超短激光脉冲二倍频理论分析

采用快速傅里叶变换（FFT）算法和Rung-Kutta法对KDP晶体中超短激光脉冲二倍频过程作了研究。详细讨论了群速度失配、群速度色散和三阶非线性相位调制效应对倍频光脉冲波形、光谱及转换效率的影响。研究结果表明：在超短激光脉冲二倍频中，三阶非线性相位调制效应可使二倍频光脉冲形状发生畸变，光谱明显展宽且转换效率降低；当入射基频光功率密度大于100GW/cm2时，三阶非线性效应是影响倍频过程的主要因素。     

啁啾脉冲堆积的宽带整形脉冲三倍频方案分析

针对啁啾脉冲堆积方法获得的宽带整形激光脉冲,采用KDP晶体Ⅰ／Ⅱ类角度失谐的三倍频方案,定量分析了影响其三倍频转换效率的主要因素,并对三倍频前后的时间波形及频谱分布进行了比较分析。在此基础上,与时间相位调制的宽带整形脉冲三倍频的相应结果进行了比较。研究结果表明,对于啁啾脉冲堆积方法获得的宽带整形激光脉冲,在实现较高的三倍频转换效率的同时,其时间波形和频谱分布基本保持不变。对于具有相同带宽和波形的时间位相调制整形脉冲,其三倍频转换效率明显低于啁啾脉冲堆积宽带整形脉冲。     

啁啾脉冲堆积宽带激光的三次谐波产生

 针对啁啾脉冲堆积方法获得的宽带激光,分析了其形成机制,并给出了其时间波形和光谱分布。采用KDP晶体Ⅰ/Ⅱ类角度失谐的三倍频方案以及Ⅰ/Ⅱ/Ⅱ晶体级联角度失谐方案,定量分析了啁啾脉冲堆积方式宽带激光三倍频转换效率随入射光强、带宽、以及晶体厚度等因素的变化,并与时间相位调制的宽带激光三倍频的相应结果进行了比较。研究结果表明,采用晶体级联方式可以大幅度提高宽带三倍频转换效率,并且啁啾脉冲堆积宽带激光的三倍频转换效率的提高比时间相位调制宽带激光更为明显。     

啁啾脉冲堆积宽带激光的三次谐波产生

针对啁啾脉冲堆积方法获得的宽带激光,分析了其形成机制,并给出了其时间波形和光谱分布。采用KDP晶体Ⅰ/Ⅱ类角度失谐的三倍频方案以及Ⅰ/Ⅱ/Ⅱ晶体级联角度失谐方案,定量分析了啁啾脉冲堆积方式宽带激光三倍频转换效率随入射光强、带宽、以及晶体厚度等因素的变化,并与时间相位调制的宽带激光三倍频的相应结果进行了比较。研究结果表明,采用晶体级联方式可以大幅度提高宽带三倍频转换效率,并且啁啾脉冲堆积宽带激光的三倍频转换效率的提高比时间相位调制宽带激光更为明显。     

啁啾脉冲激光放大中的自相位调制效应

利用光与物质相互作用的半经典理论建立了啁啾脉冲激光放大的输运方程,数值分析了多程放大过程中自相位调制对啁啾脉冲时域特性和频域特性的影响研究结果表明,由于自相位调制引起附加频率啁啾产生,啁啾脉冲瞬时频率发生变化,放大特性受到影响.通过对增益饱和状态下自相位调制效应的研究,提出利用入射脉冲的线性频率啁啾抑制自相位调制的影响. 关键词：     

高速光纤通信系统中抑制偏振模色散的新机制

重点研究了偏振模色散、群速度色散、自相位调制三者之间在高速光纤通信系统中的相互作用，从时域角度分析脉冲的演变，从频域角度分析频谱的变化，提出一定条件下，啁啾、色散、自相位调制可以部分补偿偏振模色散的思想。通过对40Gbit／s系统进行偏振模色散、群速度色散和自相位调制共同作用的仿真，从统计意义上验证了它们之间的相互影响，并找到最佳传输方案，对系统设计提供了参考。     

亚10飞秒脉冲的诊断与压缩

采用二次谐波-频谱分辨光闸法对克尔透镜锁模钛宝石激光器输出的飞秒脉冲的振幅和相位 进行诊断，测得其脉冲包络是双曲正割型的，而相位则近似为时间的三次幂形式. 通过对脉 冲频域相位的拟合，诊断出二阶群速度色散是造成脉冲啁啾、导致脉冲展宽的主要原因. 利 用棱镜对补偿二阶群速度色散，进一步将17 fs啁啾脉冲压缩至半高全宽为8.5 fs的近似转 换极限脉冲. 关键词： 飞秒脉冲 二次谐波-频谱分辨光闸法 群速度色散     

