Redistribution of Rydberg states by terahertz radiation

We present the technique and results of our study of the redistribution of Rydberg states in barium by a half cycle pulse (HCP). A survey of previous research concerning the interaction of half cycle pulses with Rydberg atoms reveals a lack of experimental data in the area of state redistribution. This is primarily due to an inability to identify the redistributed states accurately using the currently available tool: selective field ionization (SFI). We present the limitations of SFI as a state identification tool. We then present a survey of isolated core excitation as a state identification tool, along with its limitations in dealing with a mixture of states. Combining SFI and ICE overcomes these limitations and allows for the accurate characterization of a mixture of Rydberg states, as found in HCP redistribution. We present our results for the HCP redistribution of a series of ns and nd initial states, for n from 30 to 40, at various HCP intensities. These results primarily involved single photon transitions, confining us to the linear regime. Analyzing the results of these redistributions with respect to the energy difference between initial and final states yielded information about the frequency structure, after transmission to the interaction region, of the HCP. This led us to postulate on the effects of diffraction on the HCP. We conclude with several suggestions for the improvement of the experimental system, as well as a direction for future research

Supercontinuum generation in microstructured fibers and novel optical measurement techniques

Over the past few years microstructured fibers have shown potential for many practical applications and permitted significant progress in various domains such as nonlinear optics, medical science or telecommunications as the fabrication process allows for a great flexibility in the design of these fibers. In particular, small core microstructured fibers has proven to be the most efficient way for supercontinuum generation. Supercontinuum is one of the most spectacular outcome of nonlinear optics as it possesses the properties of a laser combined with an ultra-broad bandwidth spanning more than two octaves. The thesis provides a comprehensive review of the different physical mechanisms leading to the generation of these spectra in microstructured fibers. Dispersion, which reflects the dependence of the refractive index of a material on the frequency of light plays a crucial role in the pulse propagation in optical fibers or components. A novel technique to characterize the anomalous dispersion of small core microstructured fibers using short optical pulses is demonstrated. The method presents several advantages over conventional techniques and relies on the spectral modulation resulting from the evolution of the input pulse into a soliton wave. As the demand for capacity of optical networks increases, the requirements for the components employed in transmission systems becomes more stringent. In particular, the dispersion of fiber Bragg gratings or thin-film filters commonly employed in the links needs to be accurately evaluated as it may have a strong impact on the overall performance of the system. A novel method for improving the accuracy of dispersion measurements, based on the well-established phase-shift technique widely used in the characterization of optical components, is presented. The performance of diode lasers can be greatly enhanced with the use of an external cavity configuration. The wavelength tunability of the diode is increased and its linewidth considerably reduced. The behavior of the linewidth of a grating cavity laser is both theoretically and experimentally explored as the oscillation frequency of the laser is varied. Surprisingly, large changes in the linewidth value are observed, which may have an impact in applications requiring lasers with a stable and narrow linewidth. The fabrication of integrated silica-based optical components performing active functionalities is an exciting prospect for obvious reasons. Poling of silica glass is a very promising technique for the development of this type of component as it allows for the introduction of an effective second-order nonlinearity essential for performing active functions. A new technique based on the inscription of Bragg gratings for measuring the second-order nonlinearity induced by negative thermal poling is demonstrated.reviewe

Power Spectrum density of Single Side band CPM using Lorenztian frequency pulses

International audienceWe propose a new waveform for frequency-shift keying continuous phase modulation (FSK-CPM) that directly generates a single side band (SSB) spectrum. The original principle, inspired from quantum physics (levitons), is presented. From the simple analytical form of the modulation, orthonormal wave-functions and analytical expressions leading to the SSB power spectrum density are proposed. This CPM modulation displays a very compact spectrum, with 99% of the power in the primary bandwidth. Its SSB property makes this FSK-CPM scheme, hereby called SSB-FSK, a promising solution not sensitive to image frequency degradation and would allow large bit rates transmission at low carrier frequency