【作者】 袁鹏；

【导师】 钱列加；

【作者基本信息】 复旦大学 ， 光学， 2008， 博士

【摘要】 近年来,随着宽带固体激光材料的广泛应用和克尔棱镜锁模技术的成熟,超快光学技术得到了长足的发展。二阶光学非线性做为最基本(低阶)的非线性过程,用于发展激光技术已有若干成功的范例,如较早的倍频技术和光参量放大(OPA)技术,和近期的啁啾脉冲光参量放大(OPCPA)技术等。针对大型钕玻璃激光装置的需求牵引,本文着重研究了1μm波段超快激光的新型二阶非线性技术,涉及1μm波段超快激光的产生,谐波转换以及脉冲压缩。通过建立模型理论计算以及与实验研究相结合的方式,细致研究了1μm波段的飞秒光参量放大系统的设计优化;基于部分氘化KDP晶体的宽带三倍频技术:以及光栅与倍频晶体相结合的脉冲压缩方式。这些创新的非线性超快激光技术,将可能在很大程度上解决现有技术的主要缺陷,并拓展新的功能。本文主要进行了如下工作:一、研究了1μm波段飞秒光的产生方式,包括散射增强的光参量产生(OPG)技术以及泵浦脉冲预啁啾优化的光参量放大(OPA)技术。基于技术成熟的商品化的掺钛蓝宝石飞秒激光器,使用光参量技术来产生可调谐的1μm波段飞秒激光输出。经过研究发现,通过散射源激光的引入,特别是在晶体表面的散射,可以有效的降低OPG过程的泵浦光强阈值。通过实验使用铌酸锂晶体在泵浦光强远低于晶体损伤阈值的情况下我们观测到了散射增强OPG。时间同步脉冲激光以及连续激光都可以作为散射源,对于其波长,光谱形状,非共线角等都没有很严格的要求。这些结果使得有些以前不能用于飞秒OPG的晶体得到应用,尤其是在中红外波段。此外,散射增强OPG可以用于更快的寻找晶体的位相匹配状态。与可见光波段的OPA不同的是,在中红外OPA系统中,群速度色散(GVD)起着重要的作用。理论和实验结果都表明GVD的存在将显著降低飞秒OPA的转化效率。设计并且实现了一种色散预补偿的OPA,通过对泵浦光加入预啁啾可以很好补偿晶体GVD的作用。实验上证明了与傅氏变换极限的泵浦脉冲相比,色散预补偿的泵浦脉冲能得到接近2倍的转换效率,并且输出脉冲可以用棱镜对压缩至近傅氏变换极限。证明了泵浦脉冲的预啁啾为优化中红外飞秒OPA系统提供了一个新的自由度。二、研究了1μm波段超快光的宽带谐波转换,基于部分氘化KDP晶体的特性给出了新型宽带三倍频的设计。宽带三倍频在惯性约束核聚变(ICF)大型钕玻璃激光装置中有着重要的应用。主要针对宽带三倍频中的和频过程进行研究,设计一种新型分层部分氘化KDP晶体作为和频器。通过数值模拟计算,说明该结构能支持很大带宽的有效三倍频。回流作用会极大的影响了三倍频的效率,通过适当设计每层晶体长度和氘化程度(即位相匹配波长)可以有效的抑制回流产生的影响。最后提出了一种五层晶体的设计,可以满足对于4nm带宽的钕玻璃拍瓦激光脉冲的有效宽带三倍频的需求。这种新型晶体虽然还未生长成功,但是现有的晶体生长技术为新型晶体的成功生长提供了保证。三、提出了一种基于光栅和倍频晶体相结合的新型啁啾脉冲压缩器,比传统的光栅压缩器具备更高的破坏阈值,并且通过理论计算证明了其可行性。基于光栅低破坏阈值对于大型激光系统光强的限制,提出了一种使用晶体和光栅相结合的结构替代传统光栅进行脉冲压缩的方法。这种结构使用倍频晶体代替最后一级光栅完成脉冲压缩,消除了光栅的低破坏阈值对系统光强的限制。经过详细的理论计算,证实了这种方法的可行性,通过适当调整晶体以及光栅结构,可以实现压缩后无啁啾无角色散的倍频脉冲输出。并且针对1054nm的飞秒脉冲给出了一个适当的实例来说明结构的设计流程。更多还原

