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Surface plasmon resonance (SPR)-based single-core photonic crystal fiber (PCF) biosensor is investigated with external gold coating. All the geometrical parameters such as a gold layer, an analyte layer, a lattice period and cladding air holes are optimized to enhance the sensing ability of the sensor by introducing the finite element method. The designed sensor is able to achieve the highest amplitude sensitivity (AS) of 2258.95 RIU${}^{-1}$ with an acceptable refractive index sensitivity (RIS) of 6000 nm/RIU over the analyte refractive index (ARI) span of 1.31–1.40. This sensor can detect a slight index alteration in the sensing medium using a resolution of $1.66\times 10^{-5}$ and a high figure of merit (FOM) of 79.01. With the enhanced modal behavior with simple geometry, the resulting sensor can be suitable for real-time monitoring in biological, biochemical and bio-imaging applications.

A polarization filter based on simple structure photonic crystal fiber (PCF) coated with gold air holes is designed in this work. The full vector finite element method (FEM) is used to calculate the characteristic parameters of PCF. In the final numerical simulation results, the loss peak in the $y$ polarization direction is 2305.72 dB/cm and the $x$-polarized loss is as low as 75.86 dB/cm at 1.31 $\mu$m. For the fiber length of 1000 $\mu$m, the CT reaches 1936.83 dB and a wide bandwidth greater than 20 dB can provide above 800 nm. The content of this paper can provide a new idea for filter structure design.

A highly birefringent photonic crystal fiber (PCF) with large nonlinearity, low dispersion and low confinement loss is designed by introducing a solid elliptical core structure with spiral lattice of circular air holes as a cladding. The dependence of different geometrical parameters, such as pitch size, diameter of air holes and arrangement of air holes are investigated. By optimizing the available parameters, the designed elliptical–spiral PCF offers high birefringence up to 0.005264, high nonlinearity up to 8683.59W${}^{-1}$km${}^{-1}$, low chromatic dispersion of $-9072.62$ps/nm/km, and low confinement loss of 0.00305dB/km within a wide wavelength range of 1000–2000nm.

The simultaneous detection of a mixture of two gases with the aid of unique and excellent capabilities of anti-resonance hollow-core fibers (AR-HCF), is numerically investigated in this article. Low-loss transmission bands in near and mid-infrared, which correspond to the transition of methane (CH${}_4)$ and carbon dioxide (CO${}_2)$, are considered in this work. Reasonable consideration of structural parameters has caused the low-loss transmission band covering 1.57 $\mu$m and 3.33 $\mu$m wavelengths. The second harmonics caused by the interaction of the laser beam and the gas sample inside the hollow core of the fiber were analyzed utilizing the fast Fourier transform (FFT) method. The maximum amplitude for the corresponding frequency at the determined concentration has reached 3002 ${\mathrm{\text{cm}}}^{-1}$ for CH₄ and 6381 ${\mathrm{\text{cm}}}^{-1}$ for CO₂. Enhancement of the inhibited coupling between the fundamental mode and some of the cladding modes and leakage loss reduction, extra capillary tubes nested inside the silica tubes forming the fiber cladding. The simulation results confirm the suitability of the proposed structure for efficient utilization in gas sensing applications for two different spectral ranges in the near and mid-infrared. Benefiting from a large hollow core, very low background noise and anti-resonance properties, high sensitivity of about 99.6% and confinement loss of 0.127 dB/m were obtained.

Generally, fiber with a large mode area is desired for fiber laser and beam delivery applications. But the use of these fiber types is restricted to linear optical applications due to the lower value of the nonlinear coefficient. This paper investigates a single material-based photonic crystal fiber (PCF) with improved optical characteristics, applicable for linear and nonlinear optics. The light propagation in the structure is explored by applying finite element solver-based COMSOL multiphysics software. Five nonlinear optical materials are utilized as background material to get the optimized material with high performance PCF. Using this PCF, the improved optical properties of PCF such as moderate effective mode area of 115.85 $\mu {\mathrm{\text{m}}}^2$, chromatic dispersion, confinement loss of 10${}^{-6}$ dB/m, the nonlinear coefficient of 543W${}^{-1}$Km${}^{-1}$, splicing loss of 2.013 dB, and numerical aperture are numerically analyzed. This single-mode PCF also shows the low bending loss for the bending radius of 7mm. The amended PCF structure is found to be suitable for high-power fiber laser, supercontinuum generation, wavelength conversion, and slow-light generation.