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Diabetes is a metabolic disease that seriously endangers human health, and monitoring glucose levels is one of the main diagnostic methods for its prevention and treatment. In this paper, we propose a high-performance glucose sensor based on the Sagnac interferometer (SI) utilizing photonic crystal fiber (PCF) with micro-holes. By setting two columns of micro-holes in the center of the PCF, the high birefringence effect is obtained to improve the detection sensitivity. All air holes are assumed to fill with the glucose solution, and the sensing characteristics of the PCF sensor are theoretically studied by the finite element method (FEM). The results show that the sensitivity and resolution of the PCF glucose sensor can reach 4.14 nm/(g/L) and 0.48 mg/dL within the concentration range of 0–120 g/L, respectively. In addition, the effects of the partial-filling methods and PCF structural parameters on the sensing performance are also analyzed to achieve optimal performance. The proposed glucose sensor possesses high sensitivity and simple structure, which exhibits good application prospects in the field of medical monitoring. [ABSTRACT FROM AUTHOR]

Six high-performance photonic crystal fiber polarization filters based on surface plasmon resonance are proposed and studied by Matlab and Comsol Multiphysics based on finite element method. Dual D structure is adopted in order to improve birefringence and increase coupling strength between surface plasmon and fiber core light energy. The characteristics of loss, filtering, and extinction ratio were studied, including six material situations: gold, silver, aluminum, gold and graphene, silver and graphene, aluminum and graphene. The mode coupling phenomenon and the influence of metal thickness on optical fiber devices are analyzed. Research result shows that the proposed fibers have good filtering ability, and when the fiber length is 2 mm, the extinction ratio can reach 4520 dB. [ABSTRACT FROM AUTHOR]

A high‐sensitivity square‐lattice photonic crystal fiber (PCF) refractive index (RI) sensor based on an external gold coating is proposed. This design eliminates the limitations of selective coating and liquid penetration in the micrometer holes, and avoids the side polishing of D‐type fibers. The effects of air hole diameter and gold film thickness on the sensing performance are studied in detail using the full vector finite element method (FV‐FEM). In addition, the sensing performance of the gold‐coated PCF under the optimized parameters is compared with that of PCF with the same air hole diameter and a gold grating. The results show that the proposed PCF has a higher structural tolerance to the air hole diameter, which improves the manufacturing feasibility. Moreover, the gold‐coated PCF has a maximum sensitivity of 31 000 nm RIU−1 and a resolution of 3.23 × 10−6 RIU in the RI measurement range of 1.34–1.49. The maximum sensitivity of the gold‐grating PCF with lower loss is 30 000 nm RIU−1 in the RI detection range of 1.38–1.46, and the resolution is 3.33 × 10−6 RIU. Low loss is beneficial for extending the length of the fiber, which is convenient for practical implementation. Thus, it has potential applications in biomolecular and biochemical analysis. [ABSTRACT FROM AUTHOR]

A single-mode single-polarization photonic crystal fiber (SMSP-PCF) filter at the wavelengths of 1.125 and 1.31 μm based on surface plasmon resonance is proposed and studied by the finite element method. Numerical results indicate that compared with the filter with the same size of holes near the core, the filter with four small holes near the core makes its extinction ratio (ER) at the wavelengths of 1.125 and 1.31 μm twice as large. That is, when the fiber length is 500 μm, the ER at 1.125 μm increases from 33.43 to 58.62 dB, and the ER at 1.31 μm increases from 129.41 to 244.85 dB. Furthermore, the designed SMSP-PCF filter also has a good structural tolerance. The proposed optical fiber filter has excellent performance of compact structure, high ER and good structural tolerance, and can be used in an integrated communication transmission system. [ABSTRACT FROM AUTHOR]

Highlight • A high-sensitivity PCF sensor was designed for analyte detection. • This optimized sensor had an excellent linearity and good sensitivity. • The sensitivity within the refractive index range of 1.425–1.45 is 60,611 nm/RIU. • The corresponding minimum detection limit is 1.58 × 10−6 RIU. • The sensitivity within the refractive index range of 1.45–1.46 is 28,000 nm/RIU. • The corresponding minimum detection limit is 2.86 × 10−6 RIU. Abstract A high-sensitivity photonic crystal fiber (PCF) refractive index sensor based on the resonant coupling of core mode and defect mode is studied by the finite element method. The large liquid-filled is placed near to the core for easy coupling with the core mode. The simulation results show that when the refractive index of the analyte is 1.425–1.45 and 1.45–1.46, the average sensitivity is 60,611 and 28,000 nm/RIU respectively, and the corresponding minimum detection limits are 1.58 × 10−6 and 2.86 × 10−6 RIU. [ABSTRACT FROM AUTHOR]

