This PDF file contains the front matter associated with SPIE Proceedings Volume 6673, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

Single-trapped-ion frequency standards based on a 282 nm transition in ¹⁹⁹Hg+ and on a 267 nm transition in ²⁷Al⁺ 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.

The present decade has seen great advances in optical frequency standards following the development of femtosecond laser frequency combs. With this technology, combined with a stable and narrow probe laser source (50-100 Hz linewidth), we made a systematic study of the 5s ²S_1/2-4d ²D_5/2 (S-D) transition frequency of the ⁸⁸Sr⁺ ion as a function of the quantization axis in the trap. This study revealed the presence of systematic shifts caused by residual micromotion in our rf Paul trap and led to an evaluation of those shifts. Also, a method was developed based on the measurement of the Zeeman spectrum of the clock transition to cancel the electric quadrupole shift caused by patch potentials on the trap electrodes. From these results, we also determined the absolute frequency of the S-D clock transition frequency at 674 nm with a fractional frequency uncertainty of 3.4 × 10^-14. We have since made improvements to our probe laser system and have observed 5 Hz Fourier transform limited linewidths on a Zeeman component of the S-D transition of the ⁸⁸Sr⁺ ion. To observe such a narrow transition it is essential to have a known and well behaved drift of the laser frequency during the few minutes it takes to measure the ion resonance using the quantum jump signal. Our approach to minimize the drift rate and its sensitivity to temperature fluctuations has been to stabilize the cavity at the temperature where its coefficient of thermal expansion is zero. We will present an overview of our recent progress made on the frequency standard based on the laser-cooled and trapped single ion of ⁸⁸Sr⁺.

We have recently completed a breadboard ion-clock physics package based on Hg ions shuttled between a quadrupole and a 16-pole rf trap. With this architecture we have demonstrated short-term stability ~1-2x10^-13 at 1 second, averaging to 10^-15 at 1 day. This development shows that H-maser quality stabilities can be produced in a small clock package, comparable in size to an ultra-stable quartz oscillator required for holding 1-2x10^-13 at 1 second. This performance was obtained in a sealed vacuum configuration where only a getter pump was used to maintain vacuum. The vacuum tube containing the traps has now been under sealed vacuum conditions for nearly two years with no measurable degradation of ion trapping lifetimes or clock short-term performance. We have fabricated the vacuum tube, ion trap and UV windows from materials that will allow a ~ 400°C tube bake-out to prepare for tube seal-off. This approach to the vacuum follows the methods used in flight vacuum tube electronics, such as flight TWTA's where tube operation lifetime and shelf life of up to 15 years is achieved. We use neon as a buffer gas with 2-3 times less pressure induced frequency pulling than helium and, being heavier, negligible diffusion losses will occur over the operation lifetime.

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 spectrometer³ 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 mm³ and the volume of the entire spectrometer is around 0.1 cm³.

The ACES (as Atomic Clock Ensemble in Space) mission, managed by the European Space Agency, has three main objectives. The first one deals with the operation and study of the laser cooled cesium clock PHARAO (as Projet d'Horloge Atomique à Refroidissement d'Atomes en Orbite) to reach a frequency accuracy of 10^-16 in space. The second one is to perform fundamental metrology by comparing the clock signal with ground based clocks via a two way time transfer link. The third one is to perform fundamental physics tests such as a new measurement of the red shift at 2 parts per million and a search for variations of fundamental physical constants. The expected time transfer resolution is 0.3 ps at 300 seconds and 7 ps per day. An H-maser developed by the Observatoire Cantonal de Neuchatel is the second ACES clock and will be used as a stable frequency reference for mid term duration. We give an overview of the ACES mission and its operation and present the first results obtained with the engineering model of the laser cooled cesium clock PHARAO. This model is first developed to validate the flight model design.

