Quasi-phase-matched second harmonic generation in a LiTaO3 waveguide

The authors report third-order quasi-matched (QPM) second harmonic generation (SHG) in a LiTaO₃ channel waveguide. A deep domain-inverted region is first fabricated by a proton-exchange and heat treatment technique. Then a uniform and low-pass channel waveguide is fabricated by pyrophosphoric acid proton exchange. Consequently, 12 mW of blue light is obtained at 424 nm wavelength with a conversion efficiency of 6%. The observed FWHM (full-width half maximum) temperature acceptance width for SHG power is 3.2°C and FWHM wavelength acceptance bandwidth for that is 0.2 nm. It is also shown that diffraction limited focusing of the generated blue light may be obtained     

Broadening of the phase-matching bandwidth in quasi-phase-matchedsecond-harmonic generation

We report on theoretical analysis and experiments for bandwidth broadening in quasi-phase-matched (QPM) second-harmonic generation (SHG) in LiTaO₃. QPM waveguides consisting of segments, where each segment has a phase-matching condition different from the others, are proposed to obtain a broad bandwidth and simultaneously, a high SHG efficiency. The waveguides were fabricated by adjusting the phase-matching condition either by changing the grating period of the SHG coefficient or by controlling the width of the waveguide. Consequently, the bandwidth of QPM-SHG was broadened to 0.35 nm with an SHG efficiency of 57 percent/W by modulation of the waveguide width and to 1.12 nm with an SHG efficiency of 29 percent/W by modulation of the grating period. The SHG efficiency and phase-matching characteristics in both types of waveguides showed good agreement with theoretical results     

Fabrication of domain reversed gratings for SHG in LiNbO/sub 3/ by electron beam bombardment

A novel technique for the lithographic definition and the fabrication of domain reversed regions in LiNbO/sub 3/ is reported, with application to periodic structures for second harmonic generation (SHG). For the first time, to the authors' knowledge, domain reversal has been achieved on the negative c-face of the crystal. Such a structure should be useful for quasi-phase-matched SHG of infrared laser radiation.<>     

周期性极化晶体RbTiOAsO4的允许参量分析

基于准相位匹配(QPM)技术,对周期性极化RbTiOAsO4(PPRTA)晶体的允许参量进行了理论分析和数值模拟,得到了在波长为1064 nm,Ⅰ类和Ⅱ类相位匹配条件下的允许波长分别为0.25 nm/cm,0.45 nm/cm,和允许温度分别为2.30℃/cm,17.14℃/cm。并获得了室温时的允许角度为3.12°/cm1/2。数值模拟结果表明,Ⅱ类相位匹配比Ⅰ类相位匹配更加适宜于倍频过程。     

Quasi-phase-matched second-harmonic generation indiazo-dye-substituted polymer channel waveguides

First-order quasi-phase-matched (QPM) second-harmonic generation (SHG) is demonstrated in a poled diazo-dye-substituted polymer channel waveguide. The channel waveguide with a nonlinear grating was fabricated by the serial grafting technique using conventional photolithography and reactive ion etching. The dependence of the conversion efficiency on both the waveguide parameter and the grating structure was derived theoretically. A normalized internal conversion efficiency of 1.1% W^-1 cm^-2 at 1.586 μm was obtained in the fabricated waveguide with a phase-matched interaction length of 3.4 mm. The experimentally obtained conversion efficiency is compared with the theoretical value, taking into account the effect of mode-mismatching and propagation loss     

Broadband quasi-phase-matched second-harmonic generation in MgO:LiNbO3 waveguide

The quasi-phase-matched(QPM) condition of broadband second harmonic generation(SHG) in Ti-diffused MgO:LiNbO3 waveguide is theoretically simulated.The results show that the center wavelength of broadband SHG dependent on the waveguide width is around 1550 nm and the bandwidth is 50 nm.     

Broadband second-harmonic generation in a tapered PPLN waveguide

We demonstrate a period poled tapered lithium niobate waveguide and study second harmonic generation (SHG) in this device for the purpose of broadening the quasi-phase matching (QPM) acceptance bandwidth. The finite-difference beam-propagation method is used to simulate the guided modes and calculate the effective indices. The simulation results show that by tapering the width of the cross section linearly, the phase mismatch between a specific input wavelength and its SHG signal can be varied along the propagation length. Ideal SHG phase-matching conditions for a wide range of input wavelengths in communication band from 1 542.5 nm to 1 553.5 nm can be satisfied in different positions of the waveguide.     

