| 题名 | 飞秒激光的二阶非线性级联过程及其应用研究 |
| 作者 | 徐光 |
| 学位类别 | 博士 |
| 答辩日期 | 2003 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 范滇元 |
| 关键词 | 飞秒激光 二阶非线性级联 相位失配 相位匹配 时间望远镜 付氏变换极限 自聚焦 |
| 其他题名 | Cascaded quadratic nonlinear processes of femtosecond lasers and their applications |
| 中文摘要 | 从上世纪九十年代起，随着钦宝石激光晶体的成功研制及非线性克尔透镜锁模技术的发明与成熟，超短脉冲激光技术获得了革命性的发展，极大地拓宽了激光在科学研究领域的应用范围。输出脉冲宽度由几十飞秒降至十几飞秒乃至几飞秒，应用脉冲惆啾放大技术脉冲输出功率已达太瓦量级（1012W），甚至拍瓦级（1015W）。超短脉冲激光技术的这些突破性进展不但为超高能量密度物理、激光等离子体物理、强场物理、等离子体加速器及X射线激光等研究领域提供了全新的实验手段，也为惯性约束核聚变开辟了新的“快点火”技术途径。在此飞速发展的过程中，有效控制超短脉冲所具有的强非线性和大色散等效应成为了人们充分掌握和利用飞秒脉冲的基本前提条件。目前，采用光栅对、棱镜对或惆啾镜等色散补偿技术，超短脉冲的色散特性已得到了有效地控制。然而，与此同时，对于强非线性问题的处理却始终不尽人意，如自聚焦效应。正是基于此，应用二阶非线性变换所具有的灵活与丰富性，本文从理论与实验应用两方面对飞秒激光的二阶非线性级联过程进行了系统地研究与探索，获得了可用于自聚焦补偿、脉冲压缩及克尔透镜锁模的高量值级联五阶非线性租移，及具有脉冲付氏扩展功能的时间望远镜系统等创新性成果，将有力地促进超短脉冲激光技术的发展与完善。同时，在国内外重要学术刊物上发表或即」冬发表7篇相关的学术论文。一、系统地研究了飞秒激光的二阶非缪念级联过程。基于相位失配三倍频过程，首次提出了等效于χ~((5))的χ~((2)):χ~((2)):χ~((2))级联五阶非线性过程，产生了较级联三阶非线性更大且饱和性更丰富的级联五阶非线性相移。1．构建了产生五阶非线性相移的x（2）:X(2):X(2)级联非线性过程。通过χ~((2))二阶级联非线性效应在基频光上实现显著非线性相移的基本物理思想形成于20世纪纪60年代。级联三阶非线性过程是接连两次利用二阶非线性相互作用，使光波能量在基波和谐波间不断交换，从而导致位相特性发生变化的过程。以相位失配的倍频过程为例，部分基频光首先通过正过程转换为倍频光，所产生的倍频光再经过逆过程（由△k≠0导致）倒流回基频光，此时基频光将感应到非线性相移，具体物理过程可表示为：χ~((2))(ω;2ω,-ω）:χ~((2))（2ω;ω），相当于一个频率简并的X（3）（ω；ω,ω,-ω）级联三阶非线性过程。由于级联非线性相移具有量值大、可饱和性、符号可控等特性，到目前为止它忆在高维时空光孤子的产生、脉冲压缩、克尔透镜锁模等方面获得了广泛应用。基于位相失配三倍频（△k≠0）转换，我们构建了产生级联五阶非线性相移的χ~((2))级联非线性过程。三倍频光的产生通常是先通过倍频（满足相位匹配）再进行和频（满足相位匹配）χ~((2))（3ω;2ω,ω）:χ~((2))(2ω;ω,ω）二阶级联过程来实现，此过程等效于χ~((3))（3ω;ω,ω,ω）三阶非线性过程。如果在三倍频光产生过程中的第二步和频时相位失配△k摊O，那么在和频过程中也将同时存在着正过程及逆过程，倍频光和基频光经过正过程先产生三倍频光，由于相位失配三倍频光与倍频光将会再经过逆过程将能量倒流回基频光，因此在相位失配△k≠0条件下，整个三倍频过程实质χ~((2))（ω；3ω，-2ω）：χ~((2))（3ω；2ω，ω）：χ~((2))（2ω；ω，ω）二阶级联非线性过程，等效于频率简并的X（5）（ω;ωω，ω，ω，-ω，-ω）级联五阶非线性。此过程不涉及三阶等其它非线性效应，仅表现为纯的五阶非线性，所感应的非线性相移正比于入射光强的平方，较级联三阶非线性相移而言其不仅符号可控，而且具有更大的量值和更丰富的饱和特性。2．对于级联五阶非线性过程中能量损耗效应的研究以及抑制，是其能否获得实际应用的关键。针对基频光在级联五阶非线性过程中的能量损耗问题，本文提出在级联五阶非线性过程之后引入倍频光与基频光之间的差频过程。最终实现了基频光在感应到大的级联五阶非线性相移的同时无能量损耗。级联五阶非线性过程中最终所得到的基频光是由经正过程生成的三倍频光与倍频光经逆过程差频而产生的，因此在得到感应了级联五阶非线性相移的基频光的同时仍然会有一部分能量分别滞留在倍频光及三倍频光上，对基频光而言这无疑会导致其能量的衰减。与材料固有的非线性χ~((3))、χ~((5))所造成的双光子及三光子吸收损耗完全不同，级联非线性所涉及的基频光能量损耗是被转移给了其它光波，因此有可能通过相应的非线性相互作用来控制和消除。由于级联五阶非线性过程较为复杂，涉及到相位匹配的倍频过程以及相位失配的和频过程，因此除了控制相位失配量△kL以外，还需寻找新的自由度来同时抑制剩余倍频及三倍频光的能量。对此，我们提出了在级联五阶非线性过程之后引入一个倍频光与基频光之间的差频过程，最终完成了对能量损耗的控制。3．研究了飞秒光脉冲感应的级联五阶非线性相移。对于飞秒级光脉冲而言在总的级联五阶非线性过程中，基频光、倍频光及三倍频光任意两者间的群速度失配都是无法避免的，因此造成了基频光在感应到大量值的级联五阶非线性相移的同时脉冲发生了严重的畸变。为此，我们通过理论分析及数值模拟提出了使级联五阶非线性过程工作在较大相位失配条件下的解决方案，成功地消除了由群速度失配所造成的基频光脉冲畸变。这一过程的实现为级联五阶非线性在克尔透镜锁模及脉冲压缩等方面获得充分的应用奠定了坚实的基础。二、对飞秒激光二阶非线性级联过程所形成的脉冲付氏扩展功能及非线性相移开展了相应的实验应用研究，取得了非常优秀的重要成果。1．基于惆啾脉冲相位匹配的二阶级联非线性过程，理论上首次提出了具有脉冲付氏扩展功能的“时间望远镜”的概念及该系统在设计中所应遵循的基本准则。并以此为基础在实验上由70fs超短光脉冲产生了近付氏变换极限的6OPs扩展长脉冲，充分证明了在实现飞秒光脉冲扩展方面此二阶非线性方法具有转换效率高及付氏变换极限好等优点。实现超短脉冲满足付氏变换极限的扩展，对于飞秒光脉冲在超强光与物质相互作用、泵浦探测类实验及激光核聚变等方面获得更深入应用是非常关键的。在光学技术中，应用空间域望远镜系统可以方便地实现满足衍射极限的光束扩展。按照时空类比，为了获得飞秒光脉冲满足付氏变换极限的扩展，就需建立一种“时间望远镜”系统，以便对飞秒光脉冲时间扩展的同时确保相应的光谱变窄。应用惆啾脉冲的二阶级联非线性过程，我们研究了具有脉冲扩展功能的时间望远镜。理论和数值模拟结果表明，该时间望远镜具有较高的转换效率（-40％），以及群速度失配是造成扩展过程中脉冲时间一带宽积偏离付氏变换极限（“时间像差，’）和转换效率下降的主导因素。最为重要的是，根据理论分析我们提出了消除“时间像差”的时间望远镜系统在设计中所应遵循的相关准则。依据此设计准则，在实验上我们建立了用于飞秒脉冲扩展的时间望远镜系统，成功地将7Ofs超短光脉冲扩展成了近付氏变换极限的60Ps长脉冲。2．对飞秒脉冲在传输过程中所形成的全光束自聚焦及小尺度自聚焦，实验上应用相位失配的二阶级联非线性过程所产生的大量值、无能量损耗、符号可控的非线性相移，对其实施了有效的补偿。从而证实该方法在补偿高峰值功率激光的自聚焦问题方面，具有高效和方便调谐等突出优点。 |