We review the current status of the U.S. Primary Frequency Standard, NIST-F1. NIST-F1 is a laser-cooled cesium fountain based frequency standard with an inaccuracy of less than δ f / f ≤ 5x10^-16 limited mainly by the radiation field in the room-temperature fountain (blackbody shift). NIST-F1 is one of the best cesium fountains currently contributing to international atomic time, but has reached a point that it is impractical to improve its accuracy substantially. Therefore we are building a new fountain, imaginatively named NIST-F2, with a cryogenic (77 K) Ramsey interrogation zone that lowers the blackbody shift by several orders of magnitude. NIST-F2 is currently undergoing final assembly, and we will discuss our planned (hoped for) performance, which includes frequency inaccuracy of δ f / f < 1x10^-16

The frequency shift due to atomic collisions is a major, and in some cases the dominant, limitation to the accuracy of caesium fountain primary frequency standards. A correction for this shift is usually obtained by measuring the frequency of the standard as a function of atomic density and performing an extrapolation to zero density. In general this means that additional measurement time is needed to reach a given statistical resolution. Recently, we have observed that, for a certain range of fountain parameters (i.e. the initial size of the atom cloud and its temperature at launch), the collisional frequency shift varies significantly when the population of the clock states (set by the first Ramsey interaction) is varied. In particular, the collisional shift can be zero for a certain value of the population ratio. This demonstration of collisional shift cancellation offers the intriguing prospect of operating the fountain at the zero-shift point, avoiding the need for extrapolation. In this contribution we provide further experimental validation of the theoretical model describing the collisional shift variation. We also discuss requirements for and benefits of the operation at the zero shift point. In addition, we point out the possible consequences of collisional shift variation for operation of a fountain standard at elevated microwave power, a mode of operation frequently used for the evaluation of other systematic frequency shifts.

Comparisons between three Cs fountains are described. All three LNE-SYRTE fountain clocks are now using a cryogenic sapphire oscillator as a starting point to generate the microwave probe signal. As a result, all three fountains have a fractional frequency instability in the 10^-14 range at 1 s. All three fountains are also using a recently developed generation of microwave synthesizers allowing to switch the microwave probe signal without introducing detrimental phase transient. Several series of measurements have been made totalizing more than 80 days of simultaneous operation under optimized conditions of a pair of either fountains. Results of these measurements are presented. Finally, modifications of the FO2 fountain has been completed to allow simultaneous operation with Rb and Cs. Operation with Rb leads to a fractional frequency instability of 6.3 × 10^-14τ^-1/2.

Recent results from the JILA ⁸⁷Sr optical lattice clock are presented. Using the tight confinement of an optical lattice in combination wit a sub-Hz linewidth diode laser we have achieved a pulse-length limited linewidth of 1.8 Hz for the ¹S₀- ³P₀ clock transition. This corresponds to a quality factor of Q ≈ 2.4 x 10¹⁴, and is a record for coherent spectroscopy. With the addition of a small magnetic bias field, the high line Q of the clock transition has also allowed us to resolve the nuclear-spin sublevels, and make a precision measurement of the differential Landé g-factor between the ¹S₀ and ³P₀. We present the current accuracy and stability of the lattice clock, and in addition, we report on our development of precision tools for the lattice clock, including a stabilized clock laser, fs-comb based technology allowing accurate clock comparison in both the microwave and optical domains, and clock transfer over an optical fiber in an urban environment.

The present status of the development of the Yb optical lattice clock at NMIJ/AIST and future prospects are presented. Experimental equipments such as vacuum systems and laser sources are explained in detail.

We describe progress toward an optical lattice clock based on an even isotope of Yb. The ¹S₀ → ³P₀ clock resonance in ¹⁷⁴Yb is accessed through a magnetically induced spectroscopic technique. Using ≈1mT static magnetic fields and ≈10 μW of probe light power we generate Rabi frequencies of several hertz. The narrow spectroscopic features that result (< 10 Hz FWHM) require a highly stabilized laser at the clock transition wavelength of 578 nm. We describe a new all solid-state laser system that shows hertz level stability. In order to cancel the slow drift of the cavity, spectroscopy is performed on the clock transition to provide feedback to the laser. Using a Ca neutral atom frequency standard as a reference oscillator,we show high stability and an effective method for investigating clock frequency shift systematics.

We report on our progress toward the realization of a compact optical frequency standard referenced to strontium intercombination lines. Our current setup allows the production of ultracold Sr atoms in hundreds of ms. For high resolution spectroscopy of the ¹S₀-³P₀ doubly forbidden transition we have prepared a 698 nm clock laser stabilized on a high finesse, symmetrically suspended cavity and a high power 813 nm light source for the optical lattice trap at the magic wavelength. Due to their compactness, reliability, and low power consumption, semiconductor laser sources represent the best choice for the development of compact and transportable devices for application both on Earth and in Space. A new Sr trapping and cooling experimental setup is also presented.

The recent development of optical frequency standards has been performed quite rapid and the better uncertainty than that of microwave frequency standards will be realized in very near future. We are evaluating a one-dimensional ⁸⁷Sr optical lattice clock developed and located at the University of Tokyo, Tokyo, by using UTC(NMIJ) generated at NMIJ, Tsukuba. The baseline length between those two sites is about 50 km. We constructed a time and frequency transfer link using GPS carrier phase method for this link. We use GIPSY software and a newly developed one for data analysis. Our developed one works in real time using carrier phase data and broadcast navigation data which are obtained from the carrier phase receivers.

Coherent optical sources in the 1550 nm region of the spectrum have a number of applications in frequency metrology, stable frequency transfer, precision spectroscopy and remote sensing. A narrow-linewidth (~ 1 Hz) single-frequency source can be generated by phase-locking a cw fiber laser to a stable optical cavity. A comb of such narrow linewidth sources can be generated by phase-locking a mode-locked, femtosecond fiber laser to a single narrow cw source. We will discuss the current development of our narrow linewidth cw and pulsed sources at 1550 nm and some of the applications that can benefit from such coherent sources.

Antiprotonic helium is a unique atomic three-body system which is a mixture of particles and an antiparticle and yet shows metastability with lifetimes of a few microseconds before annihilation of the constituent antiproton. Using the antiprotons provided from the Antiproton Decelerator facility at CERN, Geneva, we the ASACUSA collaboration (standing for Atomic Spectroscopy And Collisions Using Slow Antiprotons) has been pursuing high precision in laser spectroscopy of this exotic atom, to test CPT invariance between matter and antimatter. Recently, we have measured 12 different transition frequencies with fractional precisions in the order of a ppb. A femtosecond optical frequency comb has found application here in measurement of absolute frequencies of a continuous-wave pulse-amplified laser light ranging from blue to infrared. Comparison with three-body QED calculations yielded an antiproton-to-electron mass ratio and antiproton-to-proton mass ratio with a precision of a few ppb, contributing to the precise determination of the fundamental constant and to the test of CPT. As a pioneering work of atomic physics with low-energy antiprotons, we have developed techniques of electromagnetic trapping of antiprotons in ultra-high vacuum and produced ultra-slow antiproton beams at energies ranging from 10 eV to 20 keV. (Note the great orders of magnitude of deceleration and cooling from the GeV energies at which antiprotons are produced at accelerator facilities.) This unique beam is used for our research on atomic collision dynamics between an antiproton and an ordinary atom, and also for future production and microwave spectroscopy of cold antihydrogen atoms.

The progress in the field of optical frequency standards is oriented to femtosecond mode-locked lasers stabilized by technique of the optical frequency synthesis. Such a laser produces a supercontinuum light, which is composed of a cluster of coherent frequency components in certain interval of wavelengths. A value of the repetition rate of femtosecond pulses determines (in the frequency domain) spacing of these coherent components. If we control the mode-locked laser by means of i.e. atomic clocks we ensure frequency of these components very stable. With respect to definition of SI unit "one meter" on basis of speed of light the stabilized mode-locked laser can be used for implementation of this definition by non-traditional way. In the work we present our proposal of a system, which converts excellent frequency stability of components generated by the mode-locked laser to a net of discrete absolute lengths represented by a distance of two mirrors of an optical resonator. On basis of theory, the optical resonator with a cavity length has a periodic frequency spectrum Similarly the frequency of i-th comb component could be written as: f_i = f_ceo + i f_rep, where f_ceo is the comb offset frequency and f_rep is the repetition rate. For the simplicity we presume the offset frequency f_ceo equals to zero. If the supercontinuum beam of the mode-locked laser illuminates the resonator and at the same time the cavity length L is adjusted to length L_p = c / (2 p f_rep ) then both spectra fit. The symbol 'p' is an integer value. It produces intensity maximum in the output of the cavity, which is detected by a photodetector and locked in the servo-loop. For absolute discrete values of cavity lengths L_p that well satisfy the condition above we obtain precise etalons of length.

We present an improved technique for detection of trace impurities in iodine-filled absorption cells for laser frequency stabilization. The results of purity investigation are compared to frequency shifts measured with a set of two iodine stabilized Nd:YAG lasers. The setup for direct fluorescence measurement with an Argon-ion laser operating at 502 nm wavelength is equipped with compensation for laser power and spectral instabilities.

In the present work, we have developed an efficient and well stablized hyper coherent diode laser light source as compact as even portable using commercially available visible 400 nm band laser diodes. The attained coherence of the present system can always be controlled at the best condition indifferent to changes in its settled environmental conditions by applying Pound-Drever-Hall technique in which the frequency of a 160mW type 405nm GaN violet laser diode is locked to a reference Fabry-Perot cavity by negative electrical feedback for the injection current of the laser diode based on FM sideband technique. In addition to this frequency stabilization system, we have also realized a stability evaluation system that can measure the Allan variance of the frequency fluctuations of our frequency stabilized laser source in real-time basis by using simple devices of a portable computer and a digital signal processing unit. As a result, we have accomplished a compact and efficient hyper coherent laser system which can always perform its optimum conditions even if the environmental conditions around the laser are to be dynamically changed when used in a field basis. The attained values of power spectral density (PSD) of FM noise calculated from the error signals of our system under controlled condition were better by about 1~2 orders than typical values of free-running conditions in the fourier frequency domain from 100Hz to 300kHz. The best achieved value of PSD was about 2.56×10⁷ [Hz²/Hz] in the fourier frequency domain from 100Hz to 1kHz, while as for the Allan variance as another measure of frequency stability, the achieved value of the minimum square root of Allan variance was 3.46×10^-11 in a 400nm type violet laser diode at integration time of 10 ms, which has been well comparable to the hyper coherent condition for the laser diode light sources.

Coefficient of thermal expansion (CTE) measurements using small Fabry-Perot etalons were conducted on high and low thermal expansion materials differing in CTE by a factor of nearly 400. The smallest detectable change in length was ~10^-12 m. The sample consisted of a mm-sized Fabry-Perot etalon equipped with spherical mirrors; the material-under-test served as the 2.5 mm-thick spacer between the mirrors. A heterodyne optical setup was used with one laser locked to an ~780 nm hyperfine line of Rb gas and the other locked to a resonance of the sample etalon; changes in the beat frequency between the two lasers as a function of temperature directly provided a CTE value. The measurement system was tested using the high-CTE SCHOTT optical glass N-KF9 (CTE = 9.5 ppm/K at 23 °C). Measurements conducted under reproducibility conditions using five identically-prepared N-KF9 etalons demonstrate a precision of 0.1 ppm/K; absolute values (accuracy) are within 2-sigma errors with those made using mechanical dilatometers with 100-mm long sample rods. Etalon-based CTE measurements were also made on a high-CTE (~10.5 ppm/K), proprietary glass-ceramic used for high peak-pressure electrical feedthroughs and revealed statistically significant differences among parts made under what were assumed to be identical conditions. Finally, CTE measurements were made on etalons constructed from SCHOTT's ultra-low CTE Zerodur^(R) glass-ceramic (CTE about -20 ppb/K at 50 °C for the material tested herein).

We have demonstrated a compact and inexpensive frequency stabilization technique for commercially available 1mW, 850nm vertical cavity surface emitting laser (VCSEL) using a Fabry-Perrot cavity as frequency standard. Recently VCSEL has been widely prevailed for uses of low cost and small sized sensors, since it may afford low power operations and manufacturing costs in comparison with edge emitting type Fabry-Perrot laser diodes. Therefore, a highly versatile and inexpensive frequency stabilized coherent light source which can be mass producible will be available if the frequency stabilization for this type of VCSEL's is carried out. Generally, it has been commonly accepted that a satisfactory degree of coherence may be easily obtained from VCSEL's without any additional frequency stabilization technique since highly reflective coatings are to be put on their laser cavity edges. Nevertheless, some VCSEL devices, especially inexpensive type commercial products show multi-mode behaviors along with polarization instabilities. In the present work, as a simple and inexpensive approach to commercially available VCSEL devices, we have demonstrated a frequency stabilization scheme using a Fabry-Perrot cavity. The error signal was derived by phase sensitive detection for the transmitted light from the Fabry-Perrot resonator. Thus, the lasing frequency of the VCSEL was locked to the zero-crossing of the error signal by negative feedback for injection current via a PID controller. As a result, we have successfully suppressed the amount of frequency fluctuations in the free-running VCSEL of as much as 2GHz to be within 500MHz at measuring time of 30sec, that is, the attained Allan variance is 4.1×10E-8.