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.     

基于角色散的非共线匹配宽带三倍频技术

宽带光打靶可以有效降低激光等离子体相互作用过程中非线性效应。提出一种基于角色散的非共线匹配宽带三倍频方案,利用宽带基频与窄带二倍频的非共线和频产生宽带三倍频,和频过程中通过特殊设计的渐变光栅实现不同频率的基频光束以特定角度入射,补偿了波长差异引入的位相失配使得全波段满足位相匹配条件。理论模拟表明,采用KDP晶体Ⅱ类位相匹配,将中心波长为1058 nm、带宽10 nm的宽带基频光与526.5 nm的二倍频光进行非共线匹配和频,可以实现高效宽带三倍频转换。     

晶体级联方式宽带三倍频方案的参数优化

 针对时间位相调制的宽带激光,采用分步离散傅里叶变换及四阶龙格－库塔算法进行数值模拟计算。讨论了采用KDP晶体级联方式时,入射基频光的光强、带宽以及晶体厚度对三倍频转换效率的影响。对采用一块РⅠ类二倍频晶体、一块Ⅱ类和一块Ⅰ类双混频晶体的级联宽带三倍频方式进行了晶体参数的优化。研究结果表明,使用两块级联的KDP晶体作为混频晶体,不仅能有效地提高带宽较宽条件下三倍频光的转换效率,还可以显著增大宽带三倍频的动态范围。经优化后,带宽1.11 nm时入射基频光强在3～8 GW/cm²范围内的三倍频转换效率可保持在60％～70％,比采用单倍频单混频方案时增大了30％～40％。     

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。     

宽带钕玻璃激光的高效三次谐波转换

提出了一种新颖的宽频带钕玻璃激光三倍频技术组合激光工作模式（宽带激光与窄带激光混频）方案。数值模拟研究表明:利用窄线宽钕玻璃激光脉冲,可以缓解宽频带激光谐波转换过程中群速度失配对转换效率的影响,从而提高宽带钕玻璃激光的转换效率;并且该方案可以与目前使用成熟的双和频晶体方案结合,从而能支持目前钕玻璃激光装置能达到的最大带宽约5 nm的高效三倍频（理论上效率达约80%）。     

Non-collinear phase-matching sum-frequency generation based on boundary total reflection in bulk KDP

Collinear phase-matching of sum-frequency generation(SFG) has been studied thoroughly previously, while non-collinear schemes are sometimes more flexible in application. However, this phase-matching type is more difficult to meet and control. We employ a convenient method to obtain harmonic generation in bulk potassium dihydrogen phosphate(KDP), using an incident wave vector and a reflected wave vector to create a triangle phase-matching relationship. With a simple, flexible set-up, we can observe 351 nm SFG, and the conversion efficiency is up to ~3.6% per reflection. Furthermore, we believe this approach has potential application value and improvement space.     

晶体级联方式的宽带三倍频方案分析

 讨论了入射基频光频谱宽度、两块和频晶体失谐角对KDP晶体Ⅰ/Ⅱ/Ⅱ类角度失谐级联方式的三倍频光功率谱分布的影响，分析了影响三倍频效率的因素。研究结果表明，第二块和频晶体得到的三倍频光的功率谱分布与基频光功率谱分布及失谐角有关，三倍频光频谱宽度近似为基频光的3倍，采用该方案的三倍频可有效抑制由于基频光时间相位调制导致的三倍频光强随时间分布的不均匀，并能有效提高大带宽条件下的三倍频转换效率。同时，采用该方案的三倍频，对级联的两块和频晶体厚度及失谐角调整精度要求不高，在实验上有很大的可操作性。     

Broadband non-collinear double QPM optical parametric amplification in mid-infrared wave

