-
[show abstract]
[hide abstract]
ABSTRACT: We report on the measurement of an anomalously large temperature sensitivity of the clock frequency in a Rb cell with buffer gas. The effect is observed in a prototype of pulsed optically pumped frequency standard which allows high resolution measurements because of its frequency stability at the level 1.7 × 10(-)13 for 1 s of measurement time. We attribute this phenomenon to the geometry of the interaction and to the presence in the cell of temperature inhomogeneities that may enhance the temperature sensitivity of the clock frequency via the buffer gas pressure coefficient. We also propose some solutions to reduce this unwanted effect that may limit the medium-long-term performances of highfrequency- stability vapor-cell clocks.
IEEE transactions on ultrasonics, ferroelectrics, and frequency control 12/2012; 59(12):2646-2654. · 1.80 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper, we describe the development of an ultrastable laser source at 578 nm, realized using frequency sum generation. This source will be used to excite the clock transition (1)S(0) - (3)P(0) in an ytterbium optical lattice clock experiment. Two independent ultrastable lasers have been realized, and the laser frequency noise and stability have been characterized.
IEEE transactions on ultrasonics, ferroelectrics, and frequency control 03/2012; 59(3):426-31. · 1.80 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper, we present the performance of a vapor-cell rubidium frequency standard working in the pulsed regime, in which the clock signal is represented by a Ramsey pattern observed on an optically detected laser absorption signal. The main experimental results agree with previously reported theoretical predictions. In particular, we measured a relative frequency stability of σy(τ) - 1.6 × 10(-13)τ-1/2 for integration times, τ, up to 200 s, which represents a record in short-term stability for a vapor-cell clock. We also discuss the most important physical phenomena that contribute to this result.
IEEE transactions on ultrasonics, ferroelectrics, and frequency control 03/2012; 59(3):457-62. · 1.80 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We stabilized the frequency of a compact planar-waveguide external cavity laser (ECL) on a Fabry-Perot cavity (FPC) through a Pound-Drever-Hall scheme. The residual frequency stability of the ECL is 10(-14), comparable to the stability achievable with a fiber laser (FL) locked to an FPC through the same scheme. We set up an optical link of 100 km, based on fiber spools, that reaches 10(-19) relative stability, and we show that its performances using the ECL or FL are comparable. Thus ECLs could serve as an excellent replacement for FLs in optical links where cost-effectiveness and robustness are important considerations.
IEEE transactions on ultrasonics, ferroelectrics, and frequency control 12/2011; 58(12):2582-7. · 1.80 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on the implementation and the metrological characterization of a
vapor-cell Rb frequency standard working in pulsed regime. The three main parts
that compose the clock, physics package, optics and electronics, are described
in detail in the paper. The prototype is designed and optimized to detect the
clock transition in the optical domain. Specifically, the reference atomic
transition, excited with a Ramsey scheme, is detected by observing the
interference pattern on a laser absorption signal.
\ The metrological analysis includes the observation and characterization of
the clock signal and the measurement of frequency stability and drift. In terms
of Allan deviation, the measured frequency stability results as low as
$1.7\times 10^{-13} \ \tau^{-1/2}$, $\tau$ being the averaging time, and
reaches the value of few units of $10^{-15}$ for $\tau=10^{4}$ s, an
unprecedent achievement for a vapor cell clock. We discuss in the paper the
physical effects leading to this result with particular care to laser and
microwave noises transferred to the clock signal. The frequency drift, probably
related to the temperature, stays below $10^{-14}$ per day, and no evidence of
flicker floor is observed.
\ We also mention some possible improvements that in principle would lead to
a clock stability below the $10^{-13}$ level at 1 s and to a drift of few units
of $10^{-15}$ per day.
11/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper, we report an analysis of the design criteria of microwave cavities for vapor cell frequency standards. Two main geometries exploited in those devices are considered: the cylindrical cavity, used, for example, in the coherent population trapping maser and in the pulsed optically pumped (POP) clock, and the spherical cavity used in the isotropically laser cooled clock. The cavity behavior is described through a lumped equivalent circuit in which the input coupling loop, the dielectric cell containing the atoms and the diodes for frequency tuning or Q control are taken into account. In particular, the effect of the cell on the cavity resonance frequency is analytically evaluated via a first-order perturbation approach. The theory is found in good agreement with the experiments performed with two different cylindrical cavities used for the POP clock; the model here developed can then be helpful in the design of the cavity system. The general principles here reported can be adapted to other standards, such as atomic fountains and hydrogen masers, and to other modes and/or geometries.
