: In this paper, a dual-core photonic crystal fiber (DC-PCF) based surface plasmon resonance (SPR) bio-compatible sensor is proposed for various bio-organic molecules and biochemical analytes refractive index (RI) detection in the visible to near-infrared region (0.5 to 2 μm). Two hexagonal ring lattice with all circular air-holes are used to simplify the sensor structure. To make the practical applications feasible, plasmonic material and analyte sensing layer both are employed at the outer surface of the fiber. Noble plasmonic material gold (Au) having a thickness of 30 nm is used to excite the surface plasmons. A thin layer of titanium oxide (TiO2) having a thickness of 5 nm is also considered as an adhesive layer between the Au and silica glass. The sensor response is investigated using the mode solver based finite element method (FEM). Numerical results indicate that the proposed sensor shows a maximum amplitude sensitivity (AS) of 6829 RIU-1, amplitude resolution (AR) of 5x10-6 RIU, maximum wavelength sensitivity (WS) of 28,000 nm/RIU, and wavelength resolution (WR) of 3.57x10-6 RIU, using the amplitude and wavelength interrogation methods, respectively. Moreover, a maximum figure of merit (FOM) of 2800 RIU-1 is obtained, which is the highest among the reported PCF-SPR sensor. Owing to the promising sensitivity and simple structure, the proposed sensor can be potentially applicable for the detection of biochemical solutions and biological samples.

In this paper, a low-loss, spiral lattice photonic crystal fiber (PCF)-based plasmonic biosensor is proposed for its application in detecting various biomolecules (i.e., sugar, protein, DNA, and mRNA) and biochemicals (i.e., serum and urine). Plasmonic material gold (Au) is employed externally to efficiently generate surface plasmon resonance (SPR) in the outer surface of the PCF. A thin layer of titanium oxide (TiO₂) is also introduced, which assists in adhering the Au layer to the silica fiber. The sensing performance is investigated using a mode solver based on the finite element method (FEM). Simulation results show a maximum wavelength sensitivity of 23,000 nm/RIU for a bio-samples refractive index (RI) detection range of 1.32–1.40. This sensor also exhibits a very low confinement loss of 0.22 and 2.87 dB/cm for the analyte at 1.32 and 1.40 RI, respectively. Because of the ultra-low propagation loss, the proposed sensor can be fabricated within several centimeters, which reduces the complexity related to splicing, and so on.

In this paper, a relatively simple and ultra-sensitive Photonic crystal fiber (PCF) based surface plasmon resonance (SPR) sensor is proposed for detecting different analyte refractive indices (RIs) ranging from 1.33 to 1.43 over a wide range of wavelength spectrum spanning $0.55~\mu $ m to $3.50~\mu $ m. The comprehensive finite-element simulations indicate that it is possible to achieve remarkable sensing performances such as wavelength sensitivity (WS) and figure of merit (FOM) as high as 123,000 nm/RIU and 683 RIU1, respectively, and extremely low value of wavelength resolution (WR) about $8.13\times 10^{-7}$ RIU. A novel artificial neural network (ANN) model is proposed which helps to accurately predict the RIs by carefully examining the simulation data. The mean square error (MSE) and prediction accuracy ( $R^{2}$ ) values for the ANN model are found about 0.0097 and 0.9987, respectively, indicating the high prediction capability of the proposed ANN model. Due to its exceptional sensitivity and precise detection capabilities, the proposed sensor has the potential to serve as a viable option for sensing analyte RI. Additionally, the sensor could be utilized for identifying cancerous cells and detecting urinary tract infections in humans.

An asymmetrical configuration of hexagonal lattice photonic crystal fiber (PCF) based surface plasmon resonance (SPR) refractive index sensor is proposed. Instead of the typical confinement loss method, the lower birefringence peak method is considered to explore the sensing performance. The asymmetry in the core region induces birefringence that enhances the coupling efficiency between the core and surface plasmon polariton (SPP) mode. To form the strong SPR effect, both the gold layer and analyte layer are deposited on the external surface of the PCF. The proposed birefringent sensor exhibits the maximum wavelength sensitivity of 22,000 nm/RIU within broad analyte refractive index (RI) from 1.33 to 1.42. The sensing characteristics are carried out with the variation of several PCF structural parameters. Owing to enhanced sensitivity, the proposed sensor can be a potential candidate for biological and biomolecular analyte detection.

Understanding the interplay between the core-guided modes and tube-modes of hollow-core anti-resonant fiber (HCARF) is essential to achieve low-loss and multi-mode guidance. In this paper, we thoroughly investigated the coupling between the core-guided modes and tube-modes of various HCARFs using extensive analytical and finite-element modeling with the aim of achieving multi-mode guidance. We found that the coupling between the core-guided modes and tube-modes can significantly be enhanced by modifying and placing the cladding tubes of nested HCARFs. We proposed a modified nested HCARF that significantly enhances the inhibited-coupling (IC) between the core-guided modes and tube-modes compared to a regular and nested HCARF design. Due to the enhanced IC between the core-guided modes and tube-modes resulting from their phase mismatch, modified nested HCARF can support as high as 50 distinct spatial modes with propagation loss $< $ 10 dB/km and also demonstrate low-bend loss upon tight bending at 1064 nm. Our study will provide a better understanding of the coupling between the core-guided modes and tube-modes for designing multi-mode HCARFs. It is anticipated that the extraordinary optical properties of the proposed fiber can be beneficial for several applications including high power laser beam delivery, short-haul communication, and ultrafast nonlinear optics.