| 英文摘要 | Since the 90s' beginning of the last century, the revolution in ultra-short pulse generation has been trigged by the emergence of Ti:Sapphire laser crystal and the realization of Kerr lens mode locking. The pulse duration is shorted from tens to several femtosecond, and the peak power 1012W and even 1015W are also achieved by use of the chirped pulse amplification. Due to the remarkable progress in ultra-short laser science, many important laser applications have been performed, for example, high-energy physics, laser-produced plasma, high-field physics, plasma accelerator, X-ray laser and "fast ignitor" in inertial-confinement fusion. In this rapid development period, how to manipulate the strong nonlinearity and dispersion, the two main features of ultra-short pulse, is the key topic to the application of femtosecond lasers. Now the dispersion is controlled efficiently by chirped mirrors, and prism or grating pairs. However, on the other hand, the way to solve the strong nonlinearity of ultra-short pulse is still under research, such as the method to deal with self-focusing. Based on the rich and flexible transformation of quadratic nonlinear process, the research and applications on the second-order nonlinear cascaded processes of the femtosecond lasers have been carried out systematically in this paper. Two novel and innovative results are acquired, the large cascaded fifth-order nonlinear phase shifts, which can be used to self-focusing compensation, pulse compression and Kerr-lens mode locking, and the time telescope system with the feature of Fourier-transform limited pulse expanding. At the same time, seven academic papers are published in the important internal and external journals. 1. The second-order nonlinear cascaded processes with femtosecond laser pulses have been systematically studied. Based on the phase mismatched frequency tripling, a special configuration is proposed firstly to produce effective fifth-order nonlinearityχ~({5}) byχ~((2)): χ~((2)): χ~((2)) cascaded processes. Compared with conventional cascaded ^(3) nonlinearity, theoretical study and numerical simulations show that this fifth-order nonlinear phase shift can be much larger in magnitude with a richer saturable feature. (1). The configuration of χ~((2)): χ~((2): χ~((2) cascaded processes to produce the fifth-order nonlinear phase shifts. The existence of the nonlinear phase shifts produced byχ~((2))cascaded processes on the fundamental wave was first discussed as early as in 1967. In phase-mismatched SHG, for example, the cascaded third-order nonlinearity is generated by the energy flowing back and forth periodically between the fundamental wave and its harmonic through two second-order nonlinear processes: frequency up-conversion via SHG and frequency down-conversion via difference-frequency generation. As a result, the fundamental wave will be impressed a nonlinear phase shift. The successive nature of this process can be expressed as: χ~((2))(ω;2ω,-ω):χ~((2))（2ω;ω,ω）which is equivalent to a third-order nonlinearityχ~((3))(ω;ω,-ω) with degenerate frequencies. Due to its large magnitude, controllable sign and saturable feature, nonlinear phase shift has found many important