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。     

高强度三次谐波转换中位相扰动的变化

 以I/II类角度失谐三倍频方案为例，详细讨论了在高强度三次谐波转换过程中入射基频光的位相扰动和振幅调制对三倍频光位相扰动的影响，定量分析了基频光和三倍频光的位相扰动之间的变化关系。模拟结果表明，三倍频光的位相扰动幅度近似为基频光的三倍；入射基频光的位相分布对三倍频光的影响比振幅调制的影响更加明显，但是当入射功率密度较大时，基频光的振幅调制对三倍频光位相分布的影响明显增大。     

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。     

利用群速度匹配的级联二阶非线性实现超短激光脉冲压缩

提出了一种采用倾斜脉冲的级联二阶非线性来实现超短激光脉冲压缩的方法. 对基于单块BBO晶体中基频光与倍频光群速度匹配的级联二阶非线性的脉冲压缩方案进行了理论分析. 对比研究了群速度匹配与失配情况下利用级联二阶非线性进行脉冲压缩的效果, 并模拟分析了基频光与倍频光的位相失配量、非线性晶体长度、 基频光初始峰值光强和初始脉宽等因素对脉冲压缩效果的影响. 结果表明, 基频光与倍频光的群速度匹配将会大幅度改善压缩脉冲的时间波形和频谱分布. 通过对位相失配量、晶体长度、初始光强等参数的优化和选取可获得较理想的压缩效果. 采用倾斜脉冲的级联二阶非线性的脉宽压缩方法, 针对中心波长800 nm、脉宽100 fs, 峰值光强为50 GW/cm²的基频光脉冲, 采用25 mm厚BBO晶体, 当基频光与倍频光位相失配量Δk=60 mm^-1(对应失谐角1.98°), 晶体外部脉冲前沿倾斜角γ₀=74°时, 计算获得了质量较好的20 fs剩余基频光, 并同时产生了14 fs的倍频光. 关键词： 倾斜波 级联二阶非线性 群速度失配 脉冲压缩     

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％。     

超短脉冲倍频中三阶非线性效应的影响及补偿

 为了提高超短脉冲二倍频的转换效率和改善出射倍频光的脉冲形状，对超短脉冲倍频中三阶非线性效应的影响进行了理论分析和数值模拟，并采用初始相位失配的方法来补偿三阶非线性效应的影响。结果表明：用KDP晶体二倍频中心波长为800 nm的超短脉冲，当入射功率密度大于100 GW/cm²时，三阶非线性效应是倍频转换效率的主要影响因素。对脉宽为50 fs，入射功率密度为250 GW/cm²的超短脉冲在KDP晶体（2 mm）中的二次谐波变换，当初始相位失谐0.9 mrad时，转换效率提高了10%，同时由三阶非线性效应引起的强度调制得到明显抑制，出射基频光和倍频光的脉冲形状得到明显改善。     

二次谐波产生的非线性相移的解析研究

给出一种分析二阶非线性级联效应的解析方法，对Ⅱ型二次谐波产生的非线性级联效应进行了详细的理论研究和分析。结果表明倍频晶体中正交输入两个振幅不相等的基波分量时，在近相位匹配下二阶非线性级联效应能够产生大的相移。利用本文的解析方法可以对任意二阶非线性相移产生的过程进行优化，从而以最小的需求输入功率来获得特定的相移。还对由非线性相移诱导的且与光强有关的偏振旋转效应和一种基于偏振旋转效应的全光偏振开关进行了分析和讨论。本文的结果对于建立一种基于Ⅱ型倍频晶体中非线性相移诱导偏振旋转效应的新全固化被动锁模飞秒激光装置具有十分重要的意义。     

