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。     

基频倍频光群速度对超短脉冲三倍频的影响分析

在超短脉冲的谐波转换过程中, 由于不同波包之间的群速度失配的存在, 非线性晶体的有效厚度、转换效率及带宽等将相互制约, 不能获得理想结果。基于耦合波方程, 在平面波和小信号近似条件下推导出了关于基频、倍频光群速度关系的解析表达式。通过对200 fs超短脉冲在II类KDP晶体中的混频过程的模拟分析, 发现在满足相位匹配的前提下, 三次谐波转换效率随着基频、倍频光群速度倒数的差值的平方呈现出指数衰减的关系, 与理论结果吻合。得出的关于基频光、倍频光群速度的表达式, 作为三倍频群速度匹配关系式的必要补充, 对寻找合适的色散晶体和选择有效的匹配措施, 实现宽带三次谐波转换具有理论指导意义。     

单模光纤中三阶色散对超短光脉冲传输的影响

基于超短光脉冲在单模光纤中传输时高阶非线性效应的影响。应用非线性薛定谔方程(HONLS)理论,考虑光纤色散三阶效应,推导出无啁啾的高斯脉冲沿光纤传输时脉冲变化的表达式,并对理论结果进行了数值模拟与分析。结果表明,三阶色散会引起光脉冲形状发生畸变,会在其前沿或后沿附近形成非对称的振荡结构。     

拉曼效应对负折射介质中超短脉冲传输的影响

通过数值法对包含拉曼延迟响应的(3+1)维非线性薛定谔方程进行求解,研究了超短脉冲激光在负折射介质中传输时拉曼效应对自聚焦传输特性的影响,着重分析其不同与常规介质的反常传输现象.结果表明:由于负的折射率影响,拉曼效应将导致超短脉冲在自聚焦过程中频谱发生蓝移现象,这与常规介质对应情形相反;而它对负折射介质中超短脉冲的自聚焦特性的影响与常规介质相同,即拉曼效应将诱导自聚焦效应首先发生在脉冲的前沿.本文研究工作对将来利用负折射介质来操控超短光脉冲串产生、自聚焦等许多实际应用领域研究具有重要的指导意义.     

强双折射光纤中超短光脉冲压缩效应研究

研究了强双折射光纤中沿偏振主轴入射的超短光脉冲压缩效应。当考虑三阶色散效应时，三阶色散与光纤非线相互作用能增强一偏振光脉冲的压缩而抑制另一偏振光脉冲的压缩。正三阶和的强慢孤子压缩，负三阶色散增强快孤子压缩，三阶色散参量越大，脉冲压缩效果越明显。     

超短脉冲三倍频中特殊群速度组合的分析

基于非线性耦合波理论,对比分析了超短脉冲三倍频过程中不同的特殊群速度组合模式下的谐波转换特性(转换效率和带宽等).利用分步傅里叶变换和四阶Runge-Kutta积分算法,对脉宽150 fs左右的超短脉冲在KDP晶体中的I类混频过程进行数值模拟,结果表明:三波包"不等速匹配"模式下的综合谐波转换性能(转换效率达到50%和转换带宽达到2 nm)整体要优于其他特殊群速度匹配模式;"三波包不等速"模式更有利于三波包间能量之间的耦合及转换.这一结论为进一步提高超短脉冲三倍频的转换效率及带宽具有理论指导意义.     

双轴晶体主平面上倍频的相位匹配参量

根据折射率椭球方程及双光轴晶体中光波的传播与偏振特性，分析双轴晶体在主平面内激光倍频相位匹配的特性与方法，导出光波在主平面上传播时倍频的相位失配关系；给出双轴晶体中容许相位匹配倍频的相位匹配角及混频的有效非线性系数deff的表达式。利用可相位匹配的类型、相位匹配角公式和有效非线性系数deff表达式的表，容易对任意一具体晶体在一给定波长求出实际能实现相位匹配的类型或偏振组合，算出相位匹配角，比较不同的相位匹配类型或偏振组合的有效非线性系数，选择最佳的相位匹配类型与方向。从相位失配关系可以计算晶体主平面内倍频的接收角、接收光谱宽度等特性参量。     

五阶非线性对啁啾超短脉冲间相互作用的影响

基于包含五次复系数的高阶Ginzburg Landau方程为模型,采用分步傅里叶方法数值研究了啁啾类超短脉冲间的相互作用。结果表明:相邻孤子之间的相互作用对五阶非线性效应非常敏感,即使参数改变很小的值,也会改变其传输特性。适当地选择五阶非线性参数值,能够很好地抑制孤子间的相互作用,提高光纤传输的比特率。当相邻孤子的初始间距为6.8,五阶非线性参数值取-0.001时,可以实现2个孤子长距离的保型传输。最后讨论了五阶非线性作用下多孤子之间的相互作用及抑制。     

高阶效应对超短艾里-高斯脉冲相互作用的影响

本文运用分步傅里叶变换,对满足高阶耦合非线性薛定谔方程的超短艾里脉冲与超短高斯脉冲,利用MATLAB数值模拟了在高阶效应下两脉冲相互作用后的演化过程以及时域上的强度变化。结果表明：负三阶色散效应使超短脉冲相互作用能传输更远距离;正三阶色散效应会减慢超短脉冲相互作用的传输。自陡峭效应通过孤子分裂现象的形式使超短脉冲相互作用产生时域位移,内拉曼效应可以将超短脉冲相互作用的能量由前沿处转移到后沿处。     

