准相位匹配（QPM）技术研究新进展

准相位匹配是一种人为地制备周期结构以实现非线性频率转换技术,它拓宽了非线性晶体应用范围,提高了频率转换效率。本文详细介绍了准相位匹配主要技术优势,并描述周期极化畴反转技术,这一当前普遍用来制备准相位匹配非线性晶体的实验方法,同时总结了这一领域国内外研究的最新进展。     

PPLN准相位匹配技术在蓝绿激光器及中红外OPO中的应用进展

以PPLN、PPMgLN、PPKTP等为代表的周期极化工艺的成熟,极大地促进了准相位匹配技术的发展,拓宽了非线性晶体应用范围,大大提高了频率转换效率。周期极化晶体与其它非线性晶体结合,利用倍频、差频、混频、OPO等技术是实现激光波长拓宽的一种非常有效的手段。本文阐述了QPMPPLN在蓝绿激光器及中红外OPO的应用进展,展望了PPLN的应用发展趋势。     

非线性光子晶体二次谐波的产生与仿真

在光子晶体的一维周期极化结构中，其频率转换只允许在单一方向上进行，而二维准相位匹配光子晶体比一维准相位匹配光子晶体具有更多方向上的倒格矢。故可实现多方向、多波长的同时倍频。文中在各向异性非线性FDTD算法的基础上。对周期为6．82方形格子二维周期性极化的LiNbO3非线性光子晶体进行了分析，给出了利用一阶准相位匹配来实现1．06μm波长的二倍频方法。     

基于周期极化晶体的QPM-SHG技术发展及其应用

随着新型高质量的非线性晶体的出现,准相位匹配技术成为非线性频率变换的最有效的方法。从周期极化晶体准相位匹配二次谐波发生器（QPM-SHG）的基本原理出发,简要介绍了实现周期极化晶体的方法和技术,总结了准相位匹配技术在光子学微结构晶体材料和波导材料中的最新研究进展和发展趋势,展望了用微结构波导QPM-SHG实现飞秒光纤光源的发展应用前景。     

飞秒激光制备非线性光子晶体研究进展

非线性光子晶体能够实现高效的非线性光学过程,其制备手段吸引了该领域研究者的高度关注。飞秒激光加工技术具有极高的精度、分辨率和灵活性,相比传统的非线性结构制备工艺具有独特的优势。总结归纳了利用飞秒激光加工技术构建非线性光子晶体的研究进展,并对涉及的准相位匹配原理进行了简要介绍。讨论了飞秒激光反转铁电畴和擦除非线性系数的加工机理,论述了这两种方式在多种维度非线性光子晶体加工方面的实验成果和应用。最后分析了目前飞秒激光加工非线性光子晶体所遇到的挑战,并展望了未来的发展前景。     

11.8 W高效率掺氧化镁的周期极化铌酸锂晶体光参量振荡2.7 μm激光器

报道了采用1064 nm激光椭圆光斑抽运掺氧化镁的周期极化铌酸锂(PPMgLN)晶体准相位匹配(QPM)技术实现2.7 μm激光输出的实验结果.理论计算了PPMgLN晶体准相位匹配周期调谐曲线,得出PPMgLN晶体周期为31.3 μm时可获得中红外波长2.7 μm激光输出.PPMgLN晶体(MgO掺杂摩尔分数为5%)单谐振光参量振荡(OPO)技术采用e→e+e相位匹配,消除了光束之间的走离效应和利用了PPMgLN晶体的最大非线性系数d33(27.4 pm/V).在1064 nm激光抽运功率78 W,声光Q开关工作频率8 kHZ的条件下,获得了平均功率11.8 W,波长2.72 μm的激光输出,斜率效率19.5%,对应闲频波长1.75 μm激光输出功率约24 W.2.7 μm激光水平方向和垂直方向光束质量M2因子分别为2.04和5.56.     

