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In this paper, we propose a hexagonal photonic crystal fiber (PCF) suitable for broadened mid-infrared supercontinuum generation. The designed PCF has five air-hole rings with three different air holes, arranged in a triangle pattern. The central air hole is filled with G e 10 A s 22 S e 68 and the inner wall of the air hole is coated with A s 2 S 3 . In the mid-infrared wavelength range of 3400 nm to 4500 nm, the supercontinuum bandwidth increases with the increase of the pump wavelength, peak power, and pulse width. When the pump light source with a peak power of 5 kW and the input pulse width is 200 fs, a 10 mm fiber can produce an extremely wide supercontinuum spectrum spanning 6000 nm. In addition, optical parameters including dispersion, confinement loss, effective mode area, and nonlinear coefficients are numerically investigated by the finite element method. The results show that at a wavelength of 4000 nm, the second order dispersion of the fiber is 8.81 × 10 - 27 s 2 m - 1 and the nonlinear coefficient is 972.5 W - 1 km - 1 . The designed photonic crystal fiber is stable, dispersion flat, and can meet the application requirements of obtaining broad supercontinuum spectrum. [ABSTRACT FROM AUTHOR]

• The sensor is capable of detecting a refractive index change of the liquid analyte ranging from 1.33 to 1.38. • The gold film and the thin layer of graphene are directly coated on the outer edge of the fiber. • After adding graphene, the average sensitivity increases from 2938.86 to 5171.51 nm/RIU. • The maximum sensitivity can reach 8600 nm/RIU. • The optical fiber can be directly immersed into the liquid analyte, so real-time detection can be realized. A novel eccentric photonic crystal fiber biosensor with gold and graphene bimetallic layers on the outer edge of the optical fiber is proposed in this paper. The effects of gold film thickness, graphene coating thickness and cladding air hole diameter on the sensor performance are analyzed in detail. The maximum sensitivity of this PCF sensor is 4657.14 nm/RIU when only gold-plated film is used, and 8600 nm/RIU after a layer of graphene is added on the gold film, the sensitivity increased by 84.66%. Compared with the traditional optical fiber sensor, this PCF sensor has the advantages of simple structure, easy manufacture and real-time detection, which will make it a potential candidate for water quality analysis, chemical detection and biomolecular detection. [ABSTRACT FROM AUTHOR]

• The sensor is capable of detecting a refractive index change of the liquid analyte ranging from 1.33 to 1.41. • The sensitivity of the sensor can reach a maximum of 22071 nm/RIU when the refractive index of liquid analyte is 1.41. • The performance of the sensor can be improved by optimizing its structural parameters. • It can easily be implemented for the manufacture of the proposed fiber sensor in actual production. A novel high-sensitivity circular dual-core photonic crystal fiber refractive index sensor is proposed in this paper. The effects of air hole spacing, liquid filled air hole diameter and cladding air hole diameter on its sensor sensitivity are analyzed in detail. The simulation results show that the liquid refractive index that the sensor can detect ranges from 1.33 to 1.41. By optimizing the geometric parameters, when the analyte refractive index is 1.33 and 1.41, respectively, the minimum sensitivity of this PCF sensor is 8929 nm/RIU, and the maximum value can reach 22071 nm/RIU. Due to its high sensitivity and simple structure, this PCF sensor can be widely used for chemical detection and biomolecule detection. [ABSTRACT FROM AUTHOR]

Abstract An octagonal photonic crystal fiber polarization beam splitter with gold-plated film is proposed. The polarization beam splitter characteristics are studied by optimizing the gold film thickness, the cladding air hole diameter and the air hole spacing. At the communication wavelength of 1.55 μm, the coupling ratio can reach the ideal value of 2. Under the optimum geometric parameters, the curve of the coupling length with the working wavelength is flat. When the gold film thickness, cladding air hole diameter and air hole spacing are 15 nm, 1.228 μm and 1.5 μm respectively, the beam splitting length and extinction ratio of the polarization beam splitter are 47.26 μm and −151.43 dB respectively. Because the polarization beam splitter has extremely short beam splitting length and high extinction ratio, it has great value in the field of large-capacity integrated optics and optical communication systems. [ABSTRACT FROM AUTHOR]

