Research On Femtosecond Filament-induced Fluorescence Measurement And Applications

The filamentation induced by femtosecond laser in the transparent media provides an important platform for exploring the interaction between filament and molecules.It has been widely used in remote sensing,velocity measurement,chemical reaction and Terahertz,etc.In general,the formation mechanism of laser-induced plasma channels(filament)is considered to be the dynamic balance between self-focusing induced by optical Kerr effect and defocusing effect of plasma.This process is accompanied by a series of complex nonlinear optical effects,such as multi-photon ionization,intensity clamping,self-shaping,pulse self-steepening,pulse self-compression,conical emission and so on.Observation of filament with ultra-fast optical imaging technology is an important aspect of researching complex optical phenomena.Fluorescence enhancement can be achieved by regulating the filament and revealing the physical process of interaction between filaments,which is of great significance for improving the sensitivity of remote sensing and signal-to-noise ratio.This thesis introduces and discusses the filamentation and the application of femtosecond laser in gas medium.The main contents are summarized as follows:1.The time-stretched dispersion Fourier transform(TS-DFT)technique is introduced into the femtosecond filamentation research,and the dynamic research of filament source and filament is realized.TS-DFT technology maps the spectrum of the pulse to a time waveform with an intensity envelope similar to the spectrum,overcoming the sampling speed and bandwidth limitations of traditional spectrometers to achieve real-time measurement of high-speed processes.In this paper,two main tasks were carried out using this technology.1)Real-time detection of transient phenomena and transient dynamics in pulse selfexcitation generation process.This study further reveals the mode-locking mechanism of lasers,laying a solid foundation for exploring the ultrashort pulse formation phenomenon that has not yet been discovered and developing high-power,highrepetition rate femtosecond fiber lasers.2)Ultra-fast real-time optical imaging of the filament.The work overcomes the interrelationship between sensitivity and frame rate between charge-coupled element(CCD)or complementary metal-oxide semiconductor(CMOS)cameras,enabling highly sensitive and high-resolution conditions to capture fast dynamic processes.2.The nonlinear transmission dynamic equation of femtosecond laser is used to numerically simulate the filament formation,which realizes the optimization and modulation of the filament.According to the simulation results,it is found that the second-order nonlinear refractive index has an important impact on the filamentation,which determines the starting point and the length of the filamentation,and can be aborted into the filamentation.In addition,the size of filament could be extended/shortened by changing the ambient temperature,air pressure,focal length of the focus lens and the secondorder dispersion.Experiments have shown that the intensity of the fluorescence signal is linearly related to the length of the filament.However,for remote sensing imaging,the longer the length of filament is,the lower the spatial resolution of the imaging achieved.Therefore,it is necessary to weigh the sensitivity and the resolution in the fluorescence imaging.3.We experimentally investigate the interaction of filaments in air.It is demonstrated that the method of enhancing the fluorescence intensity of plasma based on two collinearly counter-propagating filaments can improve the detection sensitivity and spectral signal-to-noise ratio of the filament-induced fluorescence spectroscopy.The physical process of fluorescence excitation by two collinearly counterpropagating filaments,two non-collinear filaments and one filament were compared.We find that the fluorescence enhancement achieved by the interaction of filaments is attributed to the increase of the clamped intensity,which yields more highly excited molecules(plasma).Meanwhile,more electrons are involved in multiphoton/collision ionization,slowing down the rate of electron-molecule-ion recombination and prolonging fluorescence lifetime.The fluorescence spectra,the output energy at the end of the filament,and the spectra of the excitation beams are measured by changing the relative time delay between counter-propagating beams.Our experimental results add new perspective on the modulation of the filament or ultrashort pulses.4.Remote,multi-component,multi-dimensional spectral imaging based on hyperspectral fluorescence imaging technology is realized for air/flame and gas distribution.We report a simple technique,based on filament-induced nonlinear spectroscopy,for information-rich gas imaging.Three-dimensional images with high spectral resolution(0.1 nm)and high spatial resolution(<1 mm)are obtained by point-wise imaging for target space.This technique,with backward fluorescence detection,provided a wide fluorescence fingerprint regime(e.g.,350?750 nm)and spatial information even in a non-cooperative situation.Meanwhile,quantitative equivalent ration measurements of gas mixtures could be realized by performing ratiometric fluorescence imaging using normalized spectral lines of two different radicals.Onedimensional quantitative equivalent ration measurements could be extended to three dimensions with equally high spatial resolutions.In the future,with the improvement of spatial and temporal resolution,the combination of TS-DFT and femtosecond filamentation is expected to achieve ultrafast detection of filament-induced chemical reactions,dynamic imaging of plasma density in filaments with time changes,and even spaceborne filament atmospheric pollution detection.