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Time and Frequency Noise in Electronic and Optical Systems |
Division Contact: Thomas O'Brian We are investigating the applications of ions, such as mercury (199Hg+), for high-accuracy frequency standards. The ions are stored in ion traps-devices that use electric or a combination of electric and magnetic fields to contain the ions indefinitely. We use laser radiation to cool the ions to temperatures on the order of 1 millikelvin, so the second-order Doppler shift (a serious problem in high-accuracy frequency standards) is extremely small. Single ions or small groups of ions can be cooled further-to the zero-point motion limit-using sideband-cooling methods. Frequency standards, particularly optical frequency standards, based on stored ions have the potential for higher accuracy than the best current standards, which are based on the neutral cesium atom with a transition in the microwave region, since observation times are much longer and fractional linewidths are much smaller. We recently have demonstrated an optical frequency standard, based on the 256-nanometer transition in 199Hg+, with a linewidth of 6 Hz, and the frequency comb developed in another division program (see Optical Frequency Measurements below) has been used to relate this transition frequency directly to the frequency of the primary cesium standard. Finally, we are conducting research on related topics, such as the physics of non-neutral plasmas, quantum optics, quantum measurements, laser frequency stabilization, and non-linear optical sources. Of special interest is the recent extension of concepts used in this field to the demonstration of promising quantum-logic devices. Contact: David J. Wineland The first atomic frequency standard, based on ammonia, was built in 1950 at NIST in Washington, D.C. Since then, we have constructed a series of seven standards based on cesium beams with performances improving at a rate of better than an order of magnitude every 10 years. NIST-7, introduced in 1993, is based on optical state selection and state detection rather than the more traditional magnetic methods and has an uncertainty of 5 × 10-15. A cesium-fountain frequency, called NIST-F1 is our most recent standard. This standard has an uncertainty of 1.5 × 10-15. A key part of this program is the maintenance of the NIST time scale, an ensemble of commercial cesium-beam standards and hydrogen masers. The average of the outputs of these standards is extremely stable and serves as the operational standard for all calibrations and broadcast services. In fact, the primary standards, rather than being operated continuously, serve to calibrate the NIST time scale. Finally, we have developed broad expertise in time transfer, particularly using satellite methods, which are applied to synchronization of widely distributed network nodes and serve to coordinate NIST standards with other similar standards around the world. Contact: Thomas E. Parker Noise in Electronic and Optical Systems We have developed systems for making phase-noise and amplitude-noise measurements over a broad dynamic range of carrier frequency (into the millimeter range) and Fourier frequency (up to 10 percent of the carrier frequency). The accuracy of measurement is typically one decibel or better depending on the frequency range. These systems provide the basis for specifications now arising in communication systems, radars, and other aerospace equipment. Signals at higher millimeter and optical frequencies also can be characterized by beating them against a stable optical reference. A wide range of noise measurement equipment and systems for analyzing the output data are available. Most recently, we have applied this unique measurement capability to the study of phase-modulation and amplitude-modulation noise in bipolar-junction-transistor circuits. This work has led to a better understanding of noise processes in these circuits, and design rules for low noise performance have been developed. Finally, NIST maintains an expertise in the specialized statistical analysis of noise in clocks and oscillators. Noise processes are often not white (frequency independent), so the usual variance does not converge. The two-sample variances developed to handle such noise have become standards widely used in the specification of noise in high-spectral-purity systems. Contact: David A. Howe Optical Frequency Measurements The program in this area focuses on measurements using solid-state lasers (mostly diode lasers) and mode-locked (femtosecond) lasers. Diode lasers are used widely in many applications where spectral and spatial purity are not critical, but with the addition of frequency-control and linewidth-narrowing systems, they are being increasingly applied in more demanding metrology applications such as analytical chemistry and sensing of trace impurities or pollutants as well as narrow-line sources for length standards and optical manipulation of atoms and molecules. Recognizing the broad range of measurement applications for high-performance diode lasers, we have developed a program aimed at developing methods for controlling the output characteristics of these versatile and inexpensive lasers. The program selects specific practical applications and works on the system designs needed to provide solutions. Current projects include a calcium-stabilized laser for use as a length reference, methods for synthesizing signals in the optical region, and laser-spectroscopy methods for detecting trace impurities. Most recently, we have developed mode-locked lasers that produce frequency combs across a substantial portion of the spectrum. These devices have been used to measure frequencies in the visible and even ultraviolet region (see Ion Storage Research above) relative to the cesium microwave frequency with uncertainties limited by that of the cesium standard. Contact: Leo
Hollberg
Date
created:October
1 , 2001 |