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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]

Surface plasmon resonance (SPR) sensing methods enable highly sensitive, fast response, and label-free analysis of biomolecular interactions. For SPR sensors, sensitivity and full width at half maximum (FWHM) are two incompatible performance parameters. We propose a refractive index (RI) sensor using a dual-core photonic crystal fiber (PCF) and SPR effects to achieve high sensitivity and a narrow FWHM simultaneously. The air holes of the sensor appear in a hexagonal arrangement, and polishing technology introduces two polishing planes into the cladding. A gold film is deposited on one side of the polished plane to form a highly sensitive RI sensing channel. Five gold nanowires are deposited on the other side of the polished plane to form a RI sensing channel with a narrow FWHM. We analyzed and optimized its structural parameters using the finite element method and determined the optimal structural parameters. The numerical results demonstrate that the maximum sensitivity of the sensor is 21,000 nm/RIU with the narrowest FWHM of 31 nm. Therefore, measuring the refractive index simultaneously with two sensing channels increases the detection accuracy of the measurement. In addition, the findings further indicate that variations in structural parameters do not significantly impact the sensing performance of the sensor, which makes the production of the sensor relatively simple. In conclusion, our work provides a new research method for realizing high sensitivity and a narrow FWHM simultaneously. [ABSTRACT FROM AUTHOR]

A D-shaped photonic crystal fiber (PCF) surface plasmon resonance (SPR) sensor is proposed to measure the refractive index in visible and near-infrared regions. Compared with traditional PCF, the use of D-shaped PCF can avoid filling materials and coatings in the hole. The plasma material of sensors is silver, and graphene is coated on the surface of the silver film. Graphene layer can not only prevent the oxidation of the silver but also improve the performance of the sensor. Then the influence of structural parameters on the sensor performance is studied by the finite element method (FEM). Results show that the performance of the sensor is not sensitive to the structural parameters, which makes the sensor has a good manufacturing tolerance. Moreover, when the refractive index (RI) of the analyte is 1.33 ~ 1.41, the maximum wavelength sensitivity is 13,750 nm/RIU, and the corresponding RI resolution is 7.27 × 10−6 RIU−1. In addition, the maximum figure of merit is 201 RIU−1. The proposed sensor is advanced with high sensitivity, simple structure and big fabrication tolerance, which could become a competitive candidate for water pollution monitoring, medical diagnosis, biochemical analysis, and detection fields. [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 high sensitivity plasmonic temperature sensor based on a side-polished photonic crystal fiber is proposed in this work. In order to achieve high sensitivity and high stability, the gold layer is coated on the side-polished photonic crystal fiber to support surface plasmon resonance. The mixture of ethanol and chloroform is used as the thermosensitive liquid. The performances of the proposed temperature sensor were investigated by the finite element method (FEM). Simulation results indicate that the sensitivity of the temperature sensor is as high as 7.82 nm/°C. It has good linearity (R 2 = 0.99803), the resolution of 1.1 Ă— 10âˆ'3 °C, and the amplitude sensitivity of 0.1008 °Câˆ'1. In addition, the sizes of the small air hole and polishing depth have little influence on the sensitivity. Therefore, the proposed sensor shows a high structure tolerance. The excellent performance and high structure tolerance of the sensor make it an appropriate choice for temperature measurement. [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]

An ultra-high sensitivity temperature sensor based on photonic crystal fiber (PCF) is proposed and demonstrated. The central hole and the two holes coated with nanoscale gold film are assumed to be filled with temperature sensitive liquid. The refractive index of the liquid decreases as the temperature increases with the temperature sensitive coefficient of -3.9 × 10 −4 / o C. The liquid of the central hole with high refractive index supports liquid-core mode. As the phase matching condition is satisfied, the liquid-core mode couples to the surface plasmon polariton mode and presents energy loss. The influences of the PCF structural parameters and the thickness of the gold film on the resonance wavelength, loss value and sensitivity are studied. The resonance wavelength and confinement loss increase as the temperature increases. The average sensitivity can reach to 9.89 nm/ o C ( ∼ 25359 nm/RIU) as the temperature changes from 35 °C to 45 °C and the R-square is 0.99992. By replacing the liquid core by silica core, we find the sensitivity is very low. We also compare the PCF without liquid in the holes coated with gold film by our structure and find that the sensitivity is nearly unchanged and the confinement loss is slightly decreased. [ABSTRACT FROM AUTHOR]

