半導體激發式Nd:YAG與Nd:YLF雷射在低溫環境下的特性研究

Abstract

固態雷射因具有彈性的共振腔設計及良好的光束品質等特性，已被廣泛應用於醫療、工業、國防及學術研究等各種領域。受限於雷射晶體的熱效應，固態雷射無法有效的操作在百瓦等級的高輸入功率下仍維持一定的輸出光束品質，其原因包含受熱不均造成的光學特性改變及晶體損壞閥值的限制等。過去的研究證明，隨著溫度的降低，固態雷射晶體的熱效應能有效的改善。在接近100 K左右的極低溫環境下，固態雷射晶體可承受的激發功率可大幅提升且不影響光學特性，進而可望發展出極高功率的高光束品質雷射系統。另一方面，對於在室溫輸出效率遠差於四能階雷射的準三能階雷射系統，在低溫環境下除了可有效改善再吸收損耗外，較大的量子效率亦可使之達到更高效率的輸出。除了這些改善之外，巨大的溫度改變亦會造成雷射輸出特性有許多無法預期的現象，例如晶體能階集中化的結果造成光譜強度變化等。因此，相較於已廣泛被利用來研究低溫雷射的參鐿晶體，在本研究中採用光譜更豐富、亦是最常被使用的參銣晶體來觀察雷射輸出特性在低溫環境下的改變。包含Nd:YLF雷射的連續光、自鎖模及Q開關雷射等現象的觀察，以及Nd:YAG雷射在四能階及準三能階系統的研究等。低溫雷射因降溫系統的限制，目前尚未有效的被利用在實際應用中，但隨著科技進步，勢必能發展出符合經濟價值的使用方法。雖然因研究設備的限制，本研究無法實現百瓦等級的輸出結果，但仍期許所提供的成果，對未來低溫雷射的發展有莫大的幫助。

Owning the characteristics of flexible cavity adjustment and good output beam quality, etc., solid-state lasers have been widely applied in plenty regions such as medical treatment and industry. However, suffering from the serious thermal effect, the solid-state laser faces the difficulty to be operated with up to hundreds of incident power while maintaining excellent output beam quality. The difficulty mainly comes from the thermal induced optical distortion and the limitation of crystal damage threshold. Previous researches have verified that thermal and thermo-optic properties for the laser crystal will improve with cooler temperature. At a cryogenic temperature near 100 K, the solid-state laser can sustain significantly higher input power without influences on optical properties. Furthermore, cryogenic process provides the quasi-three-level laser with less reabsorption loss and results in dramatic improvement of output efficiency. The enhanced efficiency can be even higher than the four-level system, which owns better performances at room temperature, because of the higher quantum efficiency. In spite of these features, plenty output characteristics might vary with the giant temperature difference for the cryogenic solid-state lasers, such as the emission spectrum variation caused by the sorting out of the energy level. Consequently, owning more abundant spectroscopic, the mostly used neodymium-doped crystals are employed in this dissertation instead of ytterbium-doped crystals, which have been widely utilized in cryogenic lasers due to the quasi-three-level properties. The study includes continuous wave, self-mode-locked, and Q-switched Nd:YLF lasers, as well as four-level and quasi-three-level Nd:YAG lasers. Restricted by the cooling system, the cryogenic laser is currently lack of practical applications. Nevertheless, it is expected that following the rapid technology development, an affordable and robust system is able to be developed in the future. As a result, although the research in this dissertation cannot reach extremely high power output, the study of characteristics variation is feasible to offer proper assistance in the development of cryogenic laser system.