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This paper presents a dual-core two-parameter optical fiber sensor based on the surface plasmonic resonance (SPR) effect. It is analyzed by the finite element method. The proposed sensor is a dual-channel structure designed with photonic crystal fiber (PCF) as the base material: one channel is coated with a gold film to measure the refractive index (RI) of the solution to be measured, and the other channel is coated with a gold film and polydimethylsiloxane to measure the temperature of the solution to be measured. The exposed microslot structure on both sides reduces the complexity of sensing measurements. The results show that the maximum RI sensitivity of the sensor is 19,900 nm/RIU, and the maximum sensitivity to temperature is 8.7 nm/℃. This work is conducive to realizing a PCF sensor with high sensitivity, large measurement range, real-time monitoring, and easy preparation. As a result, the sensor is expected to be widely used in fields such as biology and chemicals. [ABSTRACT FROM AUTHOR]

A negative curvature hollow core fiber (NCHCF) refractive index (RI) sensor based on localized surface plasmon resonance (LSPR) is proposed. The gold nanowires are deposited in four cladding tubes in the y-direction, with the core region serving as the analyte channel. The full vector finite element method (FEM) is used to analyze the influence of various structural parameters on this sensor, and the structural parameters are optimized and selected. Ultimately, an average sensitivity of 9356.59 nm/RIU is accomplished within a spacious refractive index detection scope of RI = 1.28–1.43. The sensor attained a maximum sensitivity of 10,220 nm/RIU at RI = 1.36. In the bargain, there is an excellent linear correlation between the resonance wavelength and the refractive index of the analyte, with a value of 0.99901 and a factor of merit (FOM) range of 119.9563–155.9432 RIU−1, achieving a resolution of 10−6 RIU. The sensor has potential applications in various fields such as environmental protection, food safety, and medical diagnostics due to its high sensitivity, spacious detection scope, and positive linear response. [ABSTRACT FROM AUTHOR]

A chalcogenide hollow-core anti-resonance fiber (HC-ARF) filter based on surface plasmon resonance (SPR) at 3 μm band is designed. The substrate materials are As40S60 and GeAsS, which are used to create the conditions for the formation of SPR, so as to realize the directional energy coupling in the specific polarization direction of the fiber. The effects of core diameter, capillary radius, and capillary wall thickness on the polarization performance of HC-ARF filter are analyzed by full-vector finite element method (FV-FEM). The numerical results show that the confinement loss of x-polarized fundamental mode (FM) reaches 1917.24 dB/m at the wavelength of 3.02 μm, while that of y-polarized FM is 23.11 dB/m. When the fiber length is 8 mm, the bandwidth with extinction ratio (ER) better than 20 dB covers the wavelength range of 320 nm. In addition, the resonance wavelength can be effectively adjusted by changing the capillary wall thickness. The proposed HC-ARF filter has potential applications in biomedicine, scientific research, and atmospheric detection. [ABSTRACT FROM AUTHOR]

In this paper, we propose a broadband mode converter based on photonic crystal fiber (PCF) starting from the method of propagation constant height fitting. Using the finite element method and the beam propagation method, three typical mode converters in the S + C + L range, LP01 to LP11, LP01 to LP21, and LP01 to LP31, are calculated. Numerical results show that the mode coupling efficiency of this mode converter is over 90% and the mode extinction ratio is over 10 dB. This converter can realize pattern multiplexing in a cascade manner. On this basis, we propose a multi-core mode multiplexer structure. On the premise of sacrificing some performance, three modes can be reused at the same time. Mode converters of PCF structure can be fabricated by PCF post-processing technology. It is expected to become an important part of the future mode division multiplexing (MDM) optical communication system. [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]

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]

A polarization filter at the two communication windows of 1.31 and 1.55 μm based on photonic crystal fiber (PCF) coated by nanoscale gold film is proposed. The effects of geometrical parameters of the PCF on the performances of polarization filter are evaluated by finite element method (FEM). Numerical simulations reveal that the polarization filter possesses ultra-short length of 400 μm and high extinction ratios of 41 and −150 dB at the communication wavelengths of 1.31 and 1.55 μm respectively. The bandwidth of extinction ratio (ER) better than 20 dB is 58 nm at the communication window of 1.31 μm. Moreover, the bandwidth of ER better than −20 dB is 164 nm at the communication window of 1.55 μm. [ABSTRACT FROM AUTHOR]

In this paper, a novel D-shaped photonic crystal fiber (D-PCF) based on surface plasmon resonance (SPR) is designed, and gold nanowire is used as the SPR active metal. A full-vector finite element method is applied to analyze the polarization filter characteristics. Phase matching between the second-order surface plasmon polariton (SPP) mode and the core-guided modes of x- and y-polarization can be met in two different wavelengths. By adjusting the diameters of the large air holes and the gold nanowire, this designed D-PCF can be used to the polarization filter in two communication bands. The loss of unwanted y-polarization mode can reach 244.9 and 292.8 dB/cm at the wavelength of 1.31 and 1.55 μm, respectively. With a length of 1 mm of the designed D-PCF, the bandwidths with the crosstalk better than 30 dB can reach to 88 and 150 nm, and the crosstalks are 208.4 and 249.5 dB the wavelength of 1.31 and 1.55 μm, respectively. [ABSTRACT FROM AUTHOR]