A dual communication band photonic crystal fiber polarizing filter based on coupling between core mode and SPP mode is proposed in this paper. By optimizing the parameters of the fiber structure, the resonant points can be tuned to the communication band. Numerical simulation results demonstrate that the resonance strength of x - and y -polarized direction can simultaneously reach 729.47 and 542 . 78 dB ⋅ cm − 1 at the communication wavelength of 1.55 and 1.31 μ m . By filling liquid analyte the confinement loss of x - and y -polarized direction can simultaneously reach 580.55 and 714 . 48 dB ⋅ cm − 1 at the wavelength of 1.55 and 1.31 μ m . Furthermore, when the fiber length of L is equal to 700 μ m , the peak value of the crosstalk can reach 421.89 and − 322 . 34 dB at the same time at the wavelength of 1.31 and 1.55 μ m , and when the length of the fiber L is 400 μ m , the bandwidths of the crosstalk better than 20 dB and less than − 20 dB are about 170 and 220 nm, respectively. These properties make it an ideal candidate for designing dual-band polarization filter equipment. [ABSTRACT FROM AUTHOR]

A square structure of dual communication band single polarizing plasma photonic crystal fiber filter is presented in this paper. The resonance point can be adjusted to the communication band by optimizing the parameters of fiber structure. High birefringence can be produced by changing the refractive index of the filled liquid, the thickness of the metal film, and the diameter of the air holes near the core. Numerical simulation results demonstrate that the resonance strength of x - and y -polarized direction can simultaneously reach 781.87 and 463.04 dB cm −1 at the communication wavelength of 1.31 and 1.55 μm. By filling liquid analyte the confinement loss of x - and y -polarized direction can simultaneously reach 804.71 and 451.69 dB cm −1 at the wavelength of 1.31 and 1.55 μm. Furthermore, when the fiber length of L is equal to 700 μm, the peak value of the crosstalk can reach 474.63 and −243.61 dB at the same time at the wavelength of 1.31 and 1.55 μm, and when the length of the fiber L is 400 μm, the bandwidths of the crosstalk better than 20 dB and less than −20 dB are about 160 and 220 nm, respectively. These features make it an ideal candidate for the design of a dual band polarization filter device. [ABSTRACT FROM AUTHOR]

The filtering characteristics and applications of photonic crystal fibers being selectively infiltrated with one aluminum rod were investigated based on the finite-element method. The aluminum rod being selectively infiltrated into one cladding air hole acted as a defect core and generated surface plasmon polaritons on its surface. As the phase matching condition was satisfied, the light transferring in fiber core coupled to the surface of the aluminum rod and the confinement losses of core modes experienced an abrupt increase. The filtering characteristics could be modified by adjusting the fiber structure parameters, such as diameters of air holes and the aluminum rod. Two applications of the photonic crystal fibers being selectively infiltrated with one aluminum rod were studied. First, a polarization filter with ultrabroad bandwidth was designed based on the cascaded resonances between fiber core modes and surface plasmon polariton modes. The bandwidth of the polarization filter was as broad as 600 nm covering wavelengths from 1.4 to 2.0 μm. Second, an intensity-type refractive index sensor was designed based on the filtering characteristics. The output power ratio and sensitivity increased as the refractive index of analyte increases and reached to 3.91 dB and 128 dB/RIU in x-polarized direction at the analyte refractive index of 1.37, respectively. [ABSTRACT FROM PUBLISHER]

