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We have used an optical polarization heterodyne technique to measure the frequencies of two OCS lines, one near 5.3 μm and the other near 5.9 μm. These sub-Doppler measurements gave 56 533 766.125 ± 0.035 MHz for the 2ν 2 + ν 3 P (16) transition and 50 627 632.013 ± 0.100 MHz for the 2ν 3 P (48) line. The P (16) measurement is in excellent agreement (20 kHz) with a new result obtained with an alternate technique by a group at the University of Bonn. The 2ν 3 measurement was combined with other frequency measurements to produce an improved calibration table for the 2ν 3 OCS band near 1700 cm −1 . These and other sub-Doppler frequency measurements in different spectral regions give improved frequencies for a number of OCS bands. These new frequencies will soon be incorporated into calibration data that will be available over the Internet.

Sub-Doppler frequency measurements have been made on three transitions of OCS in the 87-THz region (near 2900 cm−1). The CO overtone laser was used as the saturating laser. Polarization spectroscopic techniques utilizing optical heterodyne detection were used to observe the features and subsequently provide the discriminant for locking the overtone laser to the OCS transitions. A CO2 laser synthesizer was used to measure the frequency of the CO overtone laser and thereby measure the frequencies of the OCS lines. The resulting frequencies of the three new measurements are: 1001-0000 P(27), 87 117 278.496(50) MHz; 111ƒ1-011ƒ0 R(14), 87 222 001.143(70) MHz for OCS; and for the OC34S 1001-0000 P(9), 87 010 586.667(75) MHz, where the numbers in parentheses are the uncertainties in the last digits. These new numbers have been fitted along with more than 5700 other data points in our OCS data bank and improved constants have been obtained. These latest constants are used to calculate updated calibration tables containing values with much smaller uncertainties; three such tables are included.

Saturated-absorption signals on the calcium 657 nm transition are observed by direct absorption using diode lasers and a high flux atomic-beam cell. Line-widths as narrow as 65 kHz are observed with a high signal-to-noise ratio. Prospects for using this system as a compact wavelength/frequency reference are considered.

We propose a sub-millimeter-scale vapor-cell atomic-frequency reference based on a micromachined vapor cell, all-optical excitation, and advanced diode-laser technology. We analyze theoretically the performance of such a device as a function of cell size. Initial measurements on small-scale vapor cells support the theoretical treatment.

We report measurements of absolute transition frequencies and hyperfine coupling constants for the 8S_{1/2}, 9S_{1/2}, 7D_{3/2}, and 7D_{5/2} states in ^{133}Cs vapor. The stepwise excitation through either the 6P_{1/2} or 6P_{3/2} intermediate state is performed directly with broadband laser light from a stabilized femtosecond laser optical-frequency comb. The laser beam is split, counter-propagated and focused into a room-temperature Cs vapor cell. The repetition rate of the frequency comb is scanned and we detect the fluorescence on the 7P_{1/2,3/2} -> 6S_{1/2} branches of the decay of the excited states. The excitations to the different states are isolated by the introduction of narrow-bandwidth interference filters in the laser beam paths. Using a nonlinear least-squares method we find measurements of transition frequencies and hyperfine coupling constants that are in agreement with other recent measurements for the 8S state and provide improvement by two orders of magnitude over previously published results for the 9S and 7D states., 14 pages, 14 figures

We demonstrate a self-referenced 10 GHz Ti:sapphire frequency comb where the continuum is generated in microstructured fiber. In addition, we discuss optical stabilization of the comb via saturated absorption in 87Rb with a single mode.

The high power per mode of a recently developed 10 GHz femtosecond Ti:sapphire frequency comb permits nonlinear Doppler-free saturation spectroscopy in 87 Rb with a single mode of the comb. We use this access to the natural linewidth of the rubidium D2 line to effectively stabilize the optical frequencies of the comb with an instability of 7 10 �12 in 1so faveraging. The repetition rate is stabilized to a microwave reference leading to a stabilized and atomically referenced comb. The frequency stability of the 10 GHz comb is characterized using optical heterodyne with an independent self-referenced 1 GHz comb. In addition, we present alternative stabilization approaches for high repetition rate frequency combs and evaluate their expected stabilities.

