Thomann, Pierre

2014, Bucalović, Nikola, Thomann, Pierre

In the framework of this thesis, I present recent activities in the Laboratoire Temps-Fréquence (LTF) of the University of Neuchâtel, concerning the optical frequency metrology group. We developed the frequency discriminator technique for the characterization of the narrow linewidth heterodyne optical beats. The examination of the four different types of the frequency and phase discriminators has identified the appropriate tools for the analysis of the low-noise signals. We made the experimental test of the β-separation line formalism that enables easy calculation of the laser linewidth from its frequency noise power spectral density (PSD). Earlier proposed geometrical approximation has been validated in a wide linewidth range and for different laser line shapes. We developed and characterized the second ultra-stable high-finesse Perot-Fabry passive reference cavity. A novel low-noise planar waveguide extended cavity laser (PW-ECL) has been stabilized to this reference cavity in a Pound-Drever-Hall stabilization scheme. A thermal model of the cavity enclosure has been made and based on that an improvement in the temperature control system was possible, resulting in the higher flexibility of the cavity temperature change. Finally, the noise properties of a novel type of the ultrafast optical frequency comb were examined. A diode-pumped solid-state laser (DPSSL) Er:Yb:glass (ERGO) frequency comb is proven to be capable of the coherent frequency division from optical to microwave, at the level of stability of 10^-15 at 1 s. It is shown that the dynamics of the carrier-envelope offset (CEO) beat plays an important role in the stabilization of this type of the ultrafast laser.

2006, Martucci, Giovanni, Thomann, Pierre

The atmospheric aerosol, its stratification and the principal dynamics controlling the air exchange at the top and the base of the aerosol layers are of key importance for understanding critical atmospheric phenomena such as the transport and impact of air pollution, the destruction of the ozone layer and the evolution of the greenhouse effect. In particular, it is the detection of stratification within the atmospheric boundary layer, the lower Troposphere and the regions around the Tropopause that are of importance to comprehending these phenomena. Elastic backscatter lidar is a novel tool promising to complement and partially to replace established atmospheric measuring techniques for quantifying these effects. The present thesis is investigating the theoretical and practical relevance of lidar in fulfilling this promise. The thesis performs an evaluation of two lidar methods to identify the structure of the boundary layer as well as their validation with respect to traditional methods such as radiosondes employed in atmospheric measurements. The validation shows that the lidar derived structure of the boundary layer is congruent with the structure derived from radiosonde temperature measurements. This result substantiates the use of elastic backscatter lidar for considerably advanced boundary layer monitoring compared to presently accepted methods. These measurements are then used to determine the frequency of the boundary layer top height variation. It is demonstrated that this method permits the identification of gravity waves through the boundary layer, consequently allowing the use of lidar for the investigation of the vertical exchange of pollution constituents. The monitoring of the height of the boundary layer has subsequently been carried out over a one-year period of measurements above the urban site of Basel, Switzerland. This campaign demonstrates the key role of lidar in conducting measurements over such long intervals, which is otherwise not possible with established instruments. The thesis provides complementary results collected by two airborne lidars in the upper Troposphere and the lower Stratosphere. The detection of specific clouds in this atmospheric region has critical importance for two aspects in the stratospheric ozone balance: polar stratospheric clouds are responsible for initiating ozone destruction, and the ultra-thin optical cirrus are linked to the water and radiation budget as well as to large-scale transport. The significance of the present thesis lies in conclusively demonstrating that elastic backscatter lidar contributes with both unique and complementary information to the present inventory of atmospheric instruments. This is substantiated by showing that the obtained results allow straightforward interpretation of the probed atmosphere, where they are particularly advantageous in terms of simplicity, reliability and information content, with respect to traditional, established methods.

2012, Devenoges, Laurent, Mileti, Gaetano, Thomann, Pierre

Ce travail de recherche s'inscrit dans la continuité des études, menées depuis 20 ans à l'Observatoire Cantonal de Neuchâtel, pour réaliser un étalon primaire de fréquence fonctionnant avec un jet continu d'atomes froids et lents. Ces développements théoriques et expérimentaux ont conduit à la construction de deux horloges à fontaine continue FOCS-1 et FOCS-2. Ce document expose les dernières améliorations et les premières mesures d'évaluation du second étalon primaire de fréquence effectuées ces trois dernières années au Laboratoire Temps-Fréquence de l'Université de Neuchâtel.
Dans la première partie, nous présentons le développement et l'implémentation d'une nouvelle méthode de préparation d'état utilisée pour améliorer la stabilité de fréquence à court terme de l'étalon. Nous avons montré qu'en utilisant un schéma de pompage optique à deux lasers, il est possible de combiner simultanément préparation d'état et refroidissement Sisyphe. Grâce à cette technique, nous avons réussi à préparer 60% des atomes dans le niveau