扫描电镜加工铌酸锂晶体电畴反转光栅的研究

本文简单介绍了非线性晶体相位匹配和铌酸锂光波导准相位匹配倍频原理，开展了扫描电子显微镜电子束辐照反转极化实验，分析了实验结果，摸索出合适的工艺条件，证明利用扫描电镜可以加工出满足准相位匹配条件的电畴反转我栅。     

利用电场矢量叠加法对准相位匹配原理再探讨

从准相位匹配的基本原理出发,提出了一种研究周期极化介质中二阶非线性的三波互作用现象的分析方法--矢量叠加法.这种方法利用了相互作用的电场矢量叠加的特征曲线,清晰地描绘出晶体内部产生的倍频光的电场变化规律.该方法描述物理现象清晰,结果简洁直观,不仅与经典的傅里叶变换方法结果相一致,而且也可以用于分析周期极化结构中各种误差对倍频效率的影响.它还为准相位匹配技术新的实现方法奠定了理论基础.     

1064 nm激光抽运PPMgLN光参量振荡高效率2.7 μm激光器

报道了采用1064 nm激光抽运PPMgLN晶体准相位匹配(QPM)技术实现高效率2.7μm激光输出的实验结果,理论计算了PPMgLN晶体准相位匹配周期调谐曲线,得出PPMgLN品体周期为31.3μm时可获得中红外波长2.7μm激光输出.PPMgLN晶体(MgO掺杂摩尔分数为5%)单谐振光参量振荡(OPO)技术采用e→e+e相位匹配.消除了光束之间的走离效应并利用了PPMgLN晶体的最大非线性系数d33(27.4 pm/V).在1064 nm激光抽运功率为26 W,声光Q开关工作频率为7 kHz的条件下,获得平均功率为4.7 W,波长为2.72μm激光输出,斜率效率超过21%,对应闲频光波长1.75μm激光输出功率约9 W.2.7μm激光水平方向和垂商方向光束质量M2因子分别为2.05和1.84.     

适于长波红外上转换器的非线性晶体特性研究

为了发展基于参量上转换的红外成像技术,依据可将10.6 μm红外辐射转换至可见波段的上转换器的理论模型,计算模拟了GaSe、ZnGeP2和AgGaS2晶体在各种相位匹配方式下的相位匹配角、走离角、和频允许角和有效非线性系数,并对结果进行了分析和对比.ZnGeP2晶体和AgGaS2晶体均可以实现非临界相位匹配,和频允许角...     

Nonlinear Transient and Distortion Analysis via Frequency Domain Volterra Series

This paper presents a novel approach for transient and distortion analyses for time-invariant and periodically time-varying mildly nonlinear analog circuits. Our method is based on a frequency domain Volterra series representation of nonlinear circuits. It computes the nonlinear responses using a nonlinear current method that recursively solves a series of linear Volterra circuits to obtain linear and higher-order responses of a nonlinear circuit. Unlike existing approaches, where Volterra circuits are solved mainly in the time domain, the new method solves the linear Volterra circuits directly in the frequency domain via an efficient graph-based technique, which can derive transfer functions for any large linear network efficiently. As a result, both frequency domain characteristics, like harmonic and intermodulation distortion, and time domain waveforms can be computed efficiently. The new algorithm takes advantage of identical Volterra circuits for second- and higher-order responses, which results in significant savings in driving the transfer functions. Experimental results for two circuits—a low-noise amplifier and a switching mixer—are obtained and compared with SPICE3 to validate the effectiveness of this method.     

Matrix analysis for layered quasi-phase-matched media consideringmultiple reflection and pump wave depletion

A new matrix method for the distributed nondepleted pump wave approximation and its extension to include pump wave depletion are proposed for the analysis of optical wave interactions through layered nonlinear media, which takes internal multiple reflections into account and, hence, can lead to a precise analysis for both forward and backward nonlinear interactions. Using this method, several calculation results are presented for difference frequency generation and second-harmonic generation in periodically layered quasi-phase-matched media composed of Al_0.2Ga_0.8As and Al_0.38Ga_0.62As. In addition, the multiple reflection effect in quasi-phase-matched media is discussed     

AN IMPROVEMENT TO THE HARMONIC-BALANCE TECHNIQUE FOR ANALYZING NONLINEAR CIRCUITS

In this paper,an improved method,which is based on the harmonic-balance tech-nique,for speeding up the convergence iteration procedure is presented.As an example,it isapplied to the nonlinear analysis of microwave mixer,in which a simple time domain to fre-quency domain transformation technique for the mixer diode is firstly introduced.The use of thistechnique saves computing time dramatically.And the analyzed results are in good agreementwith the results published.The improved method can also be extended to the CAA of nonlinearcircuits with multi-devices.     

