Moustafa Abdel Hafiz

A coherent population trapping Cs vapor cell atomic clock based on push-pull optical pumping

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 3×10−13u2009τ−1/2 up to 100u2009s 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.

A high-performance Raman-Ramsey Cs vapor cell atomic clock

We demonstrate a high-performance coherent-population-trapping (CPT) Cs vapor cell atomic clock using the push-pull optical pumping technique in the pulsed regime, allowing the detection of high-contrast and narrow Ramsey-CPT fringes. The impact of several experimental parameters onto the clock resonance and short-term fractional frequency stability, including the laser power, the cell temperature, and the Ramsey sequence parameters, has been investigated. We observe and explain the existence of a slight dependence on laser power of the central Ramsey-CPT fringe line-width in the pulsed regime. We report also that the central fringe line-width is commonly narrower than the expected Ramsey line-width given by 1 / ( 2 T R ), with TR the free-evolution time, for short values of TR. The clock demonstrates a short-term fractional frequency stability at the level of 2.3 × 10 − 13 τ − 1 / 2 up to 100 s averaging time, mainly limited by the laser amplitude modulation noise. Comparable performances are obtained ...

Laser light routing in an elongated micromachined vapor cell with diffraction gratings for atomic clock applications.

This paper reports on an original architecture of microfabricated alkali vapor cell designed for miniature atomic clocks. The cell combines diffraction gratings with anisotropically etched single-crystalline silicon sidewalls to route a normally-incident beam in a cavity oriented along the substrate plane. Gratings have been specifically designed to diffract circularly polarized light in the first order, the latter having an angle of diffraction matching the (111) sidewalls orientation. Then, the length of the cavity where light interacts with alkali atoms can be extended. We demonstrate that a longer cell allows to reduce the beam diameter, while preserving the clock performances. As the cavity depth and the beam diameter are reduced, collimation can be performed in a tighter space. This solution relaxes the constraints on the device packaging and is suitable for wafer-level assembly. Several cells have been fabricated and characterized in a clock setup using coherent population trapping spectroscopy. The measured signals exhibit null power linewidths down to 2.23u2009kHz and high transmission contrasts up to 17%. A high contrast-to-linewidth ratio is found at a linewidth of 4.17u2009kHz and a contrast of 5.2% in a 7-mm-long cell despite a beam diameter reduced to 600u2009μm.

Characterization of Cs vapor cell coated with octadecyltrichlorosilane using coherent population trapping spectroscopy

We report the realization and characterization using coherent population trapping (CPT) spectroscopy of an octadecyltrichlorosilane (OTS)-coated centimeter-scale Cs vapor cell. The dual-structure of the resonance lineshape, with presence of a narrow structure line at the top of a Doppler-broadened structure, is clearly observed. The linewidth of the narrow resonance is compared to the linewidth of an evacuated Cs cell and of a buffer gas Cs cell of similar size. The Cs-OTS adsorption energy is measured to be (0.42u2009±u20090.03) eV, leading to a clock frequency shift rate of 2.7u2009×u200910−9/K in fractional unit. A hyperfine population lifetime, T1, and a microwave coherence lifetime, T2, of 1.6 and 0.5u2009ms are reported, corresponding to about 37 and 12 useful bounces, respectively. Atomic-motion induced Ramsey narrowing of dark resonances is observed in Cs-OTS cells by reducing the optical beam diameter. Ramsey CPT fringes are detected using a pulsed CPT interrogation scheme. Potential applications of the Cs-OTS cell ...

Double-modulation CPT cesium compact clock

Double-modulation coherent population trapping (CPT) is based on a synchronous modulation of Raman phase and laser polarization, which allows the atomic population to accumulate in a common dark state. The high contrast signal obtained on the clock transition with a relative compact and robust laser system is interesting as basis of a high performance microwave clock. Here we study the parameters of a double-modulation CPT Cs clock working in cw mode. The optimal polarization modulation frequency and cell temperature for maximum contrast of clock transition are investigated. The parameters of the detection are also studied. With the optimal parameters, we observe a CPT signal with contrast of 10% and linewidth of 492 Hz, which is well suited for implementing a cw atomic clock.

Characterization of commercially available vertical-cavity surface-emitting lasers tuned on Cs D 1 line at 894.6 nm for miniature atomic clocks

We report on the metrological characterization of novel commercially available 894.6xa0nm vertical-cavity surface-emitting lasers (VCSELs), dedicated to Cs D1 line spectroscopy experiments. The thermal behavior of the VCSELs is reported, highlighting the existence of a minimum threshold current and maximum output power in the 55°C-60°C range. The laser relative intensity noise, measured to be -108u2009u2009dB/Hz at 10xa0Hz Fourier frequency f for a laser power of 25xa0μW, is reduced with increased power. The VCSELs frequency noise is 108u2009u2009Hz2/Hz at f=100u2009u2009Hz. The spectral linewidth of the VCSELs is about 30xa0MHz. VCSELs injection current can be directly modulated at 4.596xa0GHz with microwave power in the range of -10 to +0u2009u2009dBm to generate optical sidebands. A VCSEL was used in a microcell-based Cs atomic clock based on coherent population trapping. A preliminary clock short-term fractional frequency stability of 8×10-11τ-1/2 up to about 100xa0s is reported, demonstrating the suitability of these VCSELs for miniature atomic clock applications.

Ultrahigh-contrast saturated-absorption resonance to enhance stability of CPT atom clocks

The results of detailed experimental and theoretical investigations of new nonlinear effect in the field of saturated-absorption spectroscopy of atom vapours are presented. The effect consists in observation of a high-contrast natural-linewidth absorption spike when 133Cs atoms are being irradiated by the bichromatic counterpropagating laser beams. The results obtained can be useful in various fields of modern laser physics and applications where saturated-absorption resonances are used. For instance, the effect has been already implemented in the coherent-population-trapping atom clocks to enhance stabilization of optical frequencies and to improve stability of the clocks on the whole.

Analytical expressions for parameters of the dark resonance in a vacuum vapour cell

In spite of great number of papers devoted to theoretical and experimental study of the coherent population trapping phenomenon (CPT), still there is a lack of analytical results for the properties of the CPT-caused dark resonance in some principal cases. We will present the results of our study for single and double A schemes of atomic energy levels, which are of great importance for various applications of CPT (atom clocks, magnetometers, etc.). Analytical expressions for width and amplitude of the dark resonances have been obtained, which are valid for wide range of light wave intensities. The results concern closed as well as open A schemes. We will also discuss the difference between the two A schemes and state the condition when a double scheme can be effectively treated as a single A scheme. The analytical approximate formulas obtained have been compared with exact numerical solutions based on the optical Bloch equations as well as experimental.

A CPT-based Cs vapor cell atomic clock with a short-term fractional frequency stability of 3 x 10-13 τ-1/2

This article reports on the development and short-term fractional frequency stability of a continuous-regime (CW) Cs vapor cell atomic clock based on coherent population trapping (CPT). The push-pull optical pumping technique is used to increase the number of atoms that participate to the clock transition, yielding a typical CPT resonance contrast of 25% for a CPT linewidth of about 450 Hz. The clock short-term fractional frequency stability is measured to be 3 x 10-13 τ-1/2 up to 100 seconds averaging time, in correct agreement with the signal-to-noise ratio limit. The mid-term frequency stability results are currently mainly limited by laser power effects. The detection of high-contrast narrow Raman-Ramsey fringes is demonstrated with this setup by making the atoms interact with a light pulse sequence.