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。     

Temperature-dependent second harmonic generation process based on an MgO-doped periodically poled lithium niobate waveguide

The temperature dependency of a 5-mol% MgO-doped periodically poled lithium niobate waveguide was investigated in this paper. We started with the temperature-dependent refractive index equation for the waveguide. Secondly, the temperature dependency of the second harmonic generation effect was experimentally researched under different temperatures and pump powers. The quasi-phase matched wavelengths, efficiency bandwidths and peak efficiencies of the waveguide were measured. The experimental results agreed with theoretical simulations, which are indispensable in the following all-optical sampling studies based on the cascaded second harmonic generation/difference-frequency generation process in the current device.     

飞秒激光烧蚀LiNbO3晶体的形貌特征与机理研究

采用了不同能量的单脉冲和多脉冲飞秒激光对LiNbO₃晶体进行烧蚀，并刻蚀了表面衍射型光栅.通过扫描电镜和原子力显微镜观察了烧蚀点的形貌特征，首次发现利用单束飞秒激光脉冲对LiNbO₃晶体烧蚀，可以得到超衍射极限的烧蚀点，当聚焦光斑直径约为2μm、能量为170nJ的单脉冲飞秒激光作用时，烧蚀点的直径约为400nm，100nJ，17个脉冲作用时烧蚀点的直径约为800nm.同时可以观察到在能量较低的多脉冲飞秒激光作用下， LiNbO₃晶体呈现出大约200nm周期性分布的波纹状结构.实验结果表明，选择合适参数的飞秒激光脉冲可以对LiNbO₃晶体进行超衍射极限加工，这对于利用飞秒激光制作LiNbO₃基质的微纳光电子器件有十分重要的意义.     

Three-wavelength generation from cascaded wavelength conversion in monolithic periodically poled lithium niobate

Tunable coherent emission is generated in a single-pass,cascaded wavelength conversion process from mode-locked laser-pumped monolithic periodically poled lithium niobate(PPLN).Three ranges of wavelength,including visible output from 628 nm to 639 nm,near-infrared output from 797 nm to 816 nm,and mid-infrared output from 3167 nm to 3459 nm,were obtained from the monolithic PPLN,which consists of a 10-mm section for 532-nm-pumped optical parametric generation(OPG)and a 7-mm section for 1064-nm-pumped sum frequency generation(SFG).A pump-to-signal conversion efficiency of 23.4%for OPG at 50°C and a quantum efficiency of 26.2%for SFG at 200°C were obtained.     

飞秒激光加工铌酸锂晶体：原理与应用

由于具有超短的脉冲宽度和极高的峰值强度,飞秒激光微加工是一种有效的材料加工方法, 已广泛应用于光子集成器件的加工。铌酸锂晶体具有优异的电光、非线性光学和压电特性,是集成 光学和导波光学中常见的材料。本文综述了飞秒激光对铌酸锂晶体的处理,重点介绍了飞秒激光加 工的物理原理及其制备的铌酸锂基光子器件的最新进展。飞秒激光技术使铌酸锂晶体在微纳光子学 领域具有广阔的应用前景。     

Periodically poled lithium niobate structures grown by the off-center Czochralski technique for backward and forward second harmonic generation

We report on the characterization of periodically poled lithium niobate structures grown by the off-center Czochralski technique with periods ranging between 2 and . The domains distribution along the crystal was inspected by a profilometer scan after etching the structures and carrying a suitable data processing. The second harmonic generation efficiency was predicted by numerically integrating the governing equations through to a recently proposed nonlinear bidirectional beam propagation method. The numerical analysis pointed out the feasibility of the backward second harmonic generation in the sample with the shortest domain period. The predicted second harmonic generation efficiency was finally corrected considering the phase shifts induced in the second harmonic wave by the presence of different sized domains.     

畴腐蚀掺镁铌酸锂可调阵列光分束器的研究

研究了不同畴腐蚀深度的掺镁铌酸锂二维六角可调阵列光分束器的分数Talbot效应.对不同Talbot分数β和不同畴腐蚀深度的阵列光分束器Talbot衍射像进行了数值模拟理论研究.模拟结果表明,Talbot分数β可以改变Talbot衍射像的周期及结构分布,而畴腐蚀深度可有效调制衍射像的光强分布.在理论研究的基础上,设计并制备了具有不同畴腐蚀深度的掺镁铌酸锂二维六角阵列光分束器,对其在不同Talbot分数β条件下的分数Talbot效应进行了通光实验研究,实现了畴腐蚀阵列光分束器对近场Talbot衍射光强分布的调制,实验结果与理论研究结果一致.     

