基于双光成丝过程的光参量啁啾脉冲放大器前端

Front-End of Optical Parametric Chirped-Pulse Amplifier Based on Dual Laser Filamentation

报道了一种包含两个激光成丝过程的工作在2μm波段的光参量啁啾脉冲放大(OPCPA)系统前端,该前端工作在简并模式下,2μm种子激光脉冲通过近红外和可见光波段的超连续光谱之间差频产生。可见光和近红外波段的超连续光谱分别通过泵浦脉冲激光在两个YAG晶体中通过激光成丝过程产生。经过光参量放大及合理的色散补偿后,2μm种子激光脉冲的波长可从1830 nm覆盖至2320 nm,脉冲宽度为29 fs,单脉冲能量为16.7μJ,平均功率为167 m W,功率波动小于3%。

Objective Gain narrowing and thermal effects that usually exist in a chirped-pulse amplifier can be avoided by anoptical parametric chirped-pulse amplifier(OPCPA),which relies on the optical parametric process existing in anonlinear crystal to realize pulse amplification.The 2μm OPCPA system is an ideal pumping source for waterwindow desktop X-ray light source,which has potential applications in medicine,biology,and material science.Thefront-end,which is the core of an OPCPA system,is unutilized to generate broadband and a few optical cycles seedlaser pulses.In this study,we propose a new scheme for 2μm OPCPA front-end based on dual laser filamentationprocesses.The proposed scheme can be used to realize 2μm laser seed pulses with broad spectrum,short pulseduration,and high-energy stability.Methods In the 2μm OPCPA front-end,Yb-doped laser amplifier with 500 fs pulse duration,100μJ pulse energy,and 10 k Hz pulse repetition rate is used as a pumping source.The output laser pulses are separated into two parts,which are respectively used to realize the visible and near-infrared(IR)supercontinuum generation(SCG)in YAG crystals through the dual laser filamentation processes and pump the OPA stages for amplifying 2μm seed laser pulse.Before the YAG crystal,a half-wave plate and a polarization beam splitter(PBS)are used to adjust the pulse energy incident into the YAG crystal.Then,another half-wave plate after the PBS is employed to adjust the polarization state of the incident pump laser.A CaF_(2) lens is used to adjust the beam spot size of the incident pump light on the YAG crystal.After SCG,the visible light and near-IR light are focused onto a Bi BO nonlinear crystal to generate the 2μm laser seed pulse using the difference frequency generation(DFG)process.The Bi BO nonlinear crystal is cut with a phase-matching angle of 10.7°for typeⅠphase matching in the DFG process.Results and Discussions The generated visible supercontinuum spectrum ranges from 500 to 850 nm,and the near-infrared(IR)supercontinuum spectrum ranges from 1050 to 1600 nm(Fig.2).The 2μm laser seed pulse power is further scaled to about 167 m Wb y two optical parametric amplification stages(Fig.4)based on the Mg Odoped Li NbO_(3) crystal.It possesses a wide gain bandwidth from 1800 to 2500 nm in the nonlinear process of optical parametric amplification(Fig.3).During amplification,the peak power density of the pump laser is kept near 70 GW/cm^(2) to prevent optical damage to the nonlinear crystal.The dispersion of the amplified 2μm laser pulse is managed with an acousto-optic programmable dispersive filter.The pulse energy reaches 16.7μJ after amplification.The amplified average power is monitored within 30 min,indicating that the power fluctuation is less than 6 m W,corresponding to<3%instability(Fig.5).The beam cross-section of the amplified 2μm laser pulse is measured using a CCD camera,indicating the near Gaussian distribution of the pulse energy in horizontal and vertical directions[Fig.6(a)].The near diffraction-limited beam quality M2 factor of 1.3 is achieved[Fig.6(b)].The amplified laser spectrum ranges from 1830 to 2320 nm with a spectral bandwidth of 243 nm[Fig.7(a)].Meanwhile,the pulse duration is shortened to 29 fs by proper dispersion management,approaching the Fourier transform limited value[Fig.7(b)].The calculated autocorrelation trace by taking the group dispersion delay(GDD)of 250 fs2 and thirdorder dispersion(TOD)of 156 fs2 is consistent with the measured results.This shows the existence of residual GDD and TOD in the amplified laser pulse.Conclusions In this paper,we propose a new scheme front-end of the 2μm OPCPA system.The visible and nearIR supercontinuum spectra are realized through dual laser filamentation processes based on different YAG crystals.The 2μm laser seed pulse is produced by the DFG process between the two supercontinuum spectra’s difference and amplified by two optical parametric amplification stages.The amplified spectrum of seed laser ranges from 1830 to 2320 nm with a bandwidth of 243 nm.The amplified pulse laser has average power,single pulse energy,pulse duration,and pulse repetition rate of 167 m W,16.7μJ,29 fs,and 10 k Hz,respectively.The beam quality M2 factor is less than 1.3.It is expected that the pulse energy of 2μm laser seed pulse can be further amplified to the m Jlevel-and peak power to GW-level for further driving the noble gas to generate water-window X-ray.