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European Frequency and Time Forum

Push-pull optical pumping on the Cs D1-transition

Evelina Breschi, Antoine Weis

Published Apr 1, 2012• 0 citations
Publisher

Abstract

We report on preliminarily results of push-pull optical pumping (PPOP) on the D1-transition of cesium. PPOP is based on a coherent population trapping effect prepared with polarization-modulated bichromatic light coupling the two ground-state hyperfine levels, the light polarization being modulated at the microwave frequency. With a suitable choice of polarizations, a single specific coherent superposition (dark state) formed by the two mF= 0 sublevels can be prepared under optimal conditions. This increase the contrast of the clock reference signal without significant line-broadening.

DOI
10.1109/eftf.2012.6502387

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Quantum Electronics and Laser Science Conference•May 7, 2000•1 citation
We report on preliminarily results of push-pull optical pumping (PPOP) on the D1-transition of cesium. PPOP is based on a coherent population trapping effect prepared with polarization-modulated bichromatic light coupling the two ground-state hyperfine levels, the light polarization being modulated at the microwave frequency. With a suitable choice of polarizations, a single specific coherent superposition (dark state) formed by the two mF= 0 sublevels can be prepared under optimal conditions. This increase the contrast of the clock reference signal without significant line-broadening.
We report on a theoretical study and experimental characterization of coherent-population-trapping (CPT) resonances in buffer-gas-filled vapor cells with push-pull optical pumping (PPOP) on Cs D1{D}_{1}D1​ line. We point out that the push-pull interaction scheme is identical to the so-called $\mathrm{lin}\ensuremath{\perp}\mathrm{lin}$ polarization scheme. Expressions of the relevant dark states, as well as of absorption, are reported. The experimental setup is based on the combination of a distributed feedback diode laser, a pigtailed intensity Mach-Zehnder electro-optic modulator (MZ EOM) for optical sideband generation and a Michelson-like interferometer. A microwave technique to stabilize the transfer function operating point of the MZ EOM is implemented for proper operation. A CPT resonance contrast as high as 78% is reported in a cm-scale cell for the magnetic-field-insensitive clock transition. The impact of the laser intensity on the CPT-clock signal key parameters (linewidth, contrast, linewidth/contrast ratio) is reported for three different cells with various dimensions and buffer-gas contents. The potential of the PPOP technique for the development of high-performance atomic vapor-cell clocks is discussed.
A new optical pumping method, "push-pull pumping," can produce very nearly pure, coherent superposition states between the initial and the final sublevels of the important field-independent 0-0 clock resonance of alkali-metal atoms. The key requirement for push-pull pumping is the use of D1 resonant light which alternates between left and right circular polarization at the Bohr frequency of the state. The new pumping method works for a wide range of conditions, including atomic beams with almost no collisions, and atoms in buffer gases with pressures of many atmospheres.
The D1 line spectrally selective pumping process in Doppler-broadened cesium is analyzed by solving the optical Bloch equations. The process, described by a three-level model with the Λ scheme, shows that the saturation intensity of broadened atoms is three orders of magnitude larger than that of resting atoms. The |Fg = 3〉 → |Fe = 4〉 resonance pumping can result in the ground state |Fg = 4, mF = 4〉 sublevel having a maximum population of 0.157 and the population difference would be about 0.01 in two adjacent magnetic sublevels of the hyperfine (HF) state Fg = 4. To enhance the anisotropy in the ground state, we suggest employing dichromatic optical HF pumping by adding a laser to excite D1 line |Fg = 4〉 → |Fe = 3〉 transition, in which the cesium magnetometer sensitivity increases by half a magnitude and is unaffected by the nonlinear Zeeman effect even in Earth's average magnetic field.
The D1 line spectrally selective pumping process in Doppler-broadened cesium is analyzed by solving the optical Bloch equations. The process, described by a three-level model with the A scheme, shows that the saturation intensity of broadened atoms is three orders of magnitude larger than that of resting atoms. The 丨Fg = 3） →丨Fe = 4） resonance pumping can result in the ground state丨Fg = 4, mF = 4） sublevel having a maximum population of 0.157 and the population difference would be about 0.01 in two adjacent magnetic sublevels of the hyperfine （HF） state Fg = 4. To enhance the anisotropy in the ground state, we suggest employing dichromatic optical HF pumping by adding a laser to excite D1 line 丨Fg = 4） → 丨Fe = 3） transition, in which the cesium magnetometer sensitivity increases by half a magnitude and is unaffected by the nonlinear Zeeman effect even in Earth＇s average magnetic field.
This article reports on the characterization of a compact coherent-population-trapping (CPT) Cs vapor cell atomic clock using the push-pull optical pumping technique. This method allows to detect high-contrast CPT resonances on the 0-0 magnetic-field insensitive clock transition. The clock short-term fractional frequency stability is measured to be 3×10−13 τ−1/2 up to 100 s averaging time, in correct agreement with the signal-to-noise ratio limit. These results are among the best performances ever reported for a CPT-based clock. The noise budget is detailed. A preliminary study is reported to evaluate limitations to the clock mid-term frequency stability. The clock performances are found to be presently mainly limited in the mid-term by laser power effects.
Summary form only given. We report on recent experiments to use dark line coherent population trapping (CPT) resonances in cesium vapor as the basis for a compact frequency reference. The resonance is excited by directly modulating the injection current of a VCSEL at one-half the ground state hyperfine transition of Cs (4.6 GHz) to produce RF sidebands on the optical carrier. The laser output is then passed through a room-temperature cell containing Cs at its vapor pressure together with 8.7 kPa of Neon buffer gas and the transmitted optical power is detected in a photodiode. When the laser carrier is tuned half way between the two hyperfine components of the D/sub 2/ line at 852 nm, and the RF modulation frequency is adjusted to be equal to one-half the hyperfine splitting, the absorption of the sidebands through the cell is slightly reduced due to the CPT effect. As the RF frequency is scanned over the resonance, lock-in detection is used to produce a /spl sim/1 kHz wide dispersive error signal which is in turn used to stabilize the modulation frequency. The narrow signal is the result of the buffer gas, which reduces the time-of-flight broadening of the transition.