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• A side-polished gold-coated photonic crystal fiber with a offset core is proposed. • The U-shaped air hole arrangement can enhance the leakage of the evanescent field. • The PCF integrates RI sensing and polarization filtering. • The maximum sensitivity within the RI range of 1.35–1.39 is 12,500 nm/RIU. • The proposed PCF polarization filter has bandwidth tunability. The transmission characteristics of a side-polished eccentric photonic crystal fiber (PCF) is studied by using the full-vector finite element method (FV-FEM). The proposed structure is composed of air holes similar to a U-shaped arrangement, and there is a small air hole between the side polishing plane and the core region to enhance the leakage of the evanescent field in the y polarization direction. It integrates refractive index (RI) sensing and polarization filtering. The RI sensing mainly works in the visible light band, while polarization filtering is suitable for the near-infrared band. The maximum sensitivity of the sensor is 12,500 nm/RIU, and the resolution is 8.0 × 10−6 RIU in the detection RI range of 1.35–1.39. By changing the external RI or the thickness of the gold-coated film, the bandwidth-tunable PCF polarization filter with excellent performance can be obtained. Therefore, the design of side-polished PCF will be of great significance to the development of integration and miniaturization of optical devices. [ABSTRACT FROM AUTHOR]

Highlights • The proposed PCF filter can realize three kinds of filtering effects. • Two ultra-large air holes makes the process of plating gold film more convenient. • Ultra-high crosstalks and losses can be obtained at the communication windows. • The filtering bandwidth can reach 700 nm at the communication window of 1550 nm. Abstract In this study, we present a single-polarization photonic crystal fiber filter. The metallic gold is used as the surface plasmon resonance activity material, which is deposited on the inner wall of the two ultra-large holes. The properties of the photonic crystal fiber filter are investigated by the finite element method. By adjusting the structural parameters and gold film thickness, three kinds of filtering effects can be obtained at the communication windows. When the length of the optical fiber is 1 mm, one of the filtering effects can achieve loss of 588.900 dB/cm at the communication window of 1310 nm, and the corresponding filtering bandwidth equals to 270 nm; the other two kinds of filtering effects can be applied to the communication window of 1550 nm, and the losses are 569.682 and 971.330 dB/cm and the corresponding filtering bandwidths equal to 700 and 630 nm, respectively. The proposed photonic crystal fiber filter shows great advantages compared with some previous reports on filtering. [ABSTRACT FROM AUTHOR]

Surface plasmon resonance induced tunable broadband polarization filters based on photonic crystal fibers (PCFs) were designed and investigated. The characteristics of the designed PCF were calculated by using the finite element method (FEM). The air holes in the PCFs which were arranged in hexagonal lattice were of the same size except one big air hole. The big air hole which was located on the right side of the fiber core was designed to be coated with gold film in the inner wall. Because of the arrangement of the big air hole, the surface plasmon resonance inspired on the surface of gold film was quite different for the two orthogonal polarized incident lights. As the edge of the big air hole approaching the adjacent air holes, the plasmon modes were influenced strongly by the adjacent air holes and as a results the cascaded resonances appeared. The index-matching liquid infilling in the big air hole was used to adjust the resonant wavelengths and strength. Numerical simulation results showed that the bandwidth of the polarization filter was 1200 nm covering the wavelengths from 1.0 μm to 2.2 μm with high extinction ratio (ER). [ABSTRACT FROM AUTHOR]

Photonic crystal fiber-based polarization filter has important application prospects in optical fiber communications, nonlinear optics, and sensors. In order to achieve polarization filters with optimal performance, some numerical methods have been used to search for optimal results under different geometric structure parameters of fibers. However, it is unrealistic to calculate solutions under every parameter. In addition, the search direction and scope of adjusting fiber geometric structure parameters still depend on the designer's intuition and experience. Therefore, we propose a novel design method for photonic crystal fiber polarization filters to overcome these questions. The optimization design problem of polarization filters is transformed into the single objective function optimization problem. The combination of artificial neural networks and intelligent optimization algorithms realizes the optimal value-solving problem in two-dimensional parameter space. The method efficiently and automatically searches for optimal structures in the parameter space by fine-tuning both variables simultaneously, improving the bandwidth of the polarization filter and achieving higher crosstalk at 1550 nm. The method eliminates computationally expensive wavelength parameters and greatly reduces computation time. This novel design method realizes the reverse design of photonic crystal fiber-based polarization filters. In addition, the new method can also be used to design polarization filters at other objective wavelengths, and it is potentially applicable to the design of other photonic crystal fiber devices. • Wavelength parameters are removed, drastically cutting down on computation time. • The polarization filters optimization design is transformed into function optimization. • Machine learning techniques provided optimal design parameters of polarization filters. [ABSTRACT FROM AUTHOR]