S. Guerandel

Simple-Design Low-Noise NLTL-Based Frequency Synthesizers for a CPT Cs Clock

This paper presents simple-architecture low-noise frequency synthesis chains generating a 9-GHz signal. These devices are based on the use of a nonlinear transmission line (NLTL) used as a comb generator. The residual phase noise spectra of the key components are reported. The residual phase noise performance of the chains at 9 GHz is measured to be less than -80 dBrad2/Hz at 1-Hz offset frequency. The measured fractional frequency stability of the chains is 1 times 10-14 at 1 s and better than 4 times 10-17 at one day. The contributions of these chains to phase lock two extended cavity diode lasers (ECDLs) involved in a coherent-population-trapping (CPT) Cs clock experiment are evaluated and measured.

Current Status of a Pulsed CPT Cs Cell Clock

A Raman-Ramsey Cs cell atomic clock is presented. The relaxation times of the population and the hyperfine coherences in the cell are measured. The effect on the central Ramsey fringe amplitude of the critical experimental parameters such as laser intensity, magnetic field, temperature, and Ramsey time is investigated. The existence and impact of the additional Deltam = 2 transitions involved in the pumping scheme are pointed out. Narrow resonance linewidths as low as 33 Hz with reasonable signal-to-noise ratios have been recorded. By removing a frequency drift attributed to the cell, the achieved frequency stability is 7 times 10-13 tau-1/2. The main noise contributions that limit the short-term frequency stability are reviewed and estimated.

Raman–Ramsey Interaction for Coherent Population Trapping Cs Clock

A gas cell atomic frequency standard based on coherent population trapping (CPT) with Ramsey interrogation pulses is developed. The clock resonance is detected in a Cs cell with buffer gas using two phase-locked diode lasers tuned to the Dl line. A preliminary short-term stability of 3.5 10-12t is reported. The CPT specificity allows a new interrogation-detection sequence, i.e., the continuous pulse train, which, with phase modulation, could lead to better short-term stability compared to usual continuous CPT clocks. The parameters optimizing the stability include a high Rabi frequency, a very short measurement time, and an interrogation time equal to the lifetime of the hyperfine coherence

OSCC project : A space Cs beam optically pumped atomic clock for Galileo

Thales electron devices has been pursuing since 2003 frequency standards activities. In the framework of these activities, Thales established a consortium for the development of a space Cs atomic clock for Galileo. This consortium is composed by two of the best scientific laboratories in the European Time-Frequency community : the Observatoire de Neuchatel (ON) and the SYRTE Observatoire de Paris, and by two space industrials : Oerlikon space AG (OSAG) and Thales Electron Devices. The name of the project is OSCC for Optically pumped Space Cs Clock. The first phase (phase A) of this development started in June 2006 under an ESA contract. The purpose of this phase A is a feasibility study of Cs clock technology for Galileo with the manufacturing and the test of a new compact optically pumped Cs clock breadboard. This technology is well known in laboratories but it has never been industrialized, even for ground applications. This study starts with a strong background at SYRTE and ON, but also with new industrial developments realized at Thales during the last years. Frequency stability in order of 1 to 3times10-12 . tau-1/2 has been already demonstrated in lab with different configurations. This document will first synthesize the last results obtained by each Partner, followed by the results of the existing hardware analysis performed in the first step of the project. This analysis allowed Partners to share their know-how and to identify the limits of each existing breadboards with respect to the objectives of the project. As a result of this analysis, it was possible to define the atomic resonator best configuration for each subsystem. At least, this document presents a few design drivers of the new OSCC devices.

Temperature Dependence Cancellation of the Cs Clock Frequency in the Presence of Ne Buffer Gas

The temperature dependence of the Cs clock transition frequency in a vapor cell filled with Ne buffer gas has been measured. The experimental setup is based on the coherent population trapping technique and a temporal Ramsey interrogation allowing a high resolution. A quadratic dependence of the frequency shift is shown. The temperature of the shift cancellation is evaluated. The actual Ne pressure in the cell is determined from the frequency shift of the 895-nm optical transition. We can then determine the Cs-Ne collisional temperature coefficients of the clock frequency. These results can be useful for vapor cell clocks and, particularly, for future microclocks.

Stability of the compact cold atom clock HORACE

HORACE is a compact cold cesium atom clock which is being developed in LNE-SYRTE for space applications and onboard systems. The operation of this clock is different from fountains since the laser cooling, the microwave interrogation and the detection are sequentially performed inside the spherical microwave cavity. We recently achieved short term stability of 5.5 10-13 tau-0.5 reaching the 10-14 level at 3000 sec. We report in this paper recent developments and improvements, particularly on the cooling sequence. We also study the main limitations.

Frequency Stability Measurement of a Raman-Ramsey Cs Clock

CPT clocks are studied worldwide as miniature frequency standards for portable applications. To improve their frequency stability, we have investigated the optimum operating conditions (cell temperature, laser intensity, interrogation method) and measured the corresponding frequency shifts and stability. Compared to the continuous CPT interrogation, the pulsed interrogation reduces the light shift by a factor 300 and improves the frequency stability by a factor 10. The short term frequency stability is 9 x 10 -3 at 1 s, limited to 2 x 10 -3 after 300 s.

Reaching a few 10 −13 τ −1/2 stability level with the compact cold atom clock HORACE

HORACE is a compact cold cesium atom clock which is being developed in LNE-SYRTE for space applications and onboard systems. The operation of this clock is different from fountains since the laser cooling, the microwave interrogation and the detection are sequentially performed inside the spherical microwave cavity. The entire simplified operation sequence is described. A short term relative frequency stability of 2.2 10-13 tau-1/2 is achieved. Preliminary results on mid term show that a level of 4 10-15 is reached after 5 103s of integration. Limitations are investigated.

Development of a compact cold atom clock

HORACE is a compact cold atom clock where the atoms are cooled inside the microwave interrogation cavity. About 10/sup 8/ atoms can be cooled at kinetic temperatures as low as 2.5 /spl mu/K. We report, for the first time, a Ramsey pattern observed with a 14 Hz linewidth and fringe contrast better than 80%. Since this clock is designed for space applications, some properties are extrapolated to micro-gravity operation.

A new design of ECLD for compact atomic clocks

Cold atom clocks would take great benefits of microgravity environment. In Relation with the clock developments, we work on miniature laser-cooling optical benches. In this article we describe a new design of compact external cavity laser diode, which exhibits up to 80 mW at the output of the ECLD for a diode current of 100 mA and a line width of about 150 kHz.