Technical Activities

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Most Recent Technical Activities Archive of Technical Activities

Time and Frequency Division

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

    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

    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

    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 m rather than 850 nm, since fibers at 1.3 m 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 m to 11.5 m. 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

    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 m 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 m radiation is generated by difference mixing the outputs of a diode-pumped YAG laser at 1.06 m 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 m. 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 m). 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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