利用短的線性共振腔產生多波長自鎖模雷射

Abstract

鎖模雷射是光波在時間上疊加的體現。自從2008年，實驗室在短的線性共振腔的架構下，發現了自鎖模雷射以來，實驗室已經成功地利用各種摻釹釩酸鹽晶體(Neodymium-doped vanadate crystal)，例如:Nd:YVO4、Nd:GdVO4和Nd:LuVO4等晶體，來產生自鎖模雷射。然而，過去實驗室所研究的自鎖模雷射，是著重於單波長自鎖模的研究。而我的博士研究工作則是，探討自鎖模雷射在多波長條件下的空間燒孔(spatial hole-burning)和光拍頻等兩個現象。 若要產生多波長的雷射，雷射的架構必須根據晶體的螢光光譜來設計共振腔。在我的研究工作中，我採用摻釹釩酸鹽和摻釹鋁酸鹽這兩類晶體作為增益介質。首先，我測量摻釹鋁酸鹽以及摻釹釩酸鹽晶體的螢光光譜，發現摻釹鋁酸鹽晶體相對於釩酸鹽晶體具有豐富的螢光譜線。 因為摻釹鋁酸鹽晶體的光譜特性，我採用摻釹鋁酸釔晶體並且在共振腔內放置Etalon控制光譜線之間的增益和損耗的比值(Gain-to-loss ratios)，來產生單波長和多波長的自鎖模雷射。利用空間燒孔效應，來控制單波長和多波長鎖模雷射的脈衝寬度。此外，在多波長鎖模雷射情況下，因為雷射系統自身的脈衝在時間上同步，所以可以在整體脈衝內部發現數個太兆赫(THz)的光拍頻。 雖然摻釹釩酸鹽晶體的光譜線相對單調，但是不同母材(Host)的摻釹釩酸鹽晶體之間的螢光光譜，在1.06 μm的波段稍微不同。因此可以利用雙晶體的方法，人工接合兩顆摻釹凡酸鹽晶體形成複合晶體。在1.06 μm的波段中，這樣的複合晶體具有兩個明顯的光譜線可以用來產生雙波長自鎖模雷射。在端面激發的方式下，藉由控制激發光斑與共振腔模態的模態穩合(Mode matching)程度，來控制雙波長的強度。在等強度的雙波長情況下，觀察空間燒孔和光拍頻的現象。 最後，將單晶體和雙晶體兩個自鎖模雷射系統拿來比較。實驗顯示，這兩個系統的脈衝在時間上都是同步的狀態，因此都可以觀察到光拍頻的現象，而我也利用數學模型來模擬它。有趣的是，這兩個系統在經歷空間燒孔效應之下，所產生的整體脈衝的脈衝形狀會明顯的不同。在經歷不同程度的空間燒孔效應下，單晶體的多波長雷射系統的整體脈衝形狀，除了整體脈衝寬度會跟著改變以外，整體脈衝形狀並不會改變；相反的，雙晶體的多波長雷射系統的整體脈衝形狀，不管是整體脈衝寬度或是整體脈衝形狀都會跟著改變。造成這樣差異的主因是，雙晶體的各自晶體經歷不同程度的空間燒孔效應，使得各自的脈衝寬度不同，在時間上同步後，造成整體脈衝形狀的改變。

Mode locking is a exhibition of superposition of optical wave in temporal regime. Since my laboratory members have discovered that a short linear cavity can naturally generate self-mode-locked pulse in 2008, they have already successfully exploited linear short cavities to generate self-mode-locked lasers for variety Nd-doped vanadate crystals. For example, Nd:YVO4, Nd:GdVO4, and Nd:LuVO4. However, all of these are single-spectral-band self-mode-locked lasers. Therefore, my work is to investigate the performance of multi-spectral-band self-mode locking. The fluorescence spectroscopy of laser gain media is crucial for designing multi-spectral-band lasers. In my studies, I adopted Nd-doped aluminate and vanadate crystals to be laser gain media. Accordingly, I gave priority to measure fluorescence spectroscopy of these two species of crystals. The measured results reveal that The fluorescence spectroscopy of Nd-doped aluminate crystals possess more emission lines than Nd-doped vanadate crystals do. Because Nd-doped aluminate crystals possess wealth of emission lines, I adopt Nd-YAP to be laser gain medium and exploit an intracavity etalon to control gain-to-loss among these emission lines for my laser system which can be used to generate single-spectral-band or multi-spectral-band self-mode-locked lasers. I alter pulse duration of single-spectral-band and multi-spectral-band self-mode-locked lasers by considering spatial hole-burning effect. Moreover, the temporal overlapping of the multi-spectral-band pulses is experimentally found to lead to the generation of intensity fringe pattern in the autocorrelation trace with the optically beat frequency reaching several THz. Although Nd-doped vanadate crystals possess relatively few emission lines among 4F3/2 → 4I11/2 transition, I combine two different Nd-doped vanadate crystals to form a composite gain medium that possesses two strong 1.06-μm emission lines which are generated by these two crystals respectively. In end pumping situation, I could flexibility control the intensity ratio of these two 1.06-μm lines via controlling mode matching for these two crystals. When the intensity ratio is unity, I investigate the spatial hole-burning and optical beat. Finally, I compare the experiment results of single Nd:YAP crystal laser with dual Nd-doped vanadate crystals laser. The experimental results reveal that these two lasers could generate the temporal overlapping of the multi-spectral-band pulses with intensity fringe pattern of optically beat. Interestingly, the tendency of temporal synchronize multi-spectral-band pulse of these two lasers are something different as considering the spatial hole-burning effect. When the spatial hole-burning effect gradually increases, both of the pulse duration of these two lasers is gradually decrease; however, because the two Nd-doped vanadate crystals are suffered different degree of spatial hole-burning effect but single Nd:YAP crystal are not, tendency of their pulse shapes is different. A simple mathematical model is developed to elucidate the formation of a train of optically beat pulses.