We propose a modified design for a photonic crystal fiber (PCF) polarization filter based on surface plasmon resonance (SPR). The air holes are arrayed in diamond lattices, and the diameter of the holes around the gold-coated holes are different that can separate the refractive index of the x-polarization and y-polarization second order surface plasmon polariton (SPP) modes. The influences of structural parameters of the photonic crystal fiber (PCF) on the filter characteristics are studied using the finite element method (FEM). Great changes have taken place in the results of numerical simulation by changing the thickness of the gold film and air hole diameter. Simulation results show that the resonance wavelength is communication wavelength 1550 mm, the loss of the y-polarization mode is 43,126.7 dB/m. When the length of the fiber is 500 μm, extinction ratio is more than 20 dB at the communication wavelength, and bandwidth achieve to 190 nm. It is an important property of PCF polarization filter in production. [ABSTRACT FROM AUTHOR]

A high sensitivity photonic crystal fiber (PCF) temperature sensor based on surface plasmon resonance is proposed and evaluated using the finite element method. Besides, the coupling phenomenon is studied. The gold layer deposited on the polishing surface of D-shape PCF is used as the metal to stimulate surface plasma, which can improves the sensitivity. Through exquisite design, the birefringence of the fiber is improved, which makes the loss of y-polarization far greater than the loss of x-polarization. The D-shape fiber avoids filling metal and liquid into the air-holes, which can contact with fluid directly to feel temperature. When the phase matching condition is satisfied, the core mode will couple with the surface plasma mode. The resonance position of y-polarization is very sensitive to the temperature change. The simulation shows that the PCF has high sensitivity of 36.86 nm/°C in y-polarization and wide detection that from 10 °C to 85 °C. [ABSTRACT FROM AUTHOR]

A broad bandwidth polarization filter based on photonic crystal fiber (PCF) coated with gold layer is proposed and simulated by the full vector finite element method. The surface plasmon resonance coupling theory is studied, and the factors that affect the polarization filtering effect are analyzed. By changing the structural parameters of the PCF, we realize polarization filter at 1.55 and 1.48 μm. In the designed polarization filter, the maximum loss of the unwanted y-polarization at 1.55 μm is 803.56 dB/cm and at 1.48 μm is 732.69 dB/cm. Besides, a broad bandwidth of 1000 nm with the crosstalk better than 20 dB is obtained. The suggested PCF is competitive for polarization device. [ABSTRACT FROM AUTHOR]

We design a crystal photonic fiber (PCF) polarization filter with asymmetric structure and two large air-holes coated with gold film. The influence of the thickness of metal layers and the diameter of large air-holes on the filtering characteristic is investigated using finite element method (FEM). In calculations, the sixth and the third order surface plasma modes (SPM) are presented. The resonance positions are well separated by the asymmetry of the structure in vertical direction. The loss of unwanted polarization is 829 and 1267 times than that of wanted polarization at 1.48 and 1.55 μm respectively. When the fiber length is 3 mm, the crosstalk is 1055 dB and 927 dB for 1.48 and 1.55 μm. The bandwidth with crosstalk better than 20 dB can reach to 1000 nm, covering almost all communication band. What’s more, gold excitation of SPR and silver excitation of SPR are compared and studied. [ABSTRACT FROM AUTHOR]

In recent years, photonic crystal fibers have played an important role in the field of optical communications, and the addition of metal materials to photonic crystal fibers have offered new ways to improve the transmission characteristics of optical fibers. We design a new type of gold-coated photonic crystal filter, which utilizes the surface plasmon resonance effect, and study it by using the finite element method. In this paper, we optimise the structure parameters and analyze the numerical results. The numerical results show that the thickness of metal layer and the air holes near the fiber core strongly affect the performance of the polarization filter. For the operating wavelength of 1550 nm, the loss in the y-polarization direction can be as high as 906.9 dB/cm, which is much larger than the x-polarization direction. When the fiber length is longer than 100 μm, the crosstalk in the wavelength range from 1.4 μm to 1.9 μm is greater than 20 dB. The proposed optical fiber can find application as an optical fiber polarization filter. [ABSTRACT FROM AUTHOR]

