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Global warming is an alarming issue in our daily life. As we know that CFCs and HCFCs have a great impact on ozone depletion and global warming process. So, there must be a cheaper way to detect CFCs and HCFCs from refrigerant products like refrigerator, aerosols, air-conditioner, etc. So, for the very first time, this paper proposes a novel highly sensitive dual-core bi-layer gold-coated surface plasmon resonance (DB-SPR)-based photonic crystal fiber (PCF) for sensing CFCs and HCFCs in the field of chemical and biosensing. The analysis is assumed at 20${}^{\circ }$C (293 K) for the concentration level of CFCs and HCFCs ranging between 20% and 80%. The entire numerical evaluation process is done by using finite element method (FEM) with a full vectorial software named COMSOL V-5.1 by examining the number of mesh elements of 228,120 which has better flexibility in the fabrication process. The proposed model can prove itself better than any other models by the justification of the major optical parameters, such as birefringence (B${}_i$), coupling length (L${}_c$), confinement loss (${\alpha }_c$), transmittance (T${}_x$), transmittance variance (T${}_v$), power fraction (P${}_f$), amplitude sensitivity (S${}_A$), wavelength sensitivity (S${}_w$), figure of merit (FOM), resolution (Rl), resonance (R²), etc. which respectively corresponds to their maximum performance profiles of 0.038, 40 $\mu$m, 1140 dB/cm, −180 dB, 410 dB/RIU, 51%, −3200 RIU${}^{-1}$, 30450.43 nm/RIU, 1300, 1.25×10${}^{-5}$, 0.98712. So, it is evident that proposed SPR PCF can be a tough competitor in the field of biosensing and chemical sensing as well as gas sensing.

Two photonic crystal fiber (PCF) polarization beam splitters (PBSs) featuring ultra-short length and ultra-high extinction ratios at wavelengths of 1.31 $\mu$m and 1.55 $\mu$m are designed and investigated. Non-silicon materials, such as gallium arsenide (GaAs) and nematic liquid crystal (NLC), are incorporated into the structure to ensure operation of the splitters in the communication bandwidth. Based on the full-vector finite element method (FEM), numerical simulation is carried out to optimize the structural parameters systematically. Specifically, for the splitter operating at 1.31 $\mu$m, the optimal optical fiber length, extinction ratio, and bandwidth are 27.87234 $\mu$m, $-152.40$ dB, and 152 nm, respectively. In comparison, the optimal fiber length, extinction ratio, and bandwidth of the splitter at the wavelength of 1.55 $\mu$m are 15.59356 $\mu$m, $-137.21$ dB, and 200 nm, respectively. The results reveal that the splitters have great potential in environmental monitoring, biochemical detection, and optical communication.

A multi-functional dual-core photonic crystal fiber (PCF) polarization beam splitter with a central oval hole filled with gold is designed and described. The first layer on the circular holes around the core is filled with the nematic liquid crystal (NLC) to enhance birefringence. The full-vector finite element method is employed to study the characteristics of the polarization beam splitter and derive the optimal parameters. When the fiber length is 14.55 $\mu$m, the polarized light in the $x$- and $y$-directions can be separated at a wavelength of 1.31 $\mu$m and the bandwidths are 257 and 238 nm, respectively, for extinction ratios bigger than 10 dB. When the fiber length is 10.86 $\mu$m, the polarized light in the $x$- and $y$-directions can be separated at a wavelength of 1.55 $\mu$m and the bandwidths are 239 and 174 nm, respectively for extinction ratios larger than 10 dB. The multifunctional polarization beam splitter with an ultra-short fiber length and broad bandwidth improves the sensing performance in comparison with single-function devices and our results provide insights into the design of beam splitters with multiple functions.

In this study, the coupling and the relative sensitivity characteristics of a Dual-Core photonic crystal fiber (DC-PCF)-based liquid sensor are evaluated by employing the full-vectorial finite element method (FV-FEM). The simulation results demonstrate that coupling length of the proposed sensor varies between 2mm and 0.2mm length at wide wavelength range (800–1600nm) for low index analytes (1.33–1.36). Besides, birefringence of order $\approx 10^{-4}$ is reported. It is found that analyte with 1.36 index reaches higher sensitivity levels. On the other hand, the coupling length of the proposed sensor can be optimized with the ellipticity ratio of the central core hole. Bending analysis on coupling length and optical power fluctuations is also reported. It is found that the power flow in one core is increasing almost 90% towards the bending direction. The proposed structure with simple design parameters has a great potential in various biomedical applications including DNA detection and can also be employed as fiber-based refractometer in various sensing applications.

Fiber couplers are widely used in telecommunications and industry as passive splitting power devices. The effective power coupling and transmission from one fiber to another is mainly determined by both the coupling length and the coupling coefficient. The coupling length can be calculated directly but the coupling coefficient depends upon the refractive index and the separation fiber axis. After the fusion processes of two SMF-28e^® couplers, the refractive index is unknown due to a change in the radius of the fiber (r_cladd < 40 μm and r_core < 1.5 μm). The coupling coefficient range is obtained from a distribution of the coupling ratio and compared with the empirical formula, which also enables one to calculate the refractive index. In this experiment, the coupling coefficient in the range of 0.6–0.9/mm is calculated as a function of the separation fiber axis and the refractive index of the core and cladding. The result shows a good correlation between experimental results and theoretical calculation.