applications such as spatio-temporal soliton generation, compact Kerr-lens mode locking lasers and pulse compression. Based on the phase-mismatched frequency tripling, the cascaded fifth-order nonlinearity is produced. Frequency tripling is usually accomplished by the cascading process of χ~((2))(3ω;2ω,ω): χ~((2))(2ω;ω,ω) phase-matched second harmonic generation (SHG) followed by the phase-matched sum frequency generation (SFG), which is equivalent to a cascaded third-order nonlinear process ofχ~((3))(3ω;ω,ω,ω) Obviously if the phase-mismatchingΔk≠0 is introduced in the second step of the frequency tripling, it actually will contain two processes in the frequency mixing, i.e. ( i) SFG between the fundamental and its second harmonic, (ii) beating between the second harmonic and the third harmonic to generate a wave at the fundamental frequency. This can be expressed as χ~((2))(ω;3ω,-2ω):χ~((2))（3ω;ω, ω） : χ~((2))（3ω;ω, ω） cascading, which is equivalent to the cascaded fifth-order nonlinearity χ~((5))（ω;ω, ω, ω,- ω,- ω）with degenerate frequencies. In this process, induced effective fifth-order nonlinearity dominates, which is proportional to the square of input fundamental intensity in the limits of large phase mismatch and negligible pump depletion in the first crystal. Compared with the conventional induced effective third-order nonlinearity, the induced fifth-order nonlinear phase shifts can be much larger in magnitude with a controllable sign and a richer saturable feature. (2). The key to applications of cascaded fifth-order nonlinearity lies in the elimination of the energy loss. To this end, we have proposed a configuration ofχ~((2)): χ~((2)): χ~((2)) cascading process followed by a DFG. As a consequence, the large cascaded fifth-order nonlinear phase shift induced by the fundamental wave is obtained at the same time without energy loss. The output fundamental wave from the cascaded fifth-order nonlinear processes is produced by the beating between the third harmonic and the second harmonic beam. In this course the energies held by the second and third harmonics can not flow back to the fundamental wave totally. This undoubtedly leads to the energy loss of fundamental wave. Different from the multi-photon absorption accompanied with inherent material nonlinearitiesχ~((3))andχ~((5)), the fundamental wave loss in cascaded fifth-order nonlinear processes could be controlled and even compensated, since portion of fundamental energy is just transferred to the second and third harmonic waves. Due to the complicated cascaded fifth-order nonlinear processes that involve in phase-matched SHG and phase-mismatched SFG, in addition to phase mismatchingΔkL, it requires a new degree of freedom to suppress the residual second and third harmonics. We propose a new configuration for this purpose, namely the phase mismatched frequency tripling process followed by a DFG between the fundamental and its second harmonic, and finally all the energy losses are eliminated. (3). The realization of large cascaded fifth-order nonlinear phase shifts with femtosecond laser pulses. In cascaded fifth-order nonlinear process, GVM among the fundamental pulse, its second harmonic and third harmonic is difficult