A novel ultra-wide bandwidth photonic crystal fiber (PCF) polarization filter based on surface plasmon resonance (SPR) is proposed. The PCF has two large cladding air holes in the direction of y-axis that are selectively coated with gold films to create resonance. There are two small holes in the direction of x-axis, which provides high birefringence. The polarization characteristics of PCF are studied by finite element method (FEM). The numerical simulation results indicate that the loss of y-polarized core mode is 718.87 dB/cm, while the loss of x-polarized core mode is only 0.95 dB/cm at the resonant wavelength of 1.55 µm. The extinction ratio is better than 20 dB at the wavelength ranging from 1.43 to 1.79 µm, the corresponding bandwidth is 360 nm. [ABSTRACT FROM AUTHOR]

• A side-polished gold-coated photonic crystal fiber with a offset core is proposed. • The U-shaped air hole arrangement can enhance the leakage of the evanescent field. • The PCF integrates RI sensing and polarization filtering. • The maximum sensitivity within the RI range of 1.35–1.39 is 12,500 nm/RIU. • The proposed PCF polarization filter has bandwidth tunability. The transmission characteristics of a side-polished eccentric photonic crystal fiber (PCF) is studied by using the full-vector finite element method (FV-FEM). The proposed structure is composed of air holes similar to a U-shaped arrangement, and there is a small air hole between the side polishing plane and the core region to enhance the leakage of the evanescent field in the y polarization direction. It integrates refractive index (RI) sensing and polarization filtering. The RI sensing mainly works in the visible light band, while polarization filtering is suitable for the near-infrared band. The maximum sensitivity of the sensor is 12,500 nm/RIU, and the resolution is 8.0 × 10−6 RIU in the detection RI range of 1.35–1.39. By changing the external RI or the thickness of the gold-coated film, the bandwidth-tunable PCF polarization filter with excellent performance can be obtained. Therefore, the design of side-polished PCF will be of great significance to the development of integration and miniaturization of optical devices. [ABSTRACT FROM AUTHOR]

A photonic crystal fiber (PCF) with nanoscale gold-coated and liquid-filled air-holes is proposed, which can realize anomalous single-polarization transmission. That is, the polarization filtering in the x- and y- polarization directions is achieved at wavelengths of 1.18 and 1.55 μm. The dispersion characteristics and losses of filter are analyzed by the finite element method (FEM). The results show that when the fiber length is 800 μm, the extinction ratios (ERs) at the above two wavelengths are 31.25 and -248.63 dB, and the corresponding bandwidths are 28 and 270 nm, from 1.165 to 1.193 μm and 1.43–1.70 μm, respectively. Furthermore, by changing the structural parameters, not only can the positions of resonance wavelengths in the two orthogonal polarization directions be changed simultaneously, but also the positions of resonance wavelengths in the x polarization direction can be changed singly, which is conducive to the filtering of PCF at a specific wavelength within a certain range. [ABSTRACT FROM AUTHOR]

• A single-polarization filter comprising a gold-infiltrated and liquid crystal-filled PCF-SPR is studied by the FEM. • Compared with the traditional structures, we find that our structure has higher birefringence. • The confinement losses of x-polarized mode are 99.87, 726.50, 474.60dB/cm at wavelengths of 0.88, 1.02, 1.30um. • While the confinement losses of y-polarized mode are only 0.001, 0.004 and 0.046dB/cm. • When the transmission length of PCF is 2mm, the bandwidth of ER less than −20dB is as high as 900nm, from 0.85 to 1.75um. A broadband and single-polarization filter based on high birefringence photonic crystal fiber (HB-PCF) with selectively infiltrated gold wires and liquid crystal is proposed, and its filtering characteristics are studied by the full-vector finite element method (FV-FEM). Two gold wires are selectively filled on both sides of the liquid crystal core to act as two defect cores. The results show that the confinement loss of x-polarized core mode is much larger than that of y-polarized core mode in the wavelength range of 0.85–1.75 μm. The confinement losses of x- and y-polarized core modes are 99.87 dB/cm and 0.001 dB/cm at 0.88 um, 726.50 dB/cm and 0.004 dB/cm at 1.02 um, and 474.60 dB/cm and 0.046 dB/cm at 1.30 um, respectively. The bandwidth with extinction ratio less than 20 dB is 900 nm. Moreover, compared with the traditional structures, we find that our structure has higher birefringence. High birefringence helps to separate the resonant wavelengths of x- and y-polarized core modes. Therefore, the excellent properties of the proposed filter can make it widely used in sensing systems and broadband optical communication systems. [ABSTRACT FROM AUTHOR]

