研究單晶Yb:KGW超快雷射中的自鎖模和自拉曼散射

Abstract

在過去50年間，由於科學與技術的發展，使得在超快雷射領域上急速的成長，近來對於超短時間解析、超高峰值的光強度和超快脈衝重覆率的興趣，已經轉移思考在目前小型、精巧的超快雷射當中，強調從複雜的和專門的實驗室系統，進展到小型的以及能夠實際應用在各種不同領域上的儀器，因此，我們採用了一種自組成的雷射架構，比如像是自鎖模和自拉曼雷射技術，表示無需附加元件到雷射共振腔中，並且這樣的雷射幾乎完全是自啟動的，除此之外，一個鍍膜的Yb:KGW晶體被特別的設計，我們基於在這樣單晶的架構，進一步建造了只需要約幾個毫米腔體長度的耦合共振腔雷射，去達成各種更小型的、更加精巧的超快雷射。我們首先提出了一個高功率子皮秒(sub-ps)的單晶自鎖模Yb:KGW雷射，藉由利用一個有鍍膜的Yb:KGW晶體這樣的設計架構，成功使得雷射的脈衝重覆率除了可以高達幾十億赫茲(GHz)且總晶體長度只佔了3.36 mm，接著由數值上的分析，透過鍍膜設計使耦合輸出的穿透率為1.5%，其Np和Nm偏振態的雷射輸出閾閥值可以很接近，進而產生正交地雙光梳子皮秒(sub-ps)單晶雷射，如同樣的架構，在高激發功率下且激發光點是位在晶體中心附近的位置，一個具有角動量的Laguerre-Gaussian渦旋光束(LG1,1)，透過兩偏振態對材料增益競爭的過程可以有效地產生且同時有著子皮秒(sub-ps)脈衝序列。在耦合共振腔的情況下，我們已經在理論上和實驗上證實，耦合共振腔雷射中縱模間距的各種階數的諧波可以通過調整外部腔體長度滿足相稱條件而被有系統地獲得，其最大脈衝重覆率可以高達216 GHz以及脈衝持續時間可以短到330 fs，這是基於簡潔架構下的單晶自鎖模雷射結合一個外部的法布里-博羅光(Fabry-Perot)共振腔，使雷射重覆率不斷的增值，另一方面我們同時觀察到有趣的現象，隨著一系列地掃描外部腔體長度，一個在時域上類似於塔爾博特(Talbot)分數重現的現象實現在我們的雷射系統上。此外KGW主材具有三階高非線性系數，並且這個材料是拉曼活性增益介質中最大有可為的，依此我們也實現了高效率且有著89 cm-1拉曼轉換的摻鐿(Yb-doped)自拉曼雷射，目前在摻鐿(Yb-doped)自拉曼雷射當中，我們的雷射輸出表現是最好的。最後，藉由耦合共振腔這樣的配置並利用聯級的89 cm-1自拉曼轉換來達成寬頻帶的頻率疏，在實驗上成功展現53-飛秒(fs)諧波自鎖模Yb:KGW雷射，由於89 cm-1 的拉曼位移通常會導致，在拉曼光譜和激發源之間有較小的波長差，然而再透過微微地調整兩腔體長度的比例為1：5 (Lext / L*cry)，像這樣的方法只約4.094 mm的腔體長度被需要，如此一來達到更精巧的超快雷射，不同於過去的雷射配置需要很長的腔體長度。

In the past 50 years, there has been a rapid growth of ultrafast lasers due to the advance of science and technology. More recently, with an increase in extremely short temporal resolution, highly peak optical intensities and quite fast pulse repetition rates has shifted to reflect current developments in compact ultrafast lasers which emphasize from complicated and specialized laboratory systems to compact and reliable instruments for many different applications. Consequently, we employ a kind of self-organization framework, such as self-mode-locked and self-Raman lasers, to perform without additional elements into the resonator and the laser is almost entirely self-starting. Furthermore, a specially coated Yb:KGW crystal is designed and we further construct the couple-cavity laser with several millimeter total cavity length, which is based on monolithic configuration, to achieve variously more compact ultrafast lasers. We first report on a high-power sub-picosecond monolithic self-mode-locked Yb:KGW laser with the pulse repetition rate up to several tens of gigahertz by using a coated Yb:KGW crystal in a length of 3.36 mm. With the numerical analysis, the Np and Nm polarized states are achieved the similarity of lasing threshold to generate orthogonally dual-comb sub-picosecond monolithic laser by designing the output coupling transmission of around 1.5%. Under central pumping and at a high pump power, a vortex beam of the Laguerre-Gaussian LGp,l mode with p=1 and l=1 can be efficiently generated and with a sub-picosecond pulse train through the process of the gain competition. In the couple-cavity case, we have theoretically and experimentally confirmed that various order harmonics of the mode spacing in couple-cavity laser can be systematically obtained by adjusting the external cavity length to satisfy the commensurate condition. The maximum pulse repetition rate of the laser output can be up to be 216 GHz and the pulse duration is as short as 330 fs by a monolithic self-mode-locked laser combing an external Fabry-Perot cavity with a compact scheme of repetition-rate multiplication. It is intriguingly that a similar phenomenon of Talbot fractional revivals in temporal domain has been realized in our laser system with sequentially scanning the external cavity length. Besides, KGW host has a high nonlinear susceptibility of the third order (~10-13 cgse un.) and this material is one of the most promising Raman-active mediums. The best of our knowledge, we have accomplished a high efficiency Yb-doped self-Raman laser with the Stokes shift of 89 cm-1 for the first time. Finally, we experimentally demonstrate 53-femtosecond harmonically self-mode-locked Yb:KGW laser which is based on the couple-cavity configuration by utilizing cascaded self-Raman conversion with shift of 89 cm-1 to achieve broadband frequency comb. The Raman shift of 89 cm-1 ordinarily results in the smaller wavelength difference between the Raman spectrum and its excitation source. Such this method only about 4.094 mm total cavity length is required in the couple-cavity condiguration, and that two cavity lengths are fine tuned in the ratio Lext / L*cry = 1 / 5. It is different from the previous laser configurations, which need very long cavity length, to attain more compact ultrafast laser.