准相位匹配周期极化钼酸钆制备及倍频效应

根据准相位匹配原理，对钼酸钆晶体用于准相位匹配倍频过程的周期性畴结构进行了设计，当入射基频光为l.064pm时，所需的钼酸钆晶体畴结构周期为5．54μm。利用外电场极化，当外电场为2．5kV／mm时，钼酸钆晶体的畴结构反转时间tc=500～600μs。当外电场脉冲幅值为2．5kV／mm、脉冲延续时间t=0．6～0．8tc时，在钼酸钆晶体中制备得到了周期畴结构。当Nd：YAG纳秒激光器输出准连续功率为7～10mW的l.064μm激光通过具有周期畴结构的样品时，获得了0．532μm的倍频光输出。     

准相位匹配周期极化高掺镁铌酸锂532 nm倍频准连续输出研究

对周期性极化高掺镁铌酸锂倍频过程进行了准相位匹配倍频理论研究。在室温下通过外加电场极化法,用较低的极化开关电场～5．5kV／mm,在厚为1mm、长为10mm、宽为10mm的掺镁铌酸锂基片上成功地制备了周期为5．8～7．3pm(间隔0．3pm)的一阶准相位匹配倍频周期性极化光学微结构。将温度控制在70℃左右,以波长为1．060μm的Nd：YAG激光为基频光源,对所研制的光学微结构样品进行倍频通光实验验证。当入射基频光为920mW时,可以获得约15mW的532nm准连续倍频蓝光输出．其归一化转换效率高达1．77％／W。     

准相位匹配周期极化掺镁铌酸锂490 nm倍频连续输出

在室温下通过外加电场极化法,首次用较低的极化开关电场～5.5 kV/mm,在厚为1 mm、长为20 mm、宽为18 mm的掺镁铌酸锂基片上成功的制备了周期为4.8～5.2 μm的一阶准相位匹配倍频光学微结构;并在室温下,以波长为980 nm的半导体激光器为基频光源,对所研制的微结构样品进行倍频通光实验,在入射基频光为800 mW时,产生约40 mW的490 nm的倍频光,其对应转换效率为5%,实验过程中未见绿致吸收光折变现象.     

激光热效应对PPLN晶体倍频效率的影响

从准相位失配关系出发,研究了PPLN晶体倍频效率与温度的关系,针对温度对折射率与极化周期的影响,分析了激光热效应对准相位匹配系统谐波转换过程的影响。采用有限元分析法计算了PPLN材料内部温度分布状况,并对热效应影响下晶体内部倍频效率变化情况进行了计算。计算结果表明:在不同基频光功率密度下,在倍频过程中晶体温度及通光方向截面折射率的分布不同,在倍频晶体出射面倍频效率的分布也不同;随基频光功率密度增大,晶体整体温度与折射率增大,出射端面倍频效率分布随基频光功率密度变化而变化;与理想条件倍频系统相比,激光热效应对晶体倍频效率有一定的影响。     

准相位匹配LiNbO3蓝光倍频器的研究

采用外加电场极化法,实现了ＬｉＮｂＯ３晶体的周期性极经,并制备出周期为９．５μｍ的三阶准相位匹配周期性极化ＬｉＮｂＯ３晶体（ＰＰＬＮ）;用准连续钛宝石激光器作基频光源,对准相位匹配周期性极化ＬｉＮｂＯ３进行了光学倍频实验,得到了１２μＷ的蓝光输出。     

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。     

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.     