A tunable single polarizing filter is proposed by selectively coating gold film on the air holes of photonic crystal fiber (PCF). The polarization properties of the PCF filter are evaluated by the finite-element method. Simulation results show that the loss of y-polarized core mode at 1250 and 1550 nm is 136.23 and 839.73 dB/cm, respectively. Furthermore, we innovatively combine stable modulation with flexible modulation. To be specific, the resonance wavelengths are slowly controlled in a small wavelength range by altering the diameter of the air-hole-coated gold film, while the resonance wavelengths are flexibly controlled in a wide wavelength range by altering the thickness of the gold film or the diameter of the small air holes. When the length of the PCF is 500 µm, the bandwidth of extinction ratio greater than − 20 dB is only 60 nm at the communication window of 1550 nm. It is beneficial to fabricate a narrow-band polarization filter. [ABSTRACT FROM AUTHOR]

Highlights • The PCF is simple, novel and easy to apply. • The filter can simultaneously achieve polarization filtering effects on the input signals of the 1310 and 1550 nm communication bands. • When the length of the PCF is 0.8 mm, the crosstalk is less than −20 dB or better than 20 dB in the 1550 and 1310 nm bands, the optical bandwidth can reach 220 and 150 nm, respectively. Abstract A photonic crystal fiber (PCF) polarization filter with dual communication bands is proposed in this paper. The polarization characteristics of the double-hole gold-coated PCF are numerically analyzed by utilizing the finite element method (FEM). As we know, the filter can achieve polarization filtering effect on input signals at 1310 nm (y-polarized mode) and 1550 nm (x-polarized mode) bands simultaneously in two orthogonal polarization states. In the above two communication bands, the confinement losses of the unwanted polarization mode can reach 192.7 dB/cm (y-polarized mode) and 427.5 dB/cm (x-polarized mode), whereas other polarization mode losses are very low. Furthermore, when the transmission length of the PCF is 800 μm, x and y polarization mode crosstalk can reach −275.4 and 126.9 dB in the 1550 and 1310 nm bands, respectively. Meanwhile, the crosstalk is less than −20 dB or better than 20 dB, the optical bandwidth can reach 220 and 150 nm, respectively. [ABSTRACT FROM AUTHOR]

In this paper, a dual-polarizated photonic crystal fiber filter (PCF) based on surface plasmon resonance (SPR) was proposed. Its polarization characteristics were numerically researched by finite element method (FEM). The results indicate that the polarization filtering of the filter be able to fulfill in dual communication bands of 1310 and 1550 nm. The confinement loss reaches 80,711.4 dB/m for the x polarization core mode at the wavelength of 1310 nm, while 46,614.5 dB/m for the y polarization core mode at the wavelength of 1550 nm. When the length of PCF is 2.5 mm, the extinction ratios of x and y polarization core modes can achieve 1705.27 and − 883.46 dB, respectively. For the communication windows of 1310 and 1550 nm, the filtering bandwidths can reach to 390 and 350 nm, respectively. These excellent characteristics are beneficial to the design and manufacture of filter devices. • A PCF dual windows polarization filter based on SPR is proposed. The square arrangement of air holes is easy to prepare. • The losses of the filter at dual windows can reach 80,711.4 dB/m and 46,614.5 dB/m. • When L PCF = 2.5 mm, the ER of the two orthogonal modes can reach 1705.27 and − 883.46 dB. The bandwidth can reach 390 nm and 350 nm. [ABSTRACT FROM AUTHOR]