The properties of optical parametric amplification (OPA) based on non-collinear double quasi-phase matching (NDQPM) with single periodically poled KTP (PPKTP) have been investigated theoretically. The NDQPM includes two different non-linear processes: one is optical parametric generation (OPG) and the other is difference frequency generation (DFG). The investigation of our numerical simulation focuses on the gain bandwidth of dependence upon non-collinear angle, grating period and crystal temperature. At a certain non-collinear angle and grating period with fixed temperature, there exists a broadest gain bandwidths of output mid-infrared pulse at 526 nm pump wavelength and certain signal wavelength in PPKTP. These are an optimal values of non-collinear angles and grating period. By accurately tuning the non-collinear angle or temperature near the optimal non-collinear angle, broadband mid-infrared tuning is obtained and an optimal operation of NDQPM can be realized. In this paper, the solutions of the coupled equations of the cascaded processes were discussed, and the spatial-temporal frequency (STF) band of the output idler pulse is analyzed by taking angular dispersion of amplified pulse beam into account. The idler pulse with a certain angular dispersion can improve the OPA bandwidth significantly. So, optical parametric chirped-pulse amplification can be realized in this configuration. For a broadband NDQPM both the acceptance angles and the acceptance temperature are smaller and the gain bandwidth is sensitive to non-collinear angles and temperature, it is important to control the precision of the non-collinear angles and the temperature in experiment.     

Generation of ultraviolet radiation with wide angular tolerance in cesium lithium borate crystal

Tangential phase-matching has been realised in cesium lithium borate (CLBO) crystal for the first time for the generation of fourth harmonic (266 nm) of Nd:YAG and third harmonic (226.7 nm) of a dye laser radiation by second harmonic generation and sum-frequency mixing with the angular tolerance as large as 22 mrad and 21 mrad respectively, over one of the interacting beams. An energy conversion efficiency of 15% for fourth harmonic generation is obtained with a 5.5 mm thick crystal and with the average pump powers only 170 and 70 mW. A set of Sellmeier dispersion equations for the CLBO crystal have also been formulated.     

离散效应对高强度激光三次谐波转换的影响

 使用分步傅里叶变换和四阶龙格库塔法(R-K) ，对高强度激光以Ⅰ/Ⅱ类角度失谐方式，在KDP晶体中的谐波转换进行了研究，详细讨论了离散效应对三次谐波转换的影响。结果表明，离散效应不但降低了三次谐波转换效率，而且使谐波光束质量显著降低；初始入射基频光束腰半径较小时，离散效应是二次谐波转换和三次谐波转换效率降低的主要因素，失谐角对三次谐波转换的影响较小；随着束腰半径的增加，离散效应的影响逐渐减小，失谐角对三次谐波转换的影响逐渐增加。     

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.     

时间位相调制对高强度三次谐波转换的影响

在惯性约束聚变研究中,为了利用光谱色散进行光束匀滑以及抑制KDP晶体等大口径光学元件中的横向受激布里渊散射,通常需要对激光脉冲进行时间位相调制.详细讨论了高功率条件下KDP晶体Ⅰ/Ⅱ角度失谐三倍频方案中,时间位相调制对三次谐波转换效率和脉冲频谱的影响.研究结果表明,时间位相调制会导致三次谐波转换效率明显下降.     

Efficient frequency tripling of phase modulated laser based on two KDP mixers in tandem array

By using the cascaded KDP crystals, the third harmonic generation (THG) of temporal phase modulated broadband laser is experimentally demonstrated in this paper. The spectral characteristic of the generated third harmonic waves is investigated in detail. The experimental results show that the cascaded crystals method is able to satisfy the acceptance bandwidth of broadband laser beam. In addition, the influence on the THG spectrum and THG conversion efficiency of the air spacing between the two cascaded crystals is experimentally studied and it is remarkable on THG spectrum but insignificant for THG conversion efficiency. The relatively ideal THG spectrum is obtained by the optimal parameter of air spacing and the stability of THG spectrum is very good in our experiment.     

Composite‐Assisted Phase‐Matching: Efficient Wavelength Conversion in Nonlinear Optical Composite Materials Containing Metal Nanoparticles

A novel phase‐matching scheme which is based on the dispersion compensation in the nonlinear optical composite materials containing metal nanoparticles is proposed. Anomalous dispersion originating from the plasmon resonance in metal nanoparticles compensates the dispersion of the host nonlinear material, leading to the perfect phase‐matching and high efficiency of nonlinear optical wavelength conversion. The effectiveness of this approach is theoretically demonstrated, taking third‐order nonlinear processes such as the direct third‐harmonic generation and four‐wave mixing in ZnO composites containing silica‐core–silver‐shell nanoparticles as examples. The results show that with the proposed phase‐matching scheme, unprecedentedly high conversion efficiency can be obtained compared with preceding results in third‐order nonlinear optical solid‐state materials.     

