Mohammadreza Gharavipour

Demonstration of a high-performance pulsed optically pumped Rb clock based on a compact magnetron-type microwave cavity

We demonstrate a high-performance pulsed optically pumped (POP) Rb vapor-cell clock based on a magnetron-type microwave cavity of only 44 cm3 external volume. Using optical detection, an unprecedented 35% contrast of the Ramsey signal has been obtained. Both the signal-to-noise ratio (of 30 000) and the estimated shot-noise limit of 1.7 × 10−14 τ−1/2 are at the same level as those found with a bigger cylindrical TE011 cavity (100 cm3 inner volume) and are sufficient for achieving excellent clock stability. Rabi oscillations are measured and indicate a sufficiently uniform microwave magnetic field distribution inside the cavity. The instability sources for the POP clocks performance are analyzed. A short-term stability of 2.1 × 10−13 τ−1/2 is demonstrated which is consistent with the noise budget.

High performance vapour-cell frequency standards

We report our investigations on a compact high-performance rubidium (Rb) vapour-cell clock based on microwave-optical double-resonance (DR). These studies are done in both DR continuous-wave (CW) and Ramsey schemes using the same Physics Package (PP), with the same Rb vapour cell and a magnetron-type cavity with only 45 cm3 external volume. In the CW-DR scheme, we demonstrate a DR signal with a contrast of 26% and a linewidth of 334 Hz; in Ramsey-DR mode Ramsey signals with higher contrast up to 35% and a linewidth of 160 Hz have been demonstrated. Short-term stabilities of 1.4×10-13 τ-1/2 and 2.4×10-13 τ-1/2 are measured for CW-DR and Ramsey-DR schemes, respectively. In the Ramsey-DR operation, thanks to the separation of light and microwave interactions in time, the light-shift effect has been suppressed which allows improving the long-term clock stability as compared to CW-DR operation. Implementations in miniature atomic clocks are considered.

Pulsed optical pumping in a Rb vapor cell using a compact magnetron-type microwave cavity

A compact magnetron-type microwave cavity has previously been developed for a high performance Rb atomic clock based on conventional continuous-wave double-resonance (DR) interrogation. This magnetron-type cavity shows a homogenous microwave field distribution while having a cavity volume of only 35 cm3. So it is interesting for realizing a high performance Rb cell clock based on pulsed optical pumping (POP) technique with a reduced physics package size. In this proceeding we report on the evaluation of pulsed optical pumping with our magnetron-type cavity. Using the pulsed technique and optical detection, high-contrast Ramsey fringes were observed. The central fringe of the Ramsey signal has a linewidth of 160 Hz and contrast of 35%. Under these conditions, the shot-noise limit is estimated at a level around 2×10-14 τ-1/2 which shows the great potential of a POP clock based on the compact magnetron-type cavity. The Rabi oscillation result also indicates an acceptable uniform field distribution inside the cavity for a high-performance POP clock. Some optimizations of pumping and detection power for improving the contrast are also presented.

Compact and high-performance Rb clock based on pulsed optical pumping for industrial application

We report on the development of a compact laserpumped Rb clock based on the pulsed optical pumping (POP) technique, in view of future industrial applications. The clock Physics Package (PP) is based on a compact magnetron-type microwave cavity of 45 cm3 volume, and our current clock PP has a volume of only 0.8 liters, including temperature control and magnetic shields. This clock PP is completed by a newlydeveloped frequency-stabilized laser head of 2.5 liters overall volume, with an acoustic optical modulator (AOM) integrated within the laser head for switching the laser output power. Due to the highly uniform magnetic field inside the microwave cavity, Ramsey signals with high contrast of up to 35% and with a linewidth of 160 Hz have been demonstrated. A typical shortterm clock stability of 2.4×10-13τ-1/2 is measured. Thanks to the pulsed operation, the light-shift effect has been considerably suppressed as compared to previously demonstrated continuous-wave (CW) clock operation using the same clock PP, which is expected to enable improved long-term clock stabilities down to the 10-14 level or better.

Impact of static-magnetic-field-gradients on relaxation times in a Rb vapor cell

We apply an innovative method called Optically-Detected Spin-Echo to measure the intrinsic coherence relaxation time in the buffer-gas vapor cell of a Rb atomic clock [1]. This method suppresses the influence of static-magnetic-field-gradients across the cell which is a source of relaxation processes. Such studies are of interest for high-performance Rb atomic clocks, where both intrinsic population and coherence relaxation times (T<inf>1</inf> and T<inf>2</inf>, respectively) of the “clock transition” (5²S<inf>1/2</inf>|F<inf>g</inf> = 1, m<inf>F</inf> = 0〉 ↔ |F<inf>g</inf> = 2, m<inf>F</inf> = 0〉) are relevant.

Double-resonance spectroscopy in Rubidium vapour-cells for high performance and miniature atomic clocks

We report our studies on using microwave-optical double-resonance (DR) spectroscopy for a high-performance Rb vapour-cell atomic clock in view of future industrial applications. The clock physics package is very compact with a total volume of only 0.8 dm3. It contains a recently in-house developed magnetron-type cavity and a Rb vapour cell. A homed-made frequency-stabilized laser system with an integrated acousto-optical-modulator (AOM) – for switching and controlling the light output power– is used as an optical source in a laser head (LH). The LH has the overall volume of 2.5 dm3 including the laser diode, optical elements, AOM and electronics. In our Rb atomic clock two schemes of continuous-wave DR and Ramsey-DR schemes are used, where the latter one strongly reduces the light-shift effect by separation of the interaction of light and microwave. Applications of the DR clock approach to more radically miniaturized atomic clocks are discussed.

Pulsed Optically Pumped Rb clock

INRIM demonstrated a Rb vapour cell clock based on pulsed optical pumping (POP) with unprecedented frequency stability performances, both in the short and in the medium-long term period. In the frame of a EMRP project, we are developing a new clock based on the same POP principle but adopting solutions aimed at reducing the noise sources affecting the INRIM clock. At the same time, concerning possible technological applications, particular care are devoted in the project to reduce the size and the weight of the clock, still keeping the excellent stability of the INRIM clock. The paper resumes the main results of this activity.

Compact clocks for industrial applications: The EMRP project IND 55 MClocks

Vapor cell atomic clocks are an interesting technology because they combine compactness, low power consumption and excellent relative frequency stability. Recently, due to better performing laser sources and innovative techniques to prepare and detect the atoms, several cell-based prototypes exhibiting unprecedented frequency stability have been developed. These techniques allow a reduction in the transfer of laser noise to the atoms, improvement of the signal-to-noise ratio and subsequently the clocks frequency stability. The project IND55 Mclocks funded by the European Metrological Research Programme (EMRP) proposes to develop high performances vapor cell clocks for industrial applications. Three technologies are investigated: 1) the pulsed optical pumping (POP) scheme; 2) the cold atoms approach, and 3) the Coherent Population Trapping (CPT). The results related to the first period of activity are presented.