Time and Frequency Division

Division Overview   |   Program Directions   |   Major Technical Highlights

Program Directions

• Time and Frequency Broadcast Services. The Division provides time and frequency broadcasts from stations WWV and WWVB in Fort Collins, Colorado and from WWVH in Hawaii, and a time code broadcast from NOAA's GOES weather satellites, although this satellite service is scheduled to be terminated. The Division has completed an upgrade of the equipment and power level for WWVB. At a higher output power, these LF broadcasts are substantially more useful for mobile and consumer applications, because the antenna/receiver cost and size is very small. The Division also operates telephone and network time services, the Automated Computer Time Services (ACTS), designed for setting clocks in digital systems. The network (Internet) version of these services now receives more than 200,000,000 calls per day. These broadcasts serve applications in a broad range of systems in business, telecommunications, science, transportation, and radio/TV broadcasting. Industry calibration laboratories are served by the Division's Frequency Measurement Service, a system that provides these laboratories with continuous assurance of the accuracy of their frequency measurements.

• Time Scales. The NIST Time Scale is the highly stable and reliable clock system that provides accurate time and frequency reference for services and applications and that serves as a reference for research on new standards and measurement methods. The reliability and stability of this time scale is based on the use of an ensemble of commercial cesium-beam standards and hydrogen masers combined under the control of a computer-implemented algorithm. The Division is working to advance the performance of the time scale through acquisition of more-stable clocks and improvement of electronic systems, which read the clock outputs. These improvements are critical to the successful evaluation and use of the next generation of primary standards now being developed by the Division.

• Frequency Standards. The accuracy of the time scale is derived from primary frequency standards, which provide the practical realization of the definition of the second. Over the last several years, the Division has been operating two frequency standards. The first, NIST-7, went into operation in early 1993. This atomic-beam standard is based on optical pumping methods (using diode lasers) rather than the traditional magnetic methods used for state selection and detection. The uncertainty for this standard is 5 × 10^-15. More recently, the Division has constructed and evaluated a cesium-fountain frequency standard, NIST-F1, which has an uncertainty of 1.2 × 10^-15, about three times smaller than that of NIST-7. The fountain concept, which uses laser-slowed atoms, provides longer atom observation times resulting in a narrower transition linewidth and smaller systematic frequency shifts. These two standards were operated in parallel for about two years, and were found to agree within their uncertainty statements. NIST-7 is now deemed to have served its purpose in this overlap period, and the Division will discontinue evaluations of its performance in order to place more emphasis on newer standards. Looking toward higher accuracy, the Division is studying standards based on trapped, laser-cooled atomic ions. Both microwave (40 GHz) and optical (ultraviolet-region) mercury-ion standards have been demonstrated, and while the microwave standard shows promise, the progress on the optical standard during this last year was so significant that work on the microwave standard has been slowed. When combined with the new optical-frequency-synthesis method described below, the new optical standard provides the means for measuring frequency from the near infrared through the visible portions of the spectrum. While the ion-storage program is already demonstrating prototype clocks, the work is generally treated as basic research providing the knowledge base needed for the development of future frequency standards.

• Methods of Time Transfer. Since the world operates on a unified time system, Coordinated Universal Time (UTC), highly accurate time transfer (to coordinate time internationally) is a critical ingredient in standards operations. The Division has long been a world leader in this field. The Division is working to further improve the NIST-developed, GPS common-view time-transfer method that is the standard for international time coordination. The Division plans to place more emphasis on the two-way time transfer and GPS carrier-phase time-transfer methods that are becoming significantly more important for international time coordination and for comparisons of the new generations of frequency standards.

• Optical Frequency Synthesis. Over the last year, the Time and Frequency Division has been developing improved methods for optical frequency synthesis through frequency-comb generation with mode-locked (femtosecond) lasers. The Division has now demonstrated a system, which spans the spectrum from the near infrared to the visible. The uncertainties of measurements of the frequencies of optical transitions in mercury (at 282 nm) and calcium (at 657 nm), made recently using the femtosecond comb generator, are now limited by the uncertainty of the frequency of the cesium primary standard. This means that any frequency across the visible spectrum can be measured with an uncertainty approaching that of the primary frequency standard. An important consequence of this work is that the very-high-performance optical frequency standards developed over the last few years can now generate outputs in the microwave range. This opens up the possibility of a whole new generation of frequency standards and clocks with accuracies and stabilities well beyond that of the present primary standard.

• Optical Frequency Standards and Measurements. Current optical-frequency standards such as the carbon-dioxide lasers, helium-neon lasers, and calcium-stabilized diode lasers already serve as references for supporting accurate spectroscopic measurements for industrial and scientific applications, but as indicated above, optical frequency standards will certainly have still broader impact. This is because higher-frequency transitions have better fractional-frequency uncertainty. Of particular importance will be laser-cooled ions (such as Hg⁺) and neutral atoms (such as Ca). The projected accuracy of the laser-cooled ion standard goes well beyond that achievable with microwave standards. Aside from the work on optical standards, the Division is employing the new optical measurement methods described above to making improved optical frequency measurements important for secondary wavelength standards based on atomic-and-molecular transitions, advanced optical communication, analytical instrumentation, and length measurement. An important part of this program involves the development of diode laser systems, which can have very high spectral purity, tunability, simplicity, and low cost.

• Spectral-Purity Measurements. The Division's development of spectral-purity measurements supports sound specifications for a range of aerospace and telecommunications systems. Systems capable of making highly accurate measurements of both phase-modulation (PM) and amplitude-modulation (AM) noise have been developed for carrier frequencies ranging from 5 MHz to 75 GHz. Portable systems covering this same range have also been developed and these are being used to validate measurements made in industrial and government laboratories. More recently, systems have been developed for making pulsed measurements, which are important for high-power systems such as radars. Further work will broaden the spectral coverage and simplify comparison of measurement accuracy among standards laboratories.

• Synchronization for Telecommunications. The Division has been engaged with the telecommunications industry in issues relating to synchronization of advanced generations of telecommunications networks. NIST has made useful contributions to emerging telecommunications systems, but with expansion of effort by the Division, it is clear that NIST could contribute even more significantly to this industry. The industry has requested such expansion.

• Application of Time and Frequency Technology. Finally, the Division is engaged in the application of time and frequency technology to important problems in sensing of trace impurities (pollutants) and quantum-limited measurements.