周期极化钽酸锂晶体腔内倍频实验研究

报道了基于准相位匹配(QPM)应用的周期性极化钽酸锂(PPSLT)晶体进行腔内倍频非线性光学变换实验研究.分析比对了不同长度(3.5 mm,5 mm,10 mm)晶体倍频过程.利用3.5 mm PPSLT晶体在2500 mW808 nm激光抽运下产生1445 mW 1064 nm基频光输出,经过倍频可以得到760 mW的绿光输出.同时对于制约倍频效果和效率提高的系统温度控制等问题进行了分析.     

PPLN倍频效率与温度关系分析

本文研究了准位相匹配周期性极化铌酸锂（PPLN）晶体的体器件倍频特性与晶体温度的关系，从准相位匹配位相失配关系出发，推导了PPLN光栅周期和倍频效率的表达式，并考虑到铌酸锂晶体的热膨胀因素，给出了温度调谐带宽精确表达式，理论分析与已有实验结果进行了比较，符合得较好。     

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.     

LiNbO3 waveguide quasi-phase-matching second harmonicgeneration devices with ferroelectric-domain-inverted gratings formed byelectron-beam scanning

The characteristics of the formation of LiNbO₃ ferroelectric-domain-inverted gratings using electron-beam scanning are examined and discussed for application to waveguide quasi-phase-matching second-harmonic-generation (SHG) devices. It is found that the domain inversion tends to occur in segmented regions, and the inversion width is thinner near the crystal surface than inside the crystal. The dependence of the SHG efficiency on the grating structure is examined theoretically. Prototype devices for green and blue light generation have been fabricated and the device performance examined. Normalized SHG efficiencies as high as 50%/W and 70%/W, respectively, are obtained in green and blue light generation devices of 3.3-mm length. The experimental results are compared with the theoretical results     

Blue light generation in LiNbO/sub 3/ waveguide SHG device with first order domain-inverted grating formed by EB scanning

An LiNbO/sub 3/ waveguide SHG device for blue light generation by first order quasiphase-matching was fabricated using the electron-beam scanning domain inversion technique for the first time. The period of the domain-inverted grating was 3 mu m and blue light of wavelength 0.44 mu m was obtained. The normalised SHG efficiency was 70%/W.<>     

Harmonic blue light generation in periodically poled bulk near-stoichiometric MgO:LiNbO/sub 3/

A periodically poled structure with 6.95 /spl mu/m period and 10 mm interaction length was fabricated in a 1 mm-thick 1.8 mol% MgO doped near-stoichiometric LiNbO/sub 3/ (MgSLN) by using a multi-pulse poling technique. Blue SHG characteristics of this periodically poled MgSLN were investigated, in comparison with a periodically poled 5 mol% MgO doped congruent LiNbO/sub 3/.     

All-Optical Format Conversions Using Periodically Poled Lithium Niobate Waveguides

We investigate all-optical format conversions by using cascaded second-order nonlinearities in periodically poled lithium niobate (PPLN) waveguides. Analytical solutions under non-depletion approximation with clear physical insights are derived, showing operation principles of various PPLN-based format conversions. We propose and theoretically demonstrate all-optical 40 Gb/s nonreturn-to-zero (NRZ) to carrier-suppressed return-to-zero (CSRZ), nonreturn-to-zero differential phase-shift keying (NRZ-DPSK) to return-to-zero differential phase-shift keying (RZ-DPSK), and NRZ-DPSK to carrier-suppressed return-to-zero differential phase-shift keying (CSRZ-DPSK) format conversions based on cascaded sum- and difference-frequency generation (cSFG/DFG). Tunable all-optical 20 Gb/s NRZ to return-to-zero (RZ) format conversion based on cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) is successfully confirmed in the experiment by setting NRZ signal at SHG quasi-phase matching (QPM) wavelength. Moreover, we experimentally report for the first time, PPLN-based all-optical 40 Gb/s NRZ-to-CSRZ, NRZ-to-RZ, and NRZ-DPSK-to-RZ-DPSK format conversions.      