We propose a modified design for a Core shift photonic crystal fiber (PCF) polarization filter based on surface plasmon resonance. The Core shiftting is achieved by the air holes with different diameters around the gold-coated holes, which can separate the refractive index of the x-polarization and y-polarization second order surface plasmon polariton modes. The influences of structural parameters of the PCF on the filter characteristics are studied using the finite element method. Simulation results great changes have taken place in the results of numerical simulation by changing the thickness of the gold film and air hole diameter. We obtain a resonance wavelength with 1550 nm. The y-polarization mode is supressed significantly with the loss of 58,456.7 dB/m. With the length of the fiber is 300 μm, a extinction ratio is higher than 20 dB and a bandwidth achieve to 450 nm at the communication wavelength can be got. All these properties we achieved in the core shift PCF are important properties of PCF in broadband polarization filter in application. Our design can offer some guidance to the production of the polarization filter. [ABSTRACT FROM AUTHOR]

A kind of polarized filtering photonic crystal fiber (PCF) with nanoscale gold film is studied by using the finite element method. The cross-section structure of the PCF is composed of a hexagonal lattice of air holes in which an air hole is gold coated, and the air-hole layout is modified. By our structure, the loss of x-polarized mode is further larger than the loss of y-polarized mode and the loss peak appears at the wavelength 1.31 μm. The corresponding loss in x-polarized can reach a value of 85,780 dB/m at the resonance wavelength. It can be used for the communication windows of 1.31 μm which can be used for carrying data/voice. When the length of fiber is longer than 30 μm, the extinction ratio is better than 20 dB at the wavelength 1.31 μm. When the length of fiber is longer than 210 μm, the extinction ratio is better than 20 dB with the wavelength ranging from 1.26 to 2 μm. The broadband polarized filtering PCF covering 740 nm is achieved. [ABSTRACT FROM AUTHOR]

We propose and investigate a photonic crystal fiber (PCF) refractive index sensor with triangular lattice and four large-size channels based on surface plasmon resonance. In such sensor, two gold wires obtained by chemical reaction are filled in two air holes between the upper and lower channels. Numerical results show that the refractive indexes of the analytes can be detected from 1.30 to 1.79. It's a very broad detection range that is rarely seen in the field of photonic crystal fiber sensor based on surface plasmon resonance. The average sensitivity of this photonic crystal fiber could be 100 nm/refractive index unit (RIU) in the dynamic index range from 1.30 to 1.63. In the range from 1.63 to 1.79, the highest sensitivity could reach to 3233 nm/RIU. The ability to integrate four large-size microfluidic channels for efficient analyte flow in is attractive for the development of integrated highly sensitive PCF-SPR sensors. [ABSTRACT FROM AUTHOR]

A polarization splitter based on dual-core photonic crystal fiber filled with tellurite glass in the central hole is proposed. The characteristics are studied by finite element method. The influences of the parameters $$d_{1}, d_{2}, \varLambda$$ on the coupling length and coupling length ratio (CLR) are analyzed. The curves of coupling lengths of X-polarization, Y-polarization light dependence on the operable wavelength $$\lambda$$ are flat under the optimized structural parameters which is helpful to realize broadband polarization splitter. The simulation results indicate that the CLR could reach to 2.006 at the wavelength of $$1.55\,\upmu {\hbox {m}}$$ . The fiber length is only 510 $$\upmu {\hbox {m}}$$ and the bandwidth of extinction ratio greater than 20 dB is 284 nm, i.e., from 1461 to 1745 nm which could be widely used in the large-capacity, broad-bandwidth and integrated optical communication systems. As the fiber length decreases, we find the bandwidth increases. But the extinction ratio at 1.55 $$\upmu {\hbox {m}}$$ is lower than 20 dB as the fiber length is less than 510 $$\upmu {\hbox {m}}$$ . [ABSTRACT FROM AUTHOR]

We design two kinds of plasmonic broadband polarization splitters based on dual-core photonic crystal fiber (DC-PCF) with elliptical Au or Ag nanowire in this paper. It is analyzed for the polarization independent characterestics of the designed DC-PCF by the finite element method (FEM). In order to excite the surface plasmon resonance (SPR), the metal Au and Ag are filled into elliptical central air hole. The resonance coupling between the fourth- or fifth-order surface plasmon modes (SPMs) and core-guided modes (CGMs) are founded by this numerical simulation. The device lengths of the designed splitters with Au nanowire are 2937 and 827 μm at the wavelength of 1.31 and 1.55 μm, respectively. As the extinction ratios are better than −20 dB, its bandwidths are better than 94 and 103 nm. For the designed Ag nanowire splitter, the device lengths are 3066 or 809 μm at 1.31 or 1.55 μm, respectively. The bandwidths with the extinction ratio better than −20 dB are 66 and 104 nm, respectively. [ABSTRACT FROM AUTHOR]

