摻釹氟化釔鋰晶體在連續波與被動式Q開關雷射之研究

Abstract

本論文主要是在以a-cut 摻釹氟化釔鋰(Nd:YLF)晶體作為增益介質的端面激發固態二極體雷射架構下，分成連續波雷射與被動式Q開關脈衝雷射兩大架構，並利用調整輸出耦合鏡(Output coupler) 的選頻方式，對a-cut Nd:YLF晶體的兩個主要波長(π偏振方向，1047nm與σ偏振方向，1053nm)作多面向的實驗與比較。 在連續波雷射的部份，我們分別使用0˚-wedged與3˚-wedged的a-cut Nd:YLF晶體作為增益介質，記錄兩個偏振方向的競合與表現，並討論熱效應、晶體wedge角度對其的影響。實驗發現，使用3˚-wedged時，無論是在凹平腔或平平腔的配置下，我們均可藉由調整輸出耦合鏡的方式輕易的將兩個波長分別選出，並且在σ偏振方向(1053nm)得到良好的光束品質。 在被動式Q開關脈衝雷射的部份，我們針對3˚-wedged的a-cut Nd:YLF晶體，分別配置了初始穿透率(T0)為80%與95%的Cr4+:YAG飽和吸收體作為被動式Q開關，記錄兩個偏振方向的各種表現，並探討了改變Cr4+:YAG的初始穿透率與輸出耦合鏡反射率對兩個偏振方向的影響。實驗發現，輸出耦合鏡(OC)的反射率並非與雷射的輸出功率成正比，而是在輸出鏡反射率為某一最佳值時有最大輸出功率。此外，我們亦證實了以下推論：雷射的輸出功率、脈衝重複率會隨著飽和吸收體的初始穿透率增加而增加，而平均脈衝能量、尖峰功率卻會隨著飽和吸收體的初始穿透率增加而減少。此一推論可作為設計被動式Q開關共振腔時的權衡參考。更重要的是，實驗結果顯示，在被動式Q開關的架構下，σ偏振方向(1053nm)的表現比π偏振方向(1047nm) 更穩定。

This thesis is aimed to investigate the continuous-wave (CW) and passively Q-switched (PQS) laser performance of the a-cut Nd:YLF gain medium with the wedged angle of 0˚ and 3˚, respectively. Since the Nd:YLF crystal has two emission wavelengths which are π-polarization (1047nm) and σ-polarization (1053nm) in 4F3/2 → 4I11/2, it is practically important to select pure π-polarization (1047nm) and pure σ-polarization (1053nm) independently. Although the wavelength-selection can be achieved with additional components in the laser cavity, this would inevitably lead to unnecessary loss. As a result, we decide to select pure π-polarization (1047nm) and pure σ-polarization (1053nm) simply by tilting the output coupler in this work. In CW laser configuration, we employ the Nd:YLF crystals with 0o-wedged and 3o-wedged as gain medium, and make a thorough study on the performance of π-polarization (1047nm) and σ-polarization (1053nm), respectively. Compare with the results obtained from 0o-wedged one, using 3o-wedged Nd:YLF can more efficiently select two wavelengths and obtain better beam quality in σ-polarization (1053nm). In PQS laser configuration, we utilize the Cr4+:YAG (T0=80%, 95%) saturable absorber to comparative study the PQS performance between π-polarization (1047nm) and σ-polarization in 3o-wedged Nd:YLF crystal. We discuss the influence of initial transmission of Cr4+:YAG and reflectance of output coupler (OC). Experimental results reveal that there is optimal reflectance of OC which generates highest output power for different combinations of the initial transmissions of Cr4+:YAG and lasing wavelengths. The results also show that with increasing initial transmissions of Cr4+:YAG, average output power, pulse repetition rate and pulse width would increase, but pulse energy, peak power would decrease. This means that when designing a laser cavity, we should take these characteristics into consideration so that the performance would meet our demand. More importantly, we find that pulse-to-pulse amplitude stability at σ-polarization (1053nm) is considerably superior than that at π-polarization (1047nm) in PQS laser.