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。     

空间走离和相速度补偿的高效飞秒脉冲倍频

采用两块大小均为10 mm×10 mm×0.2 mm的BBO晶体级联的实验方案对中心波长为800 nm、脉宽70 fs、重复频率10 Hz的激光进行了倍频,针对BBO晶体的空间走离和相速度失配,分别调节两块倍频晶体的光轴夹角和基频光的角度匹配失谐量.实验表明,两块倍频晶体光轴反向平行可以补偿空间走离效应,一定的角度失谐可以补偿基频光和倍频光的相速度失配.在无聚焦情况下,使用两块BBO晶体级联相互配合优化将8.1 mJ的入射基频光倍频后得到带宽6.7 nm(傅里叶转换极限脉冲宽度为35 fs)、2.8 mJ以上的倍频光.其倍频效率高达35.7%,是同等条件下单块晶体倍频转换效牢的1.7倍.     

基于Nd∶YAG/Cr∶YAG复合晶体的全固态紫外激光器

被动调Q产生1064 nm脉冲激光在腔外聚焦后入射到KTP中,产生532 nm的倍频光,再通过LBO和频产生355 nm激光。当抽运功率为3.4 W时,基频光调Q输出平均功率为350 mW,峰值功率达3.5 kW。腔外二倍频532 nm绿光输出平均功率为110 mW,用Ⅰ类相位匹配LBO晶体和频获得36 mW的355 nm的紫外激光输出,三倍频效率(1064~355 nm)达到10.2%。由于Cr∶YAG晶体达到饱和吸收后,会呈现出各向异性的特征,对基频光的偏振状态有很大影响。实验中必须合理放置复合晶体,使基频光的偏振状态为近似线偏振以提高转换效率。     

Generation of large fifth-order nonlinear phase shifts by use of lossless χ(2) cascaded nonlinear processes

It is shown that the cascaded fifth-order nonlinear phase shifts will increase with energy loss in the cascaded processes. Essentially different from the multi-photon absorption accompanied with inherent material nonlinearities, the loss of fundamental wave in a cascaded process is controllable and suppressible. By introducing difference frequencies generated from the reaction between the fundamental and its second harmonic after the cascaded processes, the fundamental wave can be free of energy loss, while the large cascaded fifth-order nonlinear phase shift is maintained.     

Generation of large fifth-order nonlinear phase shifts by use of lossless<Emphasis Type="Italic">χ</Emphasis><Superscript>(2)</Superscript> cascaded nonlinear processescascaded nonlinear processes

It is shown that the cascaded fifth-order nonlinear phase shifts will increase with energy loss in the cascaded processes. Essentially different from the multi-photon absorption accompanied with inherent material nonlinearities, the loss of fundamental wave in a cascaded process is controllable and suppressible. By introducing difference frequencies generated from the reaction between the fundamental and its second harmonic after the cascaded processes, the fundamental wave can be free of energy loss, while the large cascaded fifth-order nonlinear phase shift is maintained.     

Generation of large fifth-order nonlinear phase shifts by use of lossless χ~((2)) cascaded nonlinear processes

Theinherentthird-ordernonlinearityofmaterialc(3)isnormallyweak.Toaccumulatealargenonlinearphaseshift,highlaserintensityandlongopticalpathinmaterialareneeded.Whilethevalueofnonlinearitycouldbemadelargerneartheregionofabsorptionresonance,astronglosswillbeaccompaniedthroughtheprocessoftwo-photonabsorption.Inordertorealizestrongthird-ordernonlinearitywithcontrollableenergyloss,thec(2)(w;2w,-w)︰c(2)(2w;w,w)cascadingprocesshasbeenproposed,whichisequivalenttothethird-ordernonlinearityc(3)(w;w,w,-w)w…     

Group-velocity-matched multistep cascading in nonlinear photonic crystals

We demonstrate that simultaneous phase- and group-velocity-matched cascaded parametric processes can be achieved in two-dimensional quadratic nonlinear photonic crystals by proper matching of noncollinear processes with front tilting of the fundamental pulse. We present examples of cascaded third- and fourth-harmonic generation in a poled LiNbO3 planar structure.     

