论文标题:激光在畴反转光学超晶格中的传输、控制和频率变换研究
Study on the Laser Propagation, Control and Frequency Conversion in the Optical Superlattice
论文作者
论文导师 夏宇兴;陈险峰,论文学位 博士,论文专业 光学
论文单位 上海交通大学,点击次数 125,论文页数 124页File Size2482K
2008-05-01论文网 http://www.lw23.com/lunwen_3217777/
optical superlattice;; quasi-phase-matching;; nonlinear frequency conversion;; electro-optic effect;; Genetic Algorithm (GA)
自上世纪90年代以来,随着光学超晶格的制备工艺日趋成熟和完善,准位相匹配技术在多种材料中得以实现。光学超晶格材料的应用已从最初实现倍频发展到混频以及光脉冲整形和压缩等多种非线性光学应用领域。超晶格结构也从一维推广到二维,从周期结构推广到准周期结构,甚至是复杂的非周期结构,从而使得光学超晶格参与的非线性耦合过程变的更加复杂。且近年来,随着激光技术在现代光学技术发展中的作用越来越重要,对激光的强度、脉宽、波长范围等提出了更新、更高的要求。尤其是拓宽激光输出波长范围,最常用、最有效的方法之一就是利用光学超晶格晶体的频率变换技术。 本文根据铌酸锂光学超晶格材料的特有物理特性,即通过对其自发极化方向的改变,使其非线性系数和电光系数发生调制,通过对光学超晶格的结构设计实现对激光的偏振、强度、脉宽以及频率等参数调谐。主要内容如下: 1.晶体畴反转技术是光学超晶格制备的基础。本文从铁电晶体的微观结构出发,揭示了晶体畴反转过程的四个阶段,并详细讨论了各个阶段的机理。随后介绍了实际的畴反转晶体的制备过程,包括了周期和非周期的光学超晶格,并成功制备了非周期的光学超晶格。 2.在畴反转晶体的电光效应和非线性光学频率变换的理论基础上,推导出了在周期性极化铌酸锂(PPLN)内同时包含偏振耦合和倍频效应的耦合波方程。数值模拟结果显示在满足准位相匹配条件下,周期性极化铌酸锂晶体内偏振耦合和倍频之间存在着竞争,并分析了外加电场对晶体内三个光波的偏振和强度的影响。且发现利用外加的另外一束光波,可以改变晶体内原有的三个光波的能量耦合过程。通过对外加光波强度的控制,可以实现对三个光波的光强调制,起到光控光的效果。为了突破激光的工作波长和晶体工作温度的限制,我们设计了一种非周期的光学超晶格,能够同时满足倍频和偏振耦合的位相匹配,实现对任意波长的二次谐波的振幅和偏振调制。 3.介绍了在非均匀光栅中用倍频进行脉冲整形的原理,以及实现各种脉冲整形功能的光栅设计。在此基础上我们小组提出了一种改进的光栅设计方案,利用遗传算法寻找一种最优的光栅结构,实现脉冲压缩。为了减少上述方案中未考虑频率成分对输出压缩脉冲的影响,将位相漂白效应引入了光学超晶格结构的设计过程,选用分层遗传算法寻找最优的光栅结构分布进而实现频域上二次谐波的振幅和位相的调制,模拟结果显示这种方法能够有效的提高输出超短脉冲的质量。 4.飞秒激光脉冲频率转换中大的群速度失配会引起时域上的走离,所以只有薄的晶体可以用于飞秒脉冲的频率转换。我们将上述光栅的设计过程拓展至差频与和频过程,详细推导了差频与和频在准位相匹配光栅中耦合波的频域表达式,给出了闲置光频域输出表达式。并采用线宽很窄的的连续光和飞秒激光发生差频与和频,解决了群速度失配问题,并可以采用较长的晶体提高转化效率。
With the development of the electric poling technique at room temperature, it is possible to build various optical suprlattice in ferroelectric crystals such as LiTaO3, LiNbO3, KTiOPO4 and so on. Quasi-phase-matched(QPM) technique substantially extends the class of material available to various nonlinear optical interactions. Subsequently, optical suprlattice is widely used for optical parametric processed, pulse shaping and pulse compression. The structure of optical superlattice also changes from one-dimension to two-dimension, from single period to quasi-period, even to more complex structure. In recent years more and more requirements are demanded in Laser with the development of modern optics. To alleviate the limited wavelength coverage of available lasers, nonlinear optical frequency conversion is widely employed to extend the useful wavelength ranges of laser source. It is well known that lithium niobate crystal has a serious of outstanding properties, especially the nonlinear coefficient and the electro-optic coefficient. They change their sign due to the domain inversion. We emphasized our study on the structure design of the optical superlattice to modulate the parameters of laser. In this thesis, the main contents are following: The fabrication of optical superlattice crystal with special domain-inverted pattern is the foundation of our researches. From the micro structure of the ferroelectrics, we found four stages of the domain inversion under the room temperature in external field poling procedure. Some elements that affect the quality of the poling are discussed. Then the fabrications of the periodic and aperiodic optical superlattice are presented, and the aperiodica optical superlattice is successfully fabricated. The electro-optic effect and the nonlinear optical frequency conversion in periodically poled lithium niobate(PPLN) are studied. The wave-coupling equations including polarization coupling and frequency doubling in PPLN is proposed. Numerical solutions of the coupling equations show that there is a competition between the polarization coupling and second-harmonic generation(SHG) in a PPLN under the phase-matching condition. The influences of the external electric field on the polarizations and amplitudes of three interactive waves are studied. We found that the original energy coupling behavior among three waves can be broke by another light. The numerical results show that the magnitude of the three interactive waves can be controlled by the intensity of the controlled light. These results are beneficial in the design of optic controlled optic devices. In order to alleviate the limit of the wavelength and the crystal temperature, an aperiodic optical superlattice is engineered in the cascaded process. So both polarization and magnitude of the arbitrary second harmonic can be manipulated by an external DC electric field, which is useful when simultaneous frequency conversion and signal tuning are desired. The theory of pulse shaping by second-harmonic generation with nonuniform QPM gratings is presented. And the procedures of designing different gratings for different pulse shaping are introduced. A new procedure of designing the QPM gratings proposed by our group is introduced. For the purpose of pulse compression, Genetic algorithm is selected to search for the optimized structure of QPM gratings. In order to minimize the effect of the unconsidered frequency components on the generated compression pulses, phase-blanking effect is taken into account of designing the one-dimensional aperiodical optical superlattice. Hierarchic Genetic Algorithm(HGA) is chose to design the QPM grating’s structure, which can modulate the spectrum and phase of the generated second harmonic simultaneously. Simulation results show that the quality of the output pulse is fairly improved. A temporal walk-off effect occurs as a result of large group velocity mismatch(GVM) in the frequency conversion of ultrashort pulse. Only thin nonlinear crystal can be used for femtosecond laser frequency conversion. The procedure of design the aperiodically quasi-phase-mathced gratings was extended to the difference-frequency generation(DFG) and sum-frequency generation(SFG) for wavelength shift of optical ultrashort pulses. Under the assumptions of plane waves, undepleted pump, unamplified signal, slowly-vary envelopes, the general derivation of the ouput idler light are presented. A cw pump with sharp line-widths is employed together with a Ti: sapphire laser during the DFG and SFG process. Because SFG between two interactive pulses during SHG is avoided in this situation, the effect of GVM can be eliminated and the conversion efficency can be significantly enhanced with a relatively long crystal.
|