A single-polarization photonic crystal fiber (PCF) based on surface plasmon resonance (SPR) is proposed. Finite element method is employed in simulating the PCF with gold-coated. The resonance wavelength can be modulated by changing the thickness of gold layer. At the resonance wavelength 1.55 μm, the loss of y-polarized mode is much larger than the loss of x-polarized mode. When the fiber length is set to 2 mm, the value of extinction ratio reaches to −118.7 dB, the y-polarized mode is suppressed and only x-polarized mode can be guided. The fiber is applicable in the production of single-polarization filter. The PCF has a simple structure and a big error tolerance, it has a good practicability. [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]

The polarization filter characteristics of Au wires and liquid crystal infiltrated photonic crystal fibers are investigated by using the finite element method. The nematic liquid crystal of E7 being injected into cladding air holes is benefit to induce large birefringence under controllable electrical field. The simulation results show that the surface plasmon resonance is strongly inspired by core modes in y-polarized direction. Meanwhile, the coupling between core modes in x-polarized direction and surface plasmon polaritons modes is faint. The confinement losses can achieve 446 dB/cm in y-polarized direction and 0.8 dB/cm in x-polarized direction at wavelength of 1550 nm in one of our designed fiber. The effects of fiber structural parameters and temperature are investigated with a view of tuning and optimizing the confinement loss spectrum. Own to the large contrast of confinement losses in two orthogonal directions, the designed Au wires and liquid crystal infiltrated photonic crystal fibers promise candidate for tunable polarization filter devices. [ABSTRACT FROM AUTHOR]

A short-length polarization beam splitter is designed. It is based on a simple structure dual-core photonic crystal fiber (DC-PCF), and possesses short-length. We research the effects of DC-PCF structure parameters on performances of the splitter by finite element method (FEM). The results show that the extinction ratio of the 249 μ m - long splitter can reach −50.7 dB at the communication wavelength of 1.55 μ m . Furthermore, the extinction ratio of the 506 μ m - long splitter can reach −61.6 dB at the communication wavelength of 1.31 μ m . The bandwidths of extinction ratio better than −20 dB are about 17 nm and 12 nm at wavelength of 1.55 μ m and 1.31 μ m , respectively. [ABSTRACT FROM AUTHOR]

A novel polarization splitter based on d-shaped dual-core photonic crystal fiber with gold film is proposed and analyzed by the finite element method. The designed polarization splitter involves side-polishing of photonic crystal fiber and depositing of gold film. The numerical results reveal that the separate length is 0.782 mm and the extinction ratios are -151 and -55 dB for the x- and y-polarized modes at the communication wavelength of 1,550 nm. The bandwidths of the polarization splitter are 40 and 65 nm for the x- and y-polarized modes with extinction ratios better than -20 dB and insertion losses lower than 0.3 dB. This d-shaped dual-core photonic crystal fiber is competitive for polarization splitter. [ABSTRACT FROM AUTHOR]

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 circular ultra-short A s 2 S 3 filled double-core photonic crystal fiber polarization beam splitter is proposed. The finite element method is used to study the performance of the designed photonic crystal fiber polarization beam splitter. By filling high refractive index A s 2 S 3 into the central air hole, the coupling performance of the double-core PCF is improved. By optimizing geometric parameters, the splitting length of the circular beam splitter can be as short as 72.43 μ m, and the extinction ratio can reach −151.42 dB. The high extinction ratio makes the circular polarization beam splitter have a good beam splitting function. The designed circular double-core photonic crystal fiber has the same cladding pore diameter, which is easier to prepare than other photonic crystal fibers with complex pore structure. Due to the advantages of high extinction ratio, extremely short beam splitting function and simple structure, the designed polarization beam splitter will be widely used in all-optical networks and optical device preparation. [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 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]

We propose a Sagnac interferometer (SI) based on ethanol-filled photonic crystal fiber (PCF) with two ultra-large holes for temperature measurement. The phase birefringence of the fully-filled PCF varies significantly with the temperature which is realized by infiltrating ethanol into air-holes. Two ultra-large holes in y direction near the core result in a high birefringence of PCF. By selecting a fiber length of 2.5 cm, the average sensitivity of temperature can be up to -34 nm/°C with a resolution of 0.00029 °C in the wavelength range of 780 nm–2170 nm within temperature variation of 40 °C. The proposed PCF with the simple structure is easy to prepare in the experiment. Furthermore, due to the ultrahigh-sensitivity, the proposed sensor provides a new implementation for accurately temperature measurement. [ABSTRACT FROM AUTHOR]

