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
© Geoffrey Wheeler
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Figure 5. Tara Fortier adjusts an optical frequency comb, and a “frequency brush,” dispersed modes of the frequency comb used for
massively parallel absorption spectroscopy of iodine.
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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. The Division also collaborates with the NIST Electronics and Electrical Engineering
Laboratory to develop near-infrared femtosecond lasers for improved wavelength
and frequency references, such as in the important 1.4 μm to 1.6 μm optical
telecommunications band.
The Division led an intercomparison of four different femtosecond-laser frequency
combs from three different laboratories,
using two fundamentally different comb-generation techniques: broadband operation and nonlinear microstructure fiber. The frequency differences, determined
by optical heterodyne techniques, were measured to a relative uncertainty of
1.4 x 10-19 Hz/Hz, 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 Hz/Hz, and likely better when technical noise (mechanical and thermal fluctuations) are better controlled and longer integration times are used.
Recent Division advances in frequency comb development and applications include techniques for high-resolution, two-dimensional dispersion of the modes of the comb, into a “frequency brush.” (See Fig. 5.) This enables rapid, high-resolution
spectral fingerprinting—high-resolution
absorption spectroscopy of iodine vapor spanning 6 THz can be collected in a few milliseconds. This technique is promising for high-resolution quantum coherent quantum control and arbitrary optical waveform synthesis, areas the Division is actively pursuing.
The Division also demonstrated the use of frequency combs for ultraprecise time and frequency transfer over fiber optic networks, including a “real world” demonstration
of time transfer over 30 km
of optical fiber in an urban environment with a timing jitter better than 10-17 Hz/ Hz at 1 second of integration. Such exquisite
performance will enable the power of future optical frequency standards to be efficiently transferred and applied.
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Figure 6. Scott Diddams with a femtosecond-laser-based optical frequency synthesizer system.
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Chip-Scale Atomic Devices
Figure 7. Photomicrograph of the physics package of a NIST chip-scale atomic magnetometer (CSAM), with a schematic diagram.
(1) Laser. (2) Optics. (3) Vapor cell with rubidium atoms. (4) Detector. For scale, a CSAM is shown next to a dime and rice grain.
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The Division has become a world leader in research, metrology, and development of chip-scale atomic devices (CSADs), bringing atomically precise measurements to portable electronic applications, such as timekeeping and frequency control, measurement
of magnetic fields, and inertial navigation (gyroscopes).
The program began with development of a miniature, all-optical atomic clock, based on coherent population trapping. This stimulated DARPA interest in further developing
a chip-scale atomic clock (CSAC) to bring atomically precise timing and frequency control to portable electronic devices, such as enhanced GPS receivers and more secure communications devices. The goal is 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 Hz/Hz over one hour. This has now moved into a commercialization phase in which NIST assists with evaluations.
The Division has expanded the basic chip-scale atomic technology into other types of instruments. Atomic magnetometers, for example, have been developed with physics packages of similar size to the CSACs. Measurements indicate that the sensitivity
of these instruments can be as good as 70 fT/Hz½, which compares favorably with magnetometers based on high-Tc superconducting
quantum interference devices (SQUIDs)—but without the need for cryogenic cooling, large electronics packages,
and the power they require.
Highly sensitive gyroscopes are under development with the same power and size goals as the clocks and magnetometers. These inertial sensors are based on polarized
atomic nuclei that define a direction in space as a reference for precision measure
of rotation.
The Division has collaborated with the NIST Electronics and Electrical Engineering
Laboratory to use standard MEMS fabrication techniques in making the CSAD physics packages, suggesting that chip-scale atomic devices 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 CSADs. The Division continues to actively partner with companies and research organizations
to help commercialize CSADs and to develop new applications.
First strategic focus |
Second strategic focus |
Third strategic focus |
Fourth strategic focus
"Technical Activities 2005-2007" - Table of Contents |
