Robert L. Tjoelker

Stable Photonic Links for Frequency and Time Transfer in the Deep-Space Network and Antenna Arrays

For more than two decades, NASA deep space network (DSN) frequency and timing metrology has been a driving application for remote transfer of stable radio-frequency signals over fiber-optic cables. Precise, accurate, and stable signals are essential for deep-space communication and tracking, and syntonized and synchronized reference signals from atomic clocks calibrated to coordinated universal time must often be distributed over large distances. Fiber-optic technologies developed at the jet propulsion laboratory have resulted in several operational signal transport capabilities that enable precise spacecraft navigation and sensitive radio science experiments. These techniques are now finding further applicability in metrology applications to remotely compare ultra stable microwave and optical atomic clocks and for antenna array X- and Ka-band signal transport applications where temporal phase stability and alignment are critical. The pioneering DSN photonic link developments and capabilities are summarized, and a stabilized multiphotonic link architecture for ultrastable signal transport in antenna arrays is described.

A mercury ion frequency standard engineering prototype for the NASA deep space network

An engineering prototype linear ion trap frequency standard (LITS-4) using /sup 199/Hg/sup +/ is operational and currently under test for NASAs Deep Space Network (DSN). The DSN requires high stability and reliability with continuous operation. For practical considerations optical pumping and atomic state selection are accomplished with a /sup 202/Hg/sup +/ RF discharge lamp, and the trapped ions are cooled to near room temperature using a helium buffer gas. The standard is closely modeled from earlier research standards LITS-1 and LITS-2 which have demonstrated excellent frequency stability for uninterrupted comparison intervals up to 5 months. During an initial 135 day test in the DSN, LITS-4 operated continuously using a quartz crystal as the local oscillator. Recent signal to noise measurements indicate that a short term stability of /spl sigma//sub y/(/spl tau/)=2.0/spl times/10/sup -14///spl tau//sup 1/2/ can be achieved when operated with a sufficiently stable local oscillator.

Higher pole linear traps for atomic clock applications

We investigate experimentally and theoretically higher pole linear ion traps for frequency standard use. We have built a 12-pole trap and have successfully loaded ions into it from a linear quadrupole trap. By solving the Boltzmann equation describing large ion clouds where space charge interactions are important we show that clock frequency changes due to ion number fluctuations are much smaller in ion clocks based multipole traps than comparable clocks based on quadrupole linear traps.

Long term stability of Hg/sup +/ trapped ion frequency standards

The standards report the development of a second /sup 199/Hg/sup +/ linear ion trap (LIT) based frequency standard to provide a capability for measuring stability beyond all existing frequency standards. Increased signal using a second light collection system together with a previously demonstrated atomic line Q /spl ap/ 2 /spl times/ 10/sup 12/ yields a performance better than /spl sigma//sub y/ (/spl tau/ = 7 /spl times/ 10/sup -14///spl tau//sup 1/2/). Sensitivity to the leading perturbations are measured to identify regulation requirements to obtain a stability of 10/sup -16/.<<ETX>>

Cryo-cooled sapphire oscillator with ultra-high stability

We present test results and design details for the first short-term frequency standard to achieve ultra-high stability without the use of liquid helium. With refrigeration provided by a commercial cryocooler, the compensated sapphire oscillator (10 K CSO) makes available the superior short-term stability and phase noise performance of cryogenic oscillators without periodic interruptions for cryogen replacement. Technical features of the 10 K CSO include use of a a-stage cryocooler with vibration isolation by helium gas at atmospheric pressure, and a new sapphire/ruby resonator design giving compensated operation at 8-10 K with Q=1-2/spl times/10/sup 9/. Stability of the first unit shows an Allan Deviation of /spl sigma//sub y//spl les/2.5/spl times/10/sup -15/ for measuring times of 200 seconds /spl les//spl tau//spl les/600 seconds. We also present results showing the capability of the 10 K CSO to eliminate local oscillator degradation for atomic frequency standards. Configured as L.O. for the LITS-7 trapped mercury ion frequency standard, the CSO/LITS combination demonstrated a limiting performance of 3.0/spl times/10/sup -14///spl tau//sup 1/2/, the lowest value measured to date for a passive atomic frequency standard, and virtually identical to the value calculated from photon statistics.