We show that both photonic bandgap guidance and modified total internal reflection can simultaneously occur in a chalcogenide–tellurite photonic crystal fiber (PCF) whose cladding is composed of a high-index array with internal regions being air. The coexistence of these two guidance mechanisms is corroborated by numerical simulations based on a plane wave expansion method and an analytical model. Despite being scalar and for high-index contrast material systems, the analytical model can accurately predict the band structures of the PCF. A PCF with seven ring units removed as the fiber core is designed to guide in the 580–950-nm wavelength range by the photonic bandgap effect and in the 2.0–4.6-μm wavelength range by total internal reflection. Such a PCF can be an interesting platform for nonlinear phenomena occurring across the two guidance regions. [ABSTRACT FROM PUBLISHER]

A polarization splitter in three-core photonic crystal fiber with rectangle lattice based on resonant tunneling effect is studied by the finite element method. The transverse-electric (TE) mode polarized light is limited to the launched port and the transverse-magnetic (TM) mode polarized light can directionally couple to the other port due to the large birefringence. A polarization splitter with a coupling length of 2.592 mm, extinction ratios ofERA= –151 dB andERC= –22 dB, insertion losses ofILTE= 0.027 dB,ILTM= 2.89E–15, and 15 dB, and a bandwidth of 29 nm at the communication wavelength of 1550 nm is obtained. The designed polarization splitter exhibits nice performance characteristics under reasonable fabrication tolerance. The characteristics of the polarization splitter are influenced by the effect of surface plasmon resonance. The golden wires infilling into the air holes reduce the coupling length to 1.695 mm. [ABSTRACT FROM AUTHOR]

A photonic crystal fiber is designed with chalcogenide glass As2Se3 as the base material. As2Se3 has a high refractive index and a high nonlinear refractive index; as a result, it has a desired application for strengthening birefringence and nonlinearity. The outer layer of the fiber is a regular decagonal structure composed of four layers of circular air holes, and the inner cladding area is composed of four elliptical air holes arranged in a diamond pattern. The influence of the size of the circular air holes in the fiber cladding, the thickness between each layer, and the size and position of the elliptical air holes in the inner cladding area on the birefringence is analyzed and studied using the full vector finite element method. The results show that the birefringence value of the fundamental mode of the fibers is 0.108 at a wavelength of 1.55 μm, and at the same time, the nonlinear coefficients of the X and Y polarization states are 240 and 197 m − 1 · W − 1, respectively. [ABSTRACT FROM AUTHOR]

In this work, we propose an octagonal photonic crystal fiber (PCF) filter based on tellurite. Selective gold coating in air holes couples the core mode to a surface plasmon mode. The finite element method is used to analyze the structure in the mid-infrared band. The simulated results and analysis show that the losses in the y and x polarization modes at 2.55 µm are 2063.2 and 21 dB/cm, respectively. Therefore, the unwanted y-polarization mode is filtered out. The higher loss ratio optimizes the formant position and intensity of surface plasmon mode and core mode. When the fiber length of the filter is optimized to 500 µm, the extinction ratio of − 102 dB is achieved at 2.55 µm. Consequently, the single polarization transmission with a 500 nm width is realized. When the length of the fiber is greater than 500 µm, it realizes the 1500 nm (2.2–3.7 µm) ultra-wideband filtering. We also analyze the manufacturing feasibility of the proposed PCF and the influence of structural parameters on the limiting loss characteristics. • Introduce a new type of substrate material: TZLB, which breaks the limitation of conventional quartz glass. • The loss of the y-polarization state is much higher than the x-direction. • When the fiber length is 500 µm, the ER reaches −102 dB and the effective filtering range is 2.4‐2.9 μm. • When the fiber length is greater than 500 µm, it achieves the 1500 nm ultra-wideband filtering. [ABSTRACT FROM AUTHOR]

