Cesium Beam Tube Research Articles

Design of optically pumped cesium beam tube with hexapole magnetic system for longer lifetime and better SNR

In order to get a better tradeoff between the long lifetime and high performances of cesium beam atomic clocks, we proposed a design scheme of compact optically pumped cesium beam tube (OPCBT) based on hexapole magnetic focusing (HMF) system for the confinement of cesium. A practical optically pumped cesium beam tube without HMF system has been implemented and tested with high-stability laser by modulation transfer spectroscopy (MTS), and a signal-to-noise ratio (SNR) of 18,000 in 1 Hz bandwidth and Allan deviation of 2×10−12/τ were obtained. Based on this test, the simulation analysis results of the newly designed tube with HMF show that the predicted lifetime can reach over 20 years without degradation of the performances, and the expected frequency stability of the clock can be 6.5×10−13/τ with a lifetime of 10 years.

Beam optics analysis on magnetic-state-selected atomic clocks with optical detection

In traditional cesium beam clocks, cesium atoms are deflected by strong inhomogeneous magnetic fields. The distribution of the detectable atoms, which is critical to the short-term frequency stability, is hard to describe with analytical functions. In this paper, we numerically analyze the beam optics performance of cesium beam tubes based on the magnetic-state-selected and fluorescence detection scheme. To accurately model the cesium beam tube, we apply the Monte Carlo sampling directly inside the collimator. The finite element method is also applied to model the magnetic field. Upon the high dimensional distribution space, two key parameters are selected as indicators of the short-term performance of the cesium beam tube, the effective velocity distribution, and the normalized density difference of atoms in |F=3,mF=0⟩ and |F=4,mF=0⟩. The influence of the configuration of the collimator is analyzed. Experiments are carried out with two manufactured beam tubes. The results show good agreement with the simulation model. We also discuss limitations of the simulation method when applied to designing cesium beam tubes.

Improving the short-term frequency stability of a magnetic-state-selected cesium beam clock with optical detection.

The microwave spectrum line shape and the signal-to-noise ratio of a compact optically detected magnetic-state-selection cesium beam clock are analyzed in this paper. As the noise analysis shows, the performance is related to the atomic utilization ratio and locking parameters when the laser frequency noise is the dominant noise source. Methods are adopted for realizing better short-term frequency stability of the clock, including using a highly efficient state-selection cesium beam tube, optimizing the locking parameters, and stabilizing the microwave power to maximize the error signal. After optimization, the signal-to-noise ratio of the clock reaches 7.0 × 103 in a bandwidth of 1Hz and the clock demonstrates a short-term stability of 4.1 × 10-12 τ-1/2. The five-day Allan standard deviation reaches 7.7 × 10-15.

A linewidth locking method to control the microwave power in optically pumped cesium-beam clocks.

In this paper, we present a linewidth locking method to control the microwave power in optically pumped cesium-beam frequency standards. The responses of optically pumped cesium-beam tubes and classical cesium-beam tubes are analyzed and compared against the power of the microwave field. Due to the wide probability distribution of atomic velocity resulting from the optical state preparation and detection, the linewidth of the Ramsey pattern is sensitive to the microwave power. The results can be used to control the microwave power instead of using the traditional extremum method. The advantages of the new method are discussed, and we named this new method the linewidth locking method. When the microwave power is well controlled at a low level by the linewidth locking method, the frequency stability of cesium-beam clocks will be improved to a certain degree for the reduction of the Ramsey pattern linewidth. In experiment, using the linewidth locking method, the Allan deviation of our optically pumped cesium-beam frequency standard is 2.64×10-12/τ and continues until the averaging time exceeds 1 × 105 s, which is 17% better than that using the traditional extremum method.

Design of an atom source collimator for a compact frequency-stabilized laser.

A concise laser system for optically pumped cesium beam clocks is presented. The laser's frequency is locked by a fluorescence signal, produced by the interaction between a cesium atomic beam and laser. A cesium oven with a longer atom source collimator, formed by an array of channels, was used to reduce the divergence angle of the cesium atomic beam. The size of the cesium source collimator is 4 mm×0.6 mm, and the cesium expenditure rate is 2.4×10-1 g per year. A dense cesium atomic flux was used to interact with the laser to generate a fluorescence signal when the oven heating temperature is not too high. The cesium beam tube could have a longer lifetime. The optical structure of the laser system is compact. The measured frequency stability of the laser is ∼4×10-11 at 10,000s when the laser frequency is locked in the F=4→F'=5 transition of the cesium D2 line. The design of an atom oven and atom source may be used in other atomic beam clocks to improve their performance.