A dual-polarized microstructure fiber sensor is based on surface plasmon resonance, which is arranged of pentagonal lattice to achieve refractive index sensing in the near-infrared spectrum. The gold nanowire-filled fiber sensor exhibits excellent sensing characteristics and improved transmission properties. Finite element method is used for computational analysis and structural optimization for the fiber sensor. When the refractive index of the analyte range from 1.32 to 1.38, dual-polarizd sensing can be implemented as a self-calibration with wavelengths increasing from 700 to 1200 nm. For the proposed sensor, the highest wavelength sensitivity of 10,286 nm/RIU is exhibited with a sensor resolution of 9.72×10−6 RIU, and the corresponding amplitude sensitivity and figure of merit are as high as 544.0 RIU−1 and 146.9. Moreover, the sensing length of at least 1.14 mm ensures accurate detection of optical signals for proposed sensor. Owing to the above advantages, the proposed sensor is suitable for high integration and high precision sensitivity detection. [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]

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-bandwidth polarization splitter based on soft glass dual-core photonic crystal fiber (DC-PCF) is designed in this paper, which is analyzed through the finite element method (FEM). The coupling characteristics of the designed DC-PCF can be enhanced by a high refractive index As 2 S 3 core. Numerical results show the ultra-bandwidths of the x - and y -polarization modes can reach to 86 nm and 60 nm as the extinction ratios better than −20 dB and −30 dB at the vicinity of the wavelength of 1.31 μm. The length of the designed soft glass DC-PCF is 52.29 mm and the extinction ratios of the x - and y -polarization modes are −85.57 dB and −56.81 dB at the wavelength of 1.31 μm, respectively. In addition, the designed splitter has a tolerance of ±10 nm in its all structure parameters, which make the design not sensitive to the perturbation during the fabrication process. [ABSTRACT FROM AUTHOR]

• A polarization filter based on hollow-core anti-resonant fiber with tellurite glass as the base material was designed. • By varying the structure, a single-mode single-polarization fiber with a bandwidth of 30nm near 1550nm was achieved. • Through the Surface Plasmon Resonance (SPR) effect, a polarization filter with a bandwidth of 390 nm was realized near 1550 nm. Hollow-core anti-resonant fiber (HC-ARF) with unique light propagation through air present a promising avenue for advanced applications of polarization filters based on HC-ARF. This paper proposes a single-mode and single-polarization filters (SMSPF) around 1550 nm based on HC-ARF with tellurite glass as the substrate material. The polarization filtering characteristics of the HC-ARF with varying wall thickness and gold wire filling are simulated and analyzed using the finite element method. A single-mode single-polarization (SMSP) HC-ARF with a continuous bandwidth of 30 nm is obtained, and a polarization filter with low polarization loss and a high filtering bandwidth of 390 nm is achieved through the surface plasmon resonance (SPR) effect. [ABSTRACT FROM AUTHOR]

