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Physical Review A

Amplitude- versus frequency-modulated pumping light for coherent population trapping resonances at high buffer-gas pressure

Amber Post, Yuan‐Yu Jau, Nicholas N. Kuzma, W. Happer

Published Sep 27, 2005• 23 citations
Publisher

Abstract

Coherent population trapping (CPT) signals can be optimized with amplitude-modulated pumping light. As shown theoretically and experimentally in this paper, the optimum wave forms produce strong CPT signals at both low buffer-gas pressures, where the hyperfine structure (HFS) splitting of the optical absorption lines is well resolved, and also at high buffer-gas pressures, where the HFS is no longer optically resolved due to pressure broadening. On the other hand, CPT resonances from frequency-modulated waves are severely degraded for these high-pressure conditions. High buffer-gas pressures may offer advantages for miniature frequency standards and magnetometers, including suppression of diffusion losses in miniature cells, suppression of light shifts, and less stringent requirements for frequency stability of the pumping light sources.

DOI
10.1103/physreva.72.033417

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Coherent population trapping (CPT) signals can be optimized with amplitude-modulated pumping light. As shown theoretically and experimentally in this paper, the optimum wave forms produce strong CPT signals at both low buffer-gas pressures, where the hyperfine structure (HFS) splitting of the optical absorption lines is well resolved, and also at high buffer-gas pressures, where the HFS is no longer optically resolved due to pressure broadening. On the other hand, CPT resonances from frequency-modulated waves are severely degraded for these high-pressure conditions. High buffer-gas pressures may offer advantages for miniature frequency standards and magnetometers, including suppression of diffusion losses in miniature cells, suppression of light shifts, and less stringent requirements for frequency stability of the pumping light sources.
Coherent population trapping (CPT) on 87 Rb atoms in neon atmosphere has been studied in small-size glass absorption cells under conditions of pumping with narrow-band laser radiation at the D 2 line of the main doublet. Parameters of the absorption signal have been measured in 3-mm-diameter cells at buffer gas (Ne) pressures varied within 200–400 Torr, cell temperatures within 65–120°C, and pumping radiation power densities within 30–400 μW/cm 2 . Optimum values of the buffer gas pressures, cell temperature, and pumping power are determined at which the short-term instability of the resonance line is at minimum. Orientational shifts of the CPT resonance signal in gas-filled cells and small-size cells with antirelaxation coating have been compared.
We present the results of an experimental study of Coherent Population Trapping (CPT) in potassium, obtained by means of modulation of laser light amplitude with kHz frequency. The radiation from an external cavity diode laser, matching the D1 line of K, is modulated by an acousto-optical modulator. In the cell containing buffer gas, the CPT resonance width is reduced more than three orders of magnitude as compared to the cell containing pure potassium vapor. In K this resonance narrowing occurs with high resonance contrast; such behavior is not observed in buffered cells containing Rb or Cs, where the optical pumping to the non-interacting with the light ground level is very effective and depletes the population of the working ground Zeeman sublevels. The narrow CPT resonance of reduced fluorescence transforms to the one of enhanced fluorescence with the cell temperature rising. The transformed resonance exhibits higher contrast and lower width than those of the reduced fluorescence resonance. Hence, beside its scientific importance the resonance sign reversal can be used for the improvement of the CPT resonance parameters.
Controlling buffer-gas conditions in vapor cells is essential for frequency stabilization and atomic-clock performance. This study investigated the dependence of coherent population trapping (CPT) signal characteristics on nitrogen (N 2{}_{2}2​ ) buffer-gas pressure in micro-fabricated vapor cells containing natural-abundance rubidium (Rb) generated from Rb azide (RbN 3{}_{3}3​ ) for chip-scale atomic clocks. A wafer-level fabrication method enabling individual control of N2 pressure was developed by inkjet patterning of RbN3 and subsequent krypton fluoride (KrF) excimer laser decomposition. An increasing N2 pressure was found to reduce the CPT linewidth and cause a significant decrease in the signal amplitude, which resulted in monotonic degradation in the CPT figure of merit with higher pressures. A CPT-based laser frequency stabilization method that locked the laser wavelength to the CPT resonance peak yielded significantly larger signal amplitudes than a conventional absorption-based method in the low-pressure regime below 8 kPa. These results show that CPT performance in micro-fabricated Rb vapor cells is more restricted by the signal amplitude rather than the linewidth. They also offer practical buffer-gas design guidelines for enhancing short-term frequency stability in chip-scale atomic clocks.
The present work is dedicated to optimization of the laser radiation parameters and optically thickness of the gas cell for improvement of short-term stability of quantum frequency standard based on coherent population trapping effect in the cell with 87Rb working gas and buffer gas. The effects associated with the optical density of the medium are investigated, such as laser spectrum distortion and appearance of the light shifts of the coherent population trapping resonance due to different absorption of frequency components.
We present a pair of phase-locked lasers with a 9.2-GHz frequency difference through the injection locking of a master laser to the RF-modulation sideband of a slave diode laser. Using this laser system, a coherent population trapping (CPT) signal with a typical linewidth of ∼ 182 Hz is obtained in a cesium vapor cell filled with 30 Torr (4 kPa) of neon as the buffer gas. We investigate the influence of the partial pressure of the neon buffer gas on the CPT linewidth, amplitude, and frequency shift. The results may offer some references for CPT atomic clocks and CPT atomic magnetometers.
Characteristics of CPT (coherent population trapping) signal are investigated in small-size glass vapor cells containing 87 Rb and Ne buffer gas with narrow linewidth laser pumping on D 2 line. Parameters of CPT absorption signals are measured using small-size vapor cells with Ne buffer gas pressure in range of 200-400 Torr, cell temperature in range of 65-120 °C and values of laser pumping power of 30-400 μW/cm 2 . Optimum conditions, under which the minimal value of short-term instability of resonance line is achieved, are obtained in experiments. CPT signals using vapor cells based on integrated technologies containing 87 Rb in atmosphere of Ne are also investigated. The CPT signals with typical linewidths of 2-3 kHz and signal-to-noise ratio of 1500 in 1 Hz bandwidth are observed, which allows one to provide relative frequency instability of 10 -11 at 100 s.