相位失配法实现倍频稳定输出的模拟分析

 对BBO晶体Ⅰ类相位匹配倍频过程和KTP晶体XY平面内的Ⅱ类相位匹配倍频过程进行数值模拟，结果表明：Ⅰ类和Ⅱ类相位匹配倍频过程，都可通过引入适量的相位失配量实现稳定倍频输出；随着相位失配量的增大，实现稳定倍频输出所需的晶体长度都变小，倍频稳定输出起伏变小，倍频转换效率有不同程度的下降；相对于XY平面内的Ⅱ类相位匹配倍频过程，BBO晶体的Ⅰ类相位匹配倍频过程对相位匹配偏离角更敏感，且稳定输出时倍频转换效率更低。     

准相位匹配PPLN倍频理论研究与优化设计

对准相位匹配周期性极化铌酸锂(PPLN)倍频进行了理论研究，并对聚焦高斯光束条件下的倍频耦合波方程进行了求解，给出了准相位匹配倍频转换效率公式，分析了晶体长度与聚焦程度的关系，在此基础上对倍频谐振腔进行了优化设计，以期获得最大的倍频转换效率.本结果对准相位匹配倍频器件的设计具有一定的指导意义. 关键词： 倍频 准相位匹配 周期性极化铌酸锂     

产生蓝色相干辐射和实现Nd:YAG 1.0642μm倍频90°非临界相位匹配的Nb:KTiPO4

本文报道了用熔盐顶部籽晶法生长Nb浓度(0～13)mol%的Nb:KTP晶体的倍频的Ⅱ型相位匹配的截止波长和Nd:YAG 1.0642μm及Nd:YA1O3 1.0795μm激光在这些晶体中倍频的最佳相位匹配角的测量结果.从中可看出,由于Nb5+的引入使KTP晶体倍频的Ⅱ相位匹配的截止波长有效蓝移,目前已使截止波长蓝移至937nm且有效产生468.5nm的倍频蓝光.同时Nb5+的引入使Nd:YAG 1.0642μm激光和Nd:YA1O3倍频的最佳相位匹配方向产生很大的变化,目前已使Nb:KTP晶体倍频的最佳相位匹配方向为θ＝88.32°、Φ＝0°,非常接近于90°非临界相位匹配方向.     

分段小信号近似方法研究准相位匹配倍频效率

从PPLN波导微结构出发分析准相位匹配倍频过程中的相位补偿机理,以极化畴为单元,通过耦合波方程计算极化畴内产生的二次谐波电场,通过电场相干叠加计算倍频效率。分别在小信号近似和基频光高消耗情况下计算准相位匹配倍频效率,结果显示：倍频效率小于5％时,两种情况下的计算结果吻合得很好;随着转换效率增大,小信号近似不再适用,需要在基频光高消耗情形下计算倍频效率。研究了基频光功率密度与准相位匹配倍频效率的关系,定性分析了极化周期存在误差时,误差对倍频效率的影响。     

产生蓝色相干辐射和实现Nd:YAG 1.0642μm倍频90°非临界相位匹配的Nb:KTiPO4

本文报道了用熔盐顶部籽晶法生长Nb浓度(0～13)mol%的Nb:KTP晶体的倍频的Ⅱ型相位匹配的截止波长和Nd:YAG1.0642μm及Nd:YA1O₃1.0795μm激光在这些晶体中倍频的最佳相位匹配角的测量结果.从中可看出,由于Nb⁵⁺的引入使KTP晶体倍频的Ⅱ相位匹配的截止波长有效蓝移,目前已使截止波长蓝移至937nm且有效产生468.5nm的倍频蓝光.同时Nb⁵⁺的引入使Nd:YAG1.0642μm激光和Nd:YA1O₃倍频的最佳相位匹配方向产生很大的变化,目前已使Nb:KTP晶体倍频的最佳相位匹配方向为θ=88.32°、Φ=0°,非常接近于90°非临界相位匹配方向.     