The Review of scientific instruments 07/2011; 82(7):074703. · 1.52 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper we propose a pumping scheme to concentrate nearly all the atoms of an alkali-metal vapor into one of the ground-state Zeeman sublevels. We report theory and experiment for the case of 87Rb in a cell with buffer gas but the technique easily applies to other alkali-metal atoms. The pumping scheme consists of a suitable sequence of laser and microwave pulses after which a large fraction of atoms is transferred into the desired level. The efficiency of this technique depends mainly on the laser power and on the relaxation effects taking place inside the cell; in principle, more than 95% of the atoms can be prepared in a well-defined quantum state. Diagnostic of the resulting population distribution is made observing the free-induction decay signal at the output of a microwave cavity. This multistep laser-microwave pumping technique is particularly suited for compact vapor-cell frequency standards working in pulsed regime or in experiments where it is necessary to maintain the cylindrical symmetry of the experimental arrangement.
Phys. Rev. A. 08/2009; 80(2).
-
[show abstract]
[hide abstract]
ABSTRACT: We present a multilevel theoretical approach to describe the behavior of a pulsed optically pumped (POP) 87Rb frequency standard based on a microwave cavity-vapor cell arrangement. The full Zeeman manifold of the ground-state hyperfine levels is then considered, and the dynamics induced among them by relaxation processes (buffer gas, spin exchange, and cell-walls collisions) is taken into account. The model includes as well the absorption of the pumping laser along the cell and the cavity feedback on the atoms; the effects related to a nonuniform excitation of the atomic sample are also discussed. Theoretical predictions are proven with a laboratory prototype of POP passive maser in which the clock transition is excited by means of the Ramsey technique. The agreement between theory and experiment is very satisfactory from both a quantitative and qualitative point of view, in terms of the shape of the Ramsey fringes, microwave power delivered by the atoms, and short-term frequency stability. In particular, a frequency stability of 1.2×10−12 at one second has been measured, in very good agreement with the expected value. On this basis, the model has been extended to predict the short-term frequency stability of a POP frequency standard operating with optical detection. A shot-noise limited frequency stability (Allan deviation) of σy(τ)≈3×10−14τ−1∕2 is foreseen. This value can be degraded by microwave phase noise and laser fluctuations so that the overall predicted clock stability is σy(τ)≈1.5×10−13τ−1∕2.
Phys. Rev. A. 01/2009; 79(1).
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper we review the theory concerning the radiofrequency (RF) spectrum of a carrier phase modulated by a stochastic process with spectral density of the type |ω|−α and 0 ≤ α < 3. This problem is of particular interest in the fields of atomic frequency standards and, in general, in high resolution spectroscopy. We also extend the theory to non-integer slopes (α real) and examine the role of the low-pass filter shape in the RF lineshape, pointing out the peculiar behaviour of the Lorentzian filter. The singular features of the RF spectrum in the case of flicker phase noise are examined as well as the lineshape in the high- and low-noise limits. Analytical expressions for the RF spectrum are also given in terms of special functions for the white phase, flicker phase and white frequency noises.
Metrologia 05/2008; 45(3):313. · 1.75 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Pulsing the different operation phases of a vapor-cell clock (optical pumping, interrogation, and detection) has been recognized as one of the most effective techniques to reduce light shift and then to improve the stability perspectives of vapor cell clocks. However, in order to take full advantage of the pulsed scheme, a fast-gated electronics is required, the times involved being of the order of milliseconds. In this paper we describe the design and the implementation of the electronics that synchronizes the different phases of the clock operation, as well as of the electronics that is mainly devoted to the thermal stabilization of the clock physics package. We also report some characterization measurements, including a measurement of the clock frequency stability. In particular, in terms of Allan deviation, we measured a frequency stability of 1.2 x 10(-12) tao(-1/2) for averaging times up to tao = 10(5) s, a very interesting result by itself and also for a possible space application of such a clock.
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 10/2007; 54(9):1731-40. · 1.69 Impact Factor
-
IEEE T. Instrumentation and Measurement. 01/2007; 56:378-382.
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper we report the theoretical and experimental characterization of a pulsed optically pumped vapor-cell frequency standard based on the detection of the free-induction decay microwave signal. The features that make this standard similar to a pulsed passive maser are presented. In order to predict and optimize the frequency stability, thermal and shot noise sources are analyzed, as well as the conversions of the laser and microwave fluctuations into the output frequency. The experimental results obtained with a clock prototype based on 87Rb in buffer gas are compared with the theoretical predictions, showing the practical possibility to implement a frequency standard limited in the medium term only by thermal drift. The achieved frequency stability is σy(τ)=1.2×10−12τ−1∕2 for measurement times up to τ≈105 s. It represents one of the best results reported in literature for gas cell frequency standards and is compliant with the present day requirements for on board space applications.
