localized wavepackets. Of critical
importance in these experiments is the high level
of control of the motion of the ion, from the initial
laser cooling to the zero-point of energy to the
excitation to higher-energy coherent states of
motion in the harmonic potential. (C. Monroe)
- Quantum States of Motion of a Trapped Atom.
In a generalization of the "Schrödinger cat" work,
Dawn Meekhof, Didi Leibfried, Chris Monroe,
Brian King, Wayne Itano, and Dave Wineland
have recently reported the creation and full
determination of several quantum states of
motion of a 9Be+ ion bound in a rf (Paul) trap. The
states were coherently prepared from an ion that
was initially laser cooled to the zero-point of
motion. They have created states having both
classical and nonclassical character including,
thermal, number, coherent, squeezed, and
"Schrödinger cat" states. They have then fully
reconstructed the motional state using two novel
schemes. One determines the density matrix in
the number state basis, and the other determines
the Wigner function. Their techniques, which are
extendable to several simultaneously trapped ions
and to other quantum systems, should allow for
well-controlled experiments on decoherence and
related phenomena on the quantum-classical borderline. (W. Itano)
- Improvements in the AT1 Time Scale. Tom Parker, Jim Gray, and
Judah Levine have implemented a number of improvements in the AT1 time scale
allowing the Division to more accurately realize UTC in real time. Over the
last 17 months, UTC(NIST) has been held within 50 ns of the
international UTC maintained by the BIPM. This provides a more accurate time
scale for dissemination to high-end users in the United States, and improves
the quality of input data to the BIPM on the frequency of the
primary-frequency standard, NIST-7.
The key improvements involve the addition of new commercial hydrogen
masers and cesium-beam standards to the scale and the development of new
reset procedures in the AT1 algorithm. The drift rate of the scale was
reduced from 1 × 10-16/day to
3 × 10-17/day. The level of random
fluctuations of AT1 was also reduced by a factor
of 2 to 2 × 10-15 at 100 days. These
improvements, shown in Fig. 2, continue a long history of world
leadership in time scale operation. Further improvements are expected as two
additional masers are added to the scale. (T. Parker)
Figure 2. NIST time scale performance. UTC - UTC(NIST) is plotted
against the Modified Julian Date (MJD). The time constant for steering to
international UTC is apparent in the oscillations. The BIPM feedback of
offsets between UTC and UTC(NIST) occurs between 1 and 2 months after
data is provided to the BIPM.
- Two-Way Satellite Time and Frequency Transfer (TWSTFT). Christine
Hackman, Tom Parker, Franklin Ascarrunz, Steve Jefferts, and Victor Zhang
have advanced NIST's TWSTFT capability through four separate projects. In the
first, an empirical method was developed for correcting for effects of
unevenly spaced data. Two-way time transfer data is typically taken three days
per week. Traditional analysis methods, based on evenly spaced data, could not
properly account for the noise in the unevenly spaced data, thus limiting
knowledge of the measurement uncertainty. In a second project, a complete
analysis was made of the time transfer noise between averaging times of
1 s and 100 days. This first-ever analysis indicated a time
variance, σx(τ), of 100 ps at an averaging time of
100 s and 1 ns with averaging of 1 day clearly establishing the
value of the method. In the third project, an investigation was made of
systematic variations in the delays through the earth station caused by
variations in temperature and microwave power. These measurements provide the
basis for correcting for these effects. Finally, improved automation of
analysis of the TWSTFT data was implemented saving substantial staff effort
in handling of the large data files involved. (T. Parker)
- Multi-Channel GPS Receivers for Time Transfer.
Judah Levine of NIST, Al Gifford of the U.S.
Naval Observatory, and Tom Bartholomew of The
Analytical Sciences Corporation are collaborating
on the characterization of time transfer using
commercial multi-channel GPS receivers. These
receivers make simultaneous observations of eight
GPS satellites and support dual-wavelength
observations which can be used to estimate the
satellite-receiver delay associated with the
ionosphere. Variations in ionospheric delay are an
important contribution to uncertainty in GPS time
transfer. The current method used by NIST
involves simultaneous "common-view"
observations of the same satellite by different
observers, and subsequent processing of many
observations to remove common-mode delay
errors. This new approach uses a simpler "melting-pot"
algorithm for the time transfer process.
