基于二倍频预放大的超快光学参量放大研究

Ultrafast Optical Parametric Amplification Based on Preamplification Pumped by Second Harmonic Generation

利用钛蓝宝石飞秒激光泵浦的大能量超快光学参量放大技术在强场光学和阿秒科学领域研究中具有重要应用。现有技术利用基频激光(波长为800 nm)对其产生的超连续谱种子信号进行预放大,这对超连续谱中的红外成分转化效率提出较高要求,影响了系统的可靠性。本文在实验中利用转化效率较高的超连续谱绿光成分作为种子光,在预放大过程中使用二倍频激光泵浦得到红外闲频光,再进行第二级功率放大,得到1200~2500 nm范围的宽谱可调的近红外飞秒激光脉冲输出,脉冲能量转换效率达到25%。

Objective Ultrafast optical parametric amplifiers(OPA)providing femtosecond laser pulses with a tunable wavelength have broad applications in ultrafast sciences,high-field physics,generation of table-top X-ray,or extreme ultraviolet radiations.In a standard optical parametric amplification scheme,a microjoule laser pulse propagates in a solid transparent nonlinear medium to generate supercontinuum seed light and a specific red-shifted infrared spectral range is selected for further stages of amplifications.However,the conversion efficiency of the red-shifted component is considerably lower than that of the blue-shifted component because of the nonlinear optical effects,such as self-steepening and electron-hole plasma generation during supercontinuum generation,requiring a high-input laser intensity close to the damage threshold to obtain red-shifted spectral components with a wavelength of approximately 1600 nm.Thus,the solid nonlinear medium is vulnerable to laser damage and the system reliability is degraded.To solve this problem,we propose taking advantage of the high conversion efficiency of the blue-shifted spectral component in the supercontinuum and preamplifying it with the second harmonic generation of the pump laser pulse for infrared tunable idler wave,which is further amplified in the following power amplification stages.In this scheme,the requirement of the input laser intensity for supercontinuum seed light generation is relaxed,reducing the possibility of solid nonlinear medium damage and improving the system reliability.Methods In this study,the ultrafast OPA-based on second harmonic generation pumped-preamplification has three stages:the white light supercontinuum generation seed,second harmonic generation pumped-preamplification,and fundamental wave pumped power amplification stages.The incident laser pulse(1kHz repetition rate,800nm wavelength,37fs pulse duration,and 295μJ pulse energy)is split using beam splitters,and approximately 10%,20%,and 70%of the pulse energy are coupled into the seed,preamplification,and power amplification stages,respectively.In the seed stage,a5mm thick c-cut sapphire crystal is used for white light supercontinuum generation.In the preamplification stage,a2mm thick,type-I(cutting angle of 28.5°)barium metaborate(BBO)crystal converts the fundamental pump pulse to its second harmonic.Then,the frequency-doubled pump is further chirped through a 2mm thick SF11glass medium to reduce group velocity mismatch between the pump and signal pulses.The400nm pump pulse preamplifies the blue-shifted supercontinuum seed light in a 3mm type-I(cutting angle of 28.5°)BBO crystal for infrared idler beam.In the power amplification stage,the fundamental(800nm)laser pulse amplifies the infrared idler output of the preamplification with a 2mm type-Ⅱ(angle of 27.3°)BBO crystal.Results and Discussions The input pulse power for the supercontinuum seed stage is less than 1mW,yielding a blue-shifted plateau region(500--700nm)in the output spectrum.The white light supercontinuum generated by the sapphire crystal forms a stable plateau region in the range of 500--700 nm(Fig.2).The blue-shifted spectral component has a conversion efficiency of~5%,and the power is less than 50μW.In the preamplification stage,the second harmonic(400nm)pump pulse energy is 7μJ,which is converted from 36μJ of the 800nm fundamental pulse.The wavelength tuning is achieved by controlling the temporal delay between the second harmonic pump light and the blue-shifted supercontinuum seed.Then,different spectral components(520--610nm)of the supercontinuum seed are amplified as the signal beam and the wavelength of the corresponding idler beam ranges from 1800to 1200nm.The gain in the preamplification stage is approximately 2000times(Fig.3)and homogeneous for different phasematching conditions.Using the idler output of the preamplification stage as the seed light for the power amplification stage,the signal pulse in the spectral range of 1160--1800nm and the idler pulse in the range of 1440--2500nm are obtained(Fig.5(a)--(d)).The beam quality factors M2 of the final output signal and idler pulses are 1.91(Xdirection),1.71(Y-direction),1.92(X-direction),and 1.69(Y-direction),respectively(Fig.5(e)--(f)).For the optimized wavelengths of the signal pulse(1450nm)and idler pulse(1785nm),corresponding to the crystal cutting angle,the maximum conversion efficiency is 26.6%at 205μJ pump energy(Fig.6)and the output power stability(RMS)is approximately 1.8%.We have measured the pulse durations of the output signal(53.2±1.3)fs and idler(58.7±1.5)fs pulses from the power amplification stage(Fig.7)using a home-built scanning autocorrelator,retaining the ultrashort pulse width of the pump pulse.Conclusions We have experimentally demonstrated an ultrafast optical parametric amplification system based on a second harmonic pumped-preamplification stage,realizing the wavelength tunability of the infrared signal pulse in the range of 1160--1800nm and the idler pulse in the range of 1440--2500nm.The energy conversion efficiency is approximately 26%,and the pulse width is 50--60fs.The benefits of this scheme are three folds.First,it reduces the damage of the supercontinuum generation solid medium,improving the reliability of the entire system.Second,it leads to a uniform spectrum of the infrared idler output in the preamplification stage,thus improving the system wavelength tunability in the power amplification stage.Finally,this scheme can provide carrier-envelope phase stabilized-infrared idler output over a broadened spectral range from the power amplification stage.