Control And Synthesis Of Few-cycle Laser Pulses

Laser is a key tool for understanding the microscopic world,and ultrafast laser pulses are of great significance for exploring and controlling the motion and structure of molecules,atoms,and electrons.At present,it is difficult for single-channel laser to be compensated for the dispersion of the spectrum with more than one octave,which limits the output pulse duration.Using coherent synthesis technology to adjust the phase and amplitude of different spectra,the spectrum above one octave can be well compressed to generate few-cycle or even sub-femtosecond pulse.Based on this background,this paper focuses on the few-cycle pulse coherent synthesis technology.On the one hand,we improved the key technologies of coherent synthesis,including supercontinuum generation,precise phase locking between pulses,and pulse duration compression and measurement experiments.On the other hand,we applied coherent synthesis techniques in experiments such as high-order harmonic generation.The main research and innovative results are as follow:1,We designed and built a high-energy and few-cycle hollow fiber system,The driving source used is a commercial Ti:sapphire laser(Coherent)with output parameters of 5 mJ,35 fs,1 kHz at 800 nm.Infused into a hollow fiber with an inner diameter of 400 ?m and a length of 1 m.In the experiment,we tried the transmittance of different pumping power in vacuum,spectral broadening of He and Ne as nonlinear medium,and compressed measured pulse duration at optimal spectral broadening.Finally,the output of 2-mJ,6.7-fs high-energy ultrafast pulse is realized.2,We performed supercontinuum generation experiments.The driving source used is a commercial Ti:Sapphire laser(Femtopower Compact Pro CEP)with parameters of 0.8 mJ,35 fs,1 kHz at 800 nm.Two schemes for generating supercontinuum are used: first,an inflatable hollow fiber with an inner diameter of 250 ?m and a length of 1 m is used as a non-linear medium to broaden the spectrum to 400-980 nm,reaching to more than one octave;in another scheme,we use a thin-plates set with seven 0.1-mm fused silica plates as a non-linear medium to broaden the spectrum to 460-950 nm,which also exceeds one octave.The first scheme provides an ideal light source for coherent synthesis experiments.3,The oscillating electric field structure of the few-cycle pulse has a great influence on the carrier-envelope phase(CEP).We use the f-2f scheme to lock the CEP of the amplified pulse,and the locking result is at 3-ms integration time of the spectrometer,the rms CEP jitter is less than 150 mrad.We theoretically simulate the effect of the relative time delay between pulses on the shape of the synthesized pulse electric field,and use two schemes to lock the relative time delay between pulses.The first scheme is the balanced optical cross-correlator(BOC)with locking accuracy less than 80 as.The other is a new scheme,using a set of f-2f device to extract the relative time delay and CEP information from the obtained spectral interference pattern and lock them simultaneously.The relative time delay and CEP were locked within less than 100 as and 280 mrad,which realizes the control of the total phase between pulses.4,We built a transient-grating frequency-resolved optical gating(TG-FROG)device to measure the pulse waveform.Through this device,the relative time delay between two few-cycle pulses in the coherent synthesis system can be measured,and the measurement accuracy is less than 0.5 fs,which greatly reduces the difficulty of adjusting the relative time delay between ultrafast pulses.In addition,the pulse duration of the synthesized pulses of the coherent synthesis system was measured,and the optimized result of the synthesized pulses was 3.8 fs.5,We conducted experimental research on the high-order harmonics generation by two pulses with different spectra using a coherent synthesis device.By adjust relative time delay between two pulses,and interaction with Ar,we obtained a continuum spectrum(50-68 eV)in the HHG cutoff region,while each individual sub-pulse does not produce a continuum in this region.The use of a coherent synthesis system ultimately extends the spectral width of the continuum of the HHG cutoff region.