具腔內飽和吸收體之自起動式連續波被動鎖模鈦藍寶石雷射之研究

Abstract

本文首先探討鈦藍寶石晶體的雙折射特性。在連續波鈦藍寶石雷射中，布 魯斯特角切面的鈦藍寶石晶體本身就是一雙折射濾波器，造成雷射輸出的 暫態頻譜存在數叢分裂的頻帶，而微微秒鎖模雷射的穩定態頻譜與飛秒鎖 模雷射的暫態頻譜也都受到其影響。除了濾波器效應外，在鎖模雷射中非 線性雙折射效應提供了脈衝壓縮的力量，但是線性雙折射效應卻使脈衝變 寬，這些現象我們都配合理論計算而得到與實驗相吻合的結果。在本文之 第二部份，我們以利用飽和吸收染料DDI為起動機制的被動式鎖模鈦藍寶 石雷射為例，從脈衝建立演變過程中的脈衝壓縮速率的變化，配合理論的 分析，討論自聚焦作用，飽和吸收體，自相位調變和群速色散各個鎖模機 制的競爭消長，而了解穩定態微微秒脈衝寬度隨染料濃度呈指數函數的變 化關係以及穩定態飛秒脈衝寬度不受染料影響的原因。此外，在上述的研 究中，無論理論計算或實驗觀察都發現脈衝壓縮速率對脈衝寬度的變化函 數曲線上，有一壓縮速率最大的轉折點，其成因我們也給了合理的解釋。 To begin with, we show that the transient spectrum of a continuous wave Ti:sapphire laser during the initial stage of the buildup, exhibiting multiple clusters, is due to the spectral windowing effect of the Brewster-angle-cut Ti:sapphire crystal as a birefringent filter in the laser cavity. This effect has also been identified in the steady-state and transient spectra of picosecond and femtosecond pulses, respectively. In addition to birefringence-induced spectral windowing effect, the linear and nonlinear birefringent effects are important pulse-broadening and pulse-shortening mechanisms for a cw passively mode-locked Ti:sapphire laser, respectively. A simple analysis is used to calculate the free spectral range of this birefringent filter and the pulse- shortening speed due to the nonlinear birefringence effect. The theoretical results are in good agreements with the experimental observations. Using a self-starting cw passively mode-locked Ti:sapphire/ DDI laser as an example, we have also experimentally and theoretically investigated the pulse- shortening (or pulse- broadening) speeds owing to the self- focusing effect, saturable absorber dye, and the combined action of self-phase modulation and group-velocity dispersion as the pulses evolve to the steady- state pulse width. The results of the above investigation can be used to explain a number of experimental observations, such as: the steady-state picosecond pulse width exponentially decreases as the dye concentration increases and the steady-state femtosecond pulse width is independent of the concentration of the saturable absorber dye. Futhermore, we have also found that there is a turning point in the curve of pulse-shortening speed as a function of pulse width, at which the pulse-shortening speed is maximum. A possible reason for this phenomenon is given in this dissertation.