可调谐准相位匹配高效宽带二次谐波转换

高效宽带二次谐波转换在光通信、信号处理和光谱学等很多领域都有重要的应用.通常高效宽带二次谐波转换的研究都集中在几个波长,为了得到可调谐准相位匹配高效宽带二次谐波转换,理论分析了准相位匹配和群速度匹配条件.在此基础上,分别计算了0型和Ⅰ型准相位匹配情况下,温度对5 mol%掺杂氧化镁周期性极化铌酸锂和周期性极化铌酸锂晶体准相位匹配高效宽带二次谐波转换的影响.对于5 mol%掺杂氧化镁周期性极化铌酸锂晶体,在0型和Ⅰ型准相位匹配情况下,分别得到了调谐宽度15 nm和341 nm的可调谐准相位匹配高效宽带二次谐波转换;对于周期性极化铌酸锂晶体,在0型和Ⅰ型准相位匹配情况下,分别得到了调谐宽度44 nm和98 nm的可调谐准相位匹配高效宽带二次谐波转换.拓展了准相位匹配高效宽带二次谐波转换的波长范围.     

基于周期极化晶体MgO:PPLN的纠缠光子对制备过程的研究

基于二阶非线性效应的自发参量转换技术制备纠缠光子对过程中,以掺5 mol%MgO:PPLN周期极化晶体为研究对象,将光参量变换过程中的动量守恒和能量守恒条件与该晶体的色散方程,以及晶体极化周期随温度变化的热膨胀方程相联系,得到了355 nm、405 nm、532 nm、780 nm和1 064 nm这5个实验室常用波长点在制备纠缠光子对时的周期调谐特性和温度调谐特性。研究过程中发现,出现了晶体极化周期过小和产生两对纠缠光子对问题,总结并归纳了各波长点在一定极化周期和温度下与非线性晶体作用所产生的纠缠光波段范围。当选用其他非线性周期极化晶体进行实验时,改变QPM动量守恒条件中的极化周期项,同时根据具体使用的晶体改变色散方程。该研究方案可直接推广到使用不同非线性晶体产生通信光波段或红外光波段的纠缠光子对研究中,在制备量子光源等领域具有重要研究价值。     

温度调谐准相位匹配光参量产生的研究

对温度调谐光学参量产生的特性进行了全面的理论分析和计算,得到参量产生过程中的允许温度、温度调谐速率以及允许温度与输出参量光波长的关系曲线、信号光波长与温度调谐速率的关系曲线,同时对泵浦光的波长、非线性晶体长度、调谐温度进行了分析,以获得较高的转换效率。     

准相位匹配PPMgLN光参量振荡技术

 理论上分析了PPMgLN光参量振荡波长调谐特性，计算了泵浦阈值和转换效率。准相位匹配情况下，周期调谐是获得中红外波长调谐有效方法之一。采用高斯光束泵浦，当泵浦功率密度超过阈值泵浦功率密度约6.5倍时，可以获得最高转换效率，约71%。采用1 064 nm激光泵浦多周期PPMgLN晶体，实验上获得了波长调谐范围2.7~4.8 mm，当泵浦功率为8 W时，在波长3.7 mm处激光输出功率超过1.6 W，斜效率超过20%，相应的转换效率约为69%，与理论分析基本一致。     

