Les travaux présentés dans cette thèse portent sur le développement de références de fréquence optique, ou lasers ultra-stables. Ceux-ci sont obtenus en asservissant la fréquence d’un laser sur une cavité Fabry-Perot de haute finesse. Un premier laser est asservi sur une cavité commerciale en verre ULE et une stabilité relative de fréquence de 1; 9x 10-15 est obtenue à1 s. Ce signal optique est transféré dans le domaine des fréquences micro-ondes par un laser femto-seconde.Le signal obtenu à10 GHz dispose d’un bruit de phase de -104 dBc=Hz à1 Hz. Dans un second temps, une cavité ultra-compacte de 25 mm est développée. Des simulations par éléments finispoussées ont conduit à la fabrication d’une nouvelle géométrie de cavité, dotée de coefficients de sensibilité accélérométriques simulés inférieurs à10-12=(m=s2) selon toutes les directions. Une enceinte à vide compacte, munie d’un banc optique embarqué, a été réalisée de façon à limiter le volume du système complet à environ 40 L. Enfin, une cavité cryogénique en silicium a été conçue. Régulée à une température de 17 K, elle sera limitée par son bruit thermique à un palier de stabilité relative de fréquence de 3x10-17. Un cryogénérateur à faibles vibrations est utilisé pour atteindre cette température. Des modélisations par éléments finis de la cavité ont permis d’obtenir une sensibilité accélérométrique simulée de 4:5 x10-12=(m=s2). / The work described in this document deals with optical frequency references, or ultrastable lasers. They are obtained from the frequency locking of a laser on a high finesse Fabry-Perot cavity. A first laser is stabilized on a commercial ULE Fabry-Perot cavity and exhibits a 1:9 x10-15 fractional frequency instability at 1 s. A femtosecond laser is phased lock to this ultrastable laser and allows generating an ultra-pure microwave signal. At 10 GHz, the signal exhibits a -104 dBc/Hz phase noise at 1 Hz. In a second project, we design a 25 mm ultra-compact cavity. Extensive finite element modeling led to a new spacer geometry, with simulated acceleration sensitivities below 10-12=(m=s2) in all directions. A compact vacuum chamber with embedded optical set-up has been developed to reduce the size of the system. Finally, a cryogenic silicon cavity has been designed. It will be cooled down to 17 K. At this temperature, its thermal noise would limit its fractional frequency instability to3 x 10-17. A low vibration cryogenerator is used to reach this temperature. Finite element modeling led to 4:5x 10-12=(m=s2) acceleration sensitivity.

L'observation d'une synchronisation entre les systèmes locomoteur et respiratoire soulève de nombreuses questions concernant son utilité fonctionnelle pour l'organisme et les contraintes susceptibles de moduler son apparition. A l'heure actuelle, des contradictions subsistent dans les réponses apportées à ces interrogations, notamment en raison de la multiplicité des méthodes employées pour l'étude de ce phénomène. La théorie des oscillateurs couplés, et plus particulièrement le modèle de la sine circle map, nous permet de rendre compte de façon précise des relations de couplage entre les systèmes locomoteur et respiratoire. Les objectifs de ce travail de thèse sont de renforcer la validité de l'utilisation de ce modèle pour l'étude du couplage locomotion-respiration (CLR), d'examiner le lien entre le CLR et la performance sportive, ainsi qu'identifier les contraintes influençant les relations de couplage entre ces deux systèmes. Dans des tâches de pédalage ou de course à pied, les participants avaient pour consigne ou non de synchroniser leur locomotion ou leur respiration avec une stimulation auditive dont le tempo correspondait à leurs fréquences préférentielles (locomotrice ou respiratoire). Nos résultats montrent l'efficacité du rythme auditif pour induire une stabilisation du CLR, indépendamment de la consigne donnée aux participants et du système rythmé. Ces résultats révèlent une bidirectionnalité dans le couplage entre les systèmes locomoteur et respiratoire. De plus, nous mettons en évidence une forte corrélation entre le gain de stabilité entre les deux systèmes et le gain de consommation d'oxygène (i.e., diminution). Par conséquent, la stabilité du CLR est un facteur important à prendre en compte pour la performance sportive. Nous rapportons également une déstabilisation du CLR lors de l'éloignement d'un des deux systèmes de sa fréquence préférentielle. Ce résultat suggère l'adoption spontanée par les individus d'une synchronisation optimale entre les deux systèmes. Ainsi, il semble primordial de présenter une stimulation adaptée à chacun et adaptable aux changements de fréquences locomotrice ou respiratoire imposés par les contraintes de la tâche. Pris ensemble, nos résultats permettent une meilleure compréhension de l'évolution du CLR face aux contraintes qui lui sont imposées (e.g., fréquences locomotrice ou respiratoire, rythme auditif, modalité d'exercice) et mettent en évidence l'influence positive de sa stabilité sur la performance sportive. Nous rapportons également l'efficacité d'une stimulation visuelle pour apprendre à mieux gérer les ressources énergétiques au cours de l'effort. Ainsi, ce travail ouvre des perspectives de travaux sur l'utilisation d'une stimulation auditive ou visuelle, simple (e.g., métronome) ou complexe (e.g., musique, avatar), dans le cadre de l'entraînement et de l'amélioration de la performance sportive. / The natural synchronization between locomotor and respiratory systems raises many questions regarding its functional utility for the organism, as well as constraints that may modulate its appearance. Currently, contradictions remain in answers provided to these issues, especially because of the multiple methods used to study this phenomenon. The theory of coupled oscillators, and more specifically the sine circle map model, allows to accurately assess the coupling between locomotor and respiratory systems. This work aims at strengthening the validity of the use of this model for the study of locomotor-respiratory coupling (LRC), examining the relationship between LRC and sport performance, as well as identifying the constraints influencing the coupling between both systems. In pedalling or running tasks, participants were instructed or not to synchronize their locomotion or their breathing with an auditory rhythm which tempo matched their preferred frequencies (locomotor or respiratory). Our results show the effectiveness of auditory rhythm to induce stabilization of LRC, regardless of the instructions given to participants and the system paced. These results reflect a bidirectionality in the coupling between the locomotor and respiratory systems. Furthermore, we show a strong correlation between the increase in stability between the two systems and the gain in oxygen consumption (i.e., decrease). Therefore, the stability of LRC is an important factor to consider for sport performance. We also report a destabilization of LRC when one of the two systems is far off its preferred frequency. This result suggests that individuals spontaneously adopt an optimum synchronization between the two systems. Thus, it seems important to use a customized suitable stimulation that could be able to adapt its tempo to changes in locomotor or respiratory frequencies imposed by the constraints of the task. Overall, our results provide a better understanding of the evolution of LRC when confronted to constraints (e.g., locomotor or respiratory frequencies, auditory rhythm, exercise modality) and highlight the positive impact of its stability on sport performance. We also report the effectiveness of a visual stimulation to learn how to better manage energy resources during effort. Thus, this work opens perspectives on the use of auditory or visual stimuli, simple (e.g., metronome) or complex (e.g., music, avatar), for training and performance enhancement.