A new scheme for coherently connecting optical frequencies in a 3:1 ratio has been demonstrated. To phase lock a Nd:YAG laser at 1064 nm with a CO overtone laser at 3192 nm, we generated their difference frequency in RbTiOAsO(4) (RTA) and beat it against the second harmonic of 3192 nm that was generated in AgGaSe(2).

With simple optical geometries a separate resonant Fabry-Perot cavity can serve as an optical feedback element that forces a semiconductor laser automatically to lock its frequency optically to the cavity resonance. This method is used to stabilize laser frequencies and reduce linewidths by a factor of 1000 from 20 MHz to approximately 20 kHz.

We propose a technique for cooling optically trapped atoms to microkelvin temperatures, and lower, by using the dipole force of resonance-radiation pressure.

Introduction.—The ability to create a simple, coherent optical frequency comb has potential applications in precision measurement, short-pulse generation, microwave production, arbitrary optical and rf waveform engineering, and astronomy. In particular, generation of an optical comb with modes spaced by tens of gigahertz is especially useful because the resulting comb may be easily divided into its constituents for access and control of individual modes, and the coherent connection to radio frequencies enables low-phase-noise electronic signals. Historically, frequency combs were generated through electro-optic phase modulation of a laser [1,2] or by use of the output of a modelocked laser where the net gain is higher for a train of short pulses than for cw operation. Recent interest, however, has centered on comb generation in compact monolithic resonators. The combination of small mode volume and high quality factors (Q) makes monolithic resonators with nonlinear gain an ideal medium for efficient nonlinear optical processes. Previous work on nonlinear optics in monolithic resonators focuses on whispering-gallery modes and includes generation of frequency combs in microtoroids [3], four-wave mixing in microspheres [4] and microrings [5], and coherent backscattering and oscillation in LiNiO3 and CaF2 gain-based resonators [6]. Here, we report a substantially different monolithic

We describe recent work at NIST to develop compact, low-power instruments based on a combination of precision atomic spectroscopy, advanced diode lasers and microelectromechanical systems (MEMS). Designed to be fabricated in parallel in large numbers, these "chip-scale" atomic devices may eventually impact a wide range of applications, from the global positioning system to magnetic resonance imaging and inertial navigation. We focus here on recent work to develop compact, high-performance magnetometers.

We use a Fabry-Perot cavity to optically filter the output of a Ti:sapphire frequency comb to integer multiples of the original 1 GHz mode spacing. This effectively increases the pulse repetition rate, which is useful for several applications. In the case of low-noise microwave signal generation, such filtering leads to improved linearity of the high-speed photodiodes that detect the mode-locked laser pulse train. The result is significantly improved signal-to-noise ratio at the 10 GHz harmonic with the potential for a shot-noise limited single sideband phase noise floor near -168 dBc/Hz.

10 GHz optical pulses are generated by line-by-line phase compensation on an optical frequency comb. Residual pulse timing jitter ≤ 10 fs and high power signals at harmonics up to 80 GHz are measured.

We have developed a system for line-by-line control of a stabilized Ti:Sapphire optical frequency comb. We show individually-addressed 20 GHz comb modes around 960 nm and apply simple masks to demonstrate individual mode control.

20 GHz pulses were generated by a Fabry-Perot modulator based optical frequency comb generator. We demonstrate high-fidelity 20 GHz sub-picosecond pulses with low residual microwave AM and PM noises.

We present a review of the use of diode lasers in atomic physics with an extensive list of references. We discuss the relevant characteristics of diode lasers and explain how to purchase and use them. We also review the various techniques that have been used to control and narrow the spectral outputs of diode lasers. Finally we present a number of examples illustrating the use of diode lasers in atomic physics experiments.

Femtosecond mode-locked lasers are used to transfer the high stability of cw optical frequency references to electronic oscillators/synthesizers and generate microwave signals with exceptional frequency stability, low timing jitter and low phase-noise.

The cavity of a 10 GHz optical frequency comb generator centered at 1.55 mum was locked to a narrow-linewidth seed laser within a narrow range of less than 10 MHz for hours of operation.