高非线性光子晶体光纤中优化产生宽带紫外三次谐波

通过非线性光学频率转换技术产生不同频率的脉冲辐射是实现具有短波波长的激光光源的有效手段.近年来,光子晶体光纤技术的发展为解决传统的基于非线性晶体的频率转换系统面临的难以维护、转换效率低、不易推广等问题带来了新的解决思路.在频率转换研究领域中,紫外光波段的脉冲辐射产生一直以来都受到学者的广泛关注.国外已报道过利用超短脉冲抽运光子晶体光纤实现三次谐波产生,从而输出具有高灵敏度和高分辨率的窄带紫外脉冲辐射,但其紫外光转换效率较低,且光谱的可调谐能力有限,而这些缺陷恰恰可以由宽带紫外脉冲辐射的获得来改善.宽带紫外脉冲辐射的有效获得不仅意味着紫外光转换效率可大幅提高,并且若加以合适的滤波手段,还可获得内任意波长下的窄带宽的脉冲辐射,从而增加窄带紫外脉冲辐射的可调谐度,但目前相关报道较为有限.本文将中心波长为1035 nm,脉冲重复频率为50 MHz的飞秒激光耦合至一定长度的高非线性光子晶体光纤中,将其产生的拉曼自频移孤子作为三次谐波产生的抽运源,通过相位匹配作用,在深紫外波段产生高阶模式传导下的三次谐波.随后让超短脉冲以偏离光纤轴心一定角度入射,可进一步激发出具有更短波长的超高阶紫外光模式.通过激发多个邻近的超高阶紫外光模式,在一定连续范围内实现相位匹配,获得超高阶紫外光模式传输下紫外光转换效率为3.6%的宽带(32—360 nm)深紫外脉冲激光.实验结果与理论模拟结果相一致.     

基于光纤环形腔激光器的可调谐全光波长转换器的研究

在光纤环形腔激光器中引入周期极化反转铌酸锂(PPLN)光波导，用该激光器产生的连续光作为抽运光和控制光，使其与外加的信号光发生非线性效应实现可调谐波长转换.介绍了基于准相位匹配的PPLN光波导中的和频与差频级联型全光波长转换器的基本原理.对抽运光、信号光、控制光以及转换光的光功率随着PPLN光波导的变化进行了模拟.还对转换效率随着转换光波长的变化进行了数值计算.实验验证了该波长转换器的可调谐性. 关键词： 周期极化反转铌酸锂 和频与差频 可调谐全光波长转换 光纤环形腔激光器     

基于准相位匹配产生近中红外波段的特性分析

基于周期极化晶体的Sellmeier方程和准相位匹配理论,对准相位匹配光参变振荡器的调谐特性进行了理论分析.通过数值模拟计算,得到了PPRTA晶体光参变振荡的调谐特性与极化反转光栅周期、晶体温度和抽运光波长等参量的关系曲线.并且与PPKTP晶体和PPLN晶体光参变振荡器的调谐特性进行了比较,研究了三者的不同之处,得出了PPRTA晶体的参量调谐特性优于PPKTP晶体和PPLN晶体的结论,与国外已报道的实验数据相吻合.证实了PPRTA晶体是一种可以产生可调谐近中红外光的理想准相位匹配非线性光学晶体.     

Ba1-xB2-y-zO4SixAlyGaz晶体和频可调谐深紫外飞秒激光器

介绍了一种基于新型非线性晶体Ba_1-xB_2-y-zO₄Si_xAl_yGa_z 的可调谐深紫外飞秒激光光源. 从理论上分析了基频光和倍频光在通过非线性晶体时所造成的空间走离和群速度失配, 为了补偿空间走离以及波长调谐过程中晶体折射造成的光束偏离现象, 将两块相同的倍频晶体成镜像放置来产生二次谐波. 并调节延迟线的长度来补偿基频光和倍频光之间的群速度失配, 从而提高和频转换效率. 然后通过和频方式进行三倍频和四倍频来突破晶体相位匹配条件的限制, 产生了波长低于200 nm的深紫外飞秒激光. 利用钛宝石激光器提供基频光光源, 最终在250–300 nm, 192.5–210 nm 范围内获得了高重频、可调谐超短脉冲紫外和深紫外激光. 并在基频光波长为800 nm时, 得到的二倍频、三倍频和四倍频的功率分别为1.28 W, 194 mW和5.8 mW, 相对于前一级的转换效率依次为46.14%, 15.16%和3%. 采用互相关法测量得到266.7 nm紫外激光的脉冲宽度约为640.4 fs.     