The characteristics of a novel dual-core photonic crystal fiber are investigated. In the center of photonic crystal fiber, an energy transmission channel is introduced. The optimized photonic crystal fiber can be used for polarization splitter, which has a short length and low loss. [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]

Highlights • The proposed PCF filter can realize three kinds of filtering effects. • Two ultra-large air holes makes the process of plating gold film more convenient. • Ultra-high crosstalks and losses can be obtained at the communication windows. • The filtering bandwidth can reach 700 nm at the communication window of 1550 nm. Abstract In this study, we present a single-polarization photonic crystal fiber filter. The metallic gold is used as the surface plasmon resonance activity material, which is deposited on the inner wall of the two ultra-large holes. The properties of the photonic crystal fiber filter are investigated by the finite element method. By adjusting the structural parameters and gold film thickness, three kinds of filtering effects can be obtained at the communication windows. When the length of the optical fiber is 1 mm, one of the filtering effects can achieve loss of 588.900 dB/cm at the communication window of 1310 nm, and the corresponding filtering bandwidth equals to 270 nm; the other two kinds of filtering effects can be applied to the communication window of 1550 nm, and the losses are 569.682 and 971.330 dB/cm and the corresponding filtering bandwidths equal to 700 and 630 nm, respectively. The proposed photonic crystal fiber filter shows great advantages compared with some previous reports on filtering. [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 We propose and investigate a temperature sensor based on Sagnac interferometer with liquid crystal (LC) photonic crystal fiber (PCF) by finite element method (FEM). LC E7 which is anisotropic under electrical field is designed to be infiltrated into the cladding air holes of PCF. The birefringent PCF is then launched into a Sagnac interferometer. The interference spectrum of the Sagnac interferometer is influenced by the thermal effect of LC E7 and thus could be used to measure the temperature. The simulation results show that the temperature measuring sensitivity and resolution reach to 15 nm/K and 6.7E−4 K in the range of 290–320 K. The measuring range and sensitivity could be modified further by optimizing the PCF structure parameters. [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]

Highlights • The proposed PCF structure is a combination of circular layout and D-shaped. • The gold film is deposited on the polishing surface, the analyte is placed on the outer layer of the D-shaped PCF. • The proposed PCF sensor is feasible for the structural parameters with fabrication tolerance of ±1%. • The proposed sensor easy to fabricate and has high sensitivity. Abstract A high sensitivity D-shaped photonic crystal fiber sensor based on surface plasmon resonance is proposed for refractive index sensing. By using finite element method, the sensing properties of the proposed sensor are investigated. The numerical results show a very high average sensitivity of 14600 nm/RIU with the high resolution of 6.80 × 10−6 RIU for refractive index (RI) of analytes ranging from 1.420 to 1.435. The layout of the air holes is designed to be circular and only two layers in this D-shaped photonic crystal fiber. The metallic gold is used as the surface plasmon resonance activity material, which is deposited on the polishing surface. The analyte is placed on the outer layer of the D-shaped photonic crystal fiber. This D-shaped photonic crystal fiber solves the difficulty of gold-plated film and fill analyte in the holes. The proposed PCF structure is a combination of circular layout and D-shaped, which also leads to the proposed sensor easy to fabricate and has considerable sensitivity. [ABSTRACT FROM AUTHOR]