The ridge waveguide fabrication with periodically poled MgO-doped lithium niobate for green laser

Ridge waveguides were fabricated using an external field, a precision lapping machine and neutron loop discharge (NLD) in magnesium-oxide-doped lithium niobate. The measured quasi-phase-matching (QPM) wavelength of the second-harmonic generation (SHG) in a 30 mm long periodically poled magnesium-doped lithium niobate (PPMgLN) ridge waveguide which has a domain period of 6.8 μm is about 532 nm. A fabricated periodically poled magnesium-doped lithium niobate ridge waveguide was duty cycle of 51.9 ± 2.83% and demonstrated second-harmonic generation. By using this periodically poled magnesium-doped lithium niobate ridge waveguide, highly effective, low-cost optical devices with high power or short wavelength can be achieved.     

铌酸锂薄膜调制器的研究进展

铌酸锂薄膜调制器具有体积小、带宽高、半波电压低的优点,在光纤通讯和光纤传感领域具有重要应用价值,是近年来的研究热点。本文梳理了铌酸锂薄膜调制器的波导结构、耦合结构、电极结构的研究进展,总结了LN薄膜波导的制备工艺,并深入分析了不同结构调制器的性能。基于SOI和LNOI结构,薄膜调制器实现了V_πL<2 V·cm,双锥形耦合方案实现了耦合损耗<0.5 dB/facet,行波电极结构实现了调制带宽>100 GHz。铌酸锂薄膜调制器的性能在大多数方面优于目前商用铌酸锂调制器,随着波导工艺进一步提升,将成为铌酸锂调制器的热门方案。最后对铌酸锂薄膜调制器的发展趋势和应用前景进行了展望。     

Waveguides in lithium niobate fabricated by focused ultrashort laser pulses

We report on investigations of the bulk microstructuring of lithium niobate crystals with intense femtosecond laser pulses. In different crystal cuts, optical waveguides were produced whose properties depend strongly on the processing parameters. To explore the origin of the refractive index changes, we subjected the crystals to different conditions (like temperature, illumination, etc.) while monitoring the waveguide output. This way several mechanisms for the change in refractive index could be singled out. These include the photorefractive effect, inhomogeneous ion concentrations and stress in the crystalline lattice. As an application, we demonstrate frequency doubling of 1064 nm laser radiation in a microstructured phase-matched waveguide.     

Narrow-band terahertz wave generation and detection in one periodically poled lithium niobate crystal

In this article, we present studies on therahertz (THz) wave generation and frequency up-conversion in a periodically poled lithium niobate (PPLN) crystal. A frequency at 1.37 THz was generated as femtosecond pump pulses passed through a PPLN crystal with grating periods of 30 μm. The pump-induced THz wave interacts with the probe wave in the crystal by frequency mixing. The frequency up-converted THz wave is easily detected by a normal photodiode. A new scheme for generation and detection of THz wave in one non-linear crystal was proposed.     

Nano hillock and complex crater formation by low-energy proton implantation with incident angle into lithium niobate single crystal

The formation of nano-size hillocks and simple and complex craters was observed as a result of ion–surface collisions with a lithium niobate single crystal on proton implantation. The low-energy ion implantation process is considered as a controllable and versatile tool for surface and near-surface modifications down to an atomic scale as an alternative to the swift heavy ion irradiation effect. Lithium niobate samples implanted by proton ions with a low energy of 120 keV at various fluences (10¹⁵ and 10¹⁶ protons/cm²) were studied using atomic force microscopy (AFM). The images of surface modification appear as simple and complex crater formation in the case of incident ions at normal to the surface. Varying the angle of incidence to θ=30° with respect to the normal to the surface, hillocks and multi-hillocks were observed. The complex craters with central uplifted, cone-shaped hillocks with a height of up to 4.3 nm are surrounded by low-height (1 nm) rims. The hillock height varies from a few nanometers to 16 nm with the basal diameter from 200 to 340 nm depending on the ion implantation conditions. The complex crater and hillock formation on the lithium niobate sample surface at the collision spot with the impact of incident angle is discussed.     

散射介质中多重散射太赫兹脉冲的时域统计特性

对散射介质中多重散射太赫兹脉冲的时域统计特性进行了分析.给出了时间延迟量取不同值时电场的概率分布、散射场强度及其概率分布、散射场实部和虚部的概率分布.由于脉冲的宽带特性及瞬间特性,多重散射场概率统计分布与不同时间延迟量有关,太赫兹脉冲多重散射场的统计特性分布是非平稳过程.     

