Roland Barillet

Frequency stability of a mercury ion frequency standard

Observation with an improved signal-to-noise ratio of the hyperfine resonance pattern of199Hg+ stored in a radiofrequency trap is reported. A frequency control loop which locks the frequency of a 5MHz quartz crystal oscillator to the hyperfine transition of stored mercury ions, close to 40.5 GHz, is described. In this system, pulses delivered by the photomultiplier are processed digitally. Optimal conditions for the interrogation of the hyperfine transition are specified. A fractional frequency stability σy(τ)=3.6×10−11τ−1/2 for 10s<τ<3500s has been obtained. This frequency stability is comparble to that of commercially available cesium beam frequency standards. Prospect for improvement by a factor of at least 10 are discussed.

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 /spl sigma/=/spl 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.

Limitation of the clock frequency stability by the interrogation frequency noise: experimental results

Recent advances in atomic physics will make it possible to reach frequency stabilities in the range of 10/sup -14/-10/sup 15/ tau /sup -1/2/ for passive atomic frequency standards. However, it is recognized that the frequency noise in the interrogating microwave signal limits the frequency stability, approximately to the range 10/sup -12/-10/sup -13/ tau /sup -1/2/, depending on the spectral purity of the VCXO and on the modulation frequency f/sub M/. In order to investigate this effect experimentally, two identical microwave generators are realized at 9192 MHz. These two microwave generators enable one to measure the power spectral density (PSD) of their frequency fluctuations with or without additive frequency noise, and to characterize separately the limit of the frequency stability related to the interrogation frequency noise. The relation between the microwave frequency noise at 2nf/sub M/ and the frequency stability limit is verified: experimental results very precisely confirm the predictions of the model for slow modulation frequencies. In any case, the spurious effect is related only to the frequency noise present in the microwave interrogation at Fourier frequencies which are even harmonics of the modulation frequency. >

Electronic circuits showing a level-independent phase shift for hydrogen-maser work applications

The origin of spurious level-dependent phase shifts occuring in amplifiers is analyzed. The effects of nonlinearities in transistors and of a bandwidth limitation are stated. Rules for the design of electronic circuits in which this effect must be minimized are given. Experimental results related to a 40-dB amplifier and to an amplitude limiter, both at 5.75 kHz, are described. The tuning of a hydrogen-maser cavity with the condition that the phase of oscillation does not depend on the amplitude of oscillation is considered. A preliminary precise test of the method is reported and discussed.

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 /spl phi/ of either 0 or /spl 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 /spl phi/=/spl pi/.

Active H-Masers for V.L.S.I. Applications

Summary Three active H-masers have been manufactured at L.H.A. for french radioastronomers and geophysicists. They are transportable and compact. Including electronics, power supplies and batteries, their volume is about 0.5 m3 and their mass is 280 kg. They are operated from 220 Vac or 24 Vdc power supplies, and their internal battery makes it possible to operate them during transportation. Signal receiver and phaselocked loop have been designed to optimize the frequency stability transfer. Operating conditions are microprocessor controlled and monitored. Measurements of the short term frequency stability, and of systematic effects are presented.

Theory of Hydrogen-Maser Auto-Tuning Systems Based on the Frequency or the Phase Method

A theoretical analysis of auto-tuning systems for actively operated hydrogen (H) masers is presented. Tuning procedures based on changes in the frequency or the phase of oscillation are considered and compared. The analysis takes into account the basic results of sampling theory. The condition which should be satisfied when the reference frequency source is perturbed by white or flicker phase noise is specified. It is shown that the phase method should give better results than the frequency method when the reference source is a good quartz crystal oscillator.

Synchronisation par injection pour la détection d’ondes gravitationnelles

After a general presentation of the gravitational waves, this paper shows models which have investigated the injection-locking process in various fields, and their common result in mechanical, rf, microwave and quantum oscillators. The third chapter presents the two complementary approaches that are necessary to obtain a satifactory noise analysis of the injection-locking process, and the present state-of-the-art concerning Nd:yag lasers. Finally, the experimental results confirm the validity of the models and the feasibility of the expected performance. The limits of the models and corresponding investigations of interest are indicated.RésuméAprès une présentation générale des ondes gravitationnelles, cet article aborde le processus de synchronisation par injection dans divers domaines. Ces approches ont un résultat commun pour des oscillateurs variés, mécaniques, rf, microondes ou quantiques. Le troisième chapitre montre les deux approches complémentaires qui doivent être mises en œuvre pour procurer une analyse de bruit satisfaisante. Dans la quatrième partie, les résultats expérimentaux confirment la validité des modèles et la faisabilité des performances attendues. L’état de l’art des lasers Nd:yag est présenté. Les limites des modèles et les recherches correspondantes à effectuer sont indiquées.

Studies and Developments of H-Masers at LHA

Three active H-masers have been manufactured at LHA for french radioastronomers and geophysicists. They are transportable and compact. Including electronics, power supplies and batteries, their volume is about 0.5 m3 and their mass is 280 kg. They are operated from 220 Vac of 24 Vdc power supplies, and their internal battery makes it possible to operate them for 2 hours without external supply. The signal receiver and the P.L.L. have been designed to optimize the frequency stability transfer. Operating conditions are microprocessor controlled and monitored.

Frequency Control Loop with Digital Integrator

A frequency control loop which locks the frequency of a 5-MHz quartz crystal oscillator to a value defined by the hyperfine transition of 199Hg+ ions is described. The properties of the closed loop are studied using the z-transform technique. The loop parameters are calculated in order to optimize the response time to some usual deterministic perturbations of the VCXO frequency. The results of this analysis can be applied to other digital control loops, such as the hydrogen-maser auto-tuning system.