The current experiments are being carried out
between NIST and the Naval observatory. Making
use of the same type of commercial receivers, the
group also plans to make measurements between
NIST and NASA's Jet Propulsion Laboratory using
carrier-phase observations. Such measurements
are potentially more accurate, but pose a
challenge in resolving the carrier-phase
ambiguity, that is, in determining that both sites
have identified the same cycle of the carrier and
that this identification does not slip during the
observation period. (J. Levine)
- Telecommunications Synchronization. Marc
Weiss is leading efforts to integrate synchronization
concepts developed by the telecommunications industry
and by the time-and-frequency community. As part of this effort, NIST has now
sponsored five (annual) workshops on synchronization
with attendance growing to more than 60
participants per year, mostly from U.S. industry.
These workshops grew out of earlier NIST work on
synchronization interface standards. The first one
was held to familiarize the industry with NIST-developed
methods for characterizing synchronization systems. NIST became involved
when the industry asked for assistance in developing
more useful timing measures. The very
rapid success of this venture along with the rapid
acceptance of the measures as national and
international standards cemented a working
relationship that has stimulated the continuation
of the workshop into something that more nearly
represents a conference on telecommunications synchronization.
I
n a related project, Steve Jefferts and Marc
Weiss are developing a system for two-way time
transfer in optical-fiber (SONET) links for application
to synchronization of network nodes. The
system stability has been shown to be better than
100 ps over a period of 4 hours. This work
responds to growing international interest in
transmitting timing signals for use within the network. (M. Weiss)
- Possible Standard for SONET Time Transmission.
A format for transmitting time through
SONET systems has recently been developed. The
format involves a single byte of SONET overhead,
a part of the SONET frame that does not go
through transmission buffers which cause
variation in transmission delay. The format is
already being used by one company that is
developing SONET timing systems for the
Department of Defense. It has also been submitted
to the International Telecommunications
Union (ITU) for consideration as an international
standard. The ITU has subsequently submitted
the proposal to a study group for more detailed
consideration. (J. Levine)
- Algorithm for Improving Time Dissemination
Through Networks. In a collaborative effort,
Judah Levine along with David Mills of the
University of Delaware and Greg Woods of the
National Center for Atmospheric Research have
developed an algorithm that is now in use for
disseminating time signals through the Internet.
The algorithm separates the noise in the calibration
channel from the noise in the clock itself and
adjusts parameters of the algorithm to optimize
performance. The concept used is an application
of an earlier patent (granted to NIST) on a "Smart-Clock"
concept for maintaining synchronization of
a remote clock to a master clock with a minimal
number of transmissions. The NIST-patent
concept was developed by Dave Allan, Dick Davis,
Judah Levine, and Marc Weiss. Their system
permits time servers to be located anywhere on
the Internet and minimizes variabilities
introduced by long network paths. A new version
of the algorithm is under development. It will
improve characterization of the network delays. (J. Levine)
- An Improved Variance for Characterizing
Oscillators. Dave Howe of the Time and Frequency
Division has developed an improved variance, closely related to the Allan
variance, that provides increased confidence level at long
averaging time. This is important because long-term
data, requiring long measurement time, is
the most costly to obtain. As the method is
adopted, manufacturers of oscillators can expect
to either reduce measurement time or increase measurement accuracy.
The improvement follows from the simple
observation that the procedure for the Allan
variance, sometimes called the two-sample
variance, measures only frequency variations with
an odd symmetry at and near the longest
averaging time resulting in a bias. This work
constructs a three-sample variance of even
symmetry and combines this result with the two-sample
variance in order to minimize the bias and
thus improve the confidence interval. The process
bears a similarity to the method of complex
demodulation used in signal processing. The gain
in measurement confidence depends on the noise
type involved. For white phase-modulation noise,
the improvement for the longest averaging time in
a data set can be better than an order of
magnitude. The improvement is significant, but
not quite as dramatic, for higher order noise processes. (D. Howe)
- Fundamental Limits on the Frequency Stabilities
of Crystal Oscillators. Fred walls of NIST and
John Vig of the U.S. Army Research Laboratory
have recently published a review of the
instabilities in precision bulk-acoustic-wave
(BAW) quartz crystal oscillators. This is the most
comprehensive review of this topic in the literature.
Their examination of the fundamental limits
on achievable frequency stabilities and the degree
to which these fundamental limits have been
approached provide researchers with a road map
for improving the performance of oscillators.
