Few-Cycle Femtosecond Laser Pulse Generation,Measurement And High-order Harmonic Generation

The generation and measurement of few-cycle pulses have always been an important direction in the field of ultrafast laser research.First of all,few-cycle pulses are important driving sources for generating isolated attosecond pulses.Femtosecond lasers with an octave-spanning spectrum obtained by nonlinear spectral broadening technique can be used as reliable sources for optical parametric amplification in the mid-infrared.Also,it plays a very important role in transient spectroscopy,metrology,biology and medicine research.Therefore,it is of great significance to carry out research on few-cycle pulse technology.In this dissertation,a series of research work has been carried out on the nonlinear spectral broadening with solid thin plates,including the measurement of few-cycle pulses,carrier envelope phase locking,high-order harmonic generation and attosecond pulses generation.The main research activities and innovative results of the dissertation are as follows:1.A scheme was introduced to broaden the spectrum through multiple thin plates of solid nonlinear material,which avoid the destruction of pulse and its coherence due to excess ionization or other high-order nonlinear effects caused by self-focusing in solid materials.In the experiment,through seven 100 μm-thick fused silica plates,we obtained an octavespanning spectrum,with input pulses of 0.8 mJ,28 fs,1kHz at 800 nm central wavelength.Also based on Ti:Sapphire laser system,125μJ femtosecond pulses at 400 nm center wavelength were obtained by second harmonic generation,which was used to get a continuous spectrum covering 365～445 nm by 6 pieces of 100 μm thick silica plates.In addition,at 200 kHz repetition rate,femtosecond pulses from a Yb fiber laser with a center wavelength of 1030 nm were broadened.Four fused silica plates with thicknesses of 600 μm,600 μm,400 μm,and 600 μm were used to compress the 40 fs pulses to 18.5 fs.2.Neural network algorithm was used to realize the learning of the inversion calculation process of the transient grating Frequency-Resolved Optical Gating(TG FROG).Convolutional neural network algorithm was used to inverse the TG-FROG trace by supervised training.The relationship between the TG-FROG trace and its corresponding pulse time shape is established.After the training,the convolution neural network spends much less time inverseing the FROG trace than the traditional iterative algorithms.The FROG trace inversed by the neural network shows a high degree of similarity with the original data,which theoretically proves that accurate inversion of TG-FROG trace can be achieved through the neural network algorithm.3.Based on f-2f technology,carrier envelope phase(CEP)locking is realized with an ovtave-spanning spectrum.In the experiment,the octave spectrum(480～980 nm)was obtained by multiple thin plates,and the CEP variation was measured by the f-2f method.The feedback circuit is used to control the insertion amount of the prism in the amplifier to lock the CEP of the amplifier.The final result shows that under the 3 ms integration time,the locking result is 346 mrad.The results of CEP locking using hollow-core fiber and sapphire are 540 mrad and 570 mrad respectively.4.Based on the semiclassical theory of high-order harmonic generation(HHG),the attochirp of attosecond pulses are calculated by classical calculation.Combined with the dispersion characteristics of metal film,the atto-chirp of attosecond pulses is compensated theoretically and the width of attosecond pulses is compressed.The atto-chirp of the central photon energy of attosecond pulses at 98,120,170 eV and the dispersion characteristics of various metal films are calculated.By matching the atto-chirp with the metal film dispersion,an appropriate scheme of attosecond pulses generation and compression is selected.Under the condition of strong field approximation,we solved a one-dimensional time-dependent Schrodinger equation calculating the continuous spectrum of HHG,and matched the attochirp with the metal film dispersion.The results show that the isolated attosecond pulse of 38 as can be generated efficiently with 150 nm molybdenum film when the HHG continuum is generated at the central photon energy of 120 eV.