Surface plasmon resonance induced tunable broadband polarization filters based on photonic crystal fibers (PCFs) were designed and investigated. The characteristics of the designed PCF were calculated by using the finite element method (FEM). The air holes in the PCFs which were arranged in hexagonal lattice were of the same size except one big air hole. The big air hole which was located on the right side of the fiber core was designed to be coated with gold film in the inner wall. Because of the arrangement of the big air hole, the surface plasmon resonance inspired on the surface of gold film was quite different for the two orthogonal polarized incident lights. As the edge of the big air hole approaching the adjacent air holes, the plasmon modes were influenced strongly by the adjacent air holes and as a results the cascaded resonances appeared. The index-matching liquid infilling in the big air hole was used to adjust the resonant wavelengths and strength. Numerical simulation results showed that the bandwidth of the polarization filter was 1200 nm covering the wavelengths from 1.0 μm to 2.2 μm with high extinction ratio (ER). [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 broadband core shift photonic crystal fiber (PCF) polarization filter based on surface plasmon resonance is proposed in this paper. The fiber is consisted of an asymmetric hexagonal lattice of holes in which a small air hole is coated with gold film and another one large air hole destroyed the structure around the core. The loss of y-polarized core mode is 2232.61 dB/cm at the communication wavelength of 1.55 μm, however, the loss of x-polarized core mode is very weak. When the PCF length is 300 μm, the bandwidth of exceeding 420 nm with extinction ratio better than −20 dB is achieved. To our best knowledge, the bandwidth is the widest. As a consequence, this special fiber filter has a wider range of applications. [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]

This paper presents a method of fabricating photonic crystal fiber by secondary drawing technology combined with air pressure regulation. The internal structure of optical fiber is solidified and shaped by primary drawing. The core diameter of PCF is reduced to less than 4 μ m by adding glass sleeve in secondary drawing. Multi-core PCF is prepared, and the curvature and temperature of two-core PCF are measured. Fan-shaped PCF, band gap PCF, silver-filled PCF and V-shaped PCF are prepared, and the dispersion and loss of V-shaped PCF are simulated. The drawing parameters are fitted and compared. The parameter curve of furnace temperature presents an upward concave shape, and the parameter curve of air pressure threshold presents an upward convex shape. These fitting curves have reference value for the preparation of PCF in the future. • A method of preparing various PCFs by secondary drawing technology is proposed. • Various solid PCFs are prepared, and the core diameter is reduced to 4 um. • Two kinds of hollow-core PCFs are prepared. • Silver-filled and butterfly-shaped PCFs are prepared. [ABSTRACT FROM AUTHOR]

A high sensitivity photonic crystal fiber (PCF) magnetic field sensor based on Sagnac interferometer is proposed. All the air holes of the PCF are completely filled with the magnetic fluid (MF), which is a magnetic liquid material. The sensing performance of PCF is simulated and analyzed by using finite element method (FEM). It is found that the effects of wavelength λ, phase birefringence B(λ,H), and group birefringence Bg(λ,H) on the sensitivity are very significant. The two dip points in this spectrum show a red shift and a blue shift as the refractive index of the magnetic fluid increases, respectively. This magnetic field sensor obtains the average sensitivity of 889.5 pm/Oe (111,684.9 nm/RIU) and −994.3 pm/Oe (−124,839.5 nm/RIU) in the range from 90 to 240 Oe. [ABSTRACT FROM AUTHOR]

Two- and three-core photonic crystal fibers are prepared by using the same photonic crystal fiber preform structure. Not only the fiber diameter is reduced to standard size of 125 μ m , but also the core diameter is reduced to below 4 μ m. A curvature and temperature sensing system is built, and the curvature and temperature sensing performance of the three-core optical fiber is tested. When the length of three-core optical fiber is 150 mm, the measured curvature sensitivity is −7.27069 nm/m−1 in the range of 0 to 0.990 m−1, and the temperature sensitivity is −34 pm/°C during the temperature reduction from 75 to 45 °C. When the length of three-core photonic crystal fiber is 68 mm, the curvature sensitivity and temperature sensitivity are 2.42419 nm/m−1 and −7 pm/°C, respectively. In a small range of temperature, the sensor is insensitive to temperature, and the cross interference of temperature to curvature is very small. • Two- and three-core PCFs are prepared by using the same PCF preform structure. • The core diameter of three-core PCF is reduced to 3 μ m. • The measured curvature sensitivity is −7.27069 nm/m−1. • The cross-interference of temperature on curvature is very small. [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]

