Research On Ultrafast Laser Pulse Compression,Wavelength Extension And Contrast Improvement

The femtosecond lasers are widely applied in the fields of scientific research,material processing and medical profit its brilliant temporal properties.At the same time,the practical application also promotes the improvement of the femtosecond laser parameters.For example,the high harmonic generation and attosecond science boost the development of few cycle and high average power femtosecond laser.The precision measurement and optical frequency comb boost the development of the high repetition rate femtosecond oscillator.The strong field physics experiments like laser wake field acceleration,laboratory astrophysics and proton acceleration boost the development of high temporal contrast high peak power lasers.As can be seen,improving the temporal parameter of the femtosecond lasers will benefit for its user.The commercial femtosecond amplifiers suffer the defects of high cost,which means that the ordinary research institutions cannot afford this equipment.So,the generation of high energy femtosecond laser pulses with a lower cost is also meaningful.In this paper,a series of experimental studies for the improvement of the femtosecond laser temporal properties is demonstrated,including nonlinear compression the pulse duration of the picosecond laser down to femtosecond level,gigahertz femtosecond optical parametric oscillator(OPO) and enhancement the temporal contrast ratio of the Ti:Sapphire regenerative amplifier by using a femtosecond OPO as the seed source.The core research contents and original achievements of this paper are as follows:1.Nonlinear pulse compression of the Nd-doped picosecond laser by using Multi-pass cell(MPC)is proposed and demonstrated for the first time.The pulse duration of a 1kHz,230?J Nd:YVO₄ regenerative amplifier is compressed form 11.6ps to 172fs by cascading two stage MPC devices.The spectral bandwidth of the picosecond laser is broadened from 0.26nm to 18.06nm.The pulse energy after compression is 117?J,corresponding to a total efficiency of 51%.Further,two 3-inch diameter concave mirrors are employed as the MPC mirrors to broaden the spectrum of an 8k Hz,1.65 W picosecond laser.The spectral bandwidth of the laser is broadened from 0.192nm to 4.69nm and the pulse duration is compressed from 12.5ps down to 601 fs.So,the spectral bandwidth broadened by a factor of 24.4 and pulse duration shortened by a factor of 20.8 is experimentally achieved.This is the highest compression ratio based on the single stage MPC device with the bulk material as the nonlinear mediums.At the same time,the average power stability,and the beam quality of the picosecond laser before and after the MPC device are compared,both of which are less deteriorated by the MPC device.2.The gigahertz femtosecond OPO pumped by an all-solid-state femtosecond laser is demonstrated for the first time.By employing a ring cavity structure,the parametric oscillation at the repetition rate of 1.13GHz pumped by a 515nm laser is achieved, which is 15 times of the pump source.The signal is tunable from 700nm to 1003nm.The highest power of the signal is 430mW and the shortest pulse duration of the signal is 117fs.The beam quality of the signal is good enough.Further,the repetition rate of the signal tunable from 755MHz to 1.43GHz at an interval ofn 75.5MHz is experimentally achieved by using fractional decrease in the OPO cavity length.The 1.43GHz is the highest repetition rate of the signal pumped by 515nm laser.The average power and the central wavelength of the 1.43GHz signal is 22mW and 750nm.3.The femtosecond OPO seeded a Ti:Sapphire regenerative amplifier for enhancing the temporal contrast ratio is proposed and demonstrated for the first time.After Martinez stretcher,regenerative amplifier and compressor,a pulse with 1.8mJ energy and 96fs pulse duration is generated.The temporal contrast ratio of the amplified pulse is near 10⁷ within ±30ps delay range.Compared with the Kerr-lens mode locked Ti:Sapphire oscillator as the seed source,the contrast ratio of the amplified pulses is improved more than one order of magnitude.The broadband OPO operated near 800nm is demonstrated for shorting the pulse duration of the amplified pulses.The pulses with 1.8mJ pulse energy and 25.6fs pulse duration are generated,and its temporal contrast ratio is 10⁶.Compared with other amplifiers which use the Kerr-lens mode locked Ti:Sapphire oscillator as the seed source,the contrast ratio of the amplified pulses is improved about one order of magnitude.4.In order to further improve the temporal contrast ratio and pulse energy of the laser pulse,the effect of cross-polarization wave generation(XPW)is calculated theoretically.And the XPW conversion efficiency of two kinds of Ba F₂ crystal cutting methods((001)and(011))are compared.The calculation results indicate that both the intensity of the incident laser and the crystal thickness will change the ?angle corresponding to the maximum efficiency.The conversion efficiency of the(011)crystal is superior when the thinner crystal or the lower intense laser is employed in the experiment,and the conversion efficiency of(001)crystal is higher than that of(011)crystal when the thicker crystal or the higher intense laser is employed in the experiment.