I型准相位匹配周期性极化铌酸锂倍频蓝光输出

在周期为 14 .5 μm的周期性极化铌酸锂中 ,利用d3 1,得到了一阶I型EωYEωY-E2ωZ (oo-e)准相位匹配蓝光二次谐波。在 15 0℃下 ,由 114 μJ抽运光 ,得到了 5 2 μJ ,0 .4 73μm倍频蓝光 ,对应于平均最大转换效率 4 5 .6 %。制备了一阶和三阶周期分别为 4 .5 μm和 13.5 μm的周期性极化铌酸锂。在EωZEωZ-E2ωZ (ee -e)准相位匹配 0 .4 73μm蓝光倍频中 ,15 0℃下 ,分别得到了 4 1.3%和 19%的倍频转换效率。oo -e准相位匹配比传统的ee -e准相位匹配有较大的光栅周期 ,尤其在短波长区域倍频输出应用中 ,降低了周期性结构制作的困难 ,其较大的容许带宽在实验中提高了频率转换效率。实验结果表明了在周期性极化铌酸锂中准相位匹配倍频的偏振相关性。     

全固态飞秒激光高光束质量三倍频研究(特邀)

对全固态飞秒激光三倍频产生高光束质量343 nm飞秒激光进行了系统研究。基频光源为脉冲宽度为105 fs、重复频率为76 MHz、中心波长为1 030 nm的商用Yb:KGW锁模激光器,利用1.7 mm长LBO晶体获得60%的二倍频转换效率,然后分别研究了基于BBO晶体Ⅱ类相位匹配和Ⅰ类相位匹配的三倍频产生。在基频光功率为5 W的条件下,利用Ⅱ类相位匹配的BBO晶体,获得的最大平均功率为0.71 W,三倍频转换效率约为14%;利用Ⅰ类相位匹配的BBO晶体,获得平均功率为1.01 W的紫外激光输出,三倍频转换效率为20.2%。获得的343 nm紫外激光的光束质量优于1.3。     

产生蓝色相干辐射和实现Nd∶YAG 1.0642μm倍频90°非临界相位匹配的Nb∶KTiPO_4(英文)

本文报道了用熔盐顶部籽晶法生长Ｎｂ浓度（０～１３）ｍｏｌ％的Ｎｂ∶ＫＴＰ晶体的倍频的Ⅱ型相位匹配的截止波长和Ｎｄ∶ＹＡＧ１．０６４２μｍ及Ｎｄ∶ＹＡ１Ｏ３１．０７９５μｍ激光在这些晶体中倍频的最佳相位匹配角的测量结果．从中可看出,由于Ｎｂ５＋的引入使ＫＴＰ晶体倍频的Ⅱ相位匹配的截止波长有效蓝移,目前已使截止波长蓝移至９３７ｎｍ且有效产生４６８．５ｎｍ的倍频蓝光．同时Ｎｂ５＋的引入使Ｎｄ∶ＹＡＧ１．０６４２μｍ激光和Ｎｄ∶ＹＡ１Ｏ３倍频的最佳相位匹配方向产生很大的变化,目前已使Ｎｂ∶ＫＴＰ晶体倍频的最佳相位匹配方向为θ＝８８．３２°、Φ＝０°,非常接近于９０°非临界相位匹配方向     

新型光学晶体KABO的非线性频率变换特性

简述了新型非线性晶体KABO的光学特性.根据相位匹配角公式、非线性有效系数公式、走离角公式和允许角公式,详细计算了KABO晶体倍频时的相位匹配角、非线性有效系数、倍频时谐波走离角、允许角随波长变化的理论曲线.特别是对LD泵浦掺钕的全固态激光器532 nm输出时,得到了KABO晶体采用I类相位匹配进行四倍频的相位匹配角、非线性有效系数、倍频时谐波走离角、允许角分别为:58.1°、0.254×10^-12m/V、2.8°、1.8233 mrad·mm;并将该晶体与目前可应用于紫外倍频的晶体比较,在考虑走离效应的情况下,研究了四倍频转换效率随KABO晶体长度、基频光光斑半径的变化规律.     

双端输出外腔谐振倍频的运转特性研究

准相位匹配晶体外腔谐振倍频是获得高效率短波长激光稳定输出的技术之一.为消除高功率激光抽运时非线性级联过程的影响、提高倍频激光的输出功率,本文设计了一种双端输出倍频谐振腔.首先在实验上比较了双端和单端输出谐振倍频过程的倍频效率和输出功率;并在理论上分析了双端腔型下基频光与倍频光相对相位的变化对于倍频输出光功率的影响,通过控制相对相位可以获得高的倍频光输出功率.实验结果与理论分析基本吻合.     