Ultra-stable Hg+

Abstract We report the development of a fieldable frequency standard based on 199Hg+ ions confined in a hybrid r.f./dc linear ion trap. This trap permits storage of large numbers of ions with reduced susceptibility to the second-order Doppler effect caused by the r.f. confining fields. A 160 mHz wide atomic resonance line for the 40·5 GHz clock transition is used to steer the output of a 5 MHz crystal oscillator to obtain a stability of 2 × 10−15 for 24 000 s averaging times. For longer averaging intervals, measurements are limited by instabilities in available hydrogen maser frequency standards. Measurements with 37 mHz linewidth for the Hg+ clock transition demonstrate that the inherent stability for this frequency standard is at least as good as 1 × 10−15.

A compensated multi-pole linear ion trap mercury frequency standard for ultra-stable timekeeping

The multi-pole linear ion trap frequency standard (LITS) being developed at the Jet Propulsion Laboratory (JPL) has demonstrated excellent short- and long-term stability. The technology has now demonstrated long-term field operation providing a new capability for timekeeping standards. Recently implemented enhancements have resulted in a record line Q of 5 times 1012 for a room temperature microwave atomic transition and a short-term fractional frequency stability of 5 times 10-14/tau1/2. A scheme for compensating the second order Doppler shift has led to a reduction of the combined sensitivity to the primary LITS systematic effects below 5 times 10-17 fractional frequency. Initial comparisons to JPLs cesium fountain clock show a systematic floor of less than 2 times 10-16. The compensated multi-pole LITS at JPL was operated continuously and unattended for a 9-mo period from October 2006 to July 2007. During that time it was used as the frequency reference for the JPL geodetic receiver known as JPLT, enabling comparisons to any clock used as a reference for an International GNSS Service (IGS) site. Comparisons with the laser-cooled primary frequency standards that reported to the Bureau International des Poids et Mesures (BIPM) over this period show a frequency deviation less than 2.7 times 10-17/day. In the capacity of a stand-alone ultra-stable flywheel, such a standard could be invaluable for long-term timekeeping applications in metrology labs while its methodology and robustness make it ideal for space applications as well.

Stability measurements of a JPL multi-pole mercury trapped ion frequency standard at the USNO

Two 12-pole mercury trapped ion frequency standards were recently developed and compared at JPL. In July 2002 one of these standards was installed at the United States Naval Observatory (USNO) where it has since been continuously operating. The standard is configured with a hydrogen maser as the local oscillator and is continuously compared to approximately 10 other cavity-tuned hydrogen masers, 50 cesium standards, and the USNO master clock UTC(USNO). Stability measurements between the trapped ion standard and several USNO formulated clock ensembles over the entire 9 month operating period to date show very stable operation with a worst case differential frequency drift between any 60 day averaging period measured to be <2/spl times/10/sup -16//day.

Recent stability comparisons with the JPL linear trapped ion frequency standards

The /sup 199/Hg/sup +/ research frequency standards LITS-1 and LITS-2 were developed to provide continuous, reliable, high stability performance. For simplicity, a /sup 202/Hg lamp is used for state selection and a helium buffer gas for ion cooling. In a preliminary 9 day comparison between the trapped ion standards, the Allan deviation was /spl sigma//sub y/(/spl tau/)=1/spl times/10/sup -13///spl tau//sup 1/2/ and a fractional frequency stability of 6/spl times/10/sup -16/ measured for averaging times greater than 10/sup 5/ seconds. A 40 day comparison of LITS-2 against an auto-tuned H-maser referenced to UTC-NIST puts an upper limit on long term drift of LITS-2 of 1.2(1.4)/spl times/10/sup -16//day.<<ETX>>

Improved linear ion trap physics package

The authors describes an improvement in the architecture of the physics package used in the linear ion trap (LIT) based frequency standard developed at the Jet Propulsion Laboratory. This new design is based on the observation that ions can be moved along the axis of an LIT by applied DC voltages. The state selection/interrogation region can be separated from the more critical microwave resonance region where the multiplied local-oscillator signal is compared to the stable atomic transition. This separation relaxes many of the design constraints of the present units. Improvements include increased frequency stability, and a substantial reduction in size, mass, and cost of the final frequency standard.<<ETX>>