Phys. Rev. A. 10/2006; 74(4).
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper we calculate the effect of spin-exchange collisions in alkali-metal vapors. In the framework of the high-energy approximation, we evaluate the spin-exchange cross sections related to the line broadening and to the frequency shift of the ground state hyperfine transition. We do the calculation for the four isotopes, 23Na, 39K, 87Rb, and 133Cs. The results are used in particular to evaluate the spin-exchange frequency shift in Rb vapor cell frequency standards used in many applications. It turns out that, due to possible fluctuations in the atomic density, spin exchange may affect significantly the medium and long term frequency stability of the frequency standard.
Phys. Rev. A. 03/2006; 73(3).
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper we present the low-noise electronics for a prototype of rubidium maser based on the coherent population trapping (CPT) phenomenon. After an overview of the general architecture, we will focus our description on the main blocks of the equipment we implemented: the microwave synthesis chain, the detection apparatus, the clock servo system and the electronics devoted to control the laser and the temperature. For each part, we present the design, the implementation, and the characterization measurements we performed. The contribution to the CPT maser frequency stability of each part also has been evaluated, and the frequency stability of the clock is reported.
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 12/2005; 52(11):1923-30. · 1.69 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper we report the measurement of the frequency Stark shift of the ground-state hyperfine transition of 133Cs observed in a vapor cell through a Λ excitation scheme. The measured value of the quadratic Stark coefficient is k=(−2.05±0.04)×10−10 Hz∕(V∕m)2 when the electric field is perpendicular to the quantization axis. A comparison with the values previously reported in literature is given in view of its interest for the evaluation of the blackbody radiation shift in the primary frequency standards.
Phys. Rev. A. 06/2005; 71(6).
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper we report the accuracy evaluation of the Italian primary frequency standard IEN-CsF1. We discuss the shifts the frequency standard is corrected for and the procedure used for the accuracy evaluation. In the last section we report frequency comparisons of our fountain with those of remote laboratories and with International Atomic Time.
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 11/2004; 51(10):1216-24. · 1.69 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We examine in this paper the spectral characteristics of a coherent population trapping (CPT) maser in order to evaluate the frequency stability achievable by this kind of passive frequency standard. The peculiar features of the coherent microwave emission associated with the coherent population trapping phenomenon are theoretically examined taking into account the impact of the various noise contributions on the physical principle of operation. The theoretical predictions are then compared with the experimental results obtained with a laboratory prototype of 87Rb CPT maser. The measured frequency stability is mainly limited in the short term by the system thermal noise and in the medium term by the thermal drift and turns out to be in agreement with the computations within a factor of 2.
Phys. Rev. A. 07/2004; 70(1).
-
[show abstract]
[hide abstract]
ABSTRACT: We report the theoretical evaluations of the static scalar polarizability of the 133Cs ground state and of the blackbody radiation shift induced on the transition frequency between the two hyperfine levels with mF=0. This shift is of fundamental importance in the evaluation of the accuracy of the primary frequency standards based on atomic fountains and is employed in the realization of the SI second in the International Atomic Time scale at the level of 1×10−15. Our computed value for the polarizability is α0=(6.600±0.016)×10−39C m2∕V in agreement at the level of 1×10−3 with recent theoretical and experimental values. As regards the blackbody radiation shift we find for the relative hyperfine transition frequency β=(−1.49±0.07)×10−14 at T=300 K in agreement with frequency measurements reported by our group and by Bauch and Schröder [ Phys. Rev. Lett. 78 622 (1997)]. This value is lower by 2×10−15 than that obtained with measurements based on the dc Stark shift and than the value commonly accepted up to now.
Phys. Rev. A. 05/2004; 69(5).
-
IEEE T. Instrumentation and Measurement. 01/2003; 52:267-271.
-
[show abstract]
[hide abstract]
ABSTRACT: We consider the interaction of a three-level system with phase-modulated resonant fields in the Λ excitation scheme. We treat theoretically the case of a sinusoidal phase modulation, a phase step perturbation, and a stochastic phase modulation. The appearance of a Rabi resonance both in the spectrum of the optical transmitted signal (electromagnetically induced transparency) and in the spectrum of the microwave emission (coherent population trapping maser) is considered in detail. All the theoretical results are compared with the analogous ones reported for the two-level system and with our experimental observations obtained for the case of rubidium in a buffer gas.
Phys. Rev. A. 12/2002; 66(6).
