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实验发现飞秒激光单步刻蚀的光纤微腔两个反射壁与纤芯轴向并不完全垂直,微腔形状与飞秒激光经显微物镜聚焦后的焦点相对于光纤的位置有关。当激光经显微物镜聚焦于光纤侧表面时,刻蚀的微腔形状近似Ⅴ型。与常规的光纤法布里-珀罗(F-P)腔相比,Ⅴ型光纤微腔干涉谱出现了自由光谱范围、微腔光损耗以及干涉条纹对比度与波长有关等反常现象。通过引入倾斜因子概念,构建了Ⅴ型光纤微腔模型,推导出Ⅴ型F-P腔干涉公式,初步建立了Ⅴ型光纤微腔干涉理论。利用该理论研究了Ⅴ型光纤微腔的干涉谱特性,数值模拟了不同倾斜度对光纤微腔干涉谱的影响。理论分析结果与实验光谱测量结果相符。
It is found that the two reflective walls of the fiber microcavity with single-step etching by femtosecond laser are not perpendicular to the axial direction of the fiber core. The shape of the microcavity is related to the focus of the femtosecond laser focused by the microscope objective relative to the fiber. When the laser is focused by the microscope objective on the side of the fiber, the shape of the microcavity etched is approximately Ⅴ. Compared with the conventional fiber Fabry-Perot (F-P) cavity, the Ⅴ-type fiber microcavity interference spectrum appears in the free spectral range, the microcavity optical loss and interference fringe contrast and wavelength-related anomalies. By introducing the concept of tilt factor, the Ⅴ type optical fiber microcavity model is constructed, the Ⅴ type F-P cavity interference formula is deduced, and the Ⅴ type optical fiber microcavity interference theory is preliminarily established. The interference spectral characteristics of Ⅴ-type optical fiber microcavity were studied by using this theory. The influence of different inclination on the interference spectrum of optical microcavity was numerically simulated. The theoretical analysis is in good agreement with the experimental results.