近年來，高能極短脈衝的應用變得越來越受重視，例如能用來產生極紫外和中紅外雷射光源。而要產生比雷射輸出更窄的脈衝，需要將頻譜展寬，再將不同頻率成分的相位進行補償，展頻這方面，已經發展出許多有各自優缺點的技術。以多重薄片展頻技術(multiple-plate compression, MPC) 來說，輸入脈衝能量的上限受限於空間克爾效應和材料的損壞閾值(damage threshold)。為了提升展頻壓縮的脈衝能量，傳統上利用放大光斑大小來降低整體光強度，但會佔據更大的架構空間以及必須使用更昂貴的大尺寸光學元件。光學渦旋是有螺旋分布相位橫截面的特殊光束，在其中心產生強度為零的光學奇異點，導致脈衝等效強度會下降。
這個研究工作嘗試在MPC技術上導入了光渦旋，利用光束等效強度下降，使我們能輸入更高的脈衝的能量進行展頻壓縮。我們還特別設計光路讓展頻後的光束再通過一次同一光渦旋相位片，將光束回復成高斯模態，再進行相位補償。我們使用的初始脈衝寬度是190 fs、脈衝能量為640 μJ、重複率為12.5 kHz、中心波長為1030 nm。經過光漩渦展頻壓縮後，最後可獲得40 fs、509 μJ 的脈衝。與相同的配置下的傳統高斯模態MPC相比，輸入脈衝能量提升了5.5倍。我們也驗證了光漩渦MPC也可以維持良好的光束品質，其x軸與y軸的光譜空間均勻性分別為98.6 %及99.4 %。在未來，利用渦旋光束的MPC很有潛力成為產生高能超短脈衝的方式。

Nowadays, the application of the high-energy ultrashort laser is becoming more important. For example, it is used to generate EUV and mid-infrared light sources. To generate pulses shorter than the laser output, a broadened spectrum is necessary, and then the phase of different frequency components needs to be compensated. In the field of spectral broadening, many techniques have been developed, and each of them has its advantages and disadvantages. In the case of multiple-plate compression (MPC), the maximum value of pulse energy is limited by the spatial Kerr effect and damage threshold of the material. Traditionally, to increase the pulse energy of spectral broadening, the overall light intensity is reduced by enlarging the spot size, but it takes up more configuration space and necessitates the use of more expensive large-sized optical components. An optical vortex is a special beam with a helical phase transverse cross-section, which creates an optical singularity with zero intensity at its center, resulting in a drop in the equivalent intensity of the pulse.
We attempt to introduce MPC with an optical vortex in this research. Using the equivalent intensity reduction, we can inject more pulse energy for spectral broadening. We also specially designed the optical path so that the spectral broadening beam passes through the same spiral phase plate again to restore the beam to a Gaussian mode, and then perform phase compensation. The input pulse we used in the experiment has an FWHM pulse duration of 190 fs, a pulse energy of 640 μJ, a repetition rate of 12.5 kHz, and a center wavelength of 1030 nm. After spectral broadening by MPC and pulse compression with chirped mirrors, pulses with 40 fs and 509 μJ can finally be obtained. Comparing the results with the same setup using a conventional Gaussian beam, the pulse energy is increased by 5.5 times, and a good beam quality can also be maintained. The spatio-spectral homogeneity along the x-axis and y-axis is 98.6 % and 99.4 % respectively, showing excellent beam homogeneity. In the future, the proposed approach offers the potential for high-energy ultrashort pulse generation.

目錄
摘要 i
Abstract ii
Acknowledgement iv
List of Figures and Tables vi
Chapter 1 Introduction 1
Chapter 2 Theory 4
2.1 Spectral broadening mechanisms 4
2.1.1 Criteria of generating short pulses 4
2.1.2 Kerr effect 6
2.1.3 Self-Phase Modulation 7
2.1.4 Self-focusing 10
2.1.5 Self-steepening 10
2.2 Nonlinear pulse post-compression 12
2.2.1 Existing techniques 12
2.2.2 Spectral phase compensation 15
2.2.3 Existing pulse energy up-scaling techniques 17
2.3 Vortex beam 19
2.4 Temporal and spatial characterizations 21
2.4.1 Frequency-resolved optical gating 21
2.4.2 Beam quality 24
2.4.3 Spatial-spectral homogeneity 25
Chapter 3 Experiment 27
3.1 Experiment setup 27
3.2 Results 29
3.2.1 Short pulse generation by Vortex beam 29
3.2.2 Short pulse generation by Gaussian beam 33
Chapter 4 Conclusion and future works 36
References 38

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