Implementation of the beam reversal technique on compact cesium clocks: towards an improvement in accuracy.

We report the evaluation of the residual phase difference deltaphi in a short (18 cm) Ramsey cavity by implementing the beam reversal technique to an optically pumped cesium beam clock. Deltaphi is measured to be 21 +/- 1.5 microrad, allowing a more accurate evaluation of the frequency performances of this small cesium clock. Finally, the clock accuracy is equal to 1.1 x 10(-13).

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).

Frequency performances of a miniature optically pumped cesium beam frequency standard.

The optically pumped cesium beam clock named Cs IV is operated with a new short Ramsey cavity satisfying strict requirements on the microwave leakage level. The most relevant characteristics of the device are presented. Cs IV is presently driven by standard electronics coming from a HP 5061 B clock that provides a sinusoidal modulation of the interrogation microwave signal and a microwave power stability of about 1% at a temperature of 20+/-1 degrees C. The short- and medium-term frequency stability measurement gives sigma(y)(1 day)=2x10(-14): this value holds up to 3 days. The accuracy evaluation results in an uncertainty of 10(-12), and the repeatability is evaluated to 3x10(-13). It appears that the flicker floor is beginning at 2x10(-14) and is mainly due to both the power fluctuations of the free running microwave interrogating signal and the fluctuations of the external static magnetic field. The accuracy is limited by the lack of knowledge of the end-to-end cavity phase shift.

Frequency shifts in cesium beam clocks induced by microwave leakages.

The anomalous sensitivity of an optically pumped cesium beam clock to microwave power and the unexpected frequency shifts observed are demonstrated to result from microwave leakages outside the arms of the Ramsey cavity. A new design of the cavity associated with a careful realisation provides very weak microwave leakages and negligible related frequency offsets. We have established a theoretical model that allows us to calculate the frequency shifts due to microwave field components propagating along the beam axis in regions which are free-field in an ideal Ramsey cavity. This results in first order Doppler effect shifts. The order of magnitude of the frequency shift can be predicted and agrees with the measured one when the amplitude of the leakage magnetic field is about a 1000 times smaller than the amplitude of the microwave interrogation field in the cavity.

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.

Gas-lens effect and cavity design of some frequency-stabilized He-Ne lasers.

It is shown that there exists an optimal cavity length which should minimize the frequency shifts induced by lenslike effects in intracavity saturated absorption lasers. Ideas are developed which provide a satisfactory explanation for the dispersion in modulation shifts observed in some recent laser intercomparisons.

Helium-neon laser stabilized by saturated absorption in iodine at 612 nm.

Hyperfine structure of iodine has been studied at 612 nm by the technique of saturated absorption in an iodine cell placed in a He-Ne laser modified to operate at this wavelength. The most interesting feature of the laser is the existence of strong inverted Lamb dips at a very low vapor pressure, which provides very good short term frequency stability. Some factors which limit the reproducibility of the device are also investigated.