输出镜反射相移对倍频效率的影响

利用Macleod反射相移理论,模拟V-型腔457 nm Nd:YVO4蓝光激光器输出镜多层膜反射相位偏移。结合I类匹配LBO倍频晶体的倍频特性,讨论了输出镜反射膜对腔内基光偏振特性造成的影响,及其对I类相位匹配倍频效率的影响。研究发现,当基频光在输出镜处,入射角不大于15° 的情况下,造成的反射相移小于2° 。在近似相位匹配下,倍频效率相对于理想情况降低幅度不超过6%。     

铁电晶体畴壁增强非线性性质的研究

通过研究畴壁切伦科夫倍频,发现畴壁存在巨大增强的非线性系数.由此造成的畴壁“定域性”使其可以对非线性极化波产生相速度调制,进而影响切伦科夫倍频.提出并证实了极限切伦科夫倍频的存在,并分析了它和准相位匹配倍频产生的区别.     

利用宽带倍频特性精确测量准相位匹配晶体的极化周期

 提出了利用准相位匹配晶体进行宽带脉冲倍频，再根据准相位匹配晶体的极化周期与倍频脉冲输出中心波长的相位匹配关系来精确测量准相位匹配晶体极化周期的新方法。理论分析了准相位匹配晶体的极化周期与倍频脉冲的光谱关系、倍频容许带宽与晶体长度的关系。研究表明：宽带脉冲倍频后输出的脉冲峰值波长对应于角度的调谐曲线具有对称性；对于长度为10 mm的极化晶体，其倍频容许带宽不到0.2 nm。实验中，采用了国产PPKTP晶体试验片，晶体尺寸为10 mm×7 mm×1 mm，理想极化周期9 300 nm。结果表明：对于中心波长为537.25 nm的倍频光，其对应的实际晶体极化周期为9 303.9 nm，大于晶体加工时的理想值；当倍频脉冲光谱测量精度为0.01 nm时，准相位匹配晶体的极化周期测量精度达到0.1 nm，远高于一般光学显微镜的观测精度。     

基于周期极化化学计量比掺氧化镁钽酸锂晶体的高效率、单程、连续外腔倍频

利用长度为20mm的周期极化化学计量比掺氧化镁钽酸锂晶体（PP-MgO: SLT），实现了转换效率为11.8%的单程连续外腔准相位匹配（QPM）倍频，此时1064nm基频光的输入功率为7.69W，获得的532nm倍频光的输出功率为905mW。实验中可以发现，在不同的基频光聚焦条件和输入功率下，对应最大倍频输出的晶体温控炉的设定温度也要随之改变。此外，对允许角和允许温度也进行了研究，实验结果和理论结果符合的很好。     

Theoretical analysis of optimal quasi-phase matched second harmonic generation waveguide structure in LiTaO3 substrates

Based on the QPM structure and the coupled-mode theory, the maximum SHG conversion efficiency of QPM-SHG waveguides in LiTaO3 substrates is analyzed by optimizing the mode confinement and the mode overlap factors. For TM00()-to-TM00(2) conversion with a non-square shaped domain inversion QPM structure, the mode confinement and mode overlap factor are dependent not only on the optical field distribution for the fundamental and the SH waves in the waveguide, but also on the domain inversion shape. With the assumption that the refractive index profile of an annealed, proton exchanged LiTaO3 channel waveguide is an exponential decay function in the depth direction, and a Gaussian function in the width direction, the analytical expression of the optical field distribution for the lowest order mode can be obtained. By considering both a non-Gaussian field distribution and a non-square shaped domain inversion structure, the theoretical values for achieving the optimal QPM-SHG waveguide structure are determined.     

Efficiency pedestal in quasi-phase-matching devices with random duty-cycle errors

It is shown that random duty-cycle errors in quasi-phase-matching (QPM) nonlinear optical devices enhance the efficiency of processes far from the QPM peak. An analytical theory is shown to agree well with numerical solutions of second-harmonic generation (SHG) in disordered QPM gratings. The measured efficiency of 1550?nm band SHG in a periodically poled lithium niobate (PPLN) waveguide away from the QPM peak agrees with observations of domain disorder in a PPLN wafer by Zygo interferometry. If suppression of parasitic nonlinear interactions is important in a specific application of QPM devices, control of random duty-cycle errors is critical.     

