Research Of Contrast Improvement Of Ultra-intense Ultra-short Pulse Laser And Attosecond Pulse Technology

In recent decades,ultrashort pulse laser technology develops rapidly.The focus of attention is on the generation of pulse laser with much higher intensity and shorter duration.In terms of ultra-intense laser pulse generation,petawatt-level(10¹⁵ W)amplification systems have been built and reported by several laboratories over the world.Moreover,the laser focusing intensity has reached 10²² W/cm² level.When an ultra-intense laser pulse interacts with the material,the pre-pulse and laser pedestal can result to the formation of pre-plasma.This in turn seriously affects the physical interaction process between the main pulse and matter.Therefore,pulse contrast improvement becomes an important research topic.In the aspect of ultrashort pulse generation,since the first report and measurement of attosecond pulses in 2001,generation,measurement and application of attosecond pulse have become the focus of research.Focusing on the problem of pulse contrast improvement,a novel method named Spectro-temporal filtering technique is presented.And noise of the pulsed laser was significantly reduced by implementing the spectro-temporal filtering and the fast optical switching technique.In the aspect of ultrashort pulse generation,focusing on the construction of attosecond experimental facilities,attosecond pulse generation,spectral measurement and photoelectron spectroscopy measurement techniques are the key issues of the dissertation.Moreover,mode-locked Ti:sapphire laser directly pumped by blue and green LDs is also investigated and experimentally demonstrated.The main research contents and innovations of this dissertation include the following four aspects.1.Pulse contrast improvement based on spectro-temporal filtering method.For the chirped-pulse amplification?CPA?system,a variable bandpass filter is constructed according to the spectro-temporal characteristics of the chirped pulse.By using this filter,noises arising from the amplified spontaneous emission?ASE?process and pre-pulse are effectively reduced.The influences of the parameters such as bandwidth of the constructed filter,edge of high-voltage pulse and timing jitter on the scanning filtering performance are analyzed by numerical simulation.The generation and synchronization of fast,high-voltage pulse are studied experimentally.The dynamic scanning of the broadband filter was realized and the pulse contrast ratio measured by using Kerr-gate technique was found to be improved by an order of magnitude.Furthermore,the narrowband spectro-temporal filtering technique based on a low-voltage RTP electro-optic crystal is studied and experimentally implemented.Two modes of the photoconductive semiconductor switches?PCSS?including high resistance output and 50?coaxial output modes were adopted to drive the narrowband filter and the pulse contrast ratio is effectively improved.The innovation of this research lies in presenting a totally novel laser pulse contrast ratio improvement method.The main advantages of this method include high efficiency and easy availability of cascading use.The feasibility of the spectro-temporal filtering technique is experimentally proved for the first time and the technique can find applications in the terawatt and petawatt CPA system.2.Experimental research of pulse contrast ratio improvement through the use of fast optical switch.For ultrashort pulse laser,several-hundred-picosecond to nanosecond ASE and pre-pulse are mainly studied.and fast optical switching technology is used to enhance the pulse contrast.The pre-pulse with a time-scale of several hundred picoseconds before the main pulse is effectively reduced by driving the fast KDP optical switch with a¹ ns high-voltage pulse.The contrast enhancement effect was evaluated by constructing a pre-pulse and a strong enhancement of more than 79 times was achieved at 1.45 ns.3.Generation and measurement of attosecond pulse in noble gas.First,several key issues involved in the attosecond pulse generation in gas medium including atomic ionization rate,cutoff photon energy and phase matching process are theoretically analyzed.In the aspect of building the isolated attosecond pulse experimental facilities,attosecond driving source,double optical gating?DOG?,attosecond pulse filter and chirp compensation technology are all investigated.Second,the spectral acquisition and pump probe technique of attosecond pulse?transient absorption spectrometer?are studied.An auxiliary laser was used to realize high precision synchronization and scanning between the NIR and XUV beam with a synchronization stability of smaller than 20 as.The broadband spectrum of XUV pulse is measured based on the combination use of large-area,single-layer,microchannel plate?MCP?,fiber optic tapers and CCD.The noise in the photoelectric conversion and stray light noise is effectively reduced by using this spectral measurement technique.The bandwidth of XUV supercontinuum obtained in Ne gas target is larger than 150 eV.Finally,the principle of attosecond pulse characterization by attosecond streak camera is studied theoretically.A time of flight?TOF?spectrometer is designed and constructed.By using this spectrometer,clear photoelectron spectrum of high-order harmonic is obtained.The innovations of this research lies in the first use of large-area,single-layer,microchannel plate?MCP?and fiber optic tapers to measure the spectrum of attosecond pulse.Transform-limited pulses with a duration of smaller than 50 as can be expected from the measured XUV supercontinuum.Meanwhile,we also independently design and construct the attosecond streak camera.The researches provide theoretical and practical guidance for the generation and measurement of attosecond pulse.4.Ti:sapphire laser system directly pumped by blue and green LDs is built.The fast and low axes of the LD are collimated by using an aspherical lens and a pair of expanding cylindrical lenses.In the experiment,two 3.5 W blue LDs were used to pump the Ti:sapphire laser and the maximum average power of 82 mW CW laser was obtained by using the four mirrors cavity.Ti:sapphire laser directly pumped by a1.5 W green LD was also studied.Stable Kerr lens mode locking operation was realized by using only one LD as the pump source and a pair of GTI mirrors as the intracavity dispersion compensation components.A pulse duration of 79.4 fs and an average power of 232 mW were obtained by using double end pumping structure with two green LDs.To the best of my knowledge,it is the first time to launch this research at home.The mode locking threshold of 7 mW is the lowest one at the moment.In addition,the maximum single pulse energy of 1.59 nJ is the highest one among the similar studies.