Highlights of their study include thermodynamic
limits to temperature stability, the limits imposed
by noise generated in the electronic sustaining
stage, the effects of static and dynamic
temperature fluctuations, and the possible role of
background ionizing radiation on long-term
frequency drift. They conclude their study with a
discussion of the ideal resonator and suggest a
levitation method for suspending a resonator so
as to minimize (or eliminate) the non-ideal effects
of resonator suspensions. (F. Walls)
- Portable Microwave Phase Noise Standards.
Fred Walls has designed and constructed new
portable phase noise standards covering the
frequency range from 10 GHz to 40 GHz. These
devices generate stable and quantifiable levels of
phase noise and are used in round-robin tests of
user measurement systems. These were developed
with Department of Defense support and three of
the standards are being supplied to military
calibration laboratories. A fourth system is being
retained by NIST for its own calibration services.
The device has already been used in several tests
of commercial measurement systems and has
proven useful in establishing the accuracy of those systems. (F. Walls)
- Low Noise RF Devices. Fred Walls,
Eva Ferre-Pikal, and Steve Jefferts of the Division have
collaborated in the development of design rules
that are proving highly effective in reducing
close-to-the-carrier noise in semiconductor amplifiers
and related devices. They initially developed the
theory describing just how 1/f noise enters these
devices and then generated design rules that
would minimize the noise. Amplifiers constructed
using their methods exhibit 1/f noise that is
below thermal noise for all frequencies above a
few hertz. By comparison, in conventional
amplifiers this crossover occurs somewhere
between 100 Hz and 1 kHz. A key component of
this research effort was the development of
measurement systems for measuring very low
levels of both PM and AM noise. In order to
transfer this capability to industry, the Division
now offers special courses on low noise amplifier
design, and several have been presented. Figure 3 shows an example of
performance improvement (F. Walls)
Figure 3. Phase noise as a function of
frequency measured from the carrier for a
bipolar-junction-transistor amplifier. This
figure shows the noise performance of both a
conventional state-of-the-art amplifier and the
same amplifier after improvements have been
made using the techniques described here.
- Voltage Noise in Chemical Cells. Chadwick
Boggs and Alan Doak of the University of Colorado,
along with Fred Walls of the Division have
recently reported measurements of voltage noise
on a variety of chemical cells. Chemical cells have
often been used in electronics for their low
current drift, isolation of a particular circuit from
other circuits, and low voltage noise. While it is
clear that voltage noise on these cells is small,
actual values for the voltage noise have not been
reported, so there has been little evidence suggesting
that one type of cell might be better than another.
This work was made possible by the development
of a cross-correlation measurement system
capable of measuring voltage noise below
-200 dBV/Hz. The method involves two parallel
measurement channels. Because the measurement noises
in these two channels are completely
independent, a cross-correlation of the outputs of
the two channels effectively rejects the
measurement noise while recovering the noise on the device under test.
Nickel-cadmium cells exhibited the lowest noise of all cells tested over
the frequency range from 1 Hz to 60 kHz. An AA-size nickel-cadmium
cell showed a noise of -205 dBV/Hz at 10 kHz. This noise level is
consistent with the Johnson noise produced by the 0.2 %Omega;
internal resistance of the cell. The study involved nickel-cadmium,
alkaline, lithium, and mercury cells. A general conclusion of the study was
that the dominant broadband noise process in cells is the Johnson
noise arising from the internal resistance. This clearly indicates that
larger capacity batteries with lower internal resistance should produce
lower voltage noise. The voltage noise was found to be independent of bias
current suggesting that shot noise is not significant. The results of this
study should provide guidance in electronics applications demanding the
lowest possible levels of voltage noise. (F. Walls)
- Computer Time Services Expand. The Time
and Frequency Division operates two time
services, one through the telephone system and
one through the Internet. Both systems have
recently been expanded to accommodate
increased usage. The Automated Computer Time
Service (ACTS), which provides digital signals
through the telephone system has been expanded
to fourteen telephone lines, twelve for use with
high-speed modems and two for use at low speed
where the modem delay is more predictable. Since
this system provides a mechanism for measuring
telephone transmission delay, the latter option
provides lower time uncertainty (1 ms to 3 ms). ACTS
now handles more than 10,000 calls per day and usage is still growing.
