A novel single-polarization single-mode double-ring hollow-core anti-resonant fiber with two single-polarization regions (1545–1553 nm and 1591–1596 nm) is proposed. Single-polarization guidance is achieved by coupling a polarized fundamental mode and silica mode by using different tube thicknesses. Specifically, when the wavelength is 1550 nm, only a single x-polarized fundamental mode with a low loss of 0.04 dB/m is propagated by a polarization extinction ratio of 17662 and minimum higher-order mode extinction ratio of 393 by optimizing the structural parameters. Furthermore, this fiber also exhibits high-performance bend resistance. The x-polarized FM loss is as low as 0.11 dB/m with single-polarization single-mode guidance when the proposed fiber was bent at a bend radius of 8 cm toward the x-direction.

A novel pattern recognition method based on Empirical Mode Decomposition (EMD) and extreme gradient boosting (XGBoost) is proposed to recognize the disturbance events in phase sensitive optical time-domain reflectometer (φ-OTDR) to reduce nuisance alarm rate (NAR) and improve real-time performance in this paper. Eleven typical eigenvectors are extracted from components obtained by EMD of the disturbance signals and XGBoost is selected as a classifier to identify different type of disturbance signals. Five kinds of disturbance events, including watering, knocking, climbing, pressing and false disturbance event, can be identified, effectively. Experimental results show that NAR is 4.10% and identification time is 0.093 s. The recognition accuracy for the five patterns is 97.96%, 95.90%, 91.10%, 94.84% and 99.69%, respectively. The effectiveness of the proposed method is evaluated by using confusion matrix and decision boundary visualization. Experimental results demonstrate that our proposed pattern recognition method based on XGBoost has better performance in recognition rate and recognition time than other commonly used methods, such as support vector machine (SVM), Gradient Boosting Decision Tree (GBDT), Random Forest (RF) and Adaptive Boosting (Adaboost).

An ultrabroadband polarization splitter based on three-core photonic crystal fiber (PCF) is proposed. Two fluorine-doped cores and an elliptical modulation core are introduced to achieve an excellent performance and an ultrawide bandwidth. Numerical results demonstrate that the polarization splitter based on three-core PCF has an extinction ratio as low as $-20\mathrm{dB}$ bandwidth as great as 400 nm covering almost all communication bands ($O$, $E$, $S$, $C$, and $L$ bands). Its Gaussian-like mode-field distributions and suitable effective mode areas make it highly compatible with the standard single-mode fibers. Due to using a uniform size of circular air holes and only one elliptical central air hole, the difficulty of fabrication can be decreased to some extent.