High Spectral Power Supercontinuum Source for the Measurement of Nox and O3 after Femtosecond Filamentation in Air

摘要

The propagationof ultrashort pulses in airas self-guided filaments with formation of plasmahas numerous applications，among whicharethe laser-induced condensation of rain and the formation of snow. In this thesis,by means of abroadbandcavity-enhanced absorption spectroscopytechnique,monitoring ofthe temporal evolution of the O3 and NOx=2,3 formation after the filamentation of femtosecond laser pulses in airhas been performed，providing the first quantitative live measurement of the filament accumulated contribution anditstemporal evolution. The quantification of the generation of these species after the propagation of filaments is ofgreat importance for a complete description of the most efficient chemical means by which water and snow are formeddue tothe generation of a high concentration of hygroscopic molecules.
　　We have implemented an absorption spectroscopic technique called Supercontinuum Cavity Enhanced Absorption Spectroscopy (SC-CEAS)forthespecies quantification after the passing offemtosecondfilaments. With this technique we have achieved time-resolved，non-destructiveandlive quantification of O3 and NOx=2,3 concentration evolutionduring femtosecond filamentation in air. For many years already，different spectroscopicsystems have been used as optical solutions fortheanalysis of atmospheric components,andin this work，unlike methods like ion chromatography or chemiluminiscence，we probe the filament region directly,measuring multiple speciesat the same time，enabled bythe multi-wavelength 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 havereduced 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 energyperforming filamentation of the second harmonichave also been studied.
　　The light source of the spectroscopic technique is the supercontinuumgeneratedfrom controlled multiple filamentation of femtosecond pulses in fused silica. Several techniques ofsupercontinuum generation that providea hugespectralbroadening 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 difficultto obtain awhite light，high-repetitionsupercontinuum 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 alternativesource to filamentation in air，which hassmallergeneration efficiencyatshorter wavelengths. It also presents advantages with respect to SCgenerationfrom 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 herea high spectral power SC femtosecond source with thelevel of mW/nm in the visible from a GW laser at a 1-kHz repetition ratefromdeterministicmultiple filamentationin a solid medium.The effects ofthevariation oftheexperimental conditionsinthe efficiency and stability of the sourcewill be discussed，in order toshed 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 inthe stability of theSC generation is discussed. Also，the benefits of modifying the scheme for asupercontinuumgenerationfrom the incidence ofdouble pulse multiple filamentation (800nm+400nm)are studied.
　　The deterministic control of wavelength-dependent multifilamentation in fused silica besides playingan important role in the generation ofsupercontinuum，asmentioned above，also has applications inthe fabrication of structuresand optical components inside dielectric samples.This work studiesin depththe method of MLA focusing foraccurate control of multifilaments distribution by adjusting the diffraction pattern generated by a loosely focusing 2D periodic lens array.Microlens arrays areattractive optical elementsthat provide a high-efficiency collection of light andat the sametimegenerate diffraction patterns typical of periodic components，which enable thegenerationof many filaments per lenslet.Previous authors haveobserved that an amplitude meshwith no focusing powerwould bepreferable over a microlens arrayfor multiple filamentationbecause 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 herethe ability of microlens array toobtainingMF patterns of comparative length and filaments numberbyselectinga loosely focusing geometry.Thedistribution of filamentsiscontrolledin experiments where we simple translatethe sample alongthe propagation axis the number，showingagreement 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 thesisdemonstratesas wella 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 pulsesoriginally studied in this thesis for the purpose of generating the spectroscopic high power SC source can be extrapolated from fused silica toair，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 achievedwith a GW power level byusing a microlens array for modulation of the spatial profile and a single lens for power condensation.We have also approachedthe 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 hotspotsin the beam profileand Talbot diffraction.
　　Summarizing，theworkperformedinthis thesis showsthat 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-powersupercontinuum 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.

目录

封面

声明

英文摘要

目录

Chapter 1 Introduction.

1.1 Basics of laser-induced condensation of water

1.2Basics of femtosecond laser filamentation

Chapter 2 Deterministic control ofmultiplefilamentation

2.1 Multiple filamentation.

2.2 Methods for deterministic control of MF patterns

2.3Existing methods for deterministic control of MF patterns

2.4Array of filaments in air byfocusing with MLA and regular lens

2.5Control of MF in fused silica by using MLA

2.6Application to the writing of waveguides in fused silica

2.7Chapter conclusions

Chapter 3 Generation of high-spectral power supercontinuum

3.1Phenomena participating in supercontinuum generation

3.2 Methods for high powersuper continuum generation.

3.3Dual-wavelength,high power supercontinuum by MLA focusing.

3.4 Chapter Conclusions

Chapter 4 Supercontinuum cavity-enhanced and cavity-ringdown absorption spectroscopy

4.1Spectroscopic techniques enhanced by a high-finesse cavity

4.2SC-Cavity ring-down spectroscopy \(SC-CRDS\)

4.3SC-cavity-enhanced absorption spectroscopy \(SC-CEAS\)

4.4Analysis of the cavity.

4.5Multi-spot SC-CRDSby MLA focusing

4.6Detection of atmospheric humidity bymulti-spotSC-CRDS

4.6 Chapter conclusions

Chapter 5 Study ofthe chemical contribution of atmospheric plasma filaments by multi-spot SC-CEAS

5.1 Methods for the measurement of intermediate atmospheric species in laser induced condensation.

5.2Measurement of femtosecond filamentation contribution to NOx and O3

5.3Contribution of 800-nm and 400-nmfilaments.

5.4Chapter conclusions:

Chapter 6 General conclusions

Contributionsof this thesis

Recommendations for future work

致谢

参考文献

List of publications

Appendix