研究摻銣釩酸釔晶體自鎖模雷射與被動式Q開關雷射

Abstract

摻铷釩酸釔晶體因具有高吸收係數與適中的熱傳導率，已成為固態雷射研究常見的主要激發晶體。且此晶體亦能達成較好的、且有效率的自鎖模與被動式Q-開關雷射。為了進一步改善雷射光輸出效率與穩定性，在此論文中，我們深入研究摻铷釩酸釔晶體雷射在自鎖模與被動式Q開關操作上的物理特性。首先，我們設計了一個熱不靈敏共振腔，來探討摻铷釩酸釔晶體的自鎖模雷射其穩定性與縱模頻譜間距的影響。結論是：愈短腔可產生愈大的縱模頻譜間距，而有效防止自鎖模雷射模態間的相互競爭，使其有穩定的輸出。根據此一結果，我們進一步設計了晶體兩端面鍍膜的單晶體共振腔，來探討自鎖模拉曼雷射中的脈衝時序，並且觀察單晶體自鎖模拉曼雷射於低溫條件下的行為。實驗結果顯示，除了輸出功率有顯著的改善，更發現了當溫度低於125K時，拉曼雷射可達成單一縱模之輸出。再者，我們也進一步利用摻铷釩酸釔晶體可同時達成自拉曼及自鎖模操作的機制，成功實現雷射縱模光頻梳的擴展，進而將自鎖模雷射脈寬縮小至數皮秒。 另外，我們設計了一個線性的凹凸共振腔來探討摻铷釩酸釔晶體其被動式Q-開關雷射表現。由於此共振腔能有較大的等效空間模量，使得此一摻铷釩酸釔晶體雷射可產生高脈衝能量與高尖峰功率，並且伴隨穩定的脈衝輸出與極佳的光束品質。 除了研究摻铷釩酸釔晶體雷射在時間軸上的動力學外；在空間上，我們利用此晶體的內在雙折射效應與較大雷射增益等特性，來產生各種不同的結構光束。並且藉由柱透鏡，使得各種高階、高功率的橫向模態轉換成為具有大角動量的光渦，對於雷射在實際的應用面上應有極大的助益。

Because of the advantages of high absorption coefficients and moderate thermal conductivities, the Nd:YVO4 crystal has been confirmed to be a promising gain medium for solid-state systems to achieve good self-mode-locked (SML) and passively Q-switched (PQS) lasers effectively. To further improve the performance of output efficiency and temporal stability, the physics of the SML and PQS operation in the Nd:YVO4 lasers is deeply studied in this thesis. At the beginning, a thermally insensitive resonator is designed to explore the influence of longitudinal mode spacing on the stability of Nd:YVO4 SML lasers. It is found that a shorter cavity length can lead to larger mode spacing to prevent mode competition for realizing stable SML operation. According to this finding, the output performance of the Nd:YVO4 lasers with monolithic cavities is further investigated. By exploiting the concurrent effect of the SML and self-Raman process of the Nd:YVO4 crystal, the frequency comb expansion of the longitudinal modes is achieved to generate stable mode-locked pulses with the pulse duration of few picoseconds. On the other hand, a concave-convex linear resonator is designed to explore the criterion for achieving a good PQS Nd:YVO4 laser. With the larger effective mode volume, the Nd:YVO4 laser with the designed cavity can easily realize large-pulse-energy and high-peak-power PQS operation with stable pulse trains and excellent beam quality. In addition to investigate the temporal dynamics of the Nd:YVO4 laser, the intrinsic birefringence and large gain of the Nd:YVO4 crystal is further utilized to generate a variety of structured beams in spatial domain. By employing the astigmatic beam transformation, various high-power structured transverse modes are converted into optical vortex beam with large angular momentum for practical applications.