全硅光子晶体波导中二次谐波产生及影响因素

硅材料是半导体微细加工工艺中的常用材料,属于m3m对称点群,通常无法实现二次谐波产生,导致光子系统芯片中的非线性组件集成困难.提出一种在硅材料中可以实现二次谐波产生光子晶体波导结构.首先给出该波导结构的组成及其基本原理,然后讨论谐波产生的计算模型和计算方法,最后给出针对10.6 μm波长而设计的全硅二维光子晶体波导具体结构参数,以有限时域差分算法为基础,计算分析了谐波产生情况.研究结果表明:该结构利用光子晶体带隙边缘效应增强了硅材料的电四极极化强度从而实现二次谐波产生,在完全相位匹配条件下,当抽运波强度为1.3 MW/mm2时,转换效率为0.2%.最后,对影响谐波转换效率的因素进行了初步分析.     

倾斜入射光入射于一维非线性光子晶体中带宽加宽的二次谐波的产生(英文)

研究了置于空气中的含缺陷的一维非线性光子晶体中二次谐波的产生.由于反射的二次谐波很强而不能忽略,缓变振幅近似在系统中是不适用的.本文提出了一种不采用缓变振幅近似来处理二次谐波产生问题的方法,并应用这种方法计算相关的二次谐波转换效率.结果表明:随着入射波角度的增加,缺陷模对应的波长将变短,并且通过调整基频波入射的角度,可以产生宽带宽并且高转换效率的二次谐波.这个方法适用于任何一维非均匀系统,可以简单方便地计算出二次谐波转换效率.     

Compensation for the thermal effect in the second-harmonic generation of a Q-switched nanosecond-kilohertz Nd:YVO4 laser

We report the quantitative relationship between the phase-matching condition and the surrounding temperature in a critical phase-matching second-harmonic generation of a Q-switched Nd:YVO₄ laser with a high repetition rate. The thermal effect can be effectively compensated for by appropriately changing the phase-matching angle with respect to the surrounding temperature of a nonlinear optical crystal. A stable wavelength conversion from the 1064 nm fundamental to the 532 nm second harmonic in a type-I critical phase-matching lithium triborate crystal is experimentally achieved and the corresponding conversion efficiency up to 70% is obtained.     

Relation between efficiency of second harmonic generation and spectral properties of a one-dimensional photonic crystal

Numerical calculation of second harmonic generation in a photonic crystal consisting of alternating quarter-wave layers of ZnS and SeF₂ pumped by a femtosecond tunable laser is carried out. The results are compared with the spectral characteristics of the photonic crystal obtained with the use of a matrix method for calculating transmission coefficients. It is shown that the maximum efficiency of conversion to the second harmonic takes place at a minimum group velocity mismatch of the pump and second harmonic waves, corresponding to pump frequencies in the range below the band gap edge of the photonic crystal.     

Spectra of secondary emission during generation of optical harmonics in globular photonic crystals

Secondary emission spectra of globular silica photonic crystals when their surfaces were exposed to laser pulses 250 fs long at a power density to 1 TW/cm² have been studied. Optical harmonics and plasma emission were detected in this case. For the opal matrix containing pores filled with air, in the reflection mode, the third optical harmonic with a conversion efficiency of ～10% arises. The highest conversion efficiency for exciting radiation with wavelengths of 1026 or 513 nm is implemented when the frequencies of the exciting radiation or the second harmonic are near the stop band edge. In globular photonic crystals filled with sodium nitrite or barium titanate ferroelectrics, the second optical harmonic is observed. The exciting radiation conversion efficiency to the second optical harmonic was a few percent and depended on the frequency of exciting radiation and photonic crystal globule diameters. It is found that the plasma emission intensity increases with the exciting radiation power density. The dependences of the intensity of the second and third optical harmonics on the pump intensity are constructed for various photonic crystal globule diameters.     

