Local oscillator induced degradation of medium-term stability in passive atomic frequency standards

Abstract

As the performance of passive atomic frequency standards improves, a new limitation is encountered due to frequency fiuctuations in an ancillary local osciktor (L.O.). The efect is due to time variation in the gain of the feedback which compensates L.O. frequenq fluctuations. The high performance promised by new microwave and optical trapped ion standark may be severely compromised by this effect. We present un anabsk uf this performance limitatwn for the case of sequentially interrogated standards. The time dependence oJ the sensitivity of the interrogution process to L.O. frequenq fluctuations is evalu-ated for singde-pulse and double-puke "Ramvey " RF interrogation and also for amplitude moduluted pukes. The e$ect of these various time dependencies on performance of the standard is calculated for an L.O. with frequency fluctuations showing a typical 1 / f spectral density. A limiting 1 1 6 dependent deviation of fre-quencyjluctuutions k calculated as a function of pulse lengths, dead time, and pulse overlap. We also present conceptual und hardwure+riented solutions to this problem which achieve a much more nearly convtant sensitivity to L.O.fluctuatiofiv. Solutionr involve: we of double-puke interrogation; alternute interrogation of mulliple traps so that the "dead time " of one trap can be covered by operution of the other; and the use of double-pulse interrogation for two traps, so that during the time of the RFpulves, the increasing sensitivity of one trup tends to compensate for the decreasing sensitivity of the otht: A solution making use of amplitude-modulated pulses is aLso presented which shows nominally zero time variation.