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

Reducing PM-to-AM conversion and the light-shift in laser-pumped, vapor-cell atomic clocks

James C. Camparo, John G. Coffer, Jeremy Townsend

Published Apr 12, 2005• 3 citations
Publisher

Abstract

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.

DOI
10.1109/freq.2004.1418442

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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.
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.
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 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.
As laser light propagates through a resonant vapor, laser phase noise (PM) is converted to laser intensity noise (AM) because of the sensitivity of atomic coherence to laser phase fluctuations. In experiments reported here it is shown that this PM-to-AM conversion process is highly efficient and can cause the relative intensity noise of transmitted diode laser light to be 1 to 2 orders of magnitude larger than the laser’s intrinsic relative intensity noise. By use of a semiclassical description of the phenomenon, including the effect of optical pumping, reasonably good agreement between theory and experiment is obtained. The PM-to-AM conversion process discussed here has important consequences for atomic clock development, in which diode-laser optical pumping in thick alkali vapors holds the promise for orders-of-magnitude improvement in atomic 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.
Abstract Chip-scale atomic clocks based on coherent population trapping (CPT) face challenges due to miniaturized gas cells, which reduces the CPT resonance signal and the absorption signals of alkali atoms. To overcome this, we propose a method that simultaneously stabilizes the laser wavelength and microwave modulation frequency without relying on absorption line detection. Our approach enables stabilization at the point where the amplitude of the CPT resonance signal is maximized, making it suitable for compact atomic clocks. The system uses three lock-in amplifiers: one is used to stabilize the RF modulation frequency, and the other two are used for detecting the amplitude of the CPT resonance and stabilizing the laser wavelength. The Allan deviation measurements demonstrate a 1.4-fold improvement in short-term stability compared to conventional methods, suggesting that the proposed technique is a promising solution for enhancing the performance of small gas-cell atomic clocks.