The NIST Network Time Service (NNTS),
introduced less than three years ago, has grown
even more dramatically. Three servers are now
handling nearly 200,000 calls per day and three
new servers will be added this year bringing the
total to six. Two will be in Boulder, two in
Gaithersburg and two on the west coast. This will allow
users in the U.S. to select a server in reasonable
proximity, thus minimizing transmission delay. (J. Levine)
- Windows Software for Net and Telephone Time
Services. Judah Levine of the Division has
recently developed two windows programs, one for
use with the Automated Computer Time Service
(ACTS) and one for use with the NIST Network
Time Service (NNTS). These user friendly
programs will be available for distribution upon
completion of beta testing. The software, including
all source code is made available so that
commercial software writers can incorporate it
into systems with broader application. (J. Levine)
- Upgraded Time Broadcasts from WWVB. The
Services Group of the Division has embarked on
a major renovation and enhancement of the time
broadcasts from radio station WWVB in Fort
Collins, Colorado. These broadcasts are attractive
for a number of applications because receivers
and their antennas can be particularly simple,
small, and inexpensive, and because signal
reception within buildings is feasible. However,
the presently available signal strength has been
marginal, particularly in areas along the U.S. east
coast. One of the key objectives of this effort is to
increase the signal strength by at least 6 dB. This
will provide U.S. industry options for new
products serving both high-end timing applications and consumer needs.
A key roadblock to this project has been the
very high cost of new transmitters. This difficulty
was overcome when the Navy agreed to provide
NIST with three spare high-power transmitters
which they had in storage. Purchased new, these
transmitters would have cost NIST more than
$1M. Installation of the transmitters requires
substantial engineering modifications at the site.
These include new transmission lines to
antennas, new impedance matching networks,
and modified interfacing to the transmitters. The
present plan (barring major changes in available funding) is to complete
installation and modifications by September of 1997. (D. Hanson)
- Diode-Laser Wavelength Standard for Absolute
Distance Measurement. A major milestone has
been reached in a collaboration on wavelength
standards between Divisions of the Physics
Laboratory and the Manufacturing Engineering
Laboratory. Swept-frequency lasers, developed
jointly by staff of the Time and Frequency Division
in Boulder and the Precision Engineering Division
in Gaithersburg, are now being used in studies on
improved absolute-distance measurement being
conducted by the Precision Engineering Division.
Both Divisions contributed to the design and both
will characterize the laser for this application. The
Precision Engineering Division has responsibility
for length metrology, while the Time and
Frequency Division has responsibility for providing access to the second, in
part via laser wavelength reference standards.
The laser system for this project was
developed by Michelle Stephens and Leo Hollberg of the
Division, who converted a commercial diode laser
into a tunable laser with the desired sweep
characteristics. The laser system is now being
evaluated by Lowell Howard and Jack Stone of the
Manufacturing Engineering Laboratory. By
comparison with commercial, tunable, visible
diode lasers, the one developed at NIST can sweep
either 100 times farther than those with
comparable rate or 100 times faster than those
with a comparable range. With its unique ability
to continuously and rapidly sweep its central
wavelength near 680 nm over a range of 2 nm, the
laser will allow development of an absolute-distance
interferometry system which can overcome the
limitations of present displacement
interferometry systems which require an uninterrupted
beam. With the new interferometer system,
absolute distance between the reflectors of the
interferometer can be measured directly many
times per second rather than requiring the
position of one reflector to be physically translated
over the distance to be measured. (M. Stephens, L. Howard)
- IUPAC Sets Wavenumber Standards for the Infrared. The International
Union of Pure and Applied Chemistry (IUPAC) Working Group on "Unified
Wavenumber Standards" has recently issued a set of recommendations for
high resolution wavenumber standards for the infrared region of the spectrum.
The document provides wavenumber standards for calibration of high-resolution
infrared spectrometers where measurements have traditionally been more precise
than accurate. The most accurate standards are derived from direct
frequency-measured, saturated infrared absorptions. These are converted to
wavenumber using the 1983 redefinition of the meter in terms of the second.
The spectral lines cited by the Group cover the range from about
4 cm-1 to about 7,000 cm-1. The standards will
allow much better comparison of measurements made at different
laboratories.