【Abstract】 Over the last decade there have been spectacular developments in ultrafast laser technology,due to the introduction of broadband solid-state laser materials and of Kerr lens mode-locking techniques.Several x^（2） nonlinear process plays an important role in ultrafast optics,such as second harmonic generation（SHG）,optical parametric amplifier（OPA） and optical parametric chirp pulse amplifier（OPCPA）.Aiming at the needs in high power Nd-glass laser systems,we focus on the ultra-fast technologies based on x^（2） nonlinear process at 1μm,including the ultrafast laser generation, harmonic transformation and compression.By means of theoretical analysis, numerical simulation and experiments,We detailed studied（1） the optimization of femtosecond OPA system,（2） broadband SHG and THG based on partially deuterated KDP crystals,and（3） new pulse compress mechanism based on grating and SHG crystal.All of these results mentioned above may improve corresponding ultra-fast technologies.This paper includes the following contents:1.Investigate the generation of femtosecond pulse at 1μm,including scattering-enhanced femtosecond optical parametric generation（OPG） and dispersion pre-compensation femtosecond optical parametric amplifier（OPA）.We generated tunable femtosecond pulses at 1μm from commercial Ti:sapphire Kerr-lens mode lock laser system by parametric process.The scattering of an additional laser light,especially at the front surface of nonlinear crystal,may decrease the pump threshold drastically for a femtosecond OPG.Enhanced OPGs have been observed in LiNbO₃ at pump intensity well below the damage threshold of the crystal. Either a synchronized pulsed laser or a cw laser can be used as the scattering source, of which the wavelength,spectral shape and irradiating angle are not strictly required. These results may encourage the resuscitation of some kinds of nonlinear crystal for femtosecond OPG applications,especially in mid-infrared range.Additionally,the scattering-enhanced OPG is applicable to rapid searching of phase-matching conditions.Femtosecond OPA in MIR suffers severe effects from GVD,which is very different from the usual OPAs in the visible and near infrared where GVD effect is negligible.Both theoretical and experimental results have shown that GVDs in nonlinear crystals will significantly degrade its conversion efficiency of a femtosecond OPA.We have proposed and demonstrated a dispersion pre-compensated OPA in which the effect of GVD at the pump in crystals can be offsetted to some extent by introducing an optimal pre-chirp to the pump pulse.A lithium niobate crystal based femtosecond OPA has shown that the conversion efficiency by pumping with an optimally pre-chirped pulse is nearly twice as much as that of a FTL pulse pumping.Output pulses can be compressed to near its Fourier-transform limit by prism pairs.We have demonstrated that pre-chirp of the pump pulse is a new degree of freedom for optimizing femtosecond OPA in MIR.2.Investigate the harmonic generation about 1-μm ultra-fast pulse,including femtosecond SHG in a partially deuterated KDP around its phase match retracing point and a novel broadband THG scheme based on segmented partially deuterated KDP crystals.Broadband THG is one of important technologies in Inertial Confinement Fusion （ICF） laser system.We have proposed and numerically studied a novel THG scheme based on segmented partially deuterated KDP crystals that may support efficient conversion of a broadband laser.Back conversion is detrimental to the tripling efficiency,which can be effectively arrested by properly designing the phase match wavelength of each segment.A five-segment design is presented,which is applicable to the efficient frequency tripling of Nd:glass petawatt laser pulses with a bandwidth of～4-nm.3.Bring forward a novel pulse compressor based on grating and SHG crystal.,which may increase the damage threshold comparing with traditional grating compressor.Detailed theoretical analysis proved its feasibility.The low damage threshold of grating compressor is one of the major limitation about laser intensity in high power laser system.As the last grating in traditional grating compressor suffered the highest laser intensity,we displace it with a SHG crystal to increase damage threshold.Detailed theoretical analysis proved that by suitably choosing the grating and SHG crystal,no-chirp and no-angular dispersion SHG pulse can be obtained.An example of compressing femtosecond pulse center at 1054nm was carry out theoretically to introducing how to design the system.更多还原