Self-induced transparency and self-induced darkening with a nonlinear frequency-doubling polarization interferometer

It is shown that the system polarizer-frequency-doubling crystal(s)-analyzer has a nonlinear transmission for the fundamental wave. The intensity-dependent transmission of this device is due to the nonlinear phase shift that the fundamental beam obtains in the nonlinear crystal as a result of cascaded second-order processes. Depending on the mutual orientation of the polarizer and the analyzer such effects as self-induced transparency and self-induced darkening can be realized.     

Coupled mode analysis to study cascading in the QPM Cerenkov regime in waveguides

In this paper we use the coupled mode analysis to study nonlinear phase shifts of the fundamental beam caused by cascaded second order nonlinear effects in the Quasi Phase Matched Cerenkov (QPMC) configuration in waveguides. Under the no-pump depletion approximation which is valid for low conversion efficiencies, we obtain the nonlinear phase shift as a function of length of interaction and grating period. It is observed that the nonlinear phase shift of the fundamental beam can be maximized by choosing a grating period for which the phase matched second harmonic radiation mode is radiated parallel to the film-substrate interface, i.e., for zero Cerenkov angle. Although the phase shifts are smaller than in the case of all guided geometry, QPM Cerenkov configuration is expected to have greater tolerance towards various waveguide parameters and the fundamental wavelength.     

High-order nonlinear phase shift caused by cascaded third-order processes

We show, for the first time to our knowledge, that the fundamental beam that participates in the process of third-harmonic generation experiences an additional high-order nonlinear phase shift. The magnitude of the shift is proportional to the square of the pump intensity and the length of the sample and depends on the deviation from the exact phase-matched condition. This phase shift arises from cascaded third-order processes. Its value can exceed the value of the phase shift that originates from inherent fifth-order susceptibility of the nonlinear medium. Its sign is controllable by the sign of the phase mismatch of the third-harmonic generation process.     

Generation and weak beam control of two-dimensional multicolored arrays in a quadratic nonlinear medium

We report on the first experimental observation of 2D multicolored transverse arrays in a quadratic nonlinear medium under the pump of two crossly overlapped femtosecond beams. The 2D reproducible patterns are caused by cascaded noncollinear quadratic nonlinear couplings between the input pulses and quadratic spatial solitons originated from spatial breakup of one of the input beams with spatial ellipticity. A seed supercontinuum pulse is then diffracted and amplified with phase preservation, resulting in the formation of up-converted multicolor 2D transverse arrays. By seeding with weak second harmonic pulses, the 2D multicolored transverse patterns are suppressed due to weak beam control of the induced quadratic spatial solitons.     

Optical-frequency balanced mixer

Optical signal processing devices based on quasi-phase-matched three-wave mixing and cascaded three-wave mixing in guided-wave geometries have been demonstrated to operate efficiently at practical pump-power levels. We describe operation of such devices in a balanced mode that allows mixing without wavelength offset and separation of mixed output from pump and signal input without wavelength-selective filters. We present a design for an optical-frequency balanced mixer using quasi-phase-matched, cascaded second-order nonlinear processes. Using this design, we fabricated a balanced mixer in periodically poled lithium niobate waveguides that has the expected linear and nonlinear optical performance.     

Analytical formulae for the optimization of the process of low-power phase modulation in a quadratic nonlinear medium

We show that using the approximation of fixed intensity analytical formulae, describing the process of induced phase modulation for the beams involved in second-order nonlinear optical processes can be derived. Expressions that allow the optimization of the phase shifts experienced by the fundamental and generated waves are presented for nonlinear quadratic processes, second-harmonic generation and sum-frequency mixing. In the case of seeding at the generated wavelength, the phase shift of the fundamental wave is due to two interactions: (i) a cubic one, based on coupled second-order processes (cascade cubic nonlinearity) and (ii) single quadratic interaction with participation of the seeding wave. By comparison with the exact numerical solution, we defined the input parameters of the beams for which this analytical approach is valid. It is shown that phase shifts exceeding /2 can be correctly predicted using the expressions obtained.