飞秒激光脉冲在透明介质中的成丝现象和自压缩效应研究
文献类型：学位论文
| 作者 | 王占新 |
| 学位类别 | 博士 |
| 答辩日期 | 2010 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 徐至展 ; 刘建胜 |
| 关键词 | 飞秒激光脉冲 成丝 自压缩 超连续谱产生 频谱滤波 |
| 其他题名 | Filamentation and Self-compression of Femtosecond Laser Pulse Propagating In Transparent Media |
| 中文摘要 | 强场飞秒激光脉冲在透明介质中传输引起的成丝现象是近年来飞秒激光研究领域的热点问题。飞秒激光脉冲在透明介质中传输时，能够引起介质的高阶非线性效应，从而引发激光脉冲在介质中的自聚焦效应；当激光脉冲峰值强度增大到一定程度时，能够引发介质的较强电离，产生的低密度等离子体能对激光脉冲产生散焦效应。这种非线性自聚焦效应与等离子体散焦效应的动态平衡，导致激光脉冲在透明介质中的长距离自导传输。飞秒激光脉冲在透明介质中的成丝现象具有广泛的应用，诸如光脉冲压缩、产生白光激光、大气污染监测、人工引雷放电、激光等离子体天线以及飞秒脉冲高次谐波的产生等。飞秒激光脉冲在透明介质中的传输是超快强场激光物理领域的重要研究课题，它涉及到各种激光电离机制、非线性传输效应等问题的研究，具有重要的科学意义和潜在应用价值。 本论文针对飞秒激光脉冲在透明介质中成丝引起的脉冲自压缩和压缩效应以及超连续谱产生等现象进行了一系列数值模拟研究，主要取得了以下几点创新性的研究结果： 1. 利用飞秒激光脉冲在熔石英中的成丝效应，提出了一种频谱滤波实现脉冲压缩的方法。即使在没有谱相位补偿的情况下，采用这种方法也能够获得脉宽大约5 fs的少周期脉冲。当飞秒激光脉冲在熔石英中成丝时，自相位调制和自陡效应引起的蓝移宽带超连续谱是几乎同相的。基于这种超连续谱的特点，采用具有合适截止波长的高通滤波器，在适当的传播距离处选择这些几乎同相的蓝移频谱成分，可以实现脉冲压缩而获得少周期激光脉冲。数值计算表明：采用脉宽为50 fs、能量为2.2 μJ的输入激光脉冲，能够获得最短脉宽为4 fs（大约1.5个脉冲周期）的少周期激光脉冲。 2. 研究了脉宽为45 fs和6 fs的激光脉冲在氩气中的成丝动力学。数值计算表明：脉宽为45 fs的激光脉冲在成丝的末期能够自压缩而形成脉宽大约12 fs的时域“光子弹”；采用频谱滤波方法，这种自压缩脉冲的脉宽能够进一步缩短到7 fs。对于脉宽为6 fs的输入脉冲，成丝过程中不能产生进一步的自压缩效应，然而在这种情况下，成丝过程中能够产生十分宽的超连续谱，从200 nm扩展到1300 nm甚至更宽；而且成丝过程中能够产生一种“分离的超连续谱”，获得的分离超连续谱分布在波长220 nm 到300 nm的紫外区，其峰值位于大约255 nm，对应着脉宽大约为5 fs的紫外激光脉冲。这种“分离的超连续谱”现象以前仅在固体介质光丝中观察到。 3. 进一步提出了一种可行的实验方案来实现这种“分离超连续谱”的观察。分别采用波带板(zone plate)和透镜两种不同的控制输入脉冲波前的方法，对比研究了脉宽为6 fs、波长800 nm的激光脉冲在氩气中传输的成丝动力学。通过采用波带板来控制输入少周期激光脉冲波前，可以在成丝过程中观察到这种分离超连续谱现象，获得的分离超连续谱分布在250 nm到300 nm，峰值大约位于280 nm；如果采用透镜控制输入脉冲的波前，成丝过程中不能观察到这种分离的超连续谱现象。 |
| 英文摘要 | Filamentation and propagation of femtosecond laser pulse in transparent media have become an important topic in the field of ultra-fast high-intensity femtosecond laser. When a femtosecond laser pulse propagates in a kind of transparent media (including solid，liquid and gas), it can induce high-order nonlinear effects such as Kerr self-focusing and plasma defocusing et al.. These effects will greatly influence the propagation property of the laser pulse due to the change of the refractive index of the media. Filamentation of femtosecond laser pulses in transparent media have been widely used in pulse compression, white light production, detection of air pollution, manual inducing of lightening, laser-plasma antenna, generation of high-order harmonics, etc. It deals with the topics of laser ionization and nonlinear propagation effects, and therefore, it is valuable to the research of strong field laser physics. In this paper, we mainly investigate the effects of pulse compression and supercontinuum generation when femtosecond laser pulses filament in transparent media such as argon and fused silica. The main results are listed in the following: 1. A spectral filter method is numerically demonstrated to obtain sub-5 fs pulses by using femtosecond filamentation in fused silica. Instead of employing spectral phase compensation, by properly employing high-pass filter to select the broadened high-frequency spectra which are located almost in phase in the tailing edge of the self-compressed pulses due to self-steepening, as short as single-cycle pulses can be obtained. For instance, for an input pulse with a duration of 50 fs and energy 2.2 J, the minimum pulse duration can reach to ~ 4 fs (about 1.5 cycles) by applying a proper spectral filter. 2. Self-compression and spectral supercontinuum (SC) generated by filamentation of femtosecond laser pulses with duration from 45 fs down to 6 fs in argon gas have been numerically investigated. A 45-fs pulse can be self-compressed into a few-cycle pulse bullet with duration of 12 fs at the post-filamentation region. By properly employing a high-pass filter to select the broadening high-frequency spectra which are almost in phase, the pulse can be further shortened to about 7 fs. By contrast, a 6-fs pulse cannot be further self-compressed into a shorter pulse by filamentation. However, it can generate much broader SC extending from 200 nm to 1300 nm and even broader in this case. It is also found that a separate and strong SC in the ultraviolet (UV) region extending from 220 nm to 300 nm and peaked at about 255nm can be generated at proper propagation distances, which corresponds to a pulse with duration of about 5 fs. This kind of separate SC has been observed only in bulk media before. 3. A feasible experimental method to observe this kind of separate SC was proposed. We numerically demonstrated the filamentation dynamics of a 6-fs, 800-nm pulse focused in argon at atmospheric pressure by a zone plate and a concave mirror, respectively. In comparison with a concave mirror, the zone plate has a frequency-dependent focal length and can be used to control the wavefront of the laser beam in frequency domain. A separate supercontinuum in the ultraviolet region extending from 250 nm to 300 nm and peaked at ~ 280 nm can be generated by using a proper zone plate. When a lens was used to control the wavefront of laser pulse, this kind of separate SC cannot appear in the filamentation. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15293]  |
| 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 | 王占新. 飞秒激光脉冲在透明介质中的成丝现象和自压缩效应研究[D]. 中国科学院上海光学精密机械研究所. 2010. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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