NIST representation in this report included Joe Wells and Ken Evenson of the
Division and Art Maki of the University of Washington (formerly with the
former NIST Molecular Physics Division). A very large share of the
measurements cited in the recommendations were made at NIST. Many of these
recommendations are taken from NIST Special Publication 821,
"Wavenumber Calibration Tables from Heterodyne Frequency
Measurements" by A.G. Maki and J.S. Wells.
(K. Evenson)
- Optical Frequency Division by a Factor of
Three. Joe Wells and Leo Hollberg of the Division,
along with Olivier Pfister of the University of
Colorado, Manfred Mürtz of the University of
Bonn, and James Murray of the University of
Arizona have developed a new scheme for coherently
connecting optical frequencies in a 3:1 ratio.
They have demonstrated the method by locking
the outputs of a Nd:YAG laser at 1064 nm with a
CO overtone laser at 3192 nm. This is a
significant step in the development of simpler
frequency synthesis chains, since in combination
with divide by 2 devices, greater flexibility in
design is achieved. There are a number of
examples where division by 3 is advantageous.
For example, at NIST the very high-Q ultraviolet
transition at 282 nm in Hg+ will be excited with
the twice-frequency-doubled ND:FAP laser at
1126 nm. When divided by a factor of 3, this
yields a wavelength of 3378 nm, which is close to
the very important methane reference.
This work is part of a larger Division effort to develop new methods and
components that can contribute to the simplification of optical frequency
measurement and the construction of much simpler frequency synthesis chains.
The long term objective is a robust and simple chain linking the cesium
frequency standard to the optical region. (L. Hollberg)
- Calcium Optical Molasses. Michelle Stephens,
Chris Oates, and Leo Hollberg have cooled and
trapped calcium using frequency-doubled diode
lasers. They had previously done high-resolution
spectroscopic studies of the 657 nm line in
calcium, but this earlier work was limited by the
Ramsey-method, interaction-time linewidth of a
few kHz. The cooled and trapped atoms should
now allow them to observe the intrinsic linewidth
of 400 Hz. Furthermore, the use of diode lasers at
423 nm for cooling and trapping of the atoms and
for interrogation of the 657 nm transition results
in a relatively small system that could be made
transportable for comparisons. This is a
particularly attractive optical frequency standard
because the first-order Doppler shift is removed
by the trapping, the second-order shift is
substantially reduced by cooling, and the 657 nm line is only slightly
sensitive (in second order) to electric and magnetic fields.
The system that generates the 423 nm
radiation is particularly efficient. Infrared diode-laser
radiation is doubled to 423 nm using a
potassium niobate crystal. The excellent matching
of this laser and crystal is such that the blue
output is down in power from the IR laser by only
a factor of 3. More power is now available at the
423 nm wavelength than at 657 nm. (M. Stephens)
- Optical-Delay-Line Oscillator. John Kitching,
Leo Hollberg, and Fred Walls of the Division have
recently demonstrated a 1 GHz optical-delay-line
oscillator driven by a diode laser. This is not the
first such oscillator, but this one differs from
previous devices in its use of a directly modulated
diode laser rather than a fixed-frequency laser
and an electro-optic modulator. Light from the
diode laser travels down the fiber to strike a
detector where it is converted to an electronic
signal that is filtered, amplified, and then applied
back to the injection current of the diode laser. When the loop gain exceeds
unity, the system oscillates at a frequency consistent with phase shift
closure (a multiple of 2π) around the loop. The very low phase noise of the
system derives from the long delay time and very low loss provided by the
optical fiber.
The study concludes that optical-delay-line oscillators might be good
alternatives to crystal and dielectric-resonator oscillators if the oscillation
frequency can be increased beyond 10 GHz. At 1 GHz they observe a
single-sideband phase noise spectrum decreasing as roughly 1/f2
attaining a value of -138 dB below the carrier in a 1 Hz bandwidth
at 20 kHz offset. These oscillators are in their infancy and there is much
room for improvement. For example, the fiber-generated noise can be reduced
by running at 1.3 µm rather than 850 nm, since fibers at
1.3 µm exhibit much lower loss and scattering.