Analysis tools for the accurate evaluation of a small frequency standard

The short, optically pumped cesium beam tube developed at Laboratoire de l'Horloge Atomique has been carefully evaluated. For that purpose, we have developed a digital servo system that controls three parameters: the frequency of the ultra stable oscillator (USO), the microwave power of the signal experienced by the cesium atoms, and the static magnetic field applied to the atoms. The frequency standard shows a very satisfactory level of short- and medium-term frequency stabilities. A relative frequency offset, measured to be 4.10(-12 ), results mainly from the residual phase difference between the oscillatory fields in the two interaction regions, which is due to imperfection in cavity symmetry. We present two different means of analyzing the causes of this spurious frequency offset using theoretical and experimental considerations. First, a numerical simulation of the beam tube response is performed as a function of the microwave field amplitude for different values of the residual phase difference DeltaPhi. Results include the cavity-pulling effect. Compared with the measured frequency offset, the numerical simulation leads to a second-order Doppler shift of -3.3 mHz and a residual phase difference, DeltaPhi, between the fields interacting with the atoms in the second and first regions of the Ramsey cavity, amounting to +150 murad. Second, an experimental method of measurement of DeltaPhi without beam reversal is implemented. The latter yields DeltaPhi=155+/-17 murad. Finally, the clock accuracy is determined. It is equal to +/-14.10(-13).

Transient response following frequency or amplitude switching in a cesium beam tube

The transient responses of an optically pumped cesium beam tube to square wave frequency and amplitude modulation is considered. The frequency transient is computed assuming a phase difference phi of either 0 or pi between the two oscillatory fields. We present theoretical and experimental data showing that, contrary to the frequency transient, the amplitude transient depends on the direction of switching. The knowledge of this property is useful for the design of the servo-loop controlling the amplitude of the microwave signal applied to the atomic resonator. A justification of this asymmetrical behavior is given. Experimental results confirm the theoretical predictions in the case phi=pi.

Controlling the microwave amplitude in optically pumped cesium beam frequency standards

Assuming square wave frequency modulation, the response, versus the amplitude of the microwave field, of an optically pumped cesium beam tube is considered. The properties of the first maximum of this response are analyzed. The effect of the neighboring lines is taken into consideration, and a model of the profile of the microwave field in each interaction region is validated. A symmetry property of the response considered is pointed out. It enables us to implement a feedback control of the microwave amplitude with a large depth of the amplitude modulation. Residual frequency offsets that may occur as a consequence of a spurious amplitude modulation correlated with the frequency modulation are assessed. And, with a cavity designed such that sigma=pi between the two oscillatory fields, it also is possible to measure the microwave amplitude at the first maximum of the sole contribution of the reference atomic line.

Improvement of the Pumping Efficiency in an Optically Pumped Cesium Beam Tube with a (σ + π)-Polarized Laser

We observed that atomic population was trapped in the ground state of cesium atoms when a σ-polarized laser was illuminated to the atoms under a weak static magnetic field in an optically pumped cesium beam tube. The population trapping could be removed by applying a (σ+π)-polarized laser which was made by superposition of both σ- and π-polarizations. The pumping laser with the (σ+π)-polarization increased the pumping efficiency even in the low magnetic field and accordingly, the background noise of the Ramsey spectrum was reduced by about four times and moreover, the Ramsey signal became more stable against the power fluctuation of the pumping laser.

New design for a high performance optically pumped cesium beam tube

An optically pumped cesium beam resonator has been designed including three successive magnetic field regions. The optical interactions take place in the first and third regions, where the magnetic field has the required value of 3x10(-5) T. The microwave interaction occurs in the intermediate region, where the value of the C-field is typically set to 4x10(-6) T. It has been verified that the magnetic field profile along the cesium beam does not induce Majorana transitions. Using a single laser diode emitting at 852 nm with a linewidth of about 30 MHz, the resonator gives an excellent amplitude signal to noise ratio equal to 20000 in a 1-Hz bandwidth.

