In this paper, a hollow-core negative curvature fiber (HC-NCF) with high birefringence is proposed for low refractive index (RI) sensing based on surface plasmon resonance effect. In the design, the cladding region of the HC-NCF is composed of only one ring of eight silica tubes, and two of them are selectively filled with the gold wires. The influences of the gold wires-filled HC-NCF structure parameters on the propagation characteristic are investigated by the finite element method. Moreover, the sensing performances in the low RI range of 1.20–1.34 are evaluated by the traditional confinement loss method and novel birefringence analysis method, respectively. The simulation results show that for the confinement loss method, the obtained maximum sensitivity, resolution, and figure of merit of the gold wires-filled HC-NCF-based sensor are −5700 nm/RIU, 2.63 × 10⁻⁵ RIU, and 317 RIU⁻¹, respectively. For the birefringence analysis method, the obtained maximum sensitivity, resolution, and birefringence of the gold wires-filled HC-NCF-based sensor are −6100 nm/RIU, 2.56 × 10⁻⁵ RIU, and 1.72 × 10⁻³, respectively. It is believed that the proposed gold wires-filled HC-NCF-based low RI sensor has important applications in the fields of biochemistry and medicine.

In this paper, a surface plasmon resonance (SPR)-based photonic crystal fiber (PCF) sensor is proposed for simultaneously measuring the refractive index (RI) and temperature. In the design, the central air hole and external surface of the proposed PCF are coated with gold films, and an air hole is filled with the temperature-sensitive material (TSM). By introducing the inner and outer gold films and TSM, the RI and temperature can be measured simultaneously at different wavelength regions. The simulation results show that the average wavelength sensitivities of the proposed SPR-based PCF sensor can reach 4520 nm/RIU and 4.83 nm/°C in the RI range of 1.35~1.40 and a temperature range of 20~60 °C, respectively. Moreover, because of using the different wavelength regions for sensing, the RI and temperature detections of the proposed SPR-based PCF sensor can be achieved independently. It is believed that the proposed SPR-based PCF RI and temperature sensor has important applications in biomedicine and in environmental science.

In this paper, a novel gold film-coated V-shape dual-core photonic crystal fiber (V-DC-PCF) polarization beam splitter (PBS) based on surface plasmon resonance effect is proposed. The coupling lengths of the X-polarization (X-pol) and Y-polarization (Y-pol) and the corresponding coupling length ratio of the proposed V-DC-PCF PBS without gold film and with gold film are compared. The fiber structure parameters and thickness of the gold film are optimized through investigating their effects on the coupling lengths and coupling length ratio. As the propagation length increases, the normalized output powers of the X-pol and Y-pol of the proposed V-DC-PCF PBS at the three wavelengths 1.610, 1.631, and 1.650 $\mathsf{\mu }\mathrm{m}$ are demonstrated. The relationships between the extinction ratio ($ER$), insertion loss ($IL$) and wavelength for the three splitting lengths ($SLs$) 188, 185, and 182 $\mathsf{\mu }\mathrm{m}$ are investigated. Finally, it is demonstrated that for the proposed V-DC-PCF PBS, the optimal $SL$ is 188 $\mathsf{\mu }\mathrm{m}$, the $ILs$ of the X-pol and Y-pol are less than 0.22 dB, and the splitting bandwidth ($SB$) can cover the E + S + C + L + U band. The proposed V-DC-PCF PBS has the ultra-short $SL$, ultra-wide $SB$, and ultra-low $IL$, so it is expected to have important applications in the laser, sensing, and dense wavelength division multiplexing systems.

In this paper, a kind of super-mode orbital angular momentum microstructured fiber (SM-OAM-MSF) is proposed. By introducing 20 Ge-doped equiangular cylindrical inclusions in the ring-core region, mode coupling mechanism is employed in the formation of super-OAM (SOAM) modes. Specifically, the double degenerated out-of-phase SMs are first generated by the coupling of individual core mode, then the quadruple degenerated SOAM modes are formed by combining two components of the out-of-phase SMs with a phase difference of ±π/2. Theoretical analysis and numerical results reveal that the effective index difference (Δn_eff) between adjacent out-of-phase SM groups are strongly influenced by the parameters of the individual core except the ring-core’s width. Therefore, large mode area and SOAM modes’ index separation larger than 1.0×10⁻⁴ can be achieved simultaneously in our proposed SM-OAM-MSF. Through careful fiber design, HE_1,1 and HE_2,1 are used in the formation of SMs and SOAM modes. Simulations show that all the nine SOAM groups originating from HE_1,1 mode and the first five SOAM groups stemming from normal coupling of HE_2,1 mode can be supported above 1.0µm, that are 56 SOAM modes in total. The highest purity is 99.86% for $\textrm{SOAM}_{ \pm 2,1}^{ \pm ,5}$ mode. And the maximum mode area (A_eff) value reaches up to 638.88µm² at 1.55µm, which is nearly eight times larger compared to that of conventional ring-core MSFs.