Nd:YVO4/Cr4+:YAG雷射的連續及脈衝式運作之研究

A study of continuous-wave and pulsed operations of Nd:YVO4/Cr4+:YAG laser.

簡浩文(Hao-Wen Chien)

指導教授 ： 唐建堯

淡江大學/理學院/物理學系碩士班/碩士(2006年)

https://doi.org/10.6846/TKU.2006.00905

摘要

本論文探討Nd:YVO4/Cr4+:YAG雷射的連續波及脈衝式運作模式及兩者之間的變遷。此雷射是由一二極體雷射端面激發，具有一線型共振腔；此腔接近於半球形腔，使得腔內光束大小有明顯變化，實驗中所使用的Cr4+:YAG可飽和吸收晶體對波長1064nm的初始穿透率為87.8%。藉由變換可飽和吸收體位置可檢驗第二起振條件，由於駐波效應，使得實驗中所測得的雷射起振激發功率比忽略駐波效應的理論預期大了兩倍，而在連續波運作模式下，所測得的雷射斜率效率也低於理論預期，此結果可歸因於增益和光的重疊效應。當此雷射滿足第二起振條件時會產生脈衝式輸出；然而在不滿足第二起振條件時，也可能會產生脈衝式輸出，而產生脈衝輸出的方式有二：一種是增加激發功率，另一種是對在臨界情況下運作之連續波雷射施加一夠大的擾動。為了比較，我們使用LiF: 可飽和吸收體(在1064nm的初始穿透率為95.7%)進行實驗，發現此LiF: 較前者更不容易滿足第二起振條件而產生脈衝式輸出。我們也使用速率方程式近似來模擬Nd:YVO4/Cr4+:YAG雷射的運作，其模擬結果與實驗大致相符。然而，可能是因為模型本身過於簡化，而且計算所使用的參數有些誤差，所以無法模擬出所有實驗情況；欲獲得更精確的結果，所用的模型以及參數可能需要修正。

關鍵字

第二起振條件 ； 馳豫振盪

並列摘要

The continuous-wave (CW) and pulsed modes of operation of a Nd:YVO4/Cr4+:YAG laser and the transition between the two modes have been investigated. The laser is end-pumped by a diode laser, and it has a linear cavity which is close to a hemispherical one; thus, the beam size within the cavity varies significantly. In experiment, a Cr4+:YAG crystal with an initial transmittance of 87.8% at 1064 nm was used as a saturable absorber, and the second threshold condition can be tested by varying the position of the saturable absorber. It was found that because of the overlap of the intracavity standing wave and the gain the threshold pump power is nearly twice that of the theoretical expectation when the standing wave feature is ignored. Likewise, in CW operation, the slope efficiency of the laser was found to be lower than theory. The laser may generate pulsed output when the second threshold condition is satisfied; however, pulsed operation may also be possible when the condition is not met, and there are two ways to generate pulsed output: one is achieved by increasing the pump power, the other by tapping the saturable while the laser is under a critical condition of CW operation. For comparison, a LiF: crystal with an initial transmittance of 95.7% at 1064nm was used as saturable absorber instead. We found it was harder for the LiF: crystal than the former to satisfy the second threshold condition. We use rate equation approximation to simulate the experiment, and our calculations found good agreement with the experiment. However, maybe the model employed is oversimplified, and the parameters used in our simulation are not accurate, our simulation cannot account for the experiment in every respect. The model and the physical parameters may need further refinement for more accurate results.

並列關鍵字

YVO4/Cr4+:YAG ； second threshold condition ； relaxation oscillation

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