Extending this work to higher frequency will require high power lasers with
large modulation bandwidths and high-speed photodiodes that can handle tens of
mW of optical power. (L. Hollberg)
- 30 THz Mixing Experiments Using High-Tc Josephson
Junctions. Eric Grossman, Leila Vale, and David Rudman of the
Electromagnetic Technology Division of EEEL along with Ken
Evenson and Lyndon Zink of the Time and
Frequency Division recently published a paper
describing their experiments on high-frequency
mixing in high-Tc Josephson junctions. They
directly observed second-order difference frequencies from
10 MHz to 12.8 GHz between two
CO2 laser lines near 30 THz. Applying a third
microwave signal to the junction, they observed
laser difference frequencies up to 27 GHz. This is
the first observation of Josephson mixing at CO2
frequencies in high-Tc junctions. The hope is that
these devices can be further developed to provide
simpler means for synthesizing and measuring
optical signals at arbitrary frequencies.
The thin-film YBa2Cu3O7-δ
superconductor-normal-superconductor junctions were fabricated in EEEL's
Boulder facilities. The dc-bias dependence of the difference signal, as well
as other evidence, suggests two distinct mixing mechanisms: hot-electron
mixing in the junction banks at high dc bias, and bolometric Josephson mixing
at low dc bias. The latter mixing mechanism is superior for third and
higher-order mixing products. (K. Evenson)
- Observation of Laser Oscillation Without
Population Inversion. In a recent collaboration
with scientists from Texas A&M University and
Russia's Lebedev Institute of Physics, Leo
Hollberg and Hugh Robinson of the Time and
Frequency Division have demonstrated laser
oscillation without population inversion (LWI).
Collaborators at Texas A&M included A.S. Zibrov, M.D. Lukin,
D.E. Nikonov, and M.O. Scully. V.L. Velichansky of the Lebedev
Institute also participated in the work. The experiments, carried
out at NIST, followed the surprising theoretical
prediction that, in a V-type configuration of
energy levels, atomic coherence can result in gain
without population inversion. A strong driving
field on one transition has a significant effect on
a second transition that shares the same ground
state as the driven transition. Related coherence
effects had already been observed, but this was
the first demonstration of actual lasing without inversion.
The experiments were done in a rubidium cell using only 10 mW of drive
power from diode lasers. Measurements showed that the observed oscillation
occurred without population inversion, and that it was the presence of the
coherent drive-laser radiation that produced the conditions necessary for
oscillation. Earlier, the group reported cw amplification without inversion
in the same system, but more efficient containment of photons was needed
before the system would oscillate.
The work has possible practical implications.
In particular it suggests the possibility that short
wavelength lasers (in the ultraviolet region and
beyond) might be feasible. While the work
described here involved laser oscillation at a
wavelength not too far from the wavelength of the
drive laser, there is now the clear possibility that
the same approach could be used to produce laser
oscillation at much shorter wavelength, far removed from the wavelength of the
drive laser. Experiments to test this possibility are in progress.
(L. Hollberg)
- Improved CO2 Laser. Ken Evenson, using a new
grating produced by a local Boulder optical
manufacturer, has developed a CO2 laser that
oscillates on more than 275 lines with a maximum power
output of 30 W. Most CO2 lasers will
oscillate on only about 80 lines. The laser uses a
grating to couple power out of the laser in zero
order. The new laser, when locked to sub-Doppler
lines in CO2, is an excellent infrared frequency
and wavelength standard. The laser is also a good
source of radiation for pumping far-infrared lasers.
Three years ago, Evenson developed a new
ribbed laser tube. The ribs act as irises and
greatly increase the grating resolution of the laser
cavity. This provided a big improvement in
performance, because the ribbed structure
reduces or eliminates modes associated with
radiation bouncing off the tube walls. The latest
improvement is a direct result of the improved
grating which couples out 3 % of the power over the region from 9 µm
to 11.5 µm. The new grating, with 150 lines/mm, was developed by
a local optical manufacturer in direct response to NIST
requirements and was tested and proven at NIST.
Figure 4 below shows the laser output as the
grating angle is scanned to select the various
laser lines. The laser has a mirror separation of
2.16 m and a diameter of 18 mm. (K. Evenson)
Figure 4. Output of the new CO2 laser as a function of
wavelength. This plot was made by scanning the grating angle to select the
various laser lines.
- High Resolution Spectroscopy Supporting
Atmospheric Chemistry Studies. Diode-laser
systems developed for use with optical and
microwave frequency standards are also proving
useful for spectroscopic detection of molecular
species important in the chemistry of the upper
atmosphere. With NOAA's atmospheric scientists
sharing adjacent facilities, a natural collaboration
has developed on the application of these lasers to
their measurement problems. A notable recent
success is a collaboration between NOAA
scientists and Rich Fox of NIST on the detection
of NO3. This radical plays a key role in nighttime
atmospheric chemistry, interacting for example
with molecules involving chlorine. NO3 dissociates
in sunlight. Their recently reported measurements
quantify NO3 reactions with other key species
providing input to atmospheric models.