Feasibility of end to end phase shift correction from the outside of sealed cesium beam tubes

Summary It is well known that a frequency shift mceable to asymmetry of the microwave resonator occurs in atomic beam frequency standards making use of Ramsey’s double interaction technique. The shift is due to the phase difference which then appears between the fields interacting with the atoms in the two regions. The atoms in fact cannot distinguish the latter from the phase accumulated by a frequency offset during the time of flight between the two cavities. Beam reversal experiments can measure such end-to-end phase difference if it can be assumed constant for the two beam directions. In that case in fact a beam reversal simply changes the sign of the effect. However, this technique can only be applied with predisposed laboratory type primary standards, while in the case of unidirectional beams any accuracy assessment on end-toend phase shift has been traditionally made by relying on mechanical specifications and aimming, and on elecmcal length measurements, during cavity manufacturing. That a correction of end-to-end phase shift might be possible by acting from the outside of a sealed tube has been suggested nxently [ 13, by noting that not just the two arms of the Ramsey cavity, but the microwave structure as a whole, contribute with their asymmetry to the occurrence of the problem. A formula for the end-toend phase difference was found, which includes the asymmetry of the feeding junction at the center of the Ramsey structure, and reduces to the traditional formula in the case of a symmetric junction. It was therefore proposed that a correction of arm asymmetry could be tried by introducing a compensating asymmetry in the “symmetric” arm of the feeding T-junction. Such asymmetry should be calibrated so that the asymmemc superior cut-off modes that it excites produce at the junction the desiredphase difference between the waves launched in the two opposite directions. In this paper an electromagnetic calculation is carried out of the asymmetry induced in an E-plane waveguide T-junction by an asymmetric iris in the feeding port. A comparison with experimental results is given. It is shown that compensation of end-to-end phase shift is possible in this way, and that shifts of few parts in A discussion about how well the absolute size of the effect can be judged by measuring the structure in reflection is also given, and the relevance that this has toward accuracy statements on unidirectional beams is underlined. 1013 can be reduced to below iX1013.

Comparison of pumping a cesium beam tube with D1 and D2 lines

The operation of a cesium beam tube optically pumped with cesium D1 and D2 lines is theoretically and experimentally investigated. In a single laser diode configuration using the D2 line, the most efficient excitation scheme is found to be the 3→3σ(D2) scheme leading to a clock signal-to-noise ratio equal to 15 000 in a one-hertz bandwidth. In the case of the D1 line, the predicted best excitation schemes are the 4→3π(D1) and the 4→3σ(D1) schemes. However, these two configurations lead to an extra fluorescence noise presumably due to unpumped atoms. The 3→4π(D1) scheme is experimentally found to be the best with S/N performances comparable to the 3→3σ(D2) configuration. Nevertheless, in the case of the 3→3σ(D2) scheme, optical pumping efficiency decreases when the magnetic field is too low whereas 3→4π(D1) features are independent of the magnetic-field amplitude. Moreover, the well-separated fluorescence lines in the case of the D1 line allow a more accurate locking of the laser frequency on the top of the fluorescence line than in the case of the D2 line. A perfectly symmetrical microwave spectrum results when the 3→4π(D1) scheme is used.

Cesium gettering by graphite-improvement in the gettering efficiency

Results of experimental investigations of the cesium (Cs) gettering properties of graphite at various temperatures are reported. Possible reasons for this behavior are discussed. A model based on the grain boundary and surface diffusion of Cs in graphite is used to explain the results. Using this model, the activation energy for surface/grain boundary diffusion of Cs into polycrystalline graphite is estimated to be ~14 kcal/mole. The initial motivation was to examine the performance of graphite getters used in the cesium beam tube frequency standards. New results are presented regarding the dramatic improvement in the Cs gettering efficiency of graphite when heated to and maintained at elevated temperatures (100-130 degrees C) over the room-temperature efficiency.

Atomic trajectories in compact cesium-beam clocks

The author presents accurate calculations of atomic trajectories in conventional CBTs (cesium-beam tubes) using dispersive two-wire field magnets, and also in a novel configuration using two orthogonal one-dimensional focusing dipoles of the type described P. Kartaschoff (1974). In the first configuration, the author explains the observed differences in atomic velocity distribution in terms of small variations in the oven and/or detector offsets. The second configuration yields beam intensities that are significantly higher than the ones obtained with dispersive magnets and/or substantially lower atomic velocities, and thus could improve the frequency stability of compact cesium clocks in the white-noise regime. The present design differs from Kartaschoff's prescription in that a 1-D focusing dipole, rather than a hexapole or quadrupole magnet, is used as the A magnet in the CBT, making the tube manufacturing simpler.