混沌信号在光纤传输过程中的非线性演化

对光纤混沌传输理论及混沌信号与光纤传输媒介相互作用的物理机制进行了理论研究. 通过耦合激光混沌系统和光纤传输信道，提出光纤混沌信号传输的非线性演化物理模型. 着重分析光纤自相位调制对激光混沌信号传输与演化的作用. 结果表明：自相位调制不影响混沌信号脉冲的形状，但能产生非线性相移使混沌信号频谱展宽；自相位调制不影响混沌信号脉冲的功率分布和场强分布，但能影响混沌信号脉冲的功率频谱分布，影响混沌信号光场以及慢变场的变化. 提出混沌信号在光纤传输中的非线性演化频率啁啾和形式，数值模拟了混沌信号在光纤传输过程中的相位、频谱、场以及场的慢变部分的相空间等演化形式和特点. 关键词： 混沌 光纤 传输 演化     

色散缓变光纤啁啾演变特性

从理论上分析了色散缓变单模光纤中啁啾演变特性。采用储里叶变换法得到了只有群速度色散效应或自相位调制效应时皮秒脉立足点啁啾的解析解，利用数值法模拟了色散缓变光纤中皮秒脉立足点秋演变过程。结果表明：色散缓变光纤中的啁啾比常规光纤中的啁啾小，而且，合理选择光纤色散参数可实现脉冲波形无畸变传输。     

折射率线性调制晶体中超快倍频的数值分析

对折射率线性调制晶体中超短脉冲倍频特性进行了数值计算,在泵浦衰减的情况下,研究了转换效率和脉冲宽度随匹配带宽和晶体长度的变化.大信号时各频率成份在晶体不同位置的不平衡转换致使脉冲展宽,且由于折射率的调制,脉冲宽度和转换效率出现了周期性变化.同时,分析了谐波脉冲啁啾随晶体长度和带宽的变化,发现随着晶体长度的增大,归一化最佳啁啾系数先增大后减小,在长晶体时趋于1,其极大值位置决定于本征带宽和调制带宽.讨论了相位失配量对转换效率和脉宽的影响,结果表明失配因子y决定了中心频率的匹配位置,当y=0时,中心频率在晶体中心匹配,这有利于提高转换效率并减小脉冲宽度.     

基于频域相位共轭技术的交叉相位调制所致失真的复原

理论分析和讨论了基于频域相位共轭技术的交叉相位调制所致信号失真的复原和补偿机理，数值模拟了在交叉相位调制作用下，高斯脉冲在中距相位共轭光纤系统中的传输演化过程.结果表明，频域相位共轭技术能够抑制交叉相位调制对光纤系统中传输信号的损害，复原其所导致的信号失真，并能够同步补偿群速度色散和自相位调制非线性效应所导致的信号失真.合适的初始脉冲时延和初始脉冲啁啾有利于频域相位共轭技术对交叉相位调制所致信号失真的抑制. 关键词： 频域相位共轭 交叉相位调制 色散 自相位调制     

Effect of pulse slippage on density transition-based resonant third-harmonic generation of short-pulse laser in plasma

The resonant third-harmonic generation of a self-focusing laser in plasma with a density transition was investigated. Because of self-focusing of the fundamental laser pulse, a transverse intensity gradient was created, which generated a plasma wave at the fundamental wave frequency. Phase matching was satisfied by using a Wiggler magnetic field, which provided additional angular momentum to the third-harmonic photon to make the process resonant. An enhancement was observed in the resonant third-harmonic generation of an intense short-pulse laser in plasma embedded with a magnetic Wiggler with a density transition. A plasma density ramp played an important role in the self-focusing, enhancing the third-harmonic generation in plasma. We also examined the effect of the Wiggler magnetic field on the pulse slippage of the third-harmonic pulse in plasma. The pulse slippage was due to the group-velocity mismatch between the fundamental and third-harmonic pulses.     

Frequency doubling of phase-modulated femtosecond laser pulses in periodically poled and chirped nonlinear crystals

The second harmonic generation (SHG) at a wavelength of 0.8 μm by 50-and 10-fs pulses with and without phase modulation (PM) was systematically studied in LiNbO₃ crystals with regular domain structure and linearly varied domain thickness. The main results were obtained by numerical method, taking into account the difference between group velocities of interacting pulses and the group-velocity dispersion. In the approximation of the given field of the fundamental radiation, an analytical expression was derived for the spectral density of the second harmonic in the periodically poled nonlinear crystal (PPNC) under nonstationary excitation conditions. It was numerically found that the conversion efficiency of about 90% can be achieved by doubling the frequency of 50-fs laser pulses without PM in the LiNbO₃ PPNC. The maximum conversion efficiency for the SHG by PM pulses is achieved at a certain optimum chirp step in the crystal domain length, which depends on both the value and sign of frequency modulation.     

