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Journal of Physics B Atomic Molecular and Optical Physics

Laser-induced noise contribution due to imperfect atomic state preparation in an optically pumped caesium beam resonator

Gaëlle Lucas-Leclin, P. Cérez, N. Dimarcq

Published Jan 1, 1999• 12 citations
Publisher

Abstract

Imperfect atomic state preparation in an optically pumped beam resonator results in over-Poissonian fluctuations of the fluorescence emission of residual unpumped atoms. This excess noise is induced by the frequency noise of the pump laser. We evaluate it theoretically for a three-level atomic system interacting with a noisy laser field, and show that it is predominant within a large range of pumping efficiencies and laser linewidths for usual atomic fluxes. Experimentally, extra noise is detected from ratios of unpumped atoms as low as 0.01% of the atomic flux equal to . This noise source debases the short-term stability of an atomic clock in a flop-in configuration if the proportion of unpumped atoms exceeds 1% of the global atomic flux. In a flop-out configuration this excess noise would decrease the clock stability from a proportion of unpumped atoms of 0.01%.

DOI
10.1088/0953-4075/32/2/014

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Imperfect atomic state preparation in an optically pumped beam resonator results in over-Poissonian fluctuations of the fluorescence emission of residual unpumped atoms. This excess noise is induced by the frequency noise of the pump laser. We evaluate it theoretically for a three-level atomic system interacting with a noisy laser field, and show that it is predominant within a large range of pumping efficiencies and laser linewidths for usual atomic fluxes. Experimentally, extra noise is detected from ratios of unpumped atoms as low as 0.01% of the atomic flux equal to . This noise source debases the short-term stability of an atomic clock in a flop-in configuration if the proportion of unpumped atoms exceeds 1% of the global atomic flux. In a flop-out configuration this excess noise would decrease the clock stability from a proportion of unpumped atoms of 0.01%.
To improve the signal to noise ratio (SNR) and the short-term stability of cesium atomic fountain clocks, the work of two-laser optical pumping is presented theoretically and experimentally. The short-term stability of the NIM6 fountain clock has been improved by preparing more cold atoms in the | F = 4, mF = 0〉 clock state with a shortened cycle time. Two π-polarized laser beams overlapped in the horizontal plane have been applied after launching, one is resonant with | F = 4〉 → | F′ = 4〉 transition and the other is resonant with | F = 3〉 → | F′ = 4〉 transition. With optical pumping, the population accumulated in the | mF = 0〉 clock state is improved from 11% to 63%, and the detection signal is increased by a factor of 4.2, the SNR of the clock transition probability and the short-term stability are also improved accordingly.
We consider the problem of reducing the contributions of pump and spontaneous-emission noise to the output noise of a laser field. We consider two models based on an atomic medium, one analytic, the other involving numerical simulation of a sequence of pump cycles. The predictions of both models are shown to agree. For regular pump excitation the intensity fluctuations of a laser well above threshold may be reduced to zero at the cavity resonance frequency if the ratio of the lifetimes of the upper and lower lasing levels is sufficiently large that spontaneous emission is suppressed. When the atomic lifetimes are comparable, spontaneous emission reduces the amount of noise reduction. For example, for equal atomic lifetimes regular pump excitation will result in a maximum noise reduction in the output field of one-half the shot-noise limit.
One impediment to achieving the promise of enhanced laser-pumped gas cell clock performance resides with the light-shift effect, a phenomenon whereby diode laser optical frequency fluctuations are mapped onto the atomic clock's output frequency. The alternative to tightly stabilizing the diode laser in order to overcome this problem is to develop means of reducing the actual magnitude of the light-shift. Here, we briefly review two strategies for accomplishing this, and introduce a new one based on optical pumping with laser-induced-fluorescence (LIF). The advantage of LIF optical pumping is that the clock's output frequency is insensitive to laser frequency fluctuations. Though LIF optical pumping creates a dependence of clock frequency on laser intensity variations, it is to be noted that diode lasers have very low intensity noise. Consequently, with regards to the light-shift effect the LIF strategy takes advantage of an inherent diode laser strength.
One impediment to achieving the promise of enhanced laser-pumped gas cell clock performance resides with the light-shift effect, a phenomenon whereby diode laser optical frequency fluctuations are mapped onto the atomic clock‘s output frequency. The alternative to tightly stabilizing the diode laser in order to overcome this problem is to develop means of reducing the actual magnitude of the light-shift. Here, we briefly review two strategies for accomplishing this, and introduce a new one based on optical pumping with laser-induced-fluorescence (LIF). The advantage of LIF optical pumping is that the clock‘s output frequency is insensitive to laser frequency fluctuations. Though LIF optical pumping creates a dependence of clock frequency on laser intensity variations, it is to be noted that diode lasers have very low intensity noise. Consequently, with regards to the light-shift effect the LIF strategy takes advantage of an inherent diode laser strength.
Herein, we report the scheme of an optically pumped atomic clock based on a cold cesium atomic beam source. We propose the laser system and physical mechanism of this atomic clock, wherein the atomic beam travels in an upper parabolic trajectory, thereby eliminating the light shift effect. In the experiments, when the length of the free evolution region was 167 mm, the line width of the Ramsey fringe was 37 Hz. When the expected signal-to-noise ratio of the Ramsey fringe that can be achieved is 36,000, the expected short-term frequency stability is about 3.6 × 10–14/√τ, which is significantly higher than that of a conventional optically pumped cesium clock of similar volume.
The influence of laser frequency noise on the stability of a compact cold-atom microwave clock was investigated theoretically in this study. The phase-noisy laser was characterized by a phase-diffusion field. Given the laser linewidth, the analytical expressions of the atomic covariance function and the noise power spectral density of the relative intensity noise transferred from the frequency noise were derived based on stochastic differential equations. Our results provide an effective method to estimate the impact of laser frequency noise on the stability of an integrating sphere cold atomic clock. This method can be extended to other cold atom detection cases.