Chaitanya N Apurv, Aadhi A, Singh R P, Samanta G K
Opt Lett. 2014 Sep 15;39(18):5419-22. doi: 10.1364/OL.39.005419.
We report on experimental realization of optimum focusing condition for type-I second-harmonic generation (SHG) of high-power, ultrafast laser in "thick" nonlinear crystal. Using single-pass, frequency doubling of a 5 W Yb-fiber laser of pulse width ~260 fs at repetition rate of 78 MHz in a 5-mm-long bismuth triborate (BIBO) crystal we observed that the optimum focusing condition is more dependent on the birefringence of the crystal than its group-velocity mismatch (GVM). A theoretical fit to our experimental results reveals that even in the presence of GVM, the optimum focusing condition matches the theoretical model of Boyd and Kleinman, predicted for continuous-wave and long-pulse SHG. Using a focusing factor of ξ=1.16 close to the estimated optimum value of ξ=1.72 for our experimental conditions, we generated 2.25 W of green radiation of pulse width 176 fs with single-pass conversion efficiency as high as 46.5%. Our study also verifies the effect of pulse narrowing and broadening of angular phase-matching bandwidth of SHG at tighter focusing. This study signifies the advantage of SHG in "thick" crystal in controlling SH-pulse width by changing the focusing lens while accessing high conversion efficiency and broad angular phase-matching bandwidth.
我们报告了在“厚”非线性晶体中实现高功率超快激光的 I 型二次谐波产生（SHG）最佳聚焦条件的实验。在一块 5 毫米长的硼酸铋（BIBO）晶体中，对重复频率为 78 MHz、脉冲宽度约为 260 fs 的 5 W Yb 光纤激光器进行单通倍频，我们观察到最佳聚焦条件更多地取决于晶体的双折射，而非其群速度失配（GVM）。对我们实验结果的理论拟合表明，即使存在 GVM，最佳聚焦条件也与 Boyd 和 Kleinman 为连续波和长脉冲 SHG 预测的理论模型相匹配。在我们的实验条件下，使用接近估计最佳值 ξ = 1.72 的聚焦因子 ξ = 1.16，我们产生了 2.25 W 脉冲宽度为 176 fs 的绿色辐射，单通转换效率高达 46.5%。我们的研究还验证了在更紧密聚焦时 SHG 角相位匹配带宽的脉冲变窄和展宽效应。这项研究表明了在“厚”晶体中进行 SHG 的优势，即在获得高转换效率和宽角相位匹配带宽的同时，通过改变聚焦透镜来控制 SH 脉冲宽度。