采用光栅角色散的宽带二倍频及聚焦特性分析

针对带宽为20 nm左右、中心波长为1 053 nm的线性啁啾宽带激光,分析了采用光栅角色散方式宽带二倍频的原理,讨论了光栅角色散性能及光栅加工误差引入的基频光位相扰动对二倍频光转换效率及聚焦特性的影响.研究结果表明:采用光栅角色散方式的宽带二倍频光的转换效率、脉冲宽度和带宽均明显增大;在0～22 nm基频光带宽范围内,二倍频光均可保持80％左右的转换效率;对于入射基频光带宽为22 nm的情况,光栅角色散率在30～80 μrad/nm范围内变化时,宽带二倍频转换效率均可保持在70％以上;基频光有位相扰动时,二倍频光带宽对二倍频聚焦光斑的主瓣影响不大,但对旁瓣有一定的匀滑作用,采用光栅角色散方式宽带二倍频的聚焦光斑旁瓣的匀滑效果更为明显.     

Analysis of broadband third harmonic generation with non-collinear angular dispersion in KDP crystals

A new scheme with non-collinear sum frequency generation (SFG) process of broadband third harmonic generation (THG) with two KDP crystals (one doubler and one mixer) using angular dispersion was proposed. The principle of broadband harmonic generation with non-collinear angular dispersion was presented. The comparison between the schemes of non-collinear and collinear SFG process was performed. The effects of the angular dispersion on the conversion efficiency, the pulse shape and the spectrum of the third harmonic field were discussed. The results show that, if proper angular dispersion is added to the fundamental and the second harmonic field outside the mixing KDP crystal, respectively, the broadband third harmonic conversion efficiency can be improved significantly. However, the difficulty of this scheme arises due to the requirement of two gratings with different angular dispersion outside the mixer. With the new scheme, it can be simpler that only one grating is needed to realize the broadband phase matching in the non-collinear SFG process. Although the fundamental and the second harmonic field inside the mixing crystal are non-collinear except the center wavelength, a blazing grating with proper angular dispersion for the fundamental field (twice as that for second harmonic field) can yield the well compensation for phase mismatching in the SFG process. Consequently, the conversion efficiency and the characteristics of the third harmonic field for the broadband third harmonic generation can be improved significantly.     

Influence of third-order nonlinear and phase variations on third harmonic generation

A theoretical analysis is presented for third-harmonic generation in KDP for type I/type II angle-detuning scheme of high-intensity laser to produce third harmonic radiation near 0.35 μm. The effects of the third-order nonlinearity and the phase variations on the frequency conversion have been discussed. The results shown that the third-order nonlinear interactions decreases the tripling efficiency, and increases of the modulate strength of the output intensity of 3ω radiation. However, adjusting the angular detuning can compensate effectively the effects of third-order nonlinearity. Furthermore, 3ω conversion efficiency will drop with the increase of the degree of phase variations, and the increase 3ω conversion efficiency can decrease the influence of third-order nonlinearity on the conversion of third harmonics.     

Streaming potential analysis on the hydrodynamic transport of pressure-driven flow through a rotational microchannel

In this study, the streaming potential and electrokinetic energy conversion efficiency are discussed under the low zeta potential approximation through a microparallel channel with consideration of rotational effect. By solving Poisson-Boltzmann equation and modified Navier–Stokes equation, the analytical expressions of the streaming potential and electrokinetic energy conversion efficiency in the electrolyte solution are obtained. Combining with the numerical calculation, the influences of the dimensionless electrokinetic width K and the rotational angular velocity ω on streaming potential and the electrokinetic energy conversion efficiency are discussed. The results show that the streaming potential fields decrease both in mainstream and secondary directions with the electrokinetic width K, it decreases with the non-dimensional rotational angular velocity in the mainstream direction and it shows a first increasing then decreasing trend in the secondary flow direction. In addition, the influences of related non-dimensional parameters, including electrokinetic width, wall electric potential and rotational angular velocity, on the electrokinetic energy conversion efficiency are also discussed in detail. The rotating effect can enhance conversion efficiency comparing to the case of no rotation. The increase of wall electric potential gives rise to an augment in electrokinetic energy conversion efficiency. These theoretical results make sense to the energy harvesting in the rotating microfluidic systems.     