We report an uncertainty evaluation of an optical lattice clock based on the $^{1}{S}_{0}\ensuremath{\leftrightarrow}^{3}{P}_{0}$ transition in the bosonic isotope $^{174}\mathrm{Yb}$ by use of magnetically induced spectroscopy. The absolute frequency of the $^{1}{S}_{0}\ensuremath{\leftrightarrow}^{3}{P}_{0}$ transition has been determined through comparisons with optical and microwave standards at NIST. The weighted mean of the evaluations is $\ensuremath{\nu}(^{174}\mathrm{Yb})=518\phantom{\rule{0.2em}{0ex}}294\phantom{\rule{0.2em}{0ex}}025\phantom{\rule{0.2em}{0ex}}309\phantom{\rule{0.2em}{0ex}}217.8(0.9)\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}$. The uncertainty due to systematic effects has been reduced to less than $0.8\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}$, which represents $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ in fractional frequency.

Fabry-Perot cavity filtering of broadband optical frequency combs is studied experimentally and theoretically. Effects of dispersion, mirror coatings, and carrier envelope offset frequency are analyzed while highlighting applications to waveform generation and spectroscopic references.

Phase noise measurements of an optoelectronic oscillator (OEO) at frequencies less than 10 Ha from the carrier (10.6 GHz) as well as the measured Allan variance are presented for the first time. The system has a measured single-side-band (SSB) phase-noise of -123 dB/Hz at 10 kHz from the carrier and a /spl sigma//sub y/(/spl tau/)=10/sup -10/ for an integration time between 1 and 10 seconds. The importance of amplifier phase-noise and environmental fluctuations in determining the noise of the oscillator at these low Fourier frequencies is verified experimentally and analyzed using a generalized model of noise sources in the OEOs. This analysis then allows prediction of the oscillator performance from measured parameters of individual components in the system.

We are developing laser frequency measurement technologies that should allow us to construct an optical frequency synthesis system capable of measuring optical frequencies with a precision limited by the atomic frequency standards. The system will be used to interconnect and compare new advanced optical-frequency references (such as Ca, Hg/sup +/, and others) and eventually to connect these references to the Cs primary frequency standard. The approach we are taking is to subdivide optical frequency intervals into smaller and smaller pieces until we are able to use standard electronic-frequency-measurement technology to measure the smallest interval.

The performance of a simple microwave frequency reference based on Raman scattering in an atomic vapor is examined. This reference has the potential to be compact, low-power, and insensitive to acceleration. Several design architectures have been evaluated with a table-top experiment in order to guide the future development of a compact system. Fractional frequency deviations of /spl les/5/spl times/10/sup -11/ appear to be feasible.

We present a review of the use of diode lasers in atomic physics with an extensive list of references. We discuss the relevant characteristics of diode lasers and explain how to purchase and use them. We also review the various techniques that have been used to control and narrow the spectral outputs of diode lasers. Finally we present a number of examples illustrating the use of diode lasers in atomic physics experiments.

With a narrow linewidth seed laser, residual timing jitter on a 10 GHz optical frequency comb generator is reduced to 6 fs. We present analysis connecting spectral phase and laser linewidth to the timing jitter.

A 150-nm bandwidth frequency comb at 1.5 ?m with tailorable mode spacing is generated with a cw-pumped, 5-cm-long highly nonlinear fiber cavity. A comb with both THz and GHz spacings is produced.

We report on our recent evaluations of stability and accuracy of the JILA Sr optical lattice clock. We discuss precision tools for the lattice clock, including a stabilized clock laser with sub-Hz linewidth, fs-comb based technology allowing accurate clock comparison in both the microwave and optical domains, and clock transfer over optical fiber in an urban environment. High resolution spectroscopy (Q > 2 × 10) of lattice-confined, spin-polarized strontium atoms is used for both a high-performance optical clock and atomic structure measurement. Using a Ca optical standard for comparison, the overall systematic uncertainty of the Sr clock is reduced to < 2 × 10.

We demonstrate the scaling of the relative phase noise across hundreds of nanometers of spectra from stabilized Ti:sapphire frequency combs. We show good agreement between the predicted and measured phase noise.

Nuclear parameters can be calculated from the measured hyperfine structure. Recorded collision-free linewidths allow an estimate of the lifetime of the levels involved in a new scheme proposed for the cooling of atomic oxygen

We present an experimental study of the lattice induced light shifts on the 1S_0-3P_0 optical clock transition (v_clock~518 THz) in neutral ytterbium. The ``magic'' frequency, v_magic, for the 174Yb isotope was determined to be 394 799 475(35)MHz, which leads to a first order light shift uncertainty of 0.38 Hz on the 518 THz clock transition. Also investigated were the hyperpolarizability shifts due to the nearby 6s6p 3P_0 - 6s8p 3P_0, 6s8p 3P_2, and 6s5f 3F_2 two-photon resonances at 759.708 nm, 754.23 nm, and 764.95 nm respectively. By tuning the lattice frequency over the two-photon resonances and measuring the corresponding clock transition shifts, the hyperpolarizability shift was estimated to be 170(33) mHz for a linear polarized, 50 uK deep, lattice at the magic wavelength. In addition, we have confirmed that a circularly polarized lattice eliminates the J=0 - J=0 two-photon resonance. These results indicate that the differential polarizability and hyperpolarizability frequency shift uncertainties in a Yb lattice clock could be held to well below 10^-17., Accepted to PRL

Single-trapped-ion frequency standards based on a 282 nm transition in 199 Hg + and on a 267 nm transition in 27 Al + have been developed at NIST over the past several years. Their frequencies are measured relative to each other and to the NIST primary frequency standard, the NIST-F1 cesium fountain, by means of a self-referenced femtosecond laser frequency comb. Both ion standards have demonstrated instabilities and inaccuracies of less than 1 × 10 −16 .

Coherent population trapping (CPT) resonances usually exhibit contrasts below 10 % when interrogated with frequency modulated lasers. We discuss a relatively simple way to increase the resonance contrast to nearly 100 % generating an additional light field through a nonlinear four-wave mixing interaction in the atomic vapor 1 . A similar method can also be used to create a beat signal at the CPT resonance frequency that can injection-lock a low-power microwave oscillator at 3.4 GHz directly to the atomic resonance 2 . This could lead to chip-scale atomic clocks (CSACs) with improved performance. Furthermore, we introduce a miniature microfabricated saturated absorption spectrometer 3 that produces a signal for locking a laser frequency to optical transitions in alkali atoms. The Rb absorption spectra are comparable to signals obtained with standard table-top setups, although the rubidium vapor cell has an interior volume of only 1 mm 3 and the volume of the entire spectrometer is around 0.1 cm 3 .

High resolution spectroscopy is the foundation for many of the most challenging and productive of all astronomical observations. A highly precise, repeatable and stable wavelength calibration is especially essential for long term RV observations. The two wavelength references in wide use for visible wavelengths, iodine absorption cells and thorium/argon lamps, each have fundamental limitations which restrict their ultimate utility. We are exploring the possibility of adapting emerging laser frequency comb technology in development at the National Institute of Standards and Technology in Boulder, Colorado, to the needs of high resolution, high stability astronomical spectroscopy. This technology has the potential to extend the two current wavelength standards both in terms of spectral coverage and in terms of long term precision, ultimately enabling better than 10 cm/s astronomical radial velocity determination.

This talk presents two different types of neutral atom optical clocks and emphasizes the systems' differences in potential performance and complexity. Approaches in attaining high stability, high accuracy, and transportable optical atomic clocks are discussed in detail.

We measure the optical transition frequencies of the 6s 2S1/2 rarr 8s 2S1/2, 9s 2S1/2, and 7d 2D3/2,5/2 transitions in a 133Cs vapor cell, with an uncertainty < 100 kHz using a femtosecond laser frequency comb.

We explore methods and limitations for increasing the mode spacing of a 1 GHz optical frequency comb using a Fabry-Perot filter cavity. Applications to optical and microwave waveform generation are highlighted.

We describe recent progress in the development of millimeter-scale instruments based on alkali atom vapor cells implemented with microfabrication techniques. Because of their small size and correspondingly low power requirements, these "chip-scale" atomic clocks and magnetometers have the potential to bring atomically precise instrumentation to portable, battery-operated systems such as GPS receivers, remote sensors and wireless communication devices. In addition, wafer-level processing and assembly potentially allows for very low cost per instrument if high volumes are produced.

We demonstrate injection locking of 1 and 1.5 GHz femtosecond Ti:sapphire lasers by a second, physically separate and independent 1 GHz laser. In some cases, both the pulse envelopes and optical carriers are locked.

We report tests of local position invariance (LPI) and constancy of fundamental constants from measurements of the frequency ratio of the 282-nm 199Hg+ optical clock transition to the ground-state hyperfine splitting in 133Cs. Analysis of the frequency ratio, extending over six years at NIST, is used to place a limit on the fractional variation of the two clocks of less than 5.8times10-6 per change in normalized solar gravitational potential, and a limit on fractional variation of the fine structure constant at alpha dot/alpha < 1.3x10-16 yr-1, assuming invariance of other fundamental constants. Comparison of our results with those previously reported for the absolute optical frequency measurements of coupled 171Yb+ versus other constraint 133Cs standard yields a coupled constraint of -0.04times10-15 < alpha dot/alpha < 0.46times10-15 yr-1 and -2.39times10-15 < d/dt In muCs/muB < 0.47times10-15 yr-1.

We describe a new laser system we have developed to probe the ultra-narrow 1S0 harr 3P0 clock transition at 578 nm in neutral ytterbium. The yellow light is produced by sum frequency generation in a periodically poled waveguide. With approximately 100 mW each from a fiber laser and Nd:YAG laser, we produce 10 mW of visible light. Stabilization of the laser to a resonance of a high finesse, environmentally isolated cavity has enabled resolution of spectroscopic features as narrow as 5 Hz.

A stabilized femtosecond frequency comb has ~106 stable optical modes spanning hundreds of terahertz, making it an ideal tool for high-resolution spectroscopy. The frequency comb used in these experiments was generated with an octave-spanning, femtosecond, mode-locked laser based on Ti:sapphire with a repetition rate of ~1 GHz. We demonstrate some features of frequency-comb spectroscopy using experiments involving calcium and cesium.

We demonstrate a simple setup for observing very high contrast coherent population trapping resonances in rubidium - 87. The high contrast is achieved by eliminating the 52 P 1/2 DC-light background through polarization and spectral filtering.

A wide-band power control system based on an electro-optical modulator and transimpedance amplifier has been constructed. It is capable of reducing laser intensity fluctuations to the shot noise limit over the range of Fourier frequencies from a few tens of Hz to a few MHz with the low boundary set by the light handling capacity of a photodetector and the upper boundary imposed by the spurious resonances of a given electro-optical modulator. We discuss a general approach to the design of laser intensity control system and its noise properties.

We demonstrate self-injection locking of a low-power, compact microwave oscillator by use of feedback generated from four-wave mixing in an atomic vapor. The four-wave mixing process creates an effective microwave filter with extremely high off-resonant signal suppression. This type of locking results in a shorter locking time, increased short-term stability, improved close-in phase noise, and reduced spurious signals as compared to current techniques used in miniature atomic clocks. Other possible benefits and drawbacks are mentioned.

We demonstrate a method to suppress potentially troublesome frequency shifts in optical lattice atomic clocks based on alkaline-earth-like atoms with zero nuclear spin. Suppression of these shifts, which depend on the square of the lattice field intensity, would remove a potential barrier from the path to the next generation of atomic clocks.

We discuss various aspects of high resolution measurements of phase fluctuations at microwave frequencies. This includes methods to achieve thermal noise limited sensitivity, along with the improved immunity to oscillator amplitude noise. A few prototype measurement systems were developed to measure phase fluctuations of microwave signals extracted from the optical pulse trains generated by femtosecond lasers. This enabled first reliable measurements of the excess phase noise associated with optical-to-microwave frequency division. The spectral density of the excess phase noise was found to be -140 dBc/Hz at 100 Hz offset from the 10 GHz carrier which was almost 40 dB better than that of a high quality microwave synthesizer.

We provide an overview of our research on chip-scale atomic devices. By miniaturizing optical setups based on precision spectroscopy, we have developed small atomic sensors and atomic references such as atomic clocks, atomic magnetometers, and optical wavelength references. We have integrated microfabricated alkali vapor cells with small low-power lasers, micro-optics, and low-power microwave oscillators. As a result, we anticipate that atomic stability can be achieved with small size, low cost, battery-operated devices. Advances in fabrication methods and performance are presented.