• We propose a novel superposition-assisted structure (SAS) in few-mode multi-core fibers (FM-MCFs) to effectively suppress inter-core crosstalk (XT). • The designed SAS fiber can realize low XT characteristic while keeping good fiber transmission performances such as effective mold field area, dispersion, differential mode group delay and bending loss. • The optimal SAS FM-MCF achieve the XT below −50 dB/100 km at a cladding diameter of 125 μm, which is conducive to the docking with the existing optical fiber technology. • The designed SAS is suitable for the application of low-XT and large-capacity FM-MCFs based on space division multiplexing (SDM) technology. We propose a novel superposition-assisted structure (SAS), composed of trench and air holes, in few-mode multi-core fibers (FM-MCFs) to effectively suppress the inter-core crosstalk (XT). The proposed SAS can minimize the inter-core XT to −70.58 dB/100 km in the 7-core fibers, which is 60 dB/100 km lower than the typical trench-assisted structure and 29 dB/100 km lower than the typical hole-assisted structure, respectively. In addition, the SAS does not introduce extra fiber loss and dispersion. In this paper, the key properties of SAS MCFs are numerically simulated using the finite element method (FEM). The large effective area (A eff) over 90 μm2 enables the suppression of nonlinear effects. The values of dispersion coefficient and differential mode group delay (DMGD) are in the range of 24–42 [ps/(nm·km)] and 5–23 ps/m over C + L band, respectively, which are favorable for fiber communication systems. Such SAS 7-core 5-LP-mode fibers can be further optimized to obtain the cladding diameter of 125 μm, where five propagation modes achieve inter-core XT target below −50 dB/100 km with the bending radius less than 500 mm. Besides, the optimized relative core multiplicity factor (RCMF) can be up to 97.55. The proposed SAS provides a promising solution for the design of FM-MCFs with ultra-low inter-core XT and high capacity for space division multiplexing (SDM). [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 novel air-hole array and trench (HAT) structure is proposed. • The 6-core 2-LP fiber with a 125 μm cladding, which is compatible with the current optical fiber communication network. • This fiber exhibits an ICXT is below −38.23 dB/100 km, which is sufficiently low to neglect IMXT. • Compared to SMF, the communication capacity of this fiber has increased by 18 times. In order to be compatible with the current optical fiber communication network, we propose a 125 μm diameter heterogeneous six-core two linear polarization modes fiber with air-hole array and trench (HAT) structure. The coupled power theory and finite element method (FEM) are used to investigate inter-core crosstalk (ICXT), bending loss, and other properties. The HAT structure can effectively suppress ICXT; and the maximum ICXT is less than −38 dB/100 km in C + L band. The effective refractive index difference (Δ n eff) between modes is greater than 3 × 10-3, inter-mode crosstalk (IMXT) can be ignored. The maximum intrinsic loss is 0.187 dB/km; relative core multiplicity factor reaches 32.21. The relative spatial efficiency is 18 times that of traditional single-mode fibers. This fiber can be produced by chemical vapor deposition and existing optical fiber manufacturing technology. The designed fiber can be directly connected to the communication network to reduce the cost of equipment renewal, and the communication capacity can be increased by 18 times. [ABSTRACT FROM AUTHOR]

An ultra-bandwidth and single-mode polarization beam splitter (PBS) of dual hollow-core anti-resonant fiber (DHC-ARF) with circular outer tubes and inner tubes is proposed. Four outer tubes are introduced in the horizontal direction to obtain two hollow cores. The modal coupling is realized through the gap between the two hollow cores. The inner tubes also are inserted into two the outer tubes between the two hollow cores for realizing ultrawide bandwidth in DHC-ARF. To achieve single-mode performance, the high-order modes are coupled with the cladding modes by optimizing the diameter of outer tubes and the inner tubes. The calculation results show that an 11.0 cm long DHC-ARF can be used to develop a PBS with an operating bandwidth of 410 nm, where the polarization extinction ratio is lower than −20 dB. The high-order mode extinction ratio is 625.41, and the loss of fundamental mode is below 1.018 dB/m. • Because the size of elliptical tubes is hard to fabricate, an ultra-bandwidth and single-mode PBS of DHC-ARF completely composed of circular cladding tubes and nested tubes is proposed in the structures. • The influence factors related to bandwidth, the high-order mode extinction ratio and coupling length of PBS based on DHC-ARF are systematically studied by a full-vector finite element method. • The propagation process of light in the optical fiber is also simulated by the light speed propagation method. When the light travels to a certain distance, the two polarized lights will be separated. • Numerical simulation shows an 11.0 cm long DHC-ARF can be used to develop a PBS with an operating bandwidth of 410 nm ranging from 1.35 to 1. 76 μ m , where the polarization extinction ratio reaches −39.0 dB, the high-order mode extinction ratio is 625.41. • The PBS based on DHC-ARF with circular cladding tubes and nested tubes proposed in this paper compares the performance with other existing PBS, this PBS has the widest bandwidth. [ABSTRACT FROM AUTHOR]

• A liquid-filled temperature sensor based on Sagnac interferometer is proposed. • Light transmissions based on three liquid-filled methods were investigated. • The wavelength when φ/2π is integer corresponds to interference dip wavelength. • High sensitivity and good linearity can be obtained by the sensor. • The proposed liquid-filled methods are easier to be realized than some similar works. A temperature sensor based on Sagnac interferometer (SI) using photonic crystal fiber (PCF) with circular layout is proposed. Temperature measurement can be realized by filling the temperature sensitive liquid with high thermo-optic coefficient into the air holes. Considering the convenience of the experimental operation, three easy-to-operate liquid filling methods (FMs) were used to investigate the optical transmission characteristics of this sensor by the finite element method (FEM), the results show that two of which exhibit respective detection advantages in different aspects. One of the feasible methods is that all air holes are filled with temperature sensitive liquid. A single sensing interference dip with high detection sensitivity can be achieved, its average sensitivity reaches to 18.27 nm/°C. Another method is that only the large size air holes are filled with the temperature sensitive liquid. Two sensing interference dips with good linearity can be realized simultaneously. The linearity R2 are up to 0.99947 and 0.99877, corresponding average sensitivities are 9.47 and 8.57 nm/°C, respectively. Besides, these two proposed FMs are easier to be completed than FMs of some reported works. The proposed PCF temperature sensor with excellent detection performances, simple FMs and structural design has great application prospects to detect environmental temperature. [ABSTRACT FROM AUTHOR]