High Spectral Power Supercontinuum Source For The Measurement Of No_x And O₃ After Femtosecond Filamentation In Air

The propagation of ultrashort pulses in air as self-guided filaments with formation of plasma has numerous applications, among which are the laser-induced condensation of rain and the formation of snow. In this thesis, by means of a broadband cavity-enhanced absorption spectroscopy technique, monitoring of the temporal evolution of the O3 and NOx=2,3 formation after the filamentation of femtosecond laser pulses in air has been performed, providing the first quantitative live measurement of the filament accumulated contribution and its temporal evolution. The quantification of the generation of these species after the propagation of filaments is of great importance for a complete description of the most efficient chemical means by which water and snow are formed due to the generation of a high concentration of hygroscopic molecules.We have implemented an absorption spectroscopic technique called Supercontinuum Cavity Enhanced Absorption Spectroscopy(SC-CEAS) for the species quantification after the passing of femtosecond filaments. With this technique we have achieved time-resolved, non-destructive and live quantification of O3 and NOx=2,3 concentration evolution during femtosecond filamentation in air. For many years already, different spectroscopic systems have been used as optical solutions for the analysis of atmospheric components, and in this work, unlike methods like ion chromatography or chemiluminiscence, we probe the filament region directly, measuring multiple species at the same time, enabled by the multiwavelength feature of the technique. The choice of CEAS also allows that measurements can be made simultaneously to the production of the species. In addition, we have reduced the temporal resolution of the measurements with respect to the techniques used previously, allowing a more accurate insight into the post-filamentation evolution of the species’ concentration. As it will be shown in our results, we could handle very high absorption values without saturation and provide a high dynamic range to the measurements. Besides obtaining the contribution of IR filaments to the species generation, the effect of increasing the photon energy performing filamentation of the second harmonic have also been studied.The light source of the spectroscopic technique is the supercontinuum generated from controlled multiple filamentation of femtosecond pulses in fused silica. Several techniques of supercontinuum generation that provide a huge spectral broadening are currently very popular and commercially available. However, due to the characteristics of SC-CEAS we require a high spectral power supercontinuum, at levels that are higher than the damage threshold of fiber media normally utilized for these means. Basically, it is difficult to obtain a white light, high-repetition supercontinuum source with pulse energies higher than the mJ. The use of a microlens array allows the manipulation of the filamentation pattern under very high incident laser pulse energies without sample damage and consequently, compared to using a single focusing lens, higher power of SC generation with a similar spectral broadening can be obtained. Reducing the energy-per-spot in this experimental setup will allow an alternative source to filamentation in air, which has smaller generation efficiency at shorter wavelengths. It also presents advantages with respect to SC generation from laser propagation in hollow fibers, which can sustain mJ energy levels, but it is difficult to couple the energy into the fiber, damage is easily performed on the entrance face and fiber transmissions are comparatively small. It will be reported here a high spectral power SC femtosecond source with the level of mW/nm in the visible from a GW laser at a 1-kHz repetition rate from deterministic multiple filamentation in a solid medium. The effects of the variation of the experimental conditions in the efficiency and stability of the source will be discussed, in order to shed light into the criteria for the selection of the best experimental parameters. Moreover, the role of the interplay between diffraction pattern and proximity to the focus of the microlens array in the stability of the SC generation is discussed. Also, the benefits of modifying the scheme for a supercontinuum generation from the incidence of double pulse multiple filamentation(800 nm + 400 nm) are studied.The deterministic control of wavelength-dependent multifilamentation in fused silica besides playing an important role in the generation of supercontinuum, as mentioned above, also has applications in the fabrication of structures and optical components inside dielectric samples. This work studies in depth the method of MLA focusing for accurate control of multifilaments distribution by adjusting the diffraction pattern generated by a loosely focusing 2D periodic lens array. Microlens arrays are attractive optical elements that provide a high-efficiency collection of light and at the same time generate diffraction patterns typical of periodic components, which enable the generation of many filaments per lenslet. Previous authors have observed that an amplitude mesh with no focusing power would be preferable over a microlens array for multiple filamentation because of the strong divergence ultimately introduced by the lenslets to the beam. This, in principle, would be a limitation to the length of the filaments. Conversely, it is studied here the ability of microlens array to obtaining MF patterns of comparative length and filaments number by selecting a loosely focusing geometry. The distribution of filaments is controlled in experiments where we simple translate the sample along the propagation axis the number, showing agreement with the results of linear diffraction simulations. The loose focusing geometry allows for long filaments whose distribution is conserved along their propagation inside the sample. This thesis demonstrates as well a strong dependence of the MF distribution on the incident wavelength and sample-to-lens distance. The effect of incident energy and polarization on filament number is also studied.Patterning multiple filamentation of femtosecond pulses originally studied in this thesis for the purpose of generating the spectroscopic high power SC source can be extrapolated from fused silica to air, where it would form an array of air filaments with several applications, like the guiding of microwave energy or electric discharges. We demonstrate here that it can be achieved with a GW power level by using a microlens array for modulation of the spatial profile and a single lens for power condensation. We have also approached the problem of active control of the filament-to-filament distance(pitch) by modifying experimental conditions. In addition to this, we discuss the interaction among the main phenomena modifying the MF pattern along the beam propagation, namely the Kerr effect around hotspots in the beam profile and Talbot diffraction.Summarizing, the work performed in this thesis shows that femtosecond pulses of GW level peak power can be spatially manipulated by MLA focusing for the deterministic generation of filament arrays in air and high spectral-power supercontinuum in fused silica. This light source has been effective for the spectroscopic measurement of species like NOx and O3, appearing after filamentation in air. The choice of SC-CEAS for those measurements allowed in-situ and simultaneous quantification of the generation of such species, an important contribution for the studies that attempt to evaluate their participation in the process of condensation of water induced by filaments.