红外飞秒激光逐点刻写光纤布拉格光栅的束斑优化

Improvement of Beam-Spot Size in Fiber Bragg Grating Inscribed Point-by-Point with Infrared Femtosecond Laser

用红外飞秒激光逐点刻写低阶光纤布拉格光栅(FBG),光束精确聚焦到光纤纤芯形成很小的光斑是至关重要的。把光纤看作柱透镜,理论分析和用Zemax软件仿真了单个物镜聚焦和柱透镜-物镜组合聚焦两种情况下光纤纤芯的光斑尺寸。单个物镜聚焦时光纤纤芯的光斑尺寸理论计算为0.67μm×84μm,仿真为2.6μm×76μm;柱透镜-物镜组合聚焦时光纤纤芯的光斑尺寸理论计算为3.76μm×0.67μm,仿真为4.1μm×2.2μm。发现利用柱透镜-物镜组合聚焦在纤芯能得到很小的光斑,光纤轴向尺寸大大缩小,这为制作低阶光纤布拉格光栅提供了可能性和基础。

It is important that the light beam is focused into fiber core precisely with a small size when low-order fiber Bragg grating （FBG） is inscribed point-by-point with infrared femtosecond laser. The beam-spot size in fiber core is analyzed theoretically and simulated with Zemax software in the cases of focusing with an objective lens and focusing with an objective lens combined with a cylindrical lens separately when the optical fiber is thought as a cylindrical lens. The beam size is 0.67 μm×84 μm theoretically and 2.6 μm×76 μm simulatively separately when focusing using an objective lens, and the beam size is 3.76 μm×0.67 μm theoretically and 4.1 μm×2.2 μm simulatively separately when focusing using an objective lens combined with a cylindrical lens. It is found that the beam-spot size in the core can become very small when focusing using an objective lens combined with a cylindrical lens. This conclusion will provide possibility and basis for the fabrication of low-order FBG.