New Method For Phase Velocity Compensation Of Frequency-doubling Of Femtosecond Laser Pulses

In recent years,the ultra fast laser pulse developed constantly,and it has revealed expansive application in various areas.Presently,most of the wavelength range of ultra short lasers focuses on infrared wave band.However,ultraviolet ultra short pulse is valuable in many areas such as high resolution imaging,photochemistry, environment monitor and so on.Consequently,it is important to expand wavelength to ultraviolet.Frequency-doubling is one of the effective approaches to gain ultra short pulse with short wavelength.For intense ultra short laser pulse,there is many factors which could affect the increase of the frequency-doubling conversion efficiency, such as spatial walk-off,phase velocity mismatching(PVM),group velocity dispersive,self phase modulation,intercross phase modulation and so on.The phase-velocity mismatch leads to a temporal walk-off of fundamental and generated pulses.To avoid the temporal walk-off for laser pulses,one needs to use thinner length linear crystals.The reduction of the interaction length necessitates an appropriate increase in the fundamental pulse intensity.Consequently,this leads to an increase of high-order nonlinear optical effects.In this paper,we use multi-crystals to increase the conversion efficiency.There is no need to change the scheme using multi-crystals,just put more crystals after the original crystal.So it is simple and effective.In this thesis,high efficiency frequency-doubling of femtosecond laser pulses with multi-crystals is studied.It includes:1.The schemes of second harmonic generation of ultra short laser by using the methods of the angular spectral dispersion,the chirp-matching,the quasi-phase-matching,the spectral noncritical phase-matching and the multi-crystals have been analyzed and compared.2.The spatial walk-off and temporal walk-off has been analyzed theoretically,and the new method to compensate phase-velocity mismatching ha been proposed.3.The scheme of high efficiency frequency-doubling of femtosecond laser pulses with multi-crystals is studied detailedly.Frequency-doubling has been investigated using two BBO crystals with 70-fs,800-nm laser pulses at the repetition of 10Hz.The size of crystals are both 10mm×10mm×0.2mm.The angle between the axes of the two crystals and the phase velocity mismatching of the fundamental pulse have been adjusted respectively.The frequency-doubling pulse of which bandwidth is 6.7nm and energy is more than 2.8mJ has been achieved.The frequency-doubling conversion efficiency reaches 35.7%,which is improved by a factor of 1.7 related to a single crystal in the same condition.