R. Boudot

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.

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.

Simple-design ultra-low phase noise microwave frequency synthesizers for high-performing Cs and Rb vapor-cell atomic clocks

We report on the development and characterization of novel 4.596 GHz and 6.834 GHz microwave frequency synthesizers devoted to be used as local oscillators in high-performance Cs and Rb vapor-cell atomic clocks. The key element of the synthesizers is a custom module that integrates a high spectral purity 100 MHz oven controlled quartz crystal oscillator frequency-multiplied to 1.6 GHz with minor excess noise. Frequency multiplication, division, and mixing stages are then implemented to generate the exact output atomic resonance frequencies. Absolute phase noise performances of the output 4.596 GHz signal are measured to be -109 and -141 dB rad(2)/Hz at 100 Hz and 10 kHz Fourier frequencies, respectively. The phase noise of the 6.834 GHz signal is -105 and -138 dB rad(2)/Hz at 100 Hz and 10 kHz offset frequencies, respectively. The performances of the synthesis chains contribute to the atomic clock short term fractional frequency stability at a level of 3.1 × 10(-14) for the Cs cell clock and 2 × 10(-14) for the Rb clock at 1 s averaging time. This value is comparable with the clock shot noise limit. We describe the residual phase noise measurements of key components and stages to identify the main limitations of the synthesis chains. The residual frequency stability of synthesis chains is measured to be at the 10(-15) level for 1 s integration time. Relevant advantages of the synthesis design, using only commercially available components, are to combine excellent phase noise performances, simple-architecture, low-cost, and to be easily customized for signal output generation at 4.596 GHz or 6.834 GHz for applications to Cs or Rb vapor-cell frequency standards.

Residual Phase Noise Measurement of Optical Second Harmonic Generation in PPLN Waveguides

We report on the characterization, including residual phase noise and fractional frequency instability, of fiber-coupled periodically poled Lithium Niobate non-linear crystals. These components are devoted to frequency doubling 871-nm light from an extended-cavity diode laser to produce a 435.5-nm beam, corresponding to the ytterbium ion electric quadrupole clock transition. We measure the doubling efficiencies of up to 117.5%/W. Using a Mach–Zehnder interferometer and an original noise rejection technique, the residual phase noise of the doublers is estimated to be lower than −35 dBrad2/Hz at 1 Hz, making these modules compatible with up-to-date optical clocks and ultra-stable cavities. The influence of external parameters, such as pump laser frequency and intensity, is investigated, showing that they do not limit the stability of the frequency-doubled signal. Our results demonstrate that such compact, fiber-coupled modules are suitable for use in ultra-low phase noise metrological experiments, including transportable optical atomic clocks.

Measurement of Cs-buffer gas collisional frequency shift using pulsed CPT interrogation

In this paper we presented separate measurements of the temperature coefficients for clock transition in Cs for N2, Ar and Ne buffer gases. The inversion temperature for Ne has been determined (77°C). For N2 the inversion temperature is estimated at 157 158°C. We have reduced the uncertainties for linear temperature coefficients for Cs clock transition three buffer gases. The quadratic coefficients γ for N2 and Ne have been determined for the first time. For Ar buffer gas, the quadratic coefficient γ is much smaller and we have estimated its upper limit.

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.

Performances and progress of a compact pulsed CPT Cs clock

We report on the development of a compact Cs cell CPT frequency standard operating in the Dicke regime. Combining a double Lambda scheme and a Ramsey interrogation, high contrast fringes whose linewidth depends on the time separating two pulses, are obtained. Compared to the continuous regime, the pulsed interrogation reduces the light shift by a factor 300 and permits to reach frequency stability 10 times better. A frequency stability of 9.10-13 at 1 s has already been measured.