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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

The physics of miniature atomic clocks: 0-0 versus "end" resonances

Yuan‐Yu Jau, Amber Post, Nicholas N. Kuzma, Alan M. Braun, M. V. Romalis, W. Happer

Published Jul 8, 2004• 4 citations
Publisher

Abstract

We consider advantages of the "end" resonances for the miniaturized alkali vapor-cell atomic clock, namely a higher signal-to-noise ratio (SNR) and a narrower linewidth at high vapor densities. We report our preliminary observations of the unusual phenomenon of light narrowing, in which the rubidium end-resonance linewidth is substantially reduced and the SNR greatly improved as the laser light intensity is increased. Contributions to the total linewidth from other broadening mechanisms are discussed.

DOI
10.1109/freq.2003.1274982

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We consider advantages of the "end" resonances for the miniaturized alkali vapor-cell atomic clock, namely a higher signal-to-noise ratio (SNR) and a narrower linewidth at high vapor densities. We report our preliminary observations of the unusual phenomenon of light narrowing, in which the rubidium end-resonance linewidth is substantially reduced and the SNR greatly improved as the laser light intensity is increased. Contributions to the total linewidth from other broadening mechanisms are discussed.
The end-resonance clock uses strong hyperfine end transition to stabilize the frequency of the local oscillator. Comparing to the conventional 0-0 atomic clock, end resonance has very small spin-exchange broadening effect. The spin-exchange rate is proportional to the number density of the alkali-metal atoms. By using the end resonance, we are able to use very high dense vapor to obtain a much better signal to noise ratio. On the other hand, the end resonance suffers from the first-order magnetic field dependence. This problem, however, can be solved by simultaneously using a Zeeman end resonance to stabilize the magnetic field. Here, we report the most recent result of the end-resonance clock. In addition, we report a whole new technique, push-pull laser-atomic oscillator, which can be thought as all-photonic clock. This new clock requires no local oscillator. It acts like a photonic version of maser, which spontaneously generates modulated laser light and modulated voltage signals. The modulation serves as the clock signal, which is automatically locked to the ground-state hyperfine frequency of alkali-metal atoms.
We report on light shift and broadening in the atomic-motion-induced Ramsey narrowing of dark resonances prepared in alkali-metal vapors contained in wall-coated cells without buffer gas. The atomic-motion-induced Ramsey narrowing is due to the free motion of the polarized atomic spins in and out of the optical interaction region before spin relaxation. As a consequence of this effect, we observe a narrowing of the dark resonance linewidth as well as a reduction of the ground states' light shift when the volume of the interaction region decreases at constant optical intensity. The results can be intuitively interpreted as a dilution of the intensity effect similar to a pulsed interrogation due to the atomic motion. Finally the influence of this effect on the performance of compact atomic clocks is discussed.
We present a prospective experimental study on double-resonance Ramsey spectroscopy in a micro fabricated Rb vapor cell, in view of future miniature atomic clocks with improved stability performance. Ramsey signals with a narrow central fringe width of 1.5 kHz combined with a competitive contrast of > 5 % are observed, resulting in a clock short-term stability of ≈ 1×10-11τ−1/2. The employed pulsed optical pumping (POP) scheme bears promise for reduced light-shift effects and thus for improved clock stability on medium- to long-term timescales.
The alkali vapor-cell atomic clock, either conventional or coherent-population-trapping (CPT), offers one of the most viable approaches to an ultraminiature device. Unfortunately, this atomic clock suffers from two laser-induced noise processes, laser phase-noise (PM) to amplitude-noise (AM) conversion and ac-Stark shift fluctuations. Here, we demonstrate a method to circumvent these problems in a passive and miniaturizable manner based on pressure-broadening of the relevant optical transition. For this purpose, we have constructed a conventional atomic clock employing Rb/sup 87/ vapor confined with 100 torr of N/sub 2/. In this high-pressure environment, both PM-to-AM conversion efficiency and the ac-Stark shift are reduced. Though we employ a "phase-noisy," cleaved-facet diode laser, and lock the laser frequency to the pressure-broadened 1.6 GHz D/sub 1/ absorption line of Rb, we obtain excellent short-term frequency stability (i.e., /spl sigma//sub y/(/spl tau/) = 1.8x10/sup -12///spl tau//sup 1/2/). Moreover, since a single resonance cell generates locking signals for both the laser wavelength and the crystal oscillator, the atomic clock has real potential for miniaturization.
The alkali-vapor-cell atomic clock, of either the conventional or the coherent population trapping type, offers one of the most viable approaches to making ultraminiature and chip-scale devices. Unfortunately, this atomic clock suffers from two laser-induced noise processes: conversion of laser phase noise (PM) to amplitude noise (AM) and ac-Stark-shift fluctuations. Here we demonstrate a method for circumventing these problems in a passive and miniaturizable manner based on pressure broadening of the relevant optical transitions. For this purpose we have constructed a conventional atomic clock employing Rb^87 vapor confined with 100 Torr of N2. In this relatively high-pressure environment, both PM-to-AM conversion efficiency and the ac-Stark shift are reduced. Though we employ a phase-noisy, single-mode diode laser and lock the laser frequency to the pressure-broadened 1.6-GHz D_1 absorption line of Rb, we obtain excellent short-term frequency stability [i.e., sigma_y(tau)=1.8 × 10^−12/tau^½]. Moreover, as a single resonance cell generates locking signals for both the laser wavelength and the crystal oscillator, the atomic clock has real potential for miniaturization.