静磁波与导波光的磁光耦合理论

根据耦合模理论,分析了砂光波导中静磁波与导波光的相互作用特点。给出了导波光模式之间的耦合方程。理论分析表明,仅当垂直磁化时,发生模式转换的导波光材被全部衍射,此时衍射效率等于模式转换效率。计算了斜向场作用下ＹＩＧ薄膜中导波光的反斯托克斯相互作用衍射效率,所得结论与实验结果一致。适当倾斜静磁场,导波光的模式转换和衍射效率均可比垂直磁化时间显提高。同时改变静磁体波传播方向和斜向场的偏离方向（与垂直方向     

共焦非稳腔中ps激光的内腔倍频

考虑倍频晶体相位匹配接收角（Angular acceptance）对倍频转换效率的影响,设计了正支（p支）共焦非稳腔Nd:YAP锁模激光器。平行光通过晶体实现完全相位匹配。以BBO和KTP作为腔内倍频元件,分别获得80.6%和60.2%的高效倍频转换,及几十mJ的绿光输出。同时实验测定其它有关参数。     

光束正入射至界面时的自旋-轨道相互作用及其增强

光束正入射至均匀突变界面时的自旋-轨道相互作用表现为拓扑荷数为±2的、自旋可控的涡旋相位.然而,该涡旋相位的物理来源以及界面的性质在自旋-轨道相互作用过程中起到何种作用,这些问题还有待解决.首先建立一个简洁的菲涅耳琼斯矩阵来描述这种自旋-轨道相互作用,并揭示其中的涡旋相位其实是一种贝里(Berry)几何相位,它来源于光束本身的拓扑结构,而界面的性质影响自旋-轨道相互作用的转换效率.一般情况下,转换效率极低,限制了其应用.因此,基于上述理论,提出采用光轴平行于界面法线方向的单轴薄层材料,来极大地增强这种自旋-轨道相互作用.     

Peculiarities of self-action in second harmonic generation of laser radiation in nonlinear crystals

By using the generalized method of strong interaction of nonlinear waves and computer simulation, an analysis of the influence of self-action effects on second harmonic generation of an intense diverging picosecond pulse is performed. Using the approximation of strong wave interaction, an analytic solution is obtained for frequency conversion of the laser radiation taking into account the pump intensity depletion, the influence of the angular dispersion effect (ADE), and the higher nonlinearities. It is shown that the self-action effects, the ADE, and the linear phase mismatch can compensate for each other, increasing the efficiency of second harmonic generation. Optimum conditions for second harmonic generation in converging and diverging laser beams were found. An asymmetry of the angular dependence of the second harmonic intensity was experimentally observed and theoretically explained. The asymmetry is caused by the reverse energy conversion of the second harmonic into the pump by the influence of the ADE and Kerr nonlinearities.     

High efficiency and broad bandwidth terahertz vortex beam generation based on ultra-thin transmission Pancharatnam–Berry metasurfaces

The terahertz(THz) vortex beam generators are designed and theoretically investigated based on single-layer ultra-thin transmission metasurfaces. Noncontinuous phase changes of metasurfaces are obtained by utilizing Pancharatnam–Berry phase elements, which possess different rotation angles and are arranged on two concentric rings centered on the origin.The circularly polarized incident THz beam could be turned into a cross-polarization transmission wave, and the orbital angular momentum(OAM) varies in value by lh. The l values change from ±1 to ±5, and the maximal cross-polarization conversion efficiency that could be achieved is 23%, which nearly reaches the theoretical limit of a single-layer structure.The frequency range of the designed vortex generator is from 1.2 THz to 1.9 THz, and the generated THz vortex beam could keep a high fidelity in the operating bandwidth. The propagation behavior of the emerged THz vortex beam is analyzed in detail. Our work offers a novel way of designing ultra-thin and single-layer vortex beam generators, which have low process complexity, high conversion efficiency and broad bandwidth.     

Tunable sub-10-fs ultraviolet pulses generated by achromatic frequency doubling

Tunable UV pulses shorter than 10 fs are generated by achromatic frequency doubling of a noncollinear optical parametric amplifier. With a suitable two-prism sequence we achieve first- and second-order achromatic phase matching and increase the natural bandwidth of the nonlinear crystal by a factor of 80. Extremely broad UV spectra with a Fourier limit of 2.9 fs are generated in a 360-microm-thick beta-barium borate crystal at a conversion efficiency of 20%. We compensate for the angular dispersion and the first-order chirp of the highly stable UV pulses with a second prism sequence and fully characterize the temporal pulse shape with zero-additional-phase spectral phase interferometry for direct electric-field reconstruction (ZAP-SPIDER). Pulses as short as 7 fs are generated by controlling the higher-order chirp with a deformable mirror.