微聚焦端泵浦掺釹釩酸釔雷射在簡併共振腔下的動態與穩態行為

Abstract

我們研究一個操作在1/3簡併共振腔附近與小聚焦端泵浦的掺釹釩酸釔雷射，其共振腔結構相關的動態行為。強烈聚焦的泵浦光線會因為熱透鏡的效果，而產生巨大的相位變化進而影響共振腔的結構。為了讓數值模擬能夠符合實驗的結果，我們將熱透鏡的效應加入數值模擬。數值模擬可以指出哪些位置可以產生自脈衝的現象，為什麼我們可以在長短不同共振腔下觀察到時-空不穩脈衝現象，以及只有時間不穩定脈衝，這些結果都跟實驗非常的符合。除了在長腔的不穩區域之外，我們實驗上也觀察到橫模的光頻鎖定，及無橫模拍頻。利用基因演算法計算模態展開時，每個模態的振幅權重以及相對的相位，我們發現在完全簡併的共振腔下所有的橫模都是同相位。即使我們調離開簡併共振腔，所有的橫模還是保有相位與光頻的鎖定。即便調離簡併共振腔大約1mm，我們仍可以觀察到模形(beam profile)會隨著傳播方向改變 ，這是由於所有的橫模在一開始就是相位鎖定。 由於在簡併共振腔下所有的橫模會保有光頻與相位的鎖定，我們可以利用這個特性來控制雷射的模形。我們驗證了在一個小聚焦與端泵浦的掺釹釩酸釔雷射可以直接產生多樣的瓶型光束。只要適當的控制泵浦光斑與腔內實光欄的大小，在半共焦腔、1/3簡併腔、與1/5簡併腔下可以產生不同但是對比度極佳的瓶型光束。這樣的新發現是適用於內含任意增益介質之小聚焦端泵浦雷射。 這樣的雷射也可抑制空間燒洞(spatial hole burning)效應。由於雷射模會自動調整大小以符合泵浦光斑，雷射可以在被幫浦增益介質區域內達到很高的光強度，而使得大部分的增益被高強度的駐波消耗掉。我們利用一個縱向相關的速率方程式來描述與研究這個在平凹共振腔掺釹釩酸釔雷射的實驗。即使高於20倍的閥值，實驗上我們可發現這個抑制空間燒洞的效應。

We experimentally and numerically studied cavity-dependence of laser dynamics in an tightly-focused end-pumped Nd:YVO4 laser which operates in the vicinity of 1/3-degenerate cavity. The tightly-focused pump beam results in enormous phase distortion which influences the cavity configuration through the thermal lens effect, therefore, the thermal lens effect is considered in our simulation. The simulation results well explain our experimental observation including the regions of self-pulsation, the reasons why we should observe the temporal or the spatiotemporal dynamics in the instability region on short- or long-cavity side of the degeneracy, and the influence of thermal lens. In addition, the transverse modes are all frequency-locked over the cavity tuning except for the instability regions. By decomposing the calculated mode (similar to the observed one) into the degenerate transverse modes to obtain their mode weights and relative phases, we found that the transverse modes are all in phase at the exactly degenerate cavity. Except for the cavity configuration within the instability region on the long cavity side, all of the transverse modes are phase- and frequency-locked to one another even when the cavity is tuned away from the degeneracy. This finding consists with the experimental observation that the stationary transverse mode pattern does vary along the propagation axis due to interfere of Guoy phases from the phase-locked transverse modes even for cavity length being adjusted 1mm away from the degeneracy. Because the transverse modes which govern the laser pattern are in-phase and frequency-locked at the degeneracy, we should be able to control the laser pattern. We demonstrate various optical bottle beams can be directly generated from a tightly focused end-pumped Nd:YVO4 laser. By controlling the size of pump beam and inserting an intracavity aperture in the plano-concave cavity, we obtain good contrast optical bottles at semi-confocal, 1/3-, and 1/5-degenerate cavity configurations, respectively. This new observation is universal that is suitable for any kinds of gain media in tightly end-pumped lasers. We also found that the spatial hole-burning effect can be suppressed in this laser. Due to shrinkage of the beam waist of laser mode to match the pump beam, this laser can attain very high intensity in the pump region of gain medium and therefore most of its gain is depleted even by a standing wave. This was demonstrated by a simulation with spatial dependent rate equations and experiment results of a plano-concave Nd:YVO4 laser. The suppression effect was observed up to 20 times the pump threshold.