Research On The Supercontinuum Generation Induced By Spatially Shaped Femtosecond Laser Pulses

When femtosecond laser pulse propagates in the transparent media,it is affected by the self-focusing effect resulting in the convergence of the laser beam and the increase of the intensity of the pulse.When the intensity of the pulse increases to a certain value,the molecules or atoms in the transparent media are ionized resulting in the plasma generation,which causes divergence of the pulse.The two effects compete with each other and reach a dynamic equilibrium generating an illuminating plasma channel,which is called filament.Femtosecond filamentation is accompanied by lots of optical nonlinear phenomena,such as plasma generation,supercontinuum generation,terahertz generation,high-order harmonic generation,conical emission and fluorescence emission,etc.The supercontinuum generation/white light is a hot topic due to the potential application,such as light detection and ranging(LIDAR).Although,there are lots of the researches on the supercontinuum generated by femtosecond laser filaments,the research on filamentation induced by spatially shaped femtosecond laser pulses is still insufficient.At the same time,it is also necessary to discuss the physical mechanism of the supercontinuum generated by femtosecond optical vortex filament.In this article,we research the filament-induced supercontinuum generation using spatially shaped femtosecond laser pulses.The main contents are as follows:1.Spectral broadening is an important nonlinear effect during the interaction of intense femtosecond laser pulses with air.In this paper,we experimentally study the dependence of spectral broadening on pressure and pulse energy.It is found that increasing both pressure and pulse energy can enhance the spectral intensity,while only increasing pressure leads to obvious blue shift of spectra,and the pulse energy has little effect on it.To understand the mechanism of spectral broadening deeply,the numerical simulation relating the instantaneous frequency to the time dependent pulse intensity and time dependent plasma density in air is carried out.The results indicate that the plasma generation induced self-phase modulation which is related to the pressure plays an important role in blue shift of spectrum.Besides the transmission of the laser pulse is measured and decreases with pressure.It proves that the energy transfer of the laser pulse is promoted by pressure.This study will be helpful to understand physical mechanism of femtosecond filament induced supercontinnum generation in air deeply.This part is discussed in Chapter 3.2.We experimentally study the femtosecond supercontinuum generation by an Laguerre-Gaussian vortex beam in water.It is demonstrated that the topological charge of the optical vortex beam has little influence on spectral broadening induced by filament,while it has strong influence on spectral intensity.Despite the supercontinuum induced by filamentation of optical vortices in air possesses orbital angular momentums,the supercontinuum generated in water and sapphire loses orbital angular momentum.However,the light at central wavelength possesses orbital angular momentums after filamentation in transparent media.This study is helpful to the understanding of the physical mechanism of femtosecond supercontinuum generation by Laguerre-Gaussian vortices in water.This part is discussed in Chapter 4.3.We study the characteristic of the supercontinuum generated by Laguerre-Gaussian vortex beam and Bessel-Gaussian vortex beam.As the topological charge increases,the spectral broadening induced by both Laguerre-Gaussian vortex beam and BesselGaussian vortex beam stays constant.And the distance for the spectral broadening induced by Bessel-Gaussian vortex beam reaching its maximum increases with the topological charge.Besides,the beam profiles of Laguerre-Gaussian vortex beam and Bessel-Gaussian vortex beam shown on the board are totally different.It is also found that the angels of the conical emission for Laguerre-Gaussian vortex beam and BesselGaussian vortex beam both decrease with increasing wavelength of the supercontinuum,and have no relationship with the topological charge.It is known that the conical emission can be described by self-phase modulation(SPM)effect.The content is discussed in Chapter 5.The work in this article can make the supercontinuum generation be better used and make us understand the mechanism of the supercontinuum generation deeply.