Section-wise-exact coupled-mode theory of waveguidequasi-phase-matched second-harmonic generation

We derive exact mathematical expressions for both the amplitude and phase evolution of optical waves due to linear and nonlinear interactions in a waveguide device and propose a section-wise-exact coupled-mode theory analysis for quasi-phase-matched second-harmonic generation in the waveguide. Section-wise-exact coupled-mode theory is readily applicable to nonuniform quasi-phase-matched gratings and can be useful as a quantitative method in their analysis and design. Using the method, several numerical results are presented for second-harmonic generation in periodically poled LiNbO₃ channel waveguides. In addition, we introduce a split-step method which takes propagation losses into account and which can include linear and two-photon absorption and waveguide losses     

分析非线性电路的谐波平衡法的改进

本文根据分析微波非线性电路的基本方法之一谐波平衡法,提出了一种加速收敛过程的改进方法,并且将其应用于微波混频器的非线性分析。在分析过程中引入了混频二极管非线性特性由时域到频域的简化变换技术,大大加快了计算速度。分析结果与已发表的结果相比较,一致性较好。这种改进的分析方法也可以推广应用于包含多个非线性器件的非线性电路的计算机辅助分析。     

Experimental and theoretical characterization of a 40-Gb/s long-haul single-channel transmission system

We present a comparison between experiment and simulation of a 40-Gb/s periodically stationary dispersion-managed soliton (DMS) system in a recirculating loop. We find that we can propagate an error-free signal over 6400 km at 40 Gb/s and over 12 000 km if we lower the data rate to 10 Gb/s, keeping all other parameters constant. A careful analysis of the limiting factors shows the strong influence of nonlinear optical pulse-to-pulse interactions, causing a large increase in timing jitter. At a transmission distance of 6400 km, a large fraction of the jitter is due to pulse-to-pulse interactions. Moreover, we find that the system performance is very sensitive to parameter variations. We conclude that periodically stationary DMS systems suffer from numerous problems when increasing the data rate, suggesting that it may be impractical for wavelength-division multiplex transmission at 40 Gb/s.     

All-optical switching based on cascading of second-ordernonlinearities in a periodically poled titanium-diffused lithium niobatewaveguide

We report efficient all-optical switching by using a periodically poled titanium-diffused lithium niobate (Ti:PPLN) waveguide. The periodically domain inversion of the Ti:PPLN waveguide is achieved by electric field poling. The switching is driven by the second-order nonlinear effect of the cascading of phase-matched sum-frequency-generation and difference-frequency-generation processes. It is found that about 12% of the signal is switched by a 20-W gate power     

All-optical switching due to cascaded second-harmonic generation indirectional couplers with laterally varying phase mismatches

The local normal mode approach is employed to analyze cascaded second-harmonic generation (SHG) in directional couplers with different phase mismatches in both waveguides. The couplers are assumed to be fabricated in ferroelectric crystals, which are periodically domain inverted with different periods in both waveguide regions. These components are proposed to imitate the switching in nonlinearly asymmetric third-order directional couplers by using the cascaded second-order effect, which can mimic third-order interactions with effective third-order nonlinearities proportional to the reciprocal phase mismatch. The results for the second-order directional couplers are compared to those for the respective ideal third-order ones     

Nonlinear modes, resonant trapping, and soliton emission in engineered PPLN structures

We investigate nonlinear waveguiding effects, due to second-order nonlinearity, in quasi-phase-matching (QPM) structures in periodically poled lithium niobate (PPLN) with engineered patterns. We specifically study the existence and the propagation of pure nonlinear guided modes in QPM gratings with different domain lengths in different transverse regions of the medium, without any waveguiding effect due to linear refractive index changes. On one hand, the obtained results enlighten the possibility of soliton operation at lower intensity thresholds than in standard not-tailored PPLN structures. On the other hand, in such nonlinear engineered structures, we reveal the presence of intensity-dependent attractive and repulsive potential wells that can be exploited for soliton control (resonant trapping and emission) in quadratic nonlinear media.