A novel high birefringence gold-coated photonic crystal fiber (PCF) polarization filter based on surface plasmon resonance is proposed. The six elliptical holes arranged in a specific mode surrounding the core are utilized to gain high birefringence, and gold is coated inside the air holes to generate surface plasmons. By improving structural parameters, we achieve one state of polarization to propagate through the filter whereas the other state is severely attenuated. Specially, in the 1550nm communication band, the confinement loss of y-polarized mode can reach 442dB/cm while the corresponding loss of x-polarized mode is only 0.0316dB/cm. The distinction between two losses is over 10000 times. In addition, when the length of the designed PCF filter is 1mm, an extinction ratio of 326dB/cm is obtained. The applicable bandwidth is up to 300nm while the fiber length is merely 50um, which is better than other reported literatures. Owing to above characteristics, the proposed polarization filter can be widely applied in compact and broadband filters in the communication band.

Refractive index (RI) and temperature (T) are both critical environmental parameters for environmental monitoring, food production, and medical testing. The paper develops a D-shaped photonic crystal fiber (PCF) sensor to measure RI and T simultaneously. Its cross-sectional structure encompasses a hexagonal-hole lattice, with one hole selectively filled with toluene for temperature sensing. By coating the D-shaped surface of the PCF with a metal film and a MoS₂ film, the refractive index-detection channel is formed. Numerical results demonstrate that RI and T can be reflected in the same spectrum, without any interference caused by the two parameters with each other. At an environmental RI of 1.26–1.38, its maximum RI sensitivity is up to 5400 nm/RIU. At a temperature of 20–80 °C, its temperature sensitivity reaches −1.2 nm/°C. This design allows for a broad operational spectrum and an extensive measurement range, which makes it particularly suitable for applications requiring low-RI detection. Moreover, the resonance strength of the sensor is significantly enhanced by introducing a two-dimensional material MoS₂ on the D-surface. Specifically, it reaches 195,149 dB/m when RI = 1.34 at 30 °C. This is much higher than that of most previous studies, and the requirements for inspection equipment can be lowered in this case. These results are essential for progress in simultaneously detecting RI and T.

A metal-filled photonic crystal fiber (PCF) polarization filter based on surface plasmon resonance (SPR) is proposed and designed. The structure of the cross-section of a PCF is composed of a hexagonal lattice of air holes, in which an air hole is selectively filled with metal. We realize the polarization filter at the communication wavelengths of 1.31 and 1.55 μm with optimized structural parameters. The losses of the X polarization mode are 25126.44 and 22444.54 dB/m, while the losses of the Y polarization mode are about 1375.81 and 358.62 dB/m at the resonance wavelengths 1.31 and 1.55 μm, respectively. Therefore, the two light polarizations can be clearly separated. When the fiber length is 600 μm, cross-talk is over 20 dB at the communication wavelengths, while the achieved bandwidth is 160 and 200 nm at 1.31 and 1.55 μm, respectively. Furthermore, the distance between the resonance peaks of the two polarizations can reach 267 nm at 1.55 μm, achieving a significantly higher value than previously reported. These results are of great significance for the development of a polarization filter applicable in targeted communication bands.

Terahertz fibers are highly applicable for short-haul stable terahertz transmissions, and thus have potential use in upgrading terahertz systems. In this paper, a photonic crystal structure consisting of honeycombed tubes is proposed. Numerical studies based on the plane wave expansion method demonstrate that, in comparison to the photonic crystal consisting of honeycombed airholes or triangulated tubes, the one proposed in this paper can deliver a broader and less dispersive bandgap, which has been further confirmed by another calculation based on the finite element method. Then a fiber structure is designed to perform the single mode guidance for a broad spectrum of terahertz waves. The second derivative of mode effective area is introduced to define the effective guiding spectrum for the bandgap terahertz fiber. Mode calculations are performed by use of the finite element method, in which the imaginary part of material refractive index is directly took into account. Obtained results show that the designed fiber suffers low transmission losses over a broad spectrum centered at 1 terahertz. Further, an optimized design is worked out to exhibit the lowest loss for the frequency of 1 terahertz, coming up to 0.3 dB/cm. Not only that, this design can also provide nearly zero flat dispersions with |β₂| 1 ps²/cm over 0.81-1.12 terahertz, as well as negligible bending induced losses (< 10⁻¹¹ dB/cm) for bending radii larger than 1 cm , indicative of a good transmission characteristic for terahertz pulses. Finally, we discuss possible fabrication methods on the proposed fiber.