1.School of Science, Changchun University of Science and Technology, Changchun 130022, China 2.Shandong Provincial Engineering and Technical Center of Light Manipulations & Shandong Provincial Key Laboratory of Optics and Photonic Device, School of Physics and Electronics, Shandong Normal University, Jinan 250358, China
Received Date:06 September 2019
Accepted Date:17 December 2019
Published Online:20 February 2020
Abstract:It is important to control the femtosecond laser filamentation and the supercontinuum (SC) for their potential applications. The use of axicon is beneficial to the filamentation elongation and SC enhancement, because the axicon can convert the incident laser into a Bessel beam and forms a unique longer depth of focus region. On the other hand, the flattened laser beam which has a uniform distribution of the beam intensity, can propagate in condense media with a higher incident energy than that of Gaussian laser beam. It has unique advantages in forming a SC with high energy and high conversion efficiency. In this paper, we combine the use of axicon and the flattened laser beam to form filament and SC in fused silica. First, we study the filamentation generated by the Gaussian beam and the flattened beam, respectively, with the same incident pulse energy (672 μJ). The results show that the flattened beam can generate filament with relative uniform intensity distribution in the focal depth of the axicon and the intensity is relatively smaller than that of the Gaussian beam. It suggests that the flattened laser beam can propagate in fused silica with a higher energy than Gaussian beam. Second, we study the filamentation of the flattened beam of 1.319 mJ. In this case, the filament intensity is close to that of the Gaussian beam with 672 μJ. Moreover, the filamentation of the flattened beam with 1.319 mJ is longer and the intensity distribution is more uniform than that of the Gaussian beam with 672 μJ. Therefore, a flattened laser beam can generate the SC with a higher energy than that of the Gaussian beam in fused silica. The comparison of the spectra shows that the relative spectral intensity of flattened beam with 1.319 mJ in the range of 550–700 nm is much higher than that of the Gaussian beam with 672 μJ. The conversion efficiency of the Gaussian beam and the flattened beam is 32.58% and 39.59%, respectively. It can be seen that the flattened laser beam has advantages not only in generating long and uniform filament, but also in generating the intense SC. This approach is helpful to many applications, such as white light LIDAR and micro-nano processing. Keywords:filamentation/ flattened beam/ axicon/ supercontinuum
图 2 飞秒高斯激光((a), (c), (e))和平顶激光((b), (d), (f))的初始光斑((c), (d))及其在熔融石英中成丝的荧光图像((a), (b)), 以及成丝轴线上的荧光强度((e), (f))随传输距离的演化. 实验中使用的激光脉冲能量均为672 μJ. 图中箭头方向表示激光传输方向 Figure2. Fluorescence image ((a), (b)) and the on-axis intensity ((e), (f)) of the filament formed by Gaussian beam ((a), (e)) and flattened beam ((b), (f)) respectively, with an incident energy of 672 μJ; the intensity distributions in the cross sections of (b) Gaussian beam and (d) flattened beam. The inset arrow indicates the laser propagation direction.