The four wave mixing (FWM) characteristics of highly nonlinear tellurite photonic crystal fiber are simulated. The material of tellurite fiber is TeO 2 -Li 2 O-ZnO-P 2 O 5 (TLZP). It is calculated that there are two zero dispersion points at 1590 nm and 3880 nm in tellurite photonic crystal fiber. The linear phase mismatch, the variation of optical signal gain with fiber length and the pump power are calculated at 1538 nm and 3865 nm respectively. The results show that the fiber has good four wave mixing performance near 1550 nm communication window and mid infrared region. [ABSTRACT FROM AUTHOR]

This paper proposes a highly sensitive surface plasmon resonance (SPR) refractive index sensor based on the photonic crystal fiber (PCF). The optical properties of the PCF are investigated by modulating the refractive index of a liquid analyte. The finite element method (FEM) is used to calculate and analyze the PCF structure. After optimization, the fiber can achieve high linearity of 0.9931 and an average refractive index sensitivity of up to 14,771.4 nm/RIU over a refractive index range from 1.47 to 1.52, with the maximum wavelength sensitivity of 18,000.5 nm/RIU. The proposed structure can be used in various sensing applications, including biological monitoring, environmental monitoring, and chemical production with the modification and analysis of the proposed structure. [ABSTRACT FROM AUTHOR]

This paper proposes a Photonic Crystal Fiber (PCF) refractive index sensor model based on the surface plasmon resonance effect. The proposed PCF model also uses the full vector finite element method to transfer the structure under the anisotropic Perfect Matching Layer (PML) boundary condition. Numerical calculations were carried out on the sensor characteristics. The calculation results show that the elliptical air hole on the left side of the PCF core is coated with a gold-nano film which serves as a Surface Plasmon Resonance (SPR) sensing channel to detect the refractive index of liquid materials. Compared with other structures, the resonant peak generated by the excited SPR effect from the elliptical sensing channel has a high sensitivity to the change of the refractive index of the liquid to be measured. With the help of this attribute, it is relatively easy to adjust the sensitivity. The refractive index range of this structure is within 1.43–1.49 and the sensitivity is up to 12,719.97 nm·RIU−1. The linearity is good; R2 = 0.99927, which is very suitable for liquid sensing. [ABSTRACT FROM AUTHOR]

A surface plasmon resonance (SPR) liquid refractive index sensor based on photonic crystal fiber (PCF) is proposed. The PCF is made of the exposed core structure, and the gold film is formed by electron beam evaporation within its defects. The sensitivity of the sensor is improved by coating graphene on the surface of the gold film. The experimental results show that the sensitivity of the sensor is increased by 390 nm/RIU after the introduction of graphene, and finally to 2290 nm/RIU. The experiment and simulation have a good consistency. Significantly, the sensor can be reused, and the measurement accuracy can be maintained. [ABSTRACT FROM AUTHOR]

In this paper we propose a gold-plated photonic crystal fiber (PCF) refractive index sensor based on surface plasmon resonance (SPR), in which gold is coated on the external surface of PCF for easy fabrication and practical detection. The finite element method (FEM) is used for the performance analysis, and the numerical results show that the thickness of the gold film, the refractive index of the analyte, the radius of the air hole in the first layer, the second layer, and the central air hole can affect the sensing properties of the sensor. By optimizing the sensor structure, the maximum wavelength sensitivity can reach 11000 nm/RIU and the maximum amplitude sensitivity can reach 641 RIU−1. Due to its high sensitivity, the proposed sensor can be used for practical biological and chemical sensing. [ABSTRACT FROM AUTHOR]

In this paper, a surface plasmon resonance refractive index and temperature sensor based on V-cut photonic crystal fiber (PCF) is proposed. The proposed structure is studied and analyzed using the finite element method (FEM). The sensing area is a V-shaped cut groove, and the cases when the cut angle is positive, 0° and negative are discussed and compared. The results show that when the cutting angle is 15°, the optimal sensing performance can be obtained compared with other cutting angle. By optimizing other structural parameters when the cutting angle is 15°, in the refractive index range of 1.34–1.38, the average sensitivity of the refractive index sensor is 10,500 nm/RIU, and the maximum sensitivity is 52,500 nm/RIU. The average sensitivity is 5.57 nm/°C in the range of 85 ℃–145 ℃ as a temperature sensor. Due to the simple structure and high sensitivity of the design, it can be used for real-time monitoring of analytes in medical, biochemical and environmental monitoring. [ABSTRACT FROM AUTHOR]

