to develop new measurement
systems and methods
in support of emerging
technologies.
INTENDED OUTCOME AND
BACKGROUND
In addition to meeting current customer
needs, the Division prepares for the
future of time and frequency measurements
and calibrations. Through interactions
and discussions with constituents,
we identify important emerging
requirements and technologies. We
strive to apply our expertise and creativity
to those applications with the potential
for greatest impact on U.S. industry,
science, and the general public.
Synthesis and measurement of optical
frequencies is crucial to the future of
Division programs, and time and frequency
metrology in general. Division
expertise in developing and applying
frequency combs based on femtosecond
lasers has led to measurement of frequencies
with relative uncertainties
approaching 10-19 (0.1 aHz/Hz),
orders of magnitude better than previously
possible, and to direct comparison
of microwave and optical frequency
standards, bridging five decades in frequency.
We are working on techniques
for amplification, noise reduction, and
applications across different frequency
ranges, such as the important near-infrared
telecommunications range.
A second key thrust is development of
new tools to better measure close-to-carrier
noise in oscillators and other electronic
components. Such measurements
are crucial to development of new oscillators,
microwave and optical, used in
advanced radars, telecommunications,
high-speed digital circuits, and many
other applications. Much of this work is
conducted with significant support from
DARPA, involving NIST, industry, and research organizations.
A third major program is the development
of ultra-miniature atomic frequency
standards, to dramatically improve
the performance of small electronic
devices such as GPS receivers and wireless
communications devices. Such chip-scale
atomic clocks need not be as accurate
or stable as large laboratory standards,
but they will bring atomically
precise timekeeping and frequency
control to small, battery-powered electronic devices.
DARPA and other funding agencies
support the Division's participation in
government-industry-university collaborations,
recognizing that our core
expertise in research and metrology
accelerates the development of commercial
and military products and services
with strategic national economic and
security impacts. This support is one
important way the Division ensures that
programs are well aligned with high-priority
industrial and national needs.
Accomplishments
Improvements in Frequency Combs
Figure 5. Scott Diddams with
a femtosecond-laser-based optical
frequency synthesizer system. |
A key application of frequency combs
based on femtosecond lasers is to generate
an arbitrary optical or microwave
frequency output given an optical frequency
reference input. This remarkable
capability is crucial to the development
and dissemination of useful optical frequency
standards. As mentioned, the
Division and other laboratories have
used optical frequency combs to directly
compare the cesium fountain microwave
frequency (9.2 GHz) with optical frequencies
from the calcium atom standard
(456 THz) and the mercury ion standard (doubled 532 THz).
The Division has been continually
improving the performance and versatility
of frequency combs by exploring
new ways to broaden the femtosecond
laser output without use of microstructured
optical fibers, which are susceptible
to damage. (See Fig. 5.) The Division
also collaborates with the NIST
Electronics and Electrical Engineering
Laboratory to develop Cr:fosterite femtosecond
lasers, operating in the near-infrared
from about 1.0 µm to 2.2 µm,
including the important 1.4 µm to
1.6 µm optical telecommunications band.
An indication of progress is the recent
Division-led frequency intercomparison
of four different femtosecond laser
frequency synthesizers, from three
laboratories, using two fundamentally
different comb-generation techniques,
broadband operation and nonlinear
microstructure fiber. Three frequency
synthesizers--one each from the
International Bureau of Weights and
Measures, East China Normal
University, and the Division--were
compared to a second Division one,
referenced to a 456 THz optical frequency
standard. Comparisons were
conducted for a total of six days spread across several months.
The frequency differences, determined
by optical heterodyne techniques, were
measured to a relative uncertainty of
1.4 × 10-19, with the uncertainty arising
primarily from mechanical and thermal
effects and limits on integration
time. The results suggest optical frequency
combs can be reliably used for
frequency comparisons and synthesis to
at least a fractional uncertainty of 10-19,
and likely better when technical noise
(mechanical and thermal fluctuations)
are better controlled and longer integration times are used.
Chip-Scale Atomic Clock
Figure 6. Photograph of the physics package of a NIST chip-scale atomic clock, with a schematic diagram. |
Division research on a miniature, all-optical
atomic clock, based on coherent
population trapping, stimulated DARPA
interest in developing a chip-scale atomic
clock (CSAC). Program goals are to
bring atomically precise timing and
frequency control to applications in
portable electronic devices, such as
enhanced GPS receivers and better
performing wireless communications
devices for improved communications
security. The overall goals of the
DARPA program are to develop a
CSAC of 1 cm3 total volume, consuming
no more than 30 mW of power
with a fractional frequency stability of
about 1 × 10-11 over one hour.
NIST is part of a DARPA-funded
CSAC consortium of eight companies
and universities. The Division is charged
with providing the fundamental research
and metrology, with the expectation that
companies will develop the commercial products.
The Division has recently demonstrated
a prototype CSAC with a physics package
smaller than 0.01 cm3, a fractional
frequency instability of about
2.5 × 10-11 at 250 seconds integration,
and a power consumption of about
75 mW (Fig. 6). These parameters are
believed to be best in the world, and are
approaching the stringent DARPA
goals. The Division understands what
modifications are needed to significantly
improve the frequency stability and
reduce the power consumption, and is
optimistic that the DARPA goals can be met relatively soon.
The Division has collaborated with the
NIST Electronics and Electrical
Engineering Laboratory to use standard
MEMS fabrication techniques in making
the CSAC physics package, suggesting
that CSACs based on the Division
model could be mass-produced at
relatively low cost, using wafer-level
assembly techniques. Such a process
would enable the extremely broad application
of CSACs in portable electronic
devices, for many applications.
The Division is also exploring the possibility
of using CSAC technology to
make tiny magnetometers approaching
the sensitivity of SQUIDs, without
the need for cryogens, and for other applications.
First strategic focus |
Second strategic focus |
Third strategic focus |
Fourth strategic focus
"Technical Activities 2004" - Table of Contents |