High-power third-harmonic flat pulse laser generation

The generation of a high-power laser pulse at 266 nm that is longitudinally shaped according to a prefixed intensity profile is reported. The main features of the pulse shape modifications due to second- and third-harmonic conversions are measured, and the results are in good agreement with the theory. The UV temporal shape depends on the chirp of the fundamental pulse and on the crystal phase-matching angle. Exploiting the large stretching imposed on the third-harmonic signal, we show that the pulse intensity profile can be obtained by spectral single-shot measurements.     

Effect of Air Gap on Dual-Tripler Broadband Third-Harmonic Generation

We experimentally study the effect of the air gap on conversion efficiency and the spectrum of generated third-harmonic pulses in the dual-tripler broadband third-harmonic generation scheme. The experimental results are in good agreement with predictions that the 4-cm air gap is equivalent to a full cycle of phase mismatch among the three interacting pulses （i.e. the fundamental, second-harmonic and third-harmonic pulse）. The experimental results also show that the spectrum of the third-harmonic pulse is sensitive to the air gap. We also point out that the air gap effect can be ignored when the dual-tripler system is located in 1000 Pa atmosphere. These results will guide the design of the broadband third-harmonic generation system in high power lasers.     

Wave-coupling theory of linear electro-optic effect for ultrashort laser pulses

A wave-coupling theory of linear electro-optic (EO) effect for ultrashort laser pulses has been developed. As one of its applications, the linear EO effect of ultrashort Gaussian pulses propagating in LiNbO₃ crystal is demonstrated, with the intensity lower than the damage threshold. It is found that the EO effect is sensitive to both the azimuth angle and the polar angle of wave vector of light. And the larger the applied electric field, the shorter the crystal used for optimization of EO coupling is. The numerical results show that, with the duration of input pulse changing from 150 fs to 5 fs, the durations of output pulses are broadened due to the group-velocity dispersion, as well as the first- and second-order refractive dispersion. In addition, when input pulse duration is larger than 20 fs, in contrast with zero chirp, the output durations can be slightly compressed if a positive chirp input pulse is used. The validities of slowly varying amplitude approximation and continuous wave approximation are also discussed. The influence of self-phase modulation and cross-phase modulation on the EO effect is also discussed briefly.     

色散平坦渐减光纤中超连续谱的产生

对双正交偏振光脉冲在色散平坦渐减光纤中传输时超连续谱的产生进行了计算和分析。结果表明，由于交叉相位调制效应的作用，双光脉冲可以产生比单光脉冲明显展宽且更为平坦的超连续谱。对于双基孤子脉冲，可以得到-20dB谱宽达388nm的平坦宽带超连续谱，比单基孤子脉冲产生的超连续谱谱宽增加72nm，交叉相位调制效应对超连续谱的产生起到增强的效果。当输入脉冲的抽运功率较低时，交叉相位调制效应对超连续谱的产生的增强效果更为显著，它极大地提高了超连续谱的产生的效率。数值计算的结果还表明，与其他高阶非线性效应相比，拉曼自频率移效应对超连续谱产生的影响更为明显。     

飞秒激光作用下全向高反膜破坏的激发过程

设计和制备了全向高反膜SiO₂/TiO₂，研究了它在不同脉冲宽度、不同脉冲能量的飞秒激光作用下的破坏阈值和烧蚀深度.利用发展的抽运-探针方法，研究了抽运脉冲作用下材料中导带电子的超快激发和能量沉积过程，建立并求解了飞秒激光激发材料和材料的激发对抽运光自身反作用的耦合动力学模型.模型较好地揭示了材料破坏的激发过程. 关键词： 飞秒激光 全向高反膜 激发过程 破坏机制     

利用光纤-光栅对1.06μm皮秒光脉冲压缩

本文报道了利用光脉冲在10m单模光纤中传播所产生的自相位调制(SPM)、互相位调制(XPM)、群速色散(GVD)和受激喇曼散射(SRS)的共同作用,实现光脉冲光谱和时间加宽,经光栅对线性啁啾补偿后,将脉宽15ps的1.06μm光脉冲压缩至小于6ps,能量大于100nJ,压缩比>2.5。