A photonic crystal fiber is designed with chalcogenide glass As2Se3 as the base material. As2Se3 has a high refractive index and a high nonlinear refractive index; as a result, it has a desired application for strengthening birefringence and nonlinearity. The outer layer of the fiber is a regular decagonal structure composed of four layers of circular air holes, and the inner cladding area is composed of four elliptical air holes arranged in a diamond pattern. The influence of the size of the circular air holes in the fiber cladding, the thickness between each layer, and the size and position of the elliptical air holes in the inner cladding area on the birefringence is analyzed and studied using the full vector finite element method. The results show that the birefringence value of the fundamental mode of the fibers is 0.108 at a wavelength of 1.55 μm, and at the same time, the nonlinear coefficients of the X and Y polarization states are 240 and 197 m − 1 · W − 1, respectively. [ABSTRACT FROM AUTHOR]

In this paper, an asymmetric photonic crystal fiber (PCF) polarization filter near the gold-clad hole based on surface plasmon resonance (SPR) is proposed. The outer walls of the two air holes in the y-direction are coated with a gold film, and the two kinds of cladding air holes are introduced near the coated holes to increase the difference in the coupling strength of x and y polarization modes and SPP modes, thus separating the two polarization states. The finite element method is used to study the characteristics of the polarization filter, and the result shows that the polarization loss in the y-direction is as high as 1565.98 dB/cm at 1. 55 μ m , while the loss in the x-direction is relatively small. When the fiber length is 1 mm, the crosstalk peak reaches 1322.7 dB. From 1 μ m to 2 μ m (range: 1000 nm), the crosstalk is always greater than 20 dB. If four kinds of pores with different sizes are used in the cladding layer, the symmetry around the gold-coated holes will be destroyed, and a better filtering effect can be obtained. The excellent performance of the proposed polarization filter makes it a good candidate material for broadband polarization filters. • Propose a new idea for filter design: improve the asymmetry near the gold-clad hole. • The loss peak in y-polarization is much higher than in x-polarization at communication wavelength 1.55 μ m. • When the fiber length is 1 mm, the crosstalk peak reaches 1322.7 dB. From 1 μ m to 2 μ m, the crosstalk is always greater than 20 dB. • The higher the asymmetry near the gold-plated hole, the better the polarization filtering performance. [ABSTRACT FROM AUTHOR]

• The polarization filter we designed is novel and have a high performance. • The loss peak in Y-polarization is much higher than in X-polarization. • Air-holes except the innermost layer also influence the loss peak well. • The PCF filter can be widely used in system integration. In this study, a novel photonic crystal fiber (PCF) polarization filter has been designed. To resonate the surface plasma mode with the core mode, gold is selectively filled in the air holes. Using numerical simulations based on the finite element method, the influence of structural parameters on the filter is analyzed, and a set of rules for designing the PCF polarization filter is determined. The numerical simulation shows that the peak loss in the y-polarization direction at 1.31 μm reaches 1209.57 dB/cm, whereas the peak loss in the x-polarization is close to zero. Working at 1310 nm in the communication band, a new type of PCF is designed using these characteristics. [ABSTRACT FROM AUTHOR]