高阶效应对超短艾里-高斯脉冲相互作用的影响

本文运用分步傅里叶变换,对满足高阶耦合非线性薛定谔方程的超短艾里脉冲与超短高斯脉冲,利用MATLAB数值模拟了在高阶效应下两脉冲相互作用后的演化过程以及时域上的强度变化。结果表明：负三阶色散效应使超短脉冲相互作用能传输更远距离;正三阶色散效应会减慢超短脉冲相互作用的传输。自陡峭效应通过孤子分裂现象的形式使超短脉冲相互作用产生时域位移,内拉曼效应可以将超短脉冲相互作用的能量由前沿处转移到后沿处。     

Applications of non-collinear second harmonic generation in one-dimensional periodically poled crystals

Whereas the commonly used collinear quasi-phase-matched interaction is usually suitable for phase matching only a single nonlinear process, non-collinear interactions offer rich variety of phase matching possibilities, employing different elements of the nonlinear tensor, and various orders of quasi phase matching. By studying the dependence of the second harmonic process on the angle and the phase matching order in a periodically poled LiNbO₃ crystal, we can non-destructively derive the duty cycle of the structure. The obtained duty cycle agrees well with surface scan measurements using an atomic force microscope. Furthermore, by proper selection of the non-collinear angle, we demonstrate simultaneous second harmonic generation of two cross polarized waves.     

Wavelet-transform analysis for carrier-envelope phase extraction of amplified ultrashort optical pulses

We introduced a wavelet-transform technique into the carrier-envelope phase extraction from the spectral interferogram of amplified ultrashort optical pulses. With this technique, carrier-envelope phases were directly extracted from the ridges of the wavelet transforms. This technique did not need filters; therefore, the uncertainty introduced from filter was avoided naturally. The Wavelet transforms technique produced a two-dimensional topography on which the time-frequency information of the spectral interferogram was shown apparently.     

Properties of periodic multicrystal configurations in walk-off-compensating second harmonic generation of ultrashort pulses

This work designs a four-platelet periodic multicrystal configuration in the second harmonic generation of ultra-short pulses as a new walk-off-compensating device. It theoretically investigates a proposed active and a typical passive compensating scheme with the undepleted-pump approximation. The result shows that the angular and spectral band-widths are proportional to the number of crystal pairs as expected,but the temperature tunability is basically unaltered owing to inter-plate pulse interference. At the same time,an analysis reveals that a misuse of the phase mismatch factor is responsible for a historic controversy about pulse interference. A real design of an ultraviolet second harmonic generation (262.5 nm) is considered in a passive periodic β-Barium Borate-calcite configuration,where the inter-plate pulse interference is found to form an azimuthal tuning restriction and to lower plate length tolerance. A subsequent numerical simulation with pump depletion is in good accordance with theoretical prediction.     

A comparative study of laser second harmonic generation in some crystals

Comparison of the conversion efficiency for efficient second-harmonic generation of Nd: YAG laser radiation is reported for KDP, LAP, KTP, BBO and LBO crystals. Conversion efficiencies as high as 50% and 46% were obtained for our laboratory-grown KDP and LAP crystals respectively, for power densities well below their damage thresholds.     

Observation of the second harmonic generation pumped by microscopic to extraterrestrial incoherent light sources

I report on the experimental demonstration of the second harmonic generation in bulk nonlinear crystals excited by light emitting diode, halogen lamp and the Sun. Practical application for measurement of autocorrelation functions of incoherent non-laser driven sources via second order nonlinearity is demonstrated for the first time.     

Determination of third-order nonlinearity in ZnTe by use of femtosecond laser pulses

Third-order nonlinearity induced by femtosecond pulses in ZnTe is investigated experimentally. We employ the transmission z-scan technique, using femtosecond pulses at 810 nm, to determine Kerr-type nonlinearity in ZnTe. The implications of our results to the terahertz (THz) radiation generation are discussed.     

Tunable wavelength conversion of picosecond pulses based on cascaded sum- and difference-frequency generation in quasi-phase-matched LiNbO3 waveguides

Tunable wavelength conversion for picosecond pulses is proposed and demonstrated exploiting cascaded sum- and difference-frequency generation in quasi-phase-matched LiNbO₃ waveguides. The influences of initial pulse widths and injected pulse powers on the conversion efficiency and converted pulse width are theoretically analyzed. Arbitrarily tunable wavelength conversion is performed for the signal pulse with the temporal width of 1.57 ps and repetition rate of 40 GHz. Approximately −18.9 dB conversion efficiency and 25 nm variable region of the input signal are achieved under the lower launched signal power. The results imply that simultaneous wavelength conversion and pulse compression can be potentially obtained by using the pulsed control wave or designing longer waveguides.     

Formation of ultrashort electron pulses on scattering of an electron beam by a standing laser wave of ultrashort duration

The scattering of a well collimated electron beam by a strong standing laser wave of ultrashort duration, giving rise to a great number of scattered photons, is considered. This type of scattering is found to cause an electron beam to divide effectively into two parts. Ultrashort laser pulses are shown to be capable of forming ultrashort electron bunches whose length is governed by the laser beam diameter.     

基于空间啁啾的宽带激光倍频技术

提出一种新型的宽带倍频方案,利用时空耦合效应将宽带的时间啁啾光转换成空间啁啾光,采用多块晶体并联、各晶体独立调谐的技术途径对空间啁啾光进行谐波转换,因此倍频效率与窄带激光倍频相当。理论研究表明,采用KDP晶体I类位相匹配,对中心波长为1 053 nm的宽带基频光实现了带宽约30 nm、转换效率大于60%的高效率宽带二倍频。而且倍频光仍为线性啁啾宽带光,具备可压缩性。