to avoid with femtosecond pulse-duration. This will severely distort the fundamental pulse while the large cascaded fifth-order nonlinear phase shifts are impressed on it. From the results of systematical study we find that the deleterious effects of GVM on the fundamental with femtosecond pulse-duration can be eliminated by using the cascaded fifth-order nonlinear process at fairly large phase mismatch. Based on this the undistorted fundamental pulse with cascaded fifth-order nonlinear phase shifts is obtained finally. In addition to modelocking of lasers, the realization of large cascaded fifth-order nonlinear phase shifts with femtosecond pulse will be valuable to other applications such as pulse compression. 2. On the Fourier-transform limited pulse expanding and nonlinear phase shifts via the second-order nonlinear cascaded processes of femtosecond laser, experimental investigations have been carried out with the excellent results. (1). Based on the phase matched second-order nonlinear cascaded processes with chirped pulses, We have firstly proposed the conception of "time telescope" with the characteristics of the Fourier-transform limited pulse expanding, and the criteria fordesigning such system. A 70-fs short pulse is expanded to a 60-ps long pulse near Fourier-transform limited experimentally by this pulse expander. In this process, the high conversion efficiency and Fourier-transform limited are obvious. Such a device will find many important applications in ultra-intensive light-matter interaction, pump-probe experiments and inertial confinement laser fusion. Under Fourier-transform limited (FTL) condition in order to realize the expansion of femtosecond pulse, an optical device with functions of pulse expanding and spectral narrowing, termed as "time telescope" in analogy with beam expander in spatial domain is needed. By using the second-order nonlinear cascaded processes we have constructed this system with chirped pulses, which can provide a high conversion efficiency (-40%). Both analytical and numerical studies show that group-velocity mismatch (GVM) of the nonlinear crystal plays a big role on the derivation of time-bandwidth product from Fourier-transform limited (temporal aberration) and the decrease in conversion efficiency. Finally, the criteria for designing a near aberration-free time telescope are proposed. According to this rule, a time telescope has been setup, by which a 60-ps long pulse of narrow band based on a 70-fs Ti: sapphire laser is generated experimentally. (2). In addition, we have showed that whole-beam self-focusing and small-scale self-focusing in the propagation of femtosecond pulses in bulk nonlinear media can be conveniently compensated with the nonlinear phase shifts produced by the second-order nonlinear cascaded processes with the features of the large magnitude, controllable sign and free of energy loss. For the compensation of high peak power laser self-focusing, this method is efficient and easily tunable. |
| 语种 | 中文 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15318]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 徐光. 飞秒激光的二阶非线性级联过程及其应用研究[D]. 中国科学院上海光学精密机械研究所. 2003. |
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