W D. Lee

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).

The accuracy evaluation of NIST-7

We have performed evaluations of the major systematic errors in NIST-7 with an overall uncertainty of less than a part in 10/sup 14/. The complete evaluation process has been separated into two parts. With a computer-controlled, digital servo system and some new measurement techniques, we now perform core evaluations (second-order Zeeman and Doppler shifts, cavity pulling and phase shift, line overlap and some electronic shifts) with an overall uncertainty of less than one part in 10/sup 14/ in just a few days of measurements. The complete evaluation of all small and subtle effects in both the physics and electronics requires a few hundred days of data. But, these small effects are not variable at the 10/sup -14/ level and their infrequent evaluation does not detract from the operational accuracy of the standard. >

Environmental effects in frequency synthesizers for passive frequency standards

This paper reviews the environmental effects in synthesizers designed to support a frequency stability of 10/sup -13/ /spl tau//sup -1/2/ in short term and 10/sup -17/ in the long term. Specifically we consider the effects of temperature, pulling by spurious spectral lines, vibration effects, and pickup of spurious rf signals. We show that the temperature coefficient of the new NIST HR1 synthesizers is less than 1 ps/K and that the pulling from spectral purity is less than 3/spl times/10/sup -20/ in NIST-7, our primary thermal cesium beam standard. The pulling for slow cesium standards should be lower. We also show that the pulling due to spurious lines in Ramsey standards with narrow line widths can be manipulated to examine spectral pulling. The fractional frequency stability is better than 3/spl times/10/sup -14/ /spl tau//sup -1/2/ for measurement times out to 10/sup 4/ s and reaches 10/sup -16/ in about 15 minutes in a standard laboratory environment of roughly +/-0.5 K without the need of additional thermal regulation.

Rabi pedestal shifts as a diagnostic tool for primary frequency standards

Some of the systematic errors in primary, atomic-beam frequency standards are much larger for the Rabi pedestal part of the lineshape than for the Ramsey fringe part. We have used a digital servo system to measure the frequency offset between the Rabi pedestal and the Ramsey fringe for all seven Zeeman components of the cesium hyperfine transition. The dependence of these shifts on magnetic field, modulation amplitude, and microwave power enables us to separate three distinct causes: Rabi pulling, cavity pulling, and magnetic field inhomogeneity. From the measured pedestal shifts, we infer the corresponding shifts for the clock transition with uncertainties of the order of one part in 10/sup 15/ or less. >

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/.

Systematic errors in cesium beam frequency standards introduced by digital control of the microwave excitation

As part of the accuracy evaluation of the NIST-7 primary frequency standard, we have investigated potential sources of frequency offsets due to the electronic servo system. We present results from investigations of RF spectral purity and synthesizer switching transients.

Analysis of frequency biases and noise in sampled digital frequency servos for primary frequency standards

Unlike previous frequency standards at NIST, the main frequency servo of NIST-7, the U. S. primary frequency standard, does not lock a local oscillator to the atomic resonance. Instead it compares the frequencies of the atomic resonance and a reference oscillator. This paper presents an analysis of biases and noise in this digital servo. We show that a slight modification of the servo permits sensitive measurements for the presence of frequency biases.

DBR laser diodes for optically pumped Cs frequency standards

This note describes the use of distributed Bragg reflection (DBR) type laser diodes for use in optically pumped Cs beam frequency standards. We have used a DBR laser at 852 nm as the optical source for the fluorescence detection of the Cs clock transition in NIST-7 and find it to be equivalent to the extended cavity diode laser.

Systematic errors in NIST-7

We describe a continuing, in-depth evaluation of NISTs new optically pumped frequency standard, in which all known sources of systematic error are investigated; most by two or more independent techniques. Additionally, we have used both analog (fast sine-wave modulation) and digital (slow, square-wave modulation) servo systems during the evaluation.<<ETX>>