1.5μm波段基于级联二阶非线性的铌酸锂光波导全光波长变换的理论分析

研究了基于级联二阶非线性的铌酸锂波导全光波长变换器的特性.首先从耦合模方程出发,比较了数值分析结果与小信号近似分析的结果.其次在数值分析基础上,分析了铌酸锂晶体的温度变化导致相应基频光波波长与极化反转光栅周期的变化关系.最后分析了在不同相互作用长度下,转换的光波功率与有效基频光波波长带宽、温度调谐带宽、极化反转光栅周期带宽等关系,以对全光波长变换器件进行优化设计 关键词： 级联二阶非线性 波长变换 准位相匹配 铌酸锂光波导     

Electro-optically controlled efficiencies in a QPM coupled parametric process

We present a method for controlling the efficiencies of frequency doubling and frequency tripling in a QPM coupled parametric process based on the electro-optic effect. The numerically simulated results verify that this method is feasible and very efficient for controlling the energy exchange from the fundamental wave to the harmonic waves at any fundamental intensity. In terms of efficiencies controlled by the electro-optic effect, the asymmetric duty-cycle periodic structure is more reasonable than the symmetric one. In addition, we also show that electro-optic control is significantly better than temperature control.     

Difference-Frequency Generation Among Ultrashort Optical Pulses in Quasi-Phase-Matching Waveguides

A theoretical model of ultrashort pulse difference-frequency generation (DFG) is presented and developed with quasi-phase matching (QPM) gratings in the undepleted-pump, unamplified-signal approximation. An analytic solution to a case of ultrashort pulse pumped DFG process, including group-velocity mismatching effects and input pulses with frequency chirp and time delay, is obtained. The influences of group-velocity mismatching effects and input signal pulse parameters on the conversion efficiency and output converted pulse characteristics are analyzed and discussed. The results indicate that the conversion efficiency is increased due to the enhancement of pump and signal interaction. The output pulse is inevitable compressed under pulsed pumping and can be compensated by introducing the initial frequency chirp and time delay.     

Perfect quasi-phase matching for the third-harmonic generation using focused Gaussian beams

We propose a method to realize perfect quasi-phase matching (QPM) for nonlinear optical interactions involving Gaussian beams. Using this method, both the wave-vector mismatching and the Gouy phase shift can be compensated. Numerical simulations for the third-harmonic generation show that conversion efficiency near to 100% can be realized even if the fundamental wave is tightly focused, which is difficult or even impossible with the conventional QPM method.     

Waveguide second-harmonic generation device with broadened flat quasi-phase-matching response by use of a grating structure with located phase shifts

We report on a theoretical analysis and experiments for bandwidth broadening in quasi-phase-matched (QPM) second-harmonic generation (SHG).We used phase-shifted segments of a periodic grating to obtain a spectrally broadened, nearly flat response simultaneously with high conversion efficiency. We used an x-cut MgO:LiNbO(3) QPM waveguide in our analysis and experiments. The spectral range of the 850-nm fundamental for which SHG conversion exceeded 0.95 of the maximum value broadened from 0.02 to 0.12 nm when a 1-cm-long grating was divided into three segments with optimum phase shift. SHG conversion efficiency was 300%/W for this waveguide. The SHG efficiency and phase-matching characteristics showed good agreement with theoretical results.     

Engineerable compression of ultrashort pulses by use of second-harmonic generation in chirped-period-poled lithium niobate

We demonstrate the use of an aperiodic quasi-phase-matching (QPM) grating to generate second-harmonic pulses that are stretched or compressed relative to input pulses at the fundamental frequency. We frequency doubled an externally chirped erbium-doped fiber laser generating 17-ps (FWHM) pulses at 1560nm to produce near-transform-limited 110-fs (FWHM) pulses at 780nm by use of a 5-cm-long lithium niobate crystal poled with a QPM grating chirped from an 18.2- to a 19.8-microm period.