掺铁铌酸锂晶体中光致散射的锂组分和温度依赖关系研究

研究了单掺铁铌酸锂晶体的光致散射行为随锂组分以及温度的变化关系.实验表明,随晶体组分的升高,光致散射得到了大幅抑制;不同组分晶体光致散射被完全抑制的温度不同.由此提出掺铁铌酸锂晶体在48.9 mol%-49.3 mol%范围内可能存在一临界锂组分,当晶体组分超过这一临界组分时,晶体光致散射被大幅抑制,而光致散射行为的温度依赖关系明显改变.     

掺杂对铌酸锂晶体非挥发全息存储性能的影响

通过研究掺镁、掺锌和掺铟同成分铌酸锂晶体的紫外-红光双色全息存储性能,发现双色记录响应时间均比单色记录时明显缩短,最多的可减小3个数量级;双色记录灵敏度大幅度提高,在掺镁5 mol.%的晶体中可达到11 cm/J.在掺杂浓度超过抗光损伤阈值的铌酸锂晶体中,均可实现非挥发全息存储.但是,在掺镁、锌样品中,深、浅能级中心上的光栅反相,而在掺铟样品中则表现为同相.这是由于掺杂离子的种类不同,在铌酸锂晶体中形成的缺陷中心也不同所引起的. 关键词： 掺杂 铌酸锂晶体 非挥发 全息存储     

掺铁铌酸锂晶体的光电导衰减特性研究

利用电化学分析仪对掺铁同成分铌酸锂晶体进行瞬态光电导研究. 以不同掺铁浓度铌酸锂晶体为样品, 在不同强度的纳秒级脉冲光照射下对光电导的研究发现: 铌酸锂晶体的瞬态光电导是由较为复杂的电子迁移过程形成, 其衰减可以用一个指数函数叠加一个扩展指数来拟合. 拟合参数与光强、掺铁浓度存在以下依赖关系: 入射光强增强时, 幅值σ₁^max、σ₂^max、时间常数τ₂和扩展因子β 值增大, 在光强增大到一定时, τ₂和β出现饱和; 晶体的掺铁浓度升高时, σ₁^max、σ₂^max、τ₂ 值增大, 而β值减小. 根据实验结果, 从理论上提出了光电子导带迁移伴随光电子在小极化子上跳跃迁移的复合电荷传输模型. 该模型较好地解释了掺铁同成分铌酸锂晶体的光电导的衰减特点.     

磷化镓高功率太赫兹共线差频源的研究

在众多实现太赫兹辐射的方法中, 非线性光学共线差频能够实现高功率、宽波段、连续可调谐的太赫兹波辐射. 理论分析表明, 各向同性磷化镓晶体, 在1064 nm附近波长激光共线差频下具有毫米量级的相干长度, 能够满足高功率宽波段的太赫兹辐射条件.实验证明, 磷化镓晶体共线差频实现高功率宽波段的太赫兹光辐射, 其太赫兹光波长调谐范围为95.9–773.4 μm (0.39–3.13 THz), 最高峰值功率7 W位于频率2.0 THz处.该实验结果与理论计算基本保持一致. 关键词： 太赫兹源 磷化镓 共线差频     

The dielectric behaviour of doped near-stoichiometric lithium niobate in the terahertz range

The dielectric properties of near-stoichiometric LiNbO 3:Fe and LiNbO₃:Ce single crystals have been investigated using terahertz time domain spectroscopy in a frequency range of 0.7-1.6 THz at room temperature.When coupled with an applied external optical field,obvious photorefractive effects were observed,resulting in a modulation of the complex dielectric constant for the crystals.The variation in refractive index,| n|,had a linear relationship with the applied light intensity,accompanied by a step-like decrease at high intensity.The findings were attributed to the internal space charge field of the photorefraction and the light-induced domain reversal in the crystals.     

High-power, high-repetition-rate mid-infrared generation with PE-SRO based on a fan-out periodically poled MgO-doped lithium niobate

We present a high-average-power, pulsed mid-infrared pump-enhanced singly-resonant optical parametric oscillator (PE-SRO) using a fan-out periodically poled MgO-doped lithium niobate (MgO:PPLN). The pump laser is a Q-switched Nd:YAG laser with a repetition rate of 10 kHz. When the pump power was 22.0 W, a maximum idler output power of 3.4 W at 3781.4 nm was obtained. The thermal guiding effect caused by signal absorption was observed and the crystal heating power was measured with a new method.