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由于光纤自身固有的圆柱形结构,在飞秒激光逐点刻写光纤布拉格光栅过程中会产生柱透镜效应,使得聚焦光斑呈长条状,对刻写光栅结构产生不利影响。基于几何光学原理分析了产生长条状光斑的原因,并提出了一种有效地改善聚焦光斑质量的方法。实验中,将待写光纤夹在玻璃板之间并填充折射率匹配液,运用800 nm钛蓝宝石飞秒激光通过50×显微物镜聚焦在双包层光纤内,单脉冲曝光后得到了直径为1.5μm圆形光斑,方法简单,易于操作。光斑质量的提高扩展了飞秒激光在光纤微细结构制作中的应用。
Due to the inherent cylindrical structure of the optical fiber, a cylindrical lens effect is generated during the puncturing of the fiber Bragg grating by the femtosecond laser, so that the focusing spot is in a strip shape, which adversely affects the structure of the writing and writing grating. Based on the principle of geometrical optics, the reason of generating strip-shaped flare is analyzed, and a method of effectively improving the quality of focused spot is proposed. In the experiment, the optical fiber to be written is sandwiched between glass plates and filled with the refractive index matching liquid. The 800-nm titanium-sapphire femtosecond laser beam is focused on the double-clad fiber through a 50 × microscope objective. After single-pulse exposure, 1.5μm circular spot, the method is simple and easy to operate. The improvement of spot quality expands the application of femtosecond laser in the fabrication of optical fiber microstructure.