NO3 has a strong electronic absorption band
at 662 nm that can be reached with commercial
red diode lasers. In these experiments a solitary
diode laser is used to detect the time-dependent
concentration of NO3 in a cell containing gases of
atmospheric interest. Good detection sensitivity is
achieved with direct absorption measurements.
The interaction path is increased using reflected
multiple passage of the laser beam through the
cell. In addition, they have frequency doubled the
light from infrared diode lasers to produce
precisely tunable blue light that can be used to
detect other important atomic and molecular
resonances. For example, laser light at 425 nm,
generated by this technique, has been applied to
the high-sensitivity detection of the IO molecule. Both NO3
and IO play important roles in determining the concentration of ozone in the
atmosphere. (R. Fox)
- Detection of Methane in Air. In a collaborative
program, Frank Tittel of Rice University, Ed
Dlugokencky of NOAA, and Steve Waltman of the
Division have developed a laser-spectroscopy-based
system that can determine methane
concentration in air to 1 × 10-9. The approach is
dramatically simpler and the system much more
portable than the chemical processing and gas
chromatography methods currently used.
Furthermore, the new spectroscopic method is a
non-destructive, real-time measurement as
opposed to the laborious and destructive analytical
measurements now employed. Methane
concentrations in the atmosphere are believed to
have a substantial impact on the greenhouse
effect, so simpler methods for long-term monitoring
are especially important. The normal concentration
of methane in air is about 2 × 10-6, and
measurements over the last decade indicate that
its average concentration has been increasing at
a rate of about 1 × 10-8 per year for reasons that
are not yet well understood.
The spectroscopic system used for this detection operates at a laser
wavelength of approximately 3.3 µm on a methane line that is well
separated from a weak water line, so the measurements are not disturbed by
varying water concentration. The 3.3 µm radiation is generated by
difference mixing the outputs of a diode-pumped YAG laser at 1.06 µm and
a diode laser operating at 805 nm. The system could easily be designed to
be transportable. It could also be installed at fixed locations to provide
continuous monitoring. An additional benefit of spectroscopic monitoring is
that the system can be tuned to be sensitive only to methane containing
13C rather than the usual 12C atoms. Thus, it should be
possible to use this non-radioactive species in a method similar to
radioactive labeling to monitor the movement of 13C-labeled methane
through any type of system. (S. Waltman)
- New Observations Using LMR Spectroscopy. Ken Evenson of the Time
and Frequency Division, along with John Brown of Oxford University and
Helga Koersgen of the University of Bonn, have recently made several new
observations using laser magnetic resonance (LMR) spectroscopy. They have made
the first direct observation of the far-infrared, J=3/2 → J=1/2
transition of the Fe+ ion at 86.7 µm. Their measurement
uncertainty for this transition is about 100 times smaller than that of
indirect observations made using differences of optical-wavelength
measurements. They have also used LMR to observe the spectrum of the
FeD2 molecule near 6.9 THz (43 µm). This required
modification of their spectrometer so that it could operate at these high
frequencies. To date, these are the highest frequency FIR LMR observations
and the first FIR observations of a vibrational bending spectrum made using
LMR spectroscopy.
Iron is presumed to be an abundant interstellar species, and its spectrum is
prominent in solar observations. However, it has never been observed in
interstellar space. These new observations provide radio astronomers with
reference spectra for searches for these particular iron species. The
development of a capability for higher frequency observations opens up
significant new opportunities for measurements of importance not only to radio
astronomy, but also to upper-atmospheric research. The modified spectrometer
now covers frequencies to the upper limit used by radio astronomers. This
expanded range covers fine-structure transitions in a number of atoms and
molecules allowing for the exacting laboratory frequency measurements needed
to support searches for these atoms in space. Furthermore, ClO, an important
molecule in the upper atmosphere, has a fine structure transition at
8.2 THz. This transition might provide the best means for determining the
abundance of this species in the upper atmosphere, contributing information
on atmospheric (ozone) chemistry. (K. Evenson)
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