Parameters for efficient growth of second harmonic field in nonlinear photonic crystals

The ultrashort pulse propagation and nonlinear second harmonic generation under the undepleted pump approximation in a quadratic nonlinear photonic crystal (NPC) structure is theoretically investigated and the optimized parameters for high second harmonic generation conversion efficiency are extracted. The transfer matrix method is used for the numerical formulation for oblique angle of incidence. A unique set of material combination GaInP/InAlP is selected as alternating nonlinear and linear layers. The NPC parameters like incident angle and layer thickness are manipulated to obtain the exact phase matching using double resonance condition for a fixed number of layers with known experimental material parameters.     

Frequency doubling of phase-modulated femtosecond laser pulses in periodically poled and chirped nonlinear crystals

The second harmonic generation (SHG) at a wavelength of 0.8 μm by 50-and 10-fs pulses with and without phase modulation (PM) was systematically studied in LiNbO₃ crystals with regular domain structure and linearly varied domain thickness. The main results were obtained by numerical method, taking into account the difference between group velocities of interacting pulses and the group-velocity dispersion. In the approximation of the given field of the fundamental radiation, an analytical expression was derived for the spectral density of the second harmonic in the periodically poled nonlinear crystal (PPNC) under nonstationary excitation conditions. It was numerically found that the conversion efficiency of about 90% can be achieved by doubling the frequency of 50-fs laser pulses without PM in the LiNbO₃ PPNC. The maximum conversion efficiency for the SHG by PM pulses is achieved at a certain optimum chirp step in the crystal domain length, which depends on both the value and sign of frequency modulation.     

Wavelength conversion in a Ti:PPSMgLN channel waveguide

A periodically poled titanium (Ti)-diffusion waveguide in near-stoichiometric MgO:LiNbO₃ (SMgLN) was fabricated that exhibits a second harmonic generation (SHG) efficiency of 63%. The device shows very high resistance to photorefractive damage at room temperature. All optical wavelength conversion by difference frequency generation (DFG) has been demonstrated in a periodically poled SMgLN (PPSMgLN) with Ti-diffusion channel waveguides. The wavelength conversion efficiency was measured to be −7.3 dB with the pump power of 150 mW and the signal power of 50 mW at room temperature.     

倾斜入射光入射于一维非线性光子晶体中带宽加宽的二次谐波的产生

研究了置于空气中的含缺陷的一维非线性光子晶体中二次谐波的产生.由于反射的二次谐波很强而不能忽略,缓变振幅近似在系统中是不适用的.本文提出了一种不采用缓变振幅近似来处理二次谐波产生问题的方法,并应用这种方法计算相关的二次谐波转换效率.结果表明:随着入射波角度的增加,缺陷模对应的波长将变短,并且通过调整基频波入射的角度,可以产生宽带宽并且高转换效率的二次谐波.这个方法适用于任何一维非均匀系统,可以简单方便地计算出二次谐波转换效率.     

Computing for second harmonic generation in one-dimensional nonlinear photonic crystals

A numerical study of second harmonic generation (SHG) in one-dimensional nonlinear photonic crystals based on full nonlinear system of equations, implemented by a combination of the method of finite elements and fixed-point iterations, is reported. This model is derived from a nonlinear system of Maxwell’s equations, which partly overcomes the known shortcoming of some existing models relied on the undepleted-pump approximation. We derive a general solution of SHG in one-dimensional nonlinear photonic crystals structures. The convergence of our method is fast. Numerical simulations also show the conversion efficiency of SHG can be significantly enhanced when the frequencies of the fundamental wave are located at the photonic band edges or are assigned to the designed defect states.     

微环谐振腔实现倍频效应的带宽分析

谐振腔能以较低的输入功率获得较高的非线性转换效率,其代价是带宽方面的限制,围绕微环谐振腔实现倍频效应中影响转换效率的因素,分析了耦合系数、折射率变化、相位失配等对转换带宽的影响,讨论了基于AlGaAs的人工双折射微环谐振腔实现基频光波波长为1840 nm时倍频效应的转换带宽.