Optical frequency standards are necessary tools for accurate measurement of time and length. In practice these standards are stabilised laser systems locked to a known frequency reference. These references are typically the resonant frequencies of the atoms of an absorption medium that have been theoretically calculated to a high degree of accuracy. This thesis describes a combination of experimental and theoretical research performed on modulation transfer spectroscopy (MTS)--a technique used to frequency stabilise a laser in order to produce an accurate frequency reference--with emphasis placed on developing techniques and procedures to overcome the limitations found in existing MTS stabilised laser systems. The focus of the thesis is to generate a highly accurate frequency reference by researching the system parameters that will increase the signal to noise ratio and improve the accuracy of the reference through refinement of the signal structure. The early theoretical interpretation of MTS was effectively a low absorption approximation that occurs at low pressures. This approximation ignores the depletion of beam energy through absorption and is a distinct limitation of the theoretical model in its ability to accurately predict the influence of a range of system parameters on signal strength and structure. To overcome this limitation a 3-D (or volumetric) analysis was developed and is presented here for the first time. This volumetric model is a measure of two depleted beams interacting collinearly in an absorbing medium of iodine and is described to accurately predict the signal maximum as a function of pressure for all wavelengths. This model was found to be more accurate in predicting the influence of system parameters on the signal strength and structure, including that of pump beam intensity, pressure, saturation parameter, cell length and modulation parameters. The volumetric model is a novel approach to MTS theory but is more complex computationally than the traditional low pressure model and therefore more difficult to implement in many situations. To overcome this problem a hybrid model was developed as a combination of the low pressure and volumetric models. The comparison between the rigorous volume model and the hybrid model indicate that there is a deviation in the signal strength at high pressures. However, the agreement was very good in the pressure regimes that are commonly used to realise actual frequency references. Comparison of the hybrid model to experimental data was performed over a range of different wavelengths (532 nm, 543.5 nm, 612 nm and 633 nm) and found to be in close agreement. This gives confidence in the model to accurately predict signal strength and structure in any situation. Three mechanisms have been identified that limit the accuracy of frequency references due to the creation of residual amplitude modulation (RAM) where it shifts the frequency of the reference. The influence of RAM is included in the hybrid model as a ratio of the amplitude modulated and frequency modulated components of the saturating beam. These RAM production mechanisms result from the modulation of the saturating beam, the overlap of the beams in the medium, and the differential absorption of the sidebands in the medium. While the first mechanism has been previously reported the latter two are discussed here in detail for the first time. RAM generated by the modulators used (acousto-optic or electro-optic modulators) was typically of the order of 10% to 12%, depending on the excursion of the created sidebands. RAM generated by an asymmetric beam overlap with the modulators used was found to be as large as 30%. A combination of these two independent mechanisms can be used to provide a "RAM-free" state of the system by using one to cancel the effects of the other. The third RAM generation process--medium induced RAM--is difficult to remove but through a careful combination of absorption related parameters--namely, pump intensity, cell length, pressure and detector phase--the effects of RAM can be removed, leading to a distortion free MTS signal. Further investigation into the predictions provided by the hybrid model shows that there is a complex relationship between cell length and the optimum pressure required for maximum signal strength, such that longer cell lengths will not necessarily improve the signal strength. This is contrary to conventional thinking and is important in the MTS design process to reduce unnecessary costs and improve the signal to noise ratio and frequency accuracy. Optimisation of frequency stabilised laser systems using MTS are generally performed using trial and error. Comparison of these optimum parameter values to those predicted by the hybrid model show that for popular wavelengths such as 532 nm they are similar. In addition, the hybrid model is able to predict the frequency shifts that arise within the system parameters used and has shown that existing systems being used at 532 nm, 633 nm and 778 nm could improve their signal to noise ratio and accuracy through a variation in the parameters. A methodology based on the hybrid model is presented that can be used to calculate the optimum parameters for maximum signal strength and a "RAM-free" state for any wavelength. This systematic approach can therefore be used to guide the design of actual frequency stabilised laser systems prior to and during the design process.