Based on surface plasmon resonance (SPR), a novel refractive index (RI) sensor comprising a square photonic crystal fiber (PCF) is proposed to realize the detection of the annular analyte. Instead of hexagon structure, four large air-holes in a square array are introduced to enhance the sensitivity by allowing two polarization directions of the core mode to be more sensitive. The gold is used as the only plasmonic material. The design purpose is to reduce the difficulty in gold deposition and enhance the RI sensitivity. The guiding properties and the effects of the parameters on the performance of the sensor are numerically investigated by the Finite Element Method (FEM). By optimizing the structure, the sensor can exhibit remarkable sensitivity up to 7250 nm/RIU and resolution of [Formula: see text] RIU with only one plasmonic material, which is very competitive compared with the other reported externally coated and single-layer coated PCF-based SPR (PCF–SPR) sensors, to our best knowledge.

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A new type of V-shaped photonic crystal fiber with elliptical air-holes is proposed to realize simultaneous high birefringence and nonlinearity at a wavelength of 1.55 µm. The full vector finite element method was adopted to investigate its characteristics, including birefringence, nonlinearity, and dispersion. The PCF exhibited a very high birefringence of 2.89×10 −2 and very high nonlinear coefficient of 102.69 W −1 •km −1 . In particular, there were two zero-dispersion wavelengths (ZDWs) in the visible (X: 640-720 nm and Y : 730-760 nm) and near-infrared regions (X: 1050-1606 nm and Y : 850-1500 nm). The combination of high birefringence and nonlinearity allowed the PCF to maintain the polarization state and generate a broadband super continuum, with potential applications in nonlinear optics.

Through doping liquid crystals into the core region, we propose a kind of seven-core photonic crystal fiber (PCF) which can achieve mode shaping and temperature sensing simultaneously in the communication window of 1.1-1.7 μm. To the best of our knowledge, this is the first time that the function of seven-core PCFs as temperature sensors is investigated. By using the full vectorial finite element method, the characteristics of the fiber with the temperature, such as the effective mode area, the waveguide dispersion, and the confinement loss, are analyzed. This kind of PCF can be competitive in providing temperature sensing in multi-core PCF lasers.

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