2004, Di Domenico, Gianni, Thomann, Pierre

Ce travail de recherche s'inscrit dans le cadre du développement d'une horloge atomique à fontaine, FOCS-1, à l'Observatoire cantonal de Neuchâtel. FOCS-1 est un étalon de fréquence qui utilise un jet continu d'atomes de césium froids, en géométrie de fontaine. Depuis la découverte du refroidissement d'atomes par laser, les jets d'atomes froids et lents (quelques mK et quelques m/s) jouent un rôle encore plus important dans les expériences de haute précision, en particulier dans les horloges atomiques à fontaine. Dans ce contexte, l'approche à jet continu suivie par l'Observatoire cantonal est intéressante parce qu'elle permet de diminuer de façon considérable tous les effets indésirables liés à la densité, ainsi que l'effet Dick qui est inévitable dans les jets pulsés. Toutefois, pour profiter pleinement de l'avantage de cette approche, nous devons augmenter le flux utile, et une façon d'y parvenir est de collimater le jet atomique, ce qui fait l'objet de ce travail de recherche. Pour effectuer la collimation, nous avons fait appel aux techniques les plus récentes de refroidissement d'atomes par laser. Nous utilisons un laser puissant et très désaccordé pour créer un réseau optique de dimension 2 dans lequel nous avons successivement piégé et refroidit les atomes jusqu'au niveau de vibration fondamental. Ce réseau optique utilise une géométrie originale qui a l'avantage de combiner stabilité intrinsèque de phase, symétrie, et recyclage de la lumière. Nous avons expérimenté plusieurs mécanismes de refroidissement: Sisyphe, sideband Raman dégénéré par effet Zeeman, et sideband Raman dégénéré par effet Stark. Avec le refroidissement sideband Raman dégénéré par effet Zeeman, nous avons obtenu une excellente collimation (diminution de la température transverse de 60 mK à 1.6 mK) bien que l'efficacité de capture reste assez faible, environ 10%. D'un autre côté, le refroidissement Sisyphe s'est avéré très efficace, environ 100%, et nous avons montré qu'il peut être utilisé pour une étape de pré-collimation avant le refroidissement sideband. Finalement, nous avons mené une étude préliminaire du refroidissement sideband Raman dégénéré par effet Stark. Bien que nous n'ayons pas observé de collimation, nous avons identifié quelques pistes pour y parvenir. Ce refroidissement présente un grand intérêt pour les horloges atomiques car il prépare les atomes dans un état insensible au champ magnétique.

2011, Gaggero, Pascal Olivier, Thomann, Pierre

Electrical Impedance Tomography (EIT) calculates an image of the conductivity distribution within a body from electrical stimulation and measurements at the body surface. This work develops advances in signal acquisition hardware, optimization of stimulation patterns, and analysis of detection limits for EIT. The EIT data acquisition and image reconstruction process is systematically analyzed with respect to the influence of noise and other measurement deficiencies on image quality. A complete EIT system with 32 active electrodes has been developed, with which the theoretical predictions could be verified and practical applications could be studied.
The novel concept of distinguishability is developed for a theoretical analysis of EIT system performance. It measures the likelihood that the measured differential EIT signal is generated by actual impedance changes and not by random fluctuations. This distinguishability criterion can be considered as a signal-to-noise ratio, and it serves as a valuable benchmark to assess the performance of EIT systems. Using numerical simulations, we have studied the optimum signal acquisition strategy for differential EIT signals, in order to maximize image quality. The most favorable angles between injecting and sinking electrode are found in the range from 60 to 150 degrees. We have also studied, theoretically as well as experimentally, the miniaturization limits of EIT systems. It is concluded that EIT system miniaturization is essentially determined by the Joule heating effect and the cooling rate of the sample volume. When scaling EIT systems up to very large dimensions, electronic noise on the sample (current injection and voltage measurements) finally limits the distinguishability in reconstructed EIT images.
A prototype of the developed hardware architecture was realized and proof-of-concept studies were carried out using both thoracic-EIT and the micro-EIT setups. As a major application of the work carried out in this thesis, we have demonstrated the capability of the developed EIT system to serve as a cost-effective real-time monitor for the reliable monitoring of the ventilation and cardiac related impedance changes on patients. The advances presented in this work can help build the capability of EIT to monitor and optimize mechanical ventilation of patients in intensive care units, which has the potential of enabling safer, automatic ventilation strategies, possibly preventing the unnecessary death of tens of thousands of patients every year.