Digital synchronous detector and frequency control loop for cesium beam frequency standard

A setup that includes a digital synchronous detector and a digital frequency control loop and is designed for use in a cesium beam frequency standard is described. It is based on the TMS 32010 signal microprocessor. When associated with a high-quality HP 5061A option four cesium beam tube, it makes it possible to achieve the same levels of short-term and medium-term frequency stability as its original analog counterpart. It is thus expected that the setup will fit the improved short- and long-term frequency stability capability of optically pumped cesium beam tubes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A New Technique for the Characterization of Cesium Beam Tube Performance

Abstract : Cesium beam tube atomic standards tend to exhibit a decreasing beam tube current with time. This report describes a technique to measure noninvasively the product of electron multiplier gain and noise figure, as well as tube signal and noise performance, thus providing information to discriminate between possible causes of decreasing beam tube current. The technique characterizes cesium beam tube performance without measuring clock frequency stability. Consequently, no extensive analysis of long term data is required. The technique is also applicable to a simular evaluation of photomultiplier tubes. Keywords: Atomic clocks; Cesium beam tubes; Electron multiplier gain; Electron multiplier noise.

Effect of the Atom Transit Time on the Frequency Stability of Cesium Beam Frequency Standards

The effect of the transit time of the atoms in cesium beam tubes is analyzed in the case of fast sine-wave and square-wave fre- quency modulations and in the case of square-wave phase modulation. Analytical expressions are given for the slope of the error signal and for the frequency stability. The effect of the modulation frequency and of the separation between the second interaction region and the atom detection zone is illustrated for the interaction time distributions en- countered when optical methods are implemented for the state selec- tion and the detection of the cesium atoms.

Design and performance of a small cesium tube

AbstractThe introduction of cesium oscillators as standard clocks in communication networks of the future is expected. It is known that the frequency stability and life of a cesium oscillator mainly depends on the cesium beam tube employed. This paper describes a design method of a cesium tube considering the short‐term frequency stability ± y(T) of the oscillator. The main factors controlling the S/N ratio of the cesium beam tube are considered. Using the figure of merit relating to ± y(T), a method is presented for optimizing the basic parameters (signal current, beam current and half‐amplitude frequency) relating to the Ramsay resonance signal. If the ratio of the beam current to the signal current is constant in the atomic beam tube without an electron multiplier, it is possible to obtain the same S/N ratio as in the conventional cesium‐beam tube with an electron multiplier by increasing the beam current. The figure of merit can be increased by increasing the signal current even if the half‐amplitude frequency is wide. Analysis of the operation of a small atomic beam tube is carried out to determine the optimum arrangement of components in the tube. It is confirmed by manufacturing a prototype beam tube that the figure of merit of 0.64 can be obtained as predicted.

Atomic frequency standards: A survey

The last comprehensive survey on atomic frequency standards was done by McCoubrey [1]. This survey reviews the more recent historical background of atomic frequency standards leading to the present developments. A discussion of the underlying physical and engineering principles is given. Modern atomic frequency standards, including their performance, are compared quantitatively, and projections are attempted at likely future developments and performance characteristics. As in 1966, the standards principally used in technical and scientific applications are rubidium gas cell devices, cesium beam tubes, and hydrogen maser oscillators. However, substantial advances in physical and performance characteristics can be reported. New developments include passive hydrogen devices, saturated absorption stabilized lasers, ion storage devices, and atomic beams in the far infrared and infrared region, as well as new techniques to evaluate frequency biases such as those encountered in cesium and hydrogen standards. The survey includes a discussion of the effects of past and current developments in atomic frequency standards on the technical and scientific user community.

Corrections for Second-Order Doppler Shift and Cavity Phase Error in Cesium Atomic Beam Frequency Standards

From the measured static resonance curve of a cesium beam tube, corrections can be derived for second-order Doppler shift and cavity phase error. For the calculation Fourier transforms are used, which also produce the velocity distribution of the atoms. Data are given for the distortion of the resonance curve from which the corrections can be derived applicable in the case of very slow square wave frequency modulation. A method is also given for calculating corrections for faster modulation rates where the time-of-flight effects are no longer negligible. However, quadrature components will now occur in the beam signal, and their effect on the clock servo is discussed.