高功率高效率三次谐波二维数值模拟研究

 对大口径、高功率情况下的三次谐波转换,选择KDP晶体TypeⅠ/TypeⅡ匹配角度失谐的三倍频方案,并从非线性三波耦合方程组出发,采用离散傅立叶变换和四阶龙格库塔(R-K)积分方法,编制了二维模拟三次谐波转换的计算程序。计算了各种晶体厚度和不同失谐角条件下,二、三次谐波的转换特性和效率。并对入射基频光为六阶超高斯分布时的三倍频器进行了优化,三倍频转换效率达80%,此时具有较大的动态范围和较高的三倍频转换效率。     

振幅调制和位相扰动对三倍频参数优化的影响

 详细讨论了入射基频光的振幅调制和位相扰动对三倍频转换效率和动态范围的影响，提出了提高三倍频转换效率和增大动态范围的方法。采用I/II类角度失谐的KDP晶体三倍频方案，对倍频晶体的参数进行了优化设计。     

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。     

Efficient self phase matched third harmonic generation of ultrashort pulses in a material with positive dispersion

An ultrafast transient refractive index grating, produced in barium fluoride, a material with positive dispersion, promotes very efficient third harmonic generation. The significant enhancement of the generation up to a conversion efficiency of about 3% is due to self phase matching, involving the instantaneous grating. At the same time several diffraction orders of the third harmonic signal are observed behind the sample. Received: 17 October 2000 / Published online: 21 February 2001     

高功率激光终端KDP晶体非共线高效三倍频及远场色分离方案数值模拟分析

为满足高功率激光装置对终端光学系统的改进要求,控制3ω光路透射元件厚度以降低激光损伤风险,避免3ω非对称聚焦与色分离元件对靶场调靶产生不利影响,本文利用非共线相位匹配原理讨论了KDP晶体Ⅰ类和Ⅱ类两种和频产生351 nm(3ω)激光及其远场色分离过程.模拟结果表明,室温20?C环境中除目前常用的共线和频外,1053 nm(ω)与526.5 nm(2ω)激光可选择Ⅰ类或Ⅱ类两种非共线和频方式实现高效3ω激光输出并在激光远场实现色分离,且具有足够的高效转换失谐角容宽.计算表明,与Ⅰ类和频类似,Ⅱ类和频也存在一个非临界相位匹配过程,其匹配方向约为θ(3ω)=86.53?.可通过增加晶体厚度克服其有效非线性系数较低的缺点,实现3ω高效输出,失谐角容宽可达±20 mrad.为满足靶场需要,解决非共线角容宽苛刻带来的调节不便,并进一步使光路紧凑,将楔角为12?的熔石英楔板置于倍频晶体之后,ω与2ω激光在熔石英楔板后表面可产生约3.5 mrad分离角.经非共线和频,使用薄透镜即可实现聚焦及色分离.该方案完全满足终端光学系统的改进要求,可作为可靠的备选方案之一.     

Pulse Slippage of Resonant Third Harmonic Generation in Electron-Hole Plasma

Effect of pulse slippage on resonant third harmonic generation of a short pulse laser in electron-hole plasma in the presence of wiggler magnetic field has been investigated. The group velocity mismatch of the third harmonic pulse and the fundamental pulse is significant in electron hole plasma. As the third harmonic pulse has higher group velocity than that of fundamental pulse, therefore, it moves faster than the fundamental pulse. It gets slipped out of the domain of fundamental pulse and its amplitude saturates. Phase matching condition is satisfied by applying wiggler magnetic field,which provides additional angular momentum to the third harmonic photon to make the process resonant. Enhancement in the efficiency of third harmonic generation of an intense short pulse laser in electron-hole plasma embedded with a magnetic wiggler is seen.