Showing papers by "G.J. Dick published in 1998"

Frequency stability degradation of an oscillator slaved to a periodically interrogated atomic resonator

Abstract: Atomic frequency standards using trapped ions or cold atoms work intrinsically in a pulsed mode. Theoretically and experimentally, this mode of operation has been shown to lead to a degradation of the frequency stability due to the frequency noise of the interrogation oscillator. In this paper a physical analysis of this effect has been made by evaluating the response of a two-level atom to the interrogation oscillator phase noise in Ramsey and multi-Rabi interrogation schemes using a standard quantum mechanical approach. This response is then used to calculate the degradation of the frequency stability of a pulsed atomic frequency standard such as an atomic fountain or an ion trap standard. Comparison is made to an experimental evaluation of this effect in the LPTF Cs fountain frequency standard, showing excellent agreement.

Cryo-cooled sapphire oscillator with ultra-high stability

Abstract: 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.

L.O. limited frequency stability for passive atomic frequency standards using "square wave" frequency modulation

Abstract: Atomic frequency standards using square-wave frequency modulation effectively interrogate the atomic line by switching back and forth between two frequencies with equal atomic absorption values. For a symmetric absorption line, the slope of the responses will also be equal. In the quasi-static limit this would seem to be an ideal interrogation process-the sign reversal of frequency slope can be removed by detection electronics to give an essentially unvarying and constant sensitivity to L.O. frequency variations. Such an interrogation would seem to eliminate L.O. aliasing and so relieve stringent requirements on local oscillator phase noise. However, sign changes in the interrogation and detection processes mean that the sensitivity is actually zero at some point in the cycle. We derive consequences of this fact for white phase and flicker phase noise models by an explicitly time-dependent analysis in terms of the sensitivity function g(t). We find that an optimal strategy exists for white phase L.O. noise for which g(t) takes the form of a sequence of parabolic arches, and which gives a small (/spl times/96//spl pi//sup 4/) improvement over simple sine-wave demodulation. We also find limiting forms that could in principle eliminate L.O, aliasing for flicker-phase noise. However, in practice the improvement shows only a logarithmic dependence on available response time and bandwidth.