In this study, a novel rectangular polymer single-mode optical fiber for femtosecond (fs) laser-inscribed fiber Bragg gratings (FBGs) is proposed and demonstrated. The cylindrical geometry of the widely used circular fiber elongates the fs laser beam along the fiber axis, resulting in reduced laser intensity and requiring index-matching oil immersion during FBG inscription. However, the flat geometry and negligible surface roughness of the featured fiber significantly diminish this lensing distortion and eliminate the need for oil immersion, thereby resulting in optimal focusing of the laser beam, permitting direct and efficient inscription of FBGs within the optical fiber. The core and cladding of the rectangular fiber were fabricated using two different grades of ZEONEX, a cyclo olefin polymer, which have slightly different refractive indices. The similar glass transition temperature for core and cladding simplifies the fiber drawing process, and a rectangular single-mode optical fiber with dimensions of $213\mathrm{\mu m}\times 160\mathrm{\mu m}$ and core diameter of 9.4 μm was fabricated using an in-house fiber drawing facility. A second harmonic (520 nm) fs laser beam was used to successfully inscribe a 2-mm-long FBG in the rectangular fiber within a few seconds with a point-by-point technique. The inscription of a single FBG leads to the excitation of higher order FBG peaks at 866.8 and 1511.3 nm, corresponding to widely used wavelength bands in fiber optic sensing. The strain and temperature sensitivities of the FBG were measured to be $7.31\mathrm{nm/}\%\mathrm{\epsilon }$ ($0.731\mathrm{pm}/\mathrm{\mu \epsilon }$) and 10 pm/°C, and $12.95\mathrm{nm}/\%\mathrm{\epsilon }$ ($1.29\mathrm{pm}/\mathrm{\mu \epsilon }$) and 15 pm/°C at 866.8 nm and 1511.3 nm, respectively.

中文翻译：

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用于 FBG 飞秒激光刻字的矩形单模聚合物光纤

在这项研究中，提出并演示了一种用于飞秒 (fs) 激光刻写光纤布拉格光栅 (FBG) 的新型矩形聚合物单模光纤。广泛使用的圆形光纤的圆柱形几何形状沿光纤轴拉长了 fs 激光束，导致激光强度降低，并在 FBG 刻录过程中需要与折射率匹配的油浸。然而，特征光纤的平坦几何形状和可忽略不计的表面粗糙度显着减少了这种透镜畸变并消除了对油浸的需要，从而实现了激光束的最佳聚焦，允许在光纤内直接有效地刻录 FBG。矩形光纤的纤芯和包层是使用两种不同等级的 ZEONEX（一种环烯烃聚合物）制造的，它们的折射率略有不同。$213\mathrm{微米}\times 160\mathrm{微米}$并且使用内部光纤拉制设备制造了 9.4 μm 的纤芯直径。使用二次谐波 (520 nm) fs 激光束通过逐点技术在几秒钟内成功地在矩形光纤中刻出 2 毫米长的 FBG。单个 FBG 的铭文导致在 866.8 和 1511.3 nm 处激发更高阶的 FBG 峰，对应于光纤传感中广泛使用的波段。FBG 的应变和温度敏感性测量为$7.31\mathrm{纳米/}\%\mathrm{\epsilon }$ ($0.731\mathrm{下午}/\mathrm{\mu \epsilon }$) 和 10 pm/°C，和 $12.95\mathrm{纳米}/\%\mathrm{\epsilon }$ ($1.29\mathrm{下午}/\mathrm{\mu \epsilon }$) 和 15 pm/°C，分别为 866.8 nm 和 1511.3 nm。