Research And Application Of Spectral Broadening And Pulse Compression In Ultrafast Lasers

Pump-probe experiments based on attosecond laser technology have helped to open the door to ultrafast dynamics in atoms and electrons.Femtosecond lasers are a necessary tool for entering the field of attosecond science.Nowadays,the booming technologies of post compression of ultrashort pulses are helping to push the developments of femtosecond laser parameters.Among them is the generation of supercontinuum that breaks the bandwidth limits of the laser gain medium and gain narrowing in the amplification process.And the powerful dispersion management techniques have enabled the pulse width of femtosecond laser pulse to break the limits of the optical period.Hollow-core fibers(HCFs)and multithin plates are high-quality sources for generating millijoule-level supercontinuum,which are currently gaining attentions and used as stable driving sources for attosecond pulse generation.In addition,self-compression techniques are not limited by devices of dispersion management,which have the advantage of generating high energy,ultrashort pulses.It has been studied by many groups as a potential novel source in ultrafast science.This thesis focuses on the generation of supercontinuum,ultrashort pulses and their applications.The innovative results are achieved as follows:1.The generation of millijoule-level few-to single-cycle pulses has been investigated based on the pulse post-compressor of HCF.Pulses with energy of 1 mJ and 5.7 fs were achieved in a neon-gas-filled HCF compressor using the pressure gradient technique and one set of chirped mirrors for dispersion management.In the heliumgas-filled HCF,pulses with energy of 2.1 mJ and 6.3 fs were achieved.Using two sets of chirped mirrors,the pulse width in the neon-gas-filled HCF was further compressed to 3.55 fs,corresponding to 1.5 optical cycles at the center wavelength of 712 nm,and the pulse energy was over than 0.7 mJ.The pulse energy was further increased to 1 mJ using the pre-chirp managed technique.The pulse width is 3.8 fs.Using a balanced optical cross-correlator for active synchronization,the time jitter as low as 360 as of the two channels was demonstrated for a long test period of 40 minutes.2.Using the coherent synthesis device as the driving light source,the effects of highorder harmonic generation(HHG)with different noble gases were compared.A continuous spectrum of photon energy up to 60 eV was obtained with argon;In neon,a continuous spectrum was obtained with photon energy close to 90 eV.In addition,the solid-state HHG of two-color few-cycle pulses under time-delay modulation was studied for the first time.The enhancement of HH yields was observed in both SiO2 and MgO crystals;In MgO crystal,the asymmetry of the spectral signals was observed under time-delay modulation,and the extension of HH photon energy in the cut-off region was observed with the increasing synthesized field strength when the two-color few-cycle pulses overlapped well in time.3.Based on the induced phase modulation,the UV-enhanced supercontinuum was generated using multi-thin plates as the spectral broadening system.The front-end laser source was a Ti:sapphire regenerative amplifier with the fundamental pulse width of 37 fs and the second harmonic pulse width of 58 fs.By optimizing the temporal overlap between the fundamental and second harmonic pulses in the multithin plates,the UV-enhanced supercontinuum with pulse energy of 0.6 mJ and spectral bandwidth of 375-920 nm was demonstrated,which supported a Fourier transform limit pulse width of 1.6 fs.The experimental results were also simulated using the nonlinear Schrodinger equation,and the simulation results were consistent well with the experimental results.4.Based on the self-compression effect due to the interaction of self-focusing and ionization in positive dispersion materials,self-compressed of multi-millijoule pulses and few-cycle pulses have been demonstrated in a concave bulk material and multi-thin plates,respectively.The self-compression of pulses with energy from 15 mJ was achieved using a BK7 glass material with a focal length of f=-300 mm as the self-compression medium.The pulse width was compressed from 40 fs to less than 20 fs with an overall efficiency of 92%;Self-compression of few-cycle pulses with pulse width less than 10 fs was achieved using multi-thin plates in an annular beam spot.In addition,the above self-compression results were simulated theoretically using the three-dimensional nonlinear Schrodinger equation.