LiNbO/sub 3/ waveguide SHG device with ferroelectric-domain-inverted grating formed by electron-beam scanning

A ferroelectric-domain-inverted grating was fabricated by electron beam scanning in LiNbO/sub 3/. A waveguide second harmonic generation (SHG) device with the grating was fabricated and demonstrated for the first time. The experiments were performed using a CW-Nd:YAG laser, and normalised SHG conversion efficiency of 50%/W was obtained.<>     

Efficient Frequency Doubling of a Low-Power Femtosecond Er-Fiber Laser in BiB$_{3}$ O$_{6}$

BiB₃O₆ (BIBO) crystal has been used for efficient second-harmonic generation (SHG) of a low-power femtosecond Er-fiber laser-amplifier system operating at 56 MHz. At the maximum input power of 65 mW, an internal conversion efficiency of 23% was achieved for SHG at 782 nm, with a pulse duration of 64 fs. A comparison with beta-BaB₂O₄ reveals superior properties of BIBO for such ultrashort-pulse ultra-broadband SHG.     

Three-dimensional vector beam-propagation method for secondharmonic generation analysis

A full vectorial three-dimensional beam-propagation method (BPM) based on the finite-element method is described for the analysis of second harmonic generation (SHG). Quasi-phase-matched SHG devices with periodically domain-inverted GaAlAs- and LiTaO₃-based waveguides are analyzed. The influences of the shape of domain-inverted regions and of the inversion width on the conversion efficiencies are investigated in detail. The results of full-wave analysis are compared to those of approximate scalar analysis     

Speed limit of all-optical gate switches using cascaded second-order nonlinear effect in quasi-phase-matched LiNbO/sub 3/ devices

In all-optical gate switches that employ the cascaded second-order nonlinear effect in quasi-phase-matched (QPM) LiNbO/sub 3/ devices, walkoff between the fundamental and second harmonic pulses is very large. The authors experimentally show that crosstalk of the switch induced by such walkoff limits the switching speed, but that the switching speed can significantly be enhanced by walkoff compensation. Using a 20-mm-long QPM LiNbO/sub 3/ waveguide device, the authors switch one of twin pulses separated by 6.25 ps without crosstalk, showing the possibility of switching a 160-Gb/s signal.     

利用二次谐波产生研究Al2O3/Al(111)表面的动力学变化

主要介绍了当激光入射到单晶金属Al2O3／Al(111)表面时将发生二次谐波产生(SHG)现象的实验装置和实验方法,利用该装置可以研究SHG和从Al2O3／Al(111)表面反射的SHG在空间各向异性的变化情况,从而可以探测Al2O3／Al(111)表面结构对称性。实验中发现,当脉冲激光功率在2×106 W／cm2-9．6×106W／cm2范围内变化时,没有检测到SHG信号的各向异性变化;当使用P-极化泵浦激光时,发现Al2O3／Al(111)样品绕法线旋转360度时,P-极化的SHG信号在空间三个方向上呈现最大值相等;当使用脉冲强度为12×100 W／cm2的1064 nm P-极化泵浦激光时, Al2O3／Al(111)表面被损坏,损坏后的表面其SHG信号并不呈现对称的各向异性变化,当使用脉冲强度为11×106W／cm2的1064 nm P-极化泵浦激光时产生表面退火现象,从退火表面所产生的反射532 nm P-极化SHG信号中发现,SHG信号呈现衰减的各向异性成分。     

非对称耦合量子阱的二次谐波产生与量子阱参数关系研究

采用反射式二次谐波产生 (SHG)方法对非对称Ⅱ Ⅵ族耦合量子阱Zn1 -xCdxSe ZnSe的非线性光学特性进行了研究。非中心对称性和阱间耦合效应在很大程度上增强了材料的非线性效应。发现在入射光和反射光均为p偏振 ,以及入射光和反射光分别为s偏振和p偏振两种情况下 ,SHG信号都随Cd含量x的增大而减小。与ZnSe基体材料相比 ,非对称耦合量子阱 (ACQW)在可见光波段的SHG信号增强一个量级以上。同时发现SHG信号随入射光偏振角的变化而周期性地变化。