• An efficient hybrid design method for fiber polarization filters is proposed. • Data-driven feed-forward neural network predicts fiber characteristics fastly. • Genetic algorithm selects structural parameters with optimal performance. • Designed PCF filter exhibits high performance with significant confinement loss. In this paper, we proposed a design method for photonic crystal fiber polarization filters incorporating the artificial neural network (ANN) and the genetic algorithm (GA) to achieve high-performance polarization filters. The ANN is trained to accurately learn the relationship between the structure parameter and polarization properties of PCFs, and GA is utilized to select the structural parameters with optimal performance. Confinement loss, bandwidth, and resonance wavelength of the polarization filter serve as parameters in the multi-objective optimization process performed by the GA. Numerical results demonstrate that the designed PCF filter has high performance, with a confinement loss of 1995.75 dB/cm for the y polarization core mode at 1.55 μm, and crosstalk and bandwidth of 1720.15 dB and 932 nm, respectively. It indicates that the hybrid design method is a promising and viable approach for the development of optical applications. [ABSTRACT FROM AUTHOR]

In this paper, a D -shaped photonic crystal fiber polarization filter is optimized using a novel machine learning framework. The framework incorporates the Grey Wolf Optimizer (GWO) and convolutional neural network (CNN) to optimize the optical fiber structure parameters. The designed CNN accurately classifies optical fiber modes, while the GWO selects complex fiber structure parameters. The fitness function conducts multi-objective optimization on the confinement loss and resonance wavelength of the polarization filter. The simulation results show the y-direction confinement loss of the optimized polarization filter is 1111.51 dB/cm at a wavelength of 1.55 µm. At a fiber length of 1000 µm, the crosstalk on the communication band reaches 965.3 dB, with a bandwidth exceeding 1000 µm. Numerical analysis demonstrates that the performance of the designed PCF filter is better than that of the original and also have showed the superiority of the proposed framework, which is a promising avenue for designing other photonic devices. [ABSTRACT FROM AUTHOR]

Photonic crystal fibers (PCFs), providing ingenious microfluidic channels and intense light guidance, have many advantages in the detection of three-dimensional surface-enhanced Raman scattering (SERS). In solid-core PCFs, the light interacts with the sample in the form of an evanescent field, which is weak and limits the SERS detection performance. In this paper, we propose a new mechanism based on avoided crossings between the fiber core mode and the surface mode, which greatly enhances the laser power ratio in the liquid channels and improves the SERS sensitivity. Numerical results show that the intensity in the liquid channels can be increased from 2.8 % to 42 %. To realize this avoided-crossing effect in PCFs experimentally, the silver mirror reaction was optimized which can decorate discrete and densely distributed silver nanoparticles on the hydroxylated surface of the microfluidic channels in less than 15 min. The resulting SERS-active suspended-core PCF has ultra-high sensitivity, with a detection limit of 10−12 M, and an enhancement factor of 1010 for crystal violet solution. It also provides good stability and reproducibility with a relative standard deviation of 4.8 %. This ultra-sensitive and highly reproducible PCF-SERS probe has great potential for application in rapid in-situ detection. • Light interacts with the analyte in the form of surface mode rather than the evanescent field. • The surface mode is enhanced by the avoided crossing effect. • High density Ag nanoparticles were simply formed on the inner surface of the hydroxylated optical fiber. • The SERS performance was greatly improved, with an enhancement factor of 1010 for crystal violet solution. [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 novel polarization filter based on photonic crystal fiber with two open-ring channels is proposed. The optical device is compatible with both broadband filtering and sensing. The calculation results show that the loss of the y-polarized core mode is as high as 1147.79 dB/cm at a communication wavelength of 1.55 μm, while the loss of the x-polarized core mode is only 1.86 dB/cm. When the length of PCF is 800 μm, the optical bandwidth of crosstalk greater than 20 dB can reach 790 nm at the wavelength band of 1.55 μm, and the corresponding crosstalk peak is 796.4 dB. In addition, the simulation results show that the proposed PCF is applied in the field of sensing as long as the thickness of the gold film is changed from 22.5 nm to 40 nm. Further, when the refractive index of the tested material varies from 1.32 to 1.37, the average sensitivities of 3057 nm/RIU and 1000 nm/ RIU are obtained in x- and y-polarized modes, respectively. [ABSTRACT FROM AUTHOR]