Daphna G. Enzer

Performance of the PARCS testbed cesium fountain frequency standard

A cesium fountain frequency standard has been developed as a ground testbed for the PARCS (primary atomic reference clock in space) experiment - an experiment intended to fly on the International Space Station. We report on the performance of the fountain and describe some of the implementations motivated in large part by flight considerations, but of relevance for ground fountains. In particular, we report on a new technique for delivering cooling and trapping laser beams to the atom collection region, in which a given beam is recirculated three times effectively providing much more optical power than traditional configurations. Allan deviations down to 10/sup -15/ have been achieved with this method.

Mitigation of the light shift in laser cooled clocks without mechanical shutters

We propose an approach for keeping the light shift in laser cooled frequency standards down to the 10/sup -17/ level without the use of mechanical shutters. For laser systems using a master-slave laser configuration, cutting the injection power to a slave causes it to lase at its free-running wavelength, often two or more nanometers off from the atomic resonance. This approach does not apply to laser systems using power amplifiers.

Mercury Ion Clock for a NASA Technology Demonstration Mission

There are many different atomic frequency standard technologies but only few meet the demanding performance, reliability, size, mass, and power constraints required for space operation. The Jet Propulsion Laboratory is developing a linear ion-trap-based mercury ion clock, referred to as DSAC (DeepSpace Atomic Clock) under NASAs Technology Demonstration Mission program. This clock is expected to provide a new capability with broad application to space-based navigation and science. A one-year flight demonstration is planned as a hosted payload following an early 2017 launch. This first-generation mercury ion clock for space demonstration has a volume, mass, and power of 17 L, 16 kg, and 47 W, respectively, with further reductions planned for follow-on applications. Clock performance with a signal-to-noise ratio (SNR)*Q limited stability of 1.5E - 13/τ1/2 has been observed and a fractional frequency stability of 2E-15 at one day measured (no drift removed). Such a space-based stability enables autonomous timekeeping of Δt <; 0.2 ns/day with a technology capable of even higher stability, if desired. To date, the demonstration clock has been successfully subjected to mechanical vibration testing at the 14 grms level, thermal-vacuum operation over a range of 42 °C, and electromagnetic susceptibility tests.

Two-species cold atomic beam

We generate a bright atomic beam containing laser-cooled rubidium and cesium, and we use this beam to load a mixed-species ultrahigh-vacuum (UHV) magneto-optical trap. We have characterized our two-species atomic beam over a range of operating conditions, and we obtain similar atom fluxes for each species. Within the UHV trap, interspecies inelastic collisions are observed in the form of enhanced decay rates of a given species in the presence of a second trapped species. We analyze the trap decays to obtain a loss rate due to heteronuclear cold collisions, and we compare our result to similar measurements in vapor-cell traps Phys. Rev. A 63 , 033406 (2001).

GRAIL — A microwave ranging instrument to map out the lunar gravity field

Gravity Recovery and Interior Laboratory, or GRAIL, is a NASA mission to map out the gravity field of the moon to an unprecedented level of detail. The instrument for this mission is based on GRACE (Gravity Recovery and Climate Experiment), an earth-orbiting mission currently mapping out the gravity field of the earth. This paper will describe the similarities and differences between these two instruments with a focus on the microwave ranging measurements used to determine the gravity parameters and the testbed built at Jet Propulsion Laboratory to demonstrate micron level ranging capability. The onboard ultrastable oscillator and RF instruments will be described and noise contributions discussed.

In situ measurements of USO performance in space using the twin GRAIL spacecraft

Ultra-stable oscillators (USO) are flown on a variety of different science missions to provide stable timing and/or navigation. Typically, their performance is measured at the part in 1013 level before launch and can only be verified in space via measurements that must propagate through, and potentially be degraded by, the Earths atmosphere. To date, two missions are able to demonstrate USO performance in space, without atmospheric limitations, using twin spacecraft: Gravity Recovery and Climate Experiment (GRACE) and Gravity Recovery and Interior Laboratory (GRAIL). With the recent launch of GRAIL, a NASA mission to map out the gravity field of the Moon, the clock and timing community has access to microwave tracking data from a pair of satellites flying independent USOs. In addition, since GRAIL took a circuitous three-month journey to the Moon, there was a 30-minute opportunity to obtain USO versus USO data when the spacecraft were not in orbit around the Earth or the Moon. This paper presents GRAIL Allan deviation data obtained during this payload-checkout in September 2011 and during the Science Phase in March 2012, and also analyzes data from GRACE using these new methods.

Characterization of light shift below 10/sup -15/ in a cesium fountain frequency standard operated without the use of mechanical shutters

We characterize the light shift in the interaction region of a laser-cooled frequency standard and demonstrate an approach for its mitigation without the use of mechanical shutters. The light shift is confirmed to be below 10-15 and expected to be orders of magnitude lower. This technique makes use of a master-slave laser configuration where cutting the injection power to a slave laser causes it to lase at its free-running wavelength, often two or more nanometers off from the atomic resonance

Drifts and Environmental Disturbances in Atomic Clock Subsystems: Quantifying Local Oscillator, Control Loop, and Ion Resonance Interactions

Linear ion trap frequency standards are among the most stable continuously operating frequency references and clocks. Depending on the application, they have been operated with a variety of local oscillators (LOs), including quartz ultrastable oscillators, hydrogen-masers, and cryogenic sapphire oscillators. The short-, intermediate-, and long-term stability of the frequency output is a complicated function of the fundamental performances, the time dependence of environmental disturbances, the atomic interrogation algorithm, the implemented control loop, and the environmental sensitivity of the LO and the atomic system components. For applications that require moving these references out of controlled lab spaces and into less stable environments, such as fieldwork or spaceflight, a deeper understanding is needed of how disturbances at different timescales impact the various subsystems of the clock and ultimately the output stability. In this paper, we analyze which perturbations have an impact and to what degree. We also report on a computational model of a control loop, which keeps the microwave source locked to the ion resonance. This model is shown to agree with laboratory measurements of how well the feedback removes various disturbances and also with a useful analytic approach we developed for predicting these impacts.

Light shift measurements in a cesium fountain without the use of mechanical shutters

We present measurements confirming operation of a cesium fountain frequency standard with light shift below 10/sup -15/ (and with evidence suggesting it is several orders of magnitude below this level) but without the use of mechanical shutters. Suppression of the light shift is realized using a master-slave laser configuration by reducing the overall optical power delivered to the physics package as well as spoiling the injection of the slave, causing it to lase far off resonance (1-2 nm) as proposed by the authors several years ago (Klipstein, 2002). In the absence of any mitigation, this (AC Stark) shift, due to near-resonant laser light reaching the atoms during their microwave interrogation period, is the largest shift in such frequency standards (2 /spl times/ 10/sup -11/ for our fountain). Mechanical shutters provided adequate light attenuation but have been prone to failure.