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Development of a Conventional Laser-Pumped RB Atomic Clock: Status Report

C. Back, James C. Camparo

Published Nov 1, 2007• 0 citations
OpenAlex

Abstract

As is well known, the phase fluctuations of a laser can decrease the signal-to-noise ratio of a vapor-cell clock due to laser PM-to-AM conversion. In previous work, we demonstrated an easy means of eliminating this problem: use high-pressure resonance cells so that collisional dephasing keeps the atoms from following the laser's fluctuating phase. Though our phase-1 clock employing a 100 torr N2 cell had excellent short-term performance, the long-term performance left much to be desired, quite likely because the increased pressure in the resonance cell gave the clock a fairly large temperature sensitivity. Here, we discuss our progress towards a phase-2 laser-pumped Rb clock, where the temperature sensitivity will be much reduced.

Related papers

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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.

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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)•May 12, 2025•0 citations
As is well known, the phase fluctuations of a laser can decrease the signal-to-noise ratio of a vapor-cell clock due to laser PM-to-AM conversion. In previous work, we demonstrated an easy means of eliminating this problem: use high-pressure resonance cells so that collisional dephasing keeps the atoms from following the laser's fluctuating phase. Though our phase-1 clock employing a 100 torr N2 cell had excellent short-term performance, the long-term performance left much to be desired, quite likely because the increased pressure in the resonance cell gave the clock a fairly large temperature sensitivity. Here, we discuss our progress towards a phase-2 laser-pumped Rb clock, where the temperature sensitivity will be much reduced.
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.
The optically thick vapors employed in gas-cell atomic clocks provide for very efficient conversion of laser phase noise (PM) to amplitude noise (AM). Here, we discuss the role of PM to AM conversion in determining gas-cell clock performance. First, a brief overview of the PM to AM conversion process will be given, highlighting the fact that laser phase noise creates fluctuations in the atom's optical absorption cross-section which have an exponential influence on the laser's transmitted intensity. Experimental results will then be presented showing that the PM to AM conversion process can increase the relative intensity noise of transmitted laser light by orders of magnitude, and that this can, under certain conditions, cause laser-pumped clock performance to be poorer than lamp-pumped clock performance.
When resonant laser light passes through a vapor, the laser's intrinsic phase fluctuations induce random variations in the atomic coherence, which in turn give rise to fluctuations in the medium's absorption cross section. Hence, laser phase modulation noise (PM) is converted to transmitted laser intensity (i.e., amplitude) modulation noise (AM). Here, we consider the influence of collisional dephasing on the PM-to-AM conversion process. Specifically, we measure the relative intensity noise of a diode laser beam, resonant with the Rb D1{D}_{1}D1​ transition at 794.7 nm, after it has passed through a Rb87/N2{\mathrm{Rb}}^{87}/{\mathrm{N}}_{2}Rb87/N2​ vapor as a function of nitrogen number density. Our results demonstrate that when collisional dephasing is very rapid, the spectral density of cross-section fluctuations is reduced, so that there is a significant decrease in the efficiency of PM-to-AM conversion at low Fourier frequencies. These results imply that, in general, when laser PM-to-AM conversion is the dominant noise process, pressure broadening can actually increase spectroscopic sensitivity.
SummaryWe present first characterization results on novel Rb vapor cells in view of high-performance Rb atomic clocks with reduced environmental sensitivity. For this purpose, a new two-component buffer-gas mixture is employed, in order to achieve simultaneously sufficiently low temperature coefficients of the cell with respect to the cell (volume) temperature and the cell stem temperature, respectively, as well as low sensitivity of the cell to fluctuations in the external (atmospheric) pressure, thus opening the way for state-of-the-art clock stabilities around one-day timescales without need for a vacuum enclosure. We show how a Nitrogen-Methane gas mixture can be employed for this purpose and report on first experimental results with such cells.