Kurt R. Vogel

Absolute frequency measurements of the Hg+ and Ca optical clock transitions with a femtosecond laser.

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).

Direct comparison of two cold-atom-based optical frequency standards by using a femtosecond-laser comb

With a fiber-broadened, femtosecond-laser frequency comb, the 76-THz interval between two laser-cooled optical frequency standards was measured with a statistical uncertainty of 2x10(-13) in 5 s , to our knowledge the best short-term instability thus far reported for an optical frequency measurement. One standard is based on the calcium intercombination line at 657 nm, and the other, on the mercury ion electric-quadrupole transition at 282 nm. By linking this measurement to the known Ca frequency, we report a new frequency value for the Hg(+) clock transition with an improvement in accuracy of ~10(5) compared with its best previous measurement.

Narrow-line Doppler cooling of strontium to the recoil limit

Experimental results on narrow line cooling of strontium atoms are presented. Doppler cooling is performed on the /sup 1/S/sub 0/-/sup 3/P/sub 1/ intercombination transition at 689 nm resulting in temperatures near the recoil limit in a one-dimensional (1-D) optical molasses. Frequency chirp cooling is successfully demonstrated as a means of expanding the cooling range.

Compact femtosecond-laser-based optical clockwork

We describe in detail an optical clockwork based on a 1 GHz repetition rate femtosecond laser and silica microstructure optical fiber. This system has recently been used for the absolute frequency measurements of the Ca and Hg+ optical standards at the National Institute of Standards and Technology (NIST). The simplicity of the system makes it an ideal clockwork for dividing down high optical frequencies to the radio frequency domain where they can readily be counted and compared to the existing cesium frequency standard.

All-optical atomic clocks

We have developed two all-optical clocks in which the RF output is generated directly from the optical process within the clockwork. The frequency comb created by a femtosecond mode-locked laser and a microstructure fiber is used to phase-coherently measure the frequencies of both the Hg/sup +/ and Ca optical frequency standards with respect to the SI second as realized at NIST. We find the transition frequencies to bef/sub Hg/=1064721609899143(10) Hz and f/sub Ca/ =455986240494158(26) Hz, respectively. This work begins to reveal the high stability and potential for accuracy of optical atomic clocks. Furthermore, when combined with previous measurements, we find no time variations of these atomic frequencies within the uncertainties of 1|(/spl part/f/sub Hg///spl part/t)/f/sub Hg/| =2/spl times/10/sup -14/ yr/sup -1/ and |(/spl part/f/sub Ca///spl part/t)/f/sub Ca/| =8/spl times/10/sup -14/ yr/sup -1/.

Experiments with Strontium in a Vapor Cell Magneto-optic Trap

In a ceramic vapor cell we have created a robust Sr magneto- optical trap that stores about 108 atoms with lifetimes > 200 ms. We eliminate the 5p 1P1 yields 4d1D2 yields 5p 3P2 leak and achieve a 10-fold improvement in trap lifetime by re-pumping the 5p 3P0,2 dark states with 679 nm and 707 nm light. The observed lifetime is now limited by cold collision losses, and we have preliminary measurements of the 2-body loss rate. Direct readout of the trap velocity distribution is possible using the narrow 5s2 1S0 yields 5p 3P1 intercombination line at 689 nm. We can also cool with this narrow transition and have achieved a 40-fold 1D velocity compression for about 5 percent of the trapped atoms by applying this second-stage cooling.

Narrow line cooling of strontium to the recoil limit

Experimental results on narrow line cooling of strontium atoms is presented. Cooling is performed on the /sup 1/S/sub 0/-/sup 3/P/sub 1/ intercombination transition at 689 nm resulting in temperatures near the recoil limit in 1-D molasses.

Wavelength stability of sweepmeter-based tunable laser spectrometer

Optical fiber sensors must compete in performance with traditional electronic sensors, such as quartz crystal pressure and temperature monitors. The precision of commercial electronic sensors can reach the parts-per-billion (ppb) level. To test the precision of a laser based spectrometer system, repeated measurements of an absorption line of a molecular gas cell were made. The Allan deviation is computed, and it is shown that the laser interrogation system, built completely out of commercially available components, can achieve precision at the 10-ppb level.

A FEMTOSECOND-LASER-BASED OPTICAL CLOCKWORK

In this article we summarize our progress in developing and testing a femtosecondlaser-based optical clockwork that provides a singlostep phasocoherent connection between emerging optical frequency standards and the cesium microwave standard on which the SI second is based. This clockwork enabled absolute optical frequency measurements with statistical uncertainty 2 x and it was used in the demonstration of an optical clock with instability 5 7 x in 1 s. Recent experiments demonstrate the intrinsic fractional instability and inaccuracy of thc clockwork are less than 6.3 x in 1 s and 4 x lo-”, respectively.

Phase-locking the 3 GHz repetition rate of an ultracompact femtosecond modelocked laser

Summary form only given. Accurate measurements of optical frequencies are important for the realization of optical clocks as well as for the improved determination of certain fundamental constants. A newly-developed technique pioneered by Hansch et al. (1999) accurately controls the broad optical-frequency comb from a femtosecond laser to span the very large gaps associated with measuring optical frequencies; gaps as large as 100 THz have recently been measured. By stabilizing both the repetition rate and cavity phase of these femtosecond lasers, an accurate ultrawide frequency grid can replace the many bisection stages necessary for optical-to-microwave conversion. This approach has the potential to greatly simplify the measurements of absolute optical frequencies. We explore the use of an ultracompact femtosecond laser with a high repetition rate for generating a stable frequency comb.