A method of fabricating dual-core photonic crystal fiber(PCF) using secondary drawing technique is presented. The diameter of fiber core can be reduced to 4 μ m by placing the thin preform into glass sleeve for secondary drawing. Dual-core PCF, partial dual-core PCF and non-centrosymmetric dual-core PCF are successfully prepared by secondary drawing technology combined with air pressure regulation. A curvature sensing system is built. When the length of partial dual-core PCF is 105 mm, the measured curvature sensitivity is − 3.79944 nm/m−1. Through the water bath temperature experiment, the measured temperature sensitivity is 21 pm/ ∘ C, and the cross interference caused by temperature value on curvature value is very small. When the length of non-centrosymmetric dual-core PCF is 207 mm, the curvature sensitivity measured by forward bending and negative bending is 4.68429 nm/m−1 and − 4.68808 nm/m−1, respectively. The measured temperature sensitivity is − 13 pm/ ∘ C, and the cross interference of temperature on curvature can be ignored. • The secondary drawing technology is proposed to prepare PCF. • In the process of preparing PCF, the problem of pore collapse is solved. • Dual-core PCFs are prepared, and the core diameter is reduced to below 4 um. • The sensing system is established, and curvature and temperature are measured. • The cross-interference of temperature on curvature is very small. [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]

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]

Abstract A magnetic field sensor based on magnetic fluids (MFs) selectively infilling photonic crystal fiber (PCF) is designed and analyzed by the finite-element method (FEM). The PCF consists of three-layer air holes in the cladding which are arranged in regular hexagonal lattice. The left and right sides of the fiber core are made up of two larger circular air holes while the upper and below sides are made up of four smaller circular air holes which are benefit to enhance the birefringent effect. MFs are injected into one air hole in the second cladding layer which generates a defect core with high confinement loss. Air holes around the defect core are asymmetrically distributed which can enhance the birefringence of the transmission mode in defect core. When the phase matching condition between core modes and defect core modes is satisfied, the light transmitting in the core couples to the defect core and a sharp decrease of light transmitting energy occurs. The refractive index of defect core modes is modulated by magnetic field strength. The peak wavelengths of confinement loss spectra depicts pretty well linear relationship with magnetic field strength in the detected range of 30–130 Oe. The magnetic field measuring sensitivity reaches to 584 pm/Oe for x-polarized mode and 700 pm/Oe for y-polarized mode, respectively. [ABSTRACT FROM AUTHOR]

• This work presents a refractive index and temperature plasmonic sensor based on photonic crystal fiber. • In order to achieve the best performance, a serial of study on the theory and experimental steps is carried out, including simulation calculation, detection system, coating method and post-processing technology. • Experimental results demonstrate that an average sensitivity of 3381 nm/RIU within a linear refractive index ranging from 1.3333 to 1.3860 can be achieved, which is greater than the traditional fiber structure. • In addition, this sensor is helpful to measure temperature change and the sensitivity could be 2 nm/°C. • This study presents a lot of effective methods for preparing PCF sensor, which has enlightening significance to design optical device. In this paper, a refractive index and temperature plasmonic sensor based on photonic crystal fiber is proposed. In order to achieve the best performance, a serial of study on the theory and experimental steps is carried out, including simulation calculation, detection system, coating method and chemical etch technology. Experimental results demonstrate that a high average sensitivity of 3381 nm/RIU within a linear refractive index ranging from 1.3333 to 1.3860 can be achieved, which is greater than the traditional fiber structure. In addition, this sensor is helpful to measure temperature change and the sensitivity could be 2 nm/°C. Due to its easy and controllable fabrication, the proposed sensor has great application prospects in the field of optical device. [ABSTRACT FROM AUTHOR]

A narrowband dual-polarization filter based on photonic crystal fiber (PCF) with Au-Ag double-layer films is proposed. The cladding of PCF is composed of three layers of air holes, and Au-Ag double-layer films are coated on two symmetrical air holes to excite surface plasmon resonance (SPR) effect. The special mechanism of the Au-Ag double-layer films takes advantage of the narrower full width at half maximum (FWHM) of silver. On the other hand, gold is used to protect the silver from oxidation and improve the stability of the filter, and can also enhance SPR effect. The simulation results show that the dual-polarization filter with double-layer metal films has higher resonance absorption and better spectra than that with the single-layer metal film. When the thickness of the double-layer metal films is 22.5 nm (the thickness of silver film is 18 nm), the confinement loss of dual-polarization filter at 1430 nm is achieved to 64.08 dB/cm (y-pol core mode) and at 1530 nm is achieved to 37.26 dB/cm (x-pol core mode). The wavelength range of crosstalk (CT) greater than 20 dB and less than −20 dB are 1410–1450 nm and 1510–1550 nm for the device length of 10 mm, respectively. The CT is achieved 555.07 dB and −321.54 dB at the wavelength of 1430 nm and 1530 nm, respectively. These characteristics show that the filter has excellent performance, and will become a suitable candidate for a narrowband polarization filter. [ABSTRACT FROM AUTHