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We describe a high-performance, compact optical frequency standard based on a microfabricated Rb vapor cell and a low-noise, external cavity diode laser operating on the Rb two-photon transition at 778 nm. The optical standard achieves an instability of 1.8x10$^{-13}$/$\sqrt{\tau}$ for times less than 100 s and a flicker noise floor of 1x10$^{-14}$ out to 6000 s. At long integration times, the instability is limited by variations in optical probe power and the AC Stark shift. The retrace was measured to 5.7x10$^{-13}$ after 30 hours of dormancy. Such a simple, yet high-performance optical standard could be suitable as an accurate realization of the SI meter or, if coupled with an optical frequency comb, as a compact atomic clock comparable to a hydrogen maser., Comment: 5 pages, 5 figures

Polarization dependence of supercontinuum and third-harmonic generations in silicon-nitride waveguides is mapped from 350-1750 nm. We identify parameters suited for generating coherent signals near 500 and 1000 nm for optical atomic clocks.

© 2017 [2017 Optical Society of America.]. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited. The need for an accurate time reference on orbiting platforms motivates study of time transfer via free-space optical communication links. The impact of atmospheric turbulence on earth-to-satellite optical time transfer has not been fully characterized, however. We analyze limits to two-way laser time transfer accuracy posed by anisoplanatic non-reciprocity between uplink and downlink. We show that despite limited reciprocity, two- way time transfer can still achieve sub-picosecond accuracy in realistic propagation scenarios over a single satellite visibility period.

We demonstrate a new technique for the suppression of noise associated with the laser source in atomic clocks based on coherent population trapping (CPT). The technique uses differential detection of the transmission of linearly and circularly polarized beams that propagate through different parts of a single rubidium vapor cell filled with a buffer gas mixture. The common-mode noise associated with the laser frequency and amplitude noise is suppressed by the differential detection of the two laser beams. The CPT signal, which is present only in the circularly polarized laser beam, is unaffected. The implementation of the technique requires only a change of the polarization of part of the laser beam and an additional photodiode. The technique is simple and applicable to CPT frequency references where a major source of noise is the laser, such as compact and chip-scale devices.

We report the technical aspects of the optical-to-microwave comparison for our recent measurements of the optical frequency of the mercury single-ion frequency standard in terms of the SI second as realized by the NIST-F1 cesium fountain clock. Over the course of six years, these measurements have resulted in a determination of the mercury single-ion frequency with a fractional uncertainty of less than 7×10-16, making it the most accurately measured optical frequency to date. In this paper, we focus on the details of the comparison techniques used in the experiment and discuss the uncertainties associated with the optical-to-microwave synthesis based on a femtosecond laser frequency comb. We also present our most recent results in the context of the previous measurements of the mercury single-ion frequency and arrive at a final determination of the mercury single-ion optical frequency: f(Hg+)=1 064 721 609 899 145.30(69) Hz.

The control of the broadband frequency comb emitted from a mode-locked femtosecond laser has permitted a wide range of scientific and technological advances--ranging from the counting of optical cycles for next-generation atomic clocks to measurements of phase-sensitive high-field processes. A unique advantage of the stabilized frequency comb is that it provides, in a single laser beam, about a million optical modes with very narrow linewidths and absolute frequency positions known to better than one part in 10(15) (ref. 5). One important application of this vast array of highly coherent optical fields is precision spectroscopy, in which a large number of modes can be used to map internal atomic energy structure and dynamics. However, an efficient means of simultaneously identifying, addressing and measuring the amplitude or relative phase of individual modes has not existed. Here we use a high-resolution disperser to separate the individual modes of a stabilized frequency comb into a two-dimensional array in the image plane of the spectrometer. We illustrate the power of this technique for high-resolution spectral fingerprinting of molecular iodine vapour, acquiring in a few milliseconds absorption images covering over 6 THz of bandwidth with high frequency resolution. Our technique for direct and parallel accessing of stabilized frequency comb modes could find application in high-bandwidth spread-spectrum communications with increased security, high-resolution coherent quantum control, and arbitrary optical waveform synthesis with control at the optical radian level.

Using a Ca optical frequency standard at 657 nm, we demonstrate a method that reduces uncertainties in absolute frequency measurements of optical transitions using freely expanding neutral atoms. Working with atoms that have been laser cooled to 10 μK, we have developed and employed a new technique that combines launching of cold atom clouds with atom interferometry to measure and optimise spectroscopy beam parameters. When applied to a frequency standard with laser beams of high spatial quality, this approach can potentially reduce residual Doppler effect uncertainties to well below one part in 1016. With Doppler uncertainties greatly suppressed, we investigate other potential shifts at the 1-Hz level with a multiplexed measurement system that takes advantage of the low instability of the calcium frequency standard (4×10-15 at 1 s). The resultant fractional frequency uncertainty for the standard is 6.6×10-15, the lowest uncertainty reported to date for a neutral atom optical standard.

The demodulation of ultra-short light pulses with photodetectors is accompanied by excess phase noise at the pulse repetition rate and harmonics in the spectrum of the photocurrent. The major contribution to this noise is power fluctuations of the detected pulse train that, if not compensated for, can seriously limit the stability of frequency transfer from optical to microwave domain. By making use of an infrared femtosecond laser, we measured the spectral density of the excess phase noise, as well as power-to-phase conversion for different types of InGaAs photodetectors. Noise measurements were performed with a novel type of dual-channel readout system using a fiber coupled beam splitter. Strong suppression of the excess phase noise was observed in both channels of the measurement system when the average power of the femtosecond pulse train was stabilized. The results of this study are important for the development of low-noise microwave sources derived from optical "clocks" and optical frequency synthesis.

Surprising as it might seem, it is possible to phase-coherently track, synthesize, count and divide optical frequencies of visible laser sources. In essence, the technologies described here now allow direct connection of basically any frequency from DC to 1000 THz. Modern 'self-referenced' femtosecond mode-locked lasers have enormously simplified the required technology. These revolutionary new systems build on a long history of optical frequency metrology that spans from the early days of the laser. The latest systems rely heavily on technologies previously developed for laser frequency stabilization, optical phase-locked-loops, nonlinear mixing, ultra-fast optics and precision opto-electronic metrology. Using examples we summarize some of the heroic efforts that led to the successful development of harmonic optical frequency chains. Those systems played critical roles in defining the speed of light and in redefining the metre. We then describe the present state-of-the-art technology in femtosecond laser frequency combs, their extraordinary performance capabilities and some of the latest results.

For more than 100 years, optical atomic/molecular frequency references have played important roles in science and technology, and provide standards enabling precision measurements. Frequency-stable optical sources have been central to experimental tests of Einstein's relativity, and also serve to realize our base unit of length. The technology has evolved from atomic discharge lamps and interferometry, to narrow atomic resonances in laser-cooled atoms that are probed by frequency-stabilized cw lasers that in turn control optical frequency synthesizers (combs) based on ultra-fast mode-locked lasers. Recent technological advances have improved the performance of optical frequency references by almost four orders of magnitude in the last eight years. This has stimulated new enthusiasm for the development of optical atomic clocks, and allows new probes into nature, such as searches for time variation of fundamental constants and precision spectroscopy.

Two types of international comparisons of femtosecond laser frequency combs have been performed in France and the USA. Five combs were involved in the comparisons. Three combs, of which two are transportable, employ nonlinear photonic crystal fiber (PCF) to obtain a wide spectrum covering a full optical octave. The other two are based on broadband femtosecond lasers and require no PCF. The comparisons were performed by counting the optical heterodyne beats between pairs of combs. The frequency agreement among three combs was at the subhertz level in the 563 THz part of the comb spectrum when the combs were referenced to a hydrogen maser. When the combs were referenced to an optical standard, the frequency agreement among four combs was much improved and found to be at the /spl sim/10/sup -19/ level in the spectral range of 333-473 THz. The fact that this result is obtained by five independent measurement systems (combs) strengthens the conclusion that no systematic effects are present at this level.

Excess phase noise is observed in the spectrum of the microwave signal extracted from a photodetector illuminated by a train of ultrashort light pulses from the femtosecond laser. This noise affects the stability of frequency transfer from optical to microwave domains with the femtosecond laser. Some contributions to the excess phase noise are related to intrinsic beam-pointing fluctuations of the femtosecond laser and optical power fluctuations of the detected light. These factors contribute to excess phase noise at the harmonics of the pulse repetition rate due to power-to-phase conversion in the photodetector, spatially dependent time delays, and photodiode nonlinearities that distort the pulse shape. With spatial filtering of the laser beam and active control of its power, the additional fractional frequency fluctuations of pulse repetition rate associated with the excess noise of the photodetection process were reduced from 6/spl middot/10/sup -14/ to approximately 3/spl middot/10/sup -15/ over 1 s of averaging. The effects of other noise mechanisms, such as laser shot noise and phase noise introduced by a microwave amplifier, were also examined but were found to be at a less significant level.

The merging of continuous wave laser-based precision optical-frequency metrology with mode-locked ultrafast lasers has led to precision control of the visible and near-infrared frequency spectrum produced by mode-locked lasers. Such a phase-controlled mode-locked laser forms the foundation of a "femtosecond optical-frequency comb generator" with a regular comb of sharp lines with well-defined frequencies. For a comb with sufficiently broad bandwidth, it is now straightforward to determine the absolute frequencies of all of the comb lines. This ability has revolutionized optical-frequency metrology, synthesis, and optical atomic clocks. Precision femtosecond optical-frequency combs also have a major impact on time-domain applications, including carrier-envelope phase stabilization, synthesis of a single pulse from two independent lasers, nonlinear spectroscopy, and passive amplifiers based on empty external optical cavities. The authors first review the frequency-domain description of a mode-locked laser and the connection between the carrier-envelope phase and the frequency spectrum to provide a basis for understanding how the absolute frequencies can be determined and controlled. Using this understanding, applications in optical-frequency metrology and synthesis and optical atomic clocks are discussed. This is followed by discussions of time-domain experiments.

A method for fabricating atomic vapor cells with physical dimensions of order 1 mm3 or below is described. Cells with integrated lenses are made from hollow-core glass fiber, and fused shut at either end using highly localized heating from a CO2 laser beam. Such cells, subsequently loaded with alkali atoms and a buffer gas, could form the basis for future generations of compact frequency references or magnetometers.

We report on work directed toward the systematic evaluation of an optical frequency standard based on the /sup 2/S/sub 1/2/-/sup 2/D/sub 5/2/ transition of a single, laser-cooled, trapped /sup 199/Hg/sup +/ ion, whose resonance frequency is 1.06/spl times/10/sup 15/ Hz. For the purpose of the evaluation, a second /sup 199/Hg/sup +/ standard has been constructed. In the cooling-laser system built for the second standard, an injection-locking scheme has been applied to a CW Ti-sapphire laser. We also report optical frequency measurements of the clock transition performed over the past 21 months with the first standard. During this term, the variation of the clock transition frequency is found to be less than /spl plusmn/1/spl times/10/sup -14/.

We report on work directed toward the systematic evaluation of an optical frequency standard based on the 2 S1/2– 2 D5/2 transition of a single, laser-cooled, trapped 199 Hg + ion, whose resonance frequency is 1.06 × 10 15 Hz. For the purpose of the evaluation, two 199 Hg + standards have been constructed. In the cooling-laser system built for the second standard, an injection-locking scheme has been applied to a cw Ti:sapphire laser. We also report optical frequency measurements of the clock transition performed over the past 21 months with the first standard. During this term, the variation of the clock transition frequency is found to be less than one part in 10 14 . (Some figures in this article are in colour only in the electronic version)

We have conducted a study of potential wavelength calibration references for use as both moderate-accuracy transfer standards and high-accuracy National Institute of Standards and Technology (NIST) internal references in the 1280-1320-nm wavelength-division-multiplexing region. We found that most atomic and molecular absorption lines in this region are not ideal for use as wavelength references owing to factors such as weak absorption, complex spectra, or special requirements (for example, frequency-doubling or excitation with an additional light or discharge source). We have demonstrated one of the simpler schemes consisting of a tunable diode laser stabilized to a Doppler-broadened methane absorption line. By conducting a beat-note comparison of this reference to a calcium-based optical frequency standard, we measured the methane line center with an expanded uncertainty (2/spl sigma/) of /spl plusmn/2.3 MHz. This methane-stabilized laser now serves as a NIST internal reference.

The refractive index of coherently driven rubidium vapour is experimentally investigated in a three-level cascade con® guration using a selective reection technique. The maximum measured resonant change in the refractive index is ¢n ' 0:1. The selective reection is accompanied by a four-wave-mixing process that can reach π90% eA ciency.

Stabilized optical frequency combs (OFC) can have remarkable levels of coherence across their broad spectral bandwidth. We study the scaling of the optical noise across hundreds of nanometers of optical spectra. We measure the residual phase noise between two OFC׳s (having offset frequencies f 0 ( 1 ) and f 0 ( 2 ) ) referenced to a common cavity-stabilized narrow linewidth CW laser. Their relative offset frequency Δ f 0 = f 0 ( 2 ) − f 0 ( 1 ) , which appears across their entire spectra, provides a convenient measure of the phase noise. By comparing Δ f 0 at different spectral regions, we demonstrate that the observed scaling of the residual phase noise is in very good agreement with the noise predicted from the standard frequency comb equation.

We describe the performance characteristics and frequency measurements of two high-accuracy high-stability laser-cooled atomic frequency standards. One is a 657-nm (456-THz) reference using magneto-optically trapped Ca atoms, and the other is a 282-nm (1064-THz) reference based on a single Hg/sup +/ ion confined in an RF-Paul trap. A femtosecond mode-locked laser combined with a nonlinear microstructure fiber produces a broad and stable comb of optical modes that is used to measure the frequencies of the reference lasers locked to the atomic standards. The measurement system is referenced to the primary frequency standard NIST F-1, a Cs atomic fountain clock. Both optical standards demonstrate exceptional short-term instability (/spl ap/5/spl times/10/sup -15/ at 1 s), as well as excellent reproducibility over time. In light of our expectations for the future of optical frequency standards, we consider the present performance of the femtosecond optical frequency comb, along with its limitations and future requirements.

The frequency comb created by a femtosecond mode-locked laser and a microstructured fiber is used to phase coherently measure the frequencies of both the Hg+ and Ca optical standards with respect to the SI second. We find the transition frequencies to be f(Hg) = 1 064 721 609 899 143(10) Hz and f(Ca) = 455 986 240 494 158(26) Hz, respectively. In addition to the unprecedented precision demonstrated here, this work is the precursor to all-optical atomic clocks based on the Hg+ and Ca standards. Furthermore, when combined with previous measurements, we find no time variations of these atomic frequencies within the uncertainties of the absolute value of( partial differential f(Ca)/ partial differential t)/f(Ca) < or =8 x 10(-14) yr(-1) and the absolute value of(partial differential f(Hg)/ partial differential t)/f(Hg) < or =30 x 10(-14) yr(-1).

A new modification of external optical feedback OFB was used to narrow the line of a diode laser DL . A 'whispering . gallery' WG mode of a high- Q microsphere was excited by means of frustrated total internal reflection while the feedback for optical locking of the laser was provided by the intracavity Rayleigh backscattering. The ; 600 MHz beatnote of the two laser diodes optically locked to a pair of orthogonally polarized modes of the same microresonator had the indicated spectral width of 20 kHz, and the stability of 2 = 10 y6 over averaging time of 10 s. The feasibility of miniature sub-kHz-line width

High resolution atomic absorption measurements of lead at 283 nm in a vapor cell were performed by frequency doubling an 850 nm laser diode to obtain 425 nm light, followed by sum frequency generation of the harmonic radiation with a second 850 nm laser diode.

Spectroscopic properties of coherently prepared, optically dense atomic media are studied experimentally and analyzed theoretically. It is shown that in such media the power broadening of the resonances can be substantially reduced. A density-dependent spectral narrowing of the electromagnetically induced transparency (EIT) window and novel, even narrower, resonances superimposed on the EIT line are observed in dense Rb vapor. A nonlinear two-photon spectroscopic technique based on coherent atomic media and combining high resolution with a large signal-to-noise ratio seems feasible.

volume multi-pass absorption cell with an 18-m path length. The methane mixing ratio was determined by comparing the direct optical absorption measured in the sample with that measured in a reference gas at 100 torr and room temperature. Relative accuracy of better than 1 ppb (parts in 109, by mole fraction) was achieved in measurements of natural air that contained 1700–1900 ppb methane. The typical measurement time for each sample was 60 seconds. The accuracy was limited by residual interference fringes in the multi-pass cell that resulted from scattering. Without the use of reference samples, the relative accuracy was 20 ppb; it was limited by the long-term reproducibility of the spectroscopic baseline, which was affected by drift in the optical alignment coupled to changes in the ambient temperature. This work demonstrates the use of diode-pumped difference-frequency generation (DFG) in PPLN in a high-precision infrared spectrometer. Compact, room-temperature solid-state gas sensors can be built based on this technology, for accurate real-time measurements of trace gases in the 3–5 μm spectroscopic region.

A femtosecond laser–based optical frequency synthesizer is referenced to an optical standard, and we use it to demonstrate the generation and control of the frequency of electromagnetic fields over 100 terahertz of bandwidth with fractional uncertainties approaching 1 part in 10 19 . The reproducibility of this performance is verified by comparison of different types of femtosecond laser–based frequency synthesizers from three laboratories.

Spectroscopic detection of the methane in natural air using an 800 nm diode laser and a diode-pumped 1064 nm Nd:YAG laser to produce tunable light near 3.2 µm is reported. The lasers were pump sources for ring-cavity-enhanced tunable difference-frequency mixing in AgGaS2. IR frequency tuning between 3076 and 3183 cm−1 was performed by crystal rotation and tuning of the extended-cavity diode laser. Feedback stabilization of the IR power reduced intensity noise below the detector noise level. Direct absorption and wavelength-modulation (2f) spectroscopy of the methane in natural air at 10.7 kPa (80 torr) were performed in a 1 m single-pass cell with 1 µW probe power. Methane has also been detected using a 3.2 µm confocal build-up cavity in conjunction with an intracavity absorption cell. The best methane detection limit observed was 12 ppb m (Hz.)−1/2.

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 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 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.

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.

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.

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.