3.结果与讨论图2(a)和图2(b)分别为脉冲能量为672 μJ的高斯激光和平顶激光经过锥角为170°的圆锥透镜聚焦后在熔融石英中形成的等离子体细丝的荧光图. 从图中可以看出, 高斯激光形成的细丝强度明显比平顶激光的大. 为了更加清晰地比较细丝的强度随传输距离的演化, 图2(e)和图2(f)分别给出了两种激光形成的细丝在光轴上的荧光强度随传输距离的演化. 在这里, 把熔融石英的入射面定为坐标原点, 激光的传播方向为正方向. 从图2(e)可以看到, 随着传输距离的增加, 等离子体细丝的强度快速增加, 在达到最大值以后, 其强度逐渐衰减, 在经过几次再聚焦过程之后细丝结束. 而对于相同能量的平顶激光, 由图2(f)可以看到, 其成丝相对强度基本维持在600上下(该强度为CCD的强度计数), 分布相对均匀. 但是, 平顶光束的成丝传输过程仍然存在一定的强度起伏, 这主要是激光成丝过程中多种非线性效应(主要包括克尔自聚焦、等离子体散焦、多光子电离、自相位调制等)共同作用的结果. 这种强度起伏, 被称为多次自聚焦现象, 是激光成丝的一个重要特点[25-28]. 经过比较两种光束的演化, 我们还可以看出, 在相同能量下, 平顶激光形成的细丝的强度较小, 但是强度分布更为均匀. 要产生强度相近的细丝, 平顶激光比高斯飞秒激光需要更高的激光能量. 这也说明, 在不引起介质永久损伤的前提下[10,20], 熔融石英中允许传输能量更高的平顶飞秒激光. 为了更好地说明这一点, 我们逐步增大平顶激光的脉冲能量, 当能量增加到1.319 mJ时, 获得了与672 μJ的高斯光束情况下强度相近的等离子体细丝, 成丝荧光图像和成丝强度随传输距离的演化如图3所示. 图 3 入射能量为1.319 mJ的平顶激光经圆锥透镜在熔融石英中形成的细丝荧光图像(a)及其光轴上的强度随传输距离的演化(b) Figure3. Fluorescence image (a) and the on-axis intensity of the filament (b) formed by flattened beam with incident energy of 1.319 mJ.
图3(a)为入射能量1.319 mJ的平顶光束产生的等离子体细丝的侧面荧光图像, 图3(b)为细丝光轴上的强度随传输距离的演化. 对比图2(e)和图3(b)可以发现, 1.319 mJ的平顶激光与672 μJ的高斯激光所产生的等离子体细丝的最大强度接近, 但平顶激光产生的细丝的分布更为均匀, 细丝也更长. 平顶飞秒激光的这些成丝特征为产生更高能量的超连续辐射奠定了基础. 我们进一步研究了两种激光成丝产生的超连续辐射. 首先, 对图2中能量均为672 μJ的高斯和平顶激光的超连续辐射进行了光谱测量, 结果如图4所示. 从图中可以看出, 当使用相同的能量672 μJ时, 高斯光束成丝产生的超连续辐射的光谱强度明显高于平顶光束的情况. 以650 nm处为例, 高斯光束产生的超连续光谱能量密度为0.31 μJ/nm, 而平顶光束的仅为0.18 μJ/nm. 我们还计算了这两种情况下的超连续辐射转换效率, 其定义为除去基频(770—830 nm)以外的光谱的积分与整个光谱区域积分的比值[28]. 通过计算得到, 高斯激光产生的超连续辐射的转换效率为32.58%, 而相同能量的平顶激光的仅为25.83%. 形成这种差别的原因主要是, 在相同激光能量下, 虽然高斯激光和平顶激光的成丝长度没有明显区别, 但是前者的成丝强度明显大于后者, 因此, 在这种条件下, 高斯激光的超连续辐射强度及其转换效率均会高于平顶激光的情况[28]. 图 4 入射能量为672 μJ的高斯光束与672 μJ, 1.319 mJ的平顶光束成丝产生的超连续辐射光谱 Figure4. Supercontinuum spectra from filamentation of the Gaussian beam with an incident energy of 672 μJ and flattened beam with an incident energy of 672 μJ and 1.319 mJ, respectively.
图 1 实验装置示意图
图 2 飞秒高斯激光((a), (c), (e))和平顶激光((b), (d), (f))的初始光斑((c), (d))及其在熔融石英中成丝的荧光图像((a), (b)), 以及成丝轴线上的荧光强度((e), (f))随传输距离的演化. 实验中使用的激光脉冲能量均为672 μJ. 图中箭头方向表示激光传输方向
图 3 入射能量为1.319 mJ的平顶激光经圆锥透镜在熔融石英中形成的细丝荧光图像(a)及其光轴上的强度随传输距离的演化(b)
图 4 入射能量为672 μJ的高斯光束与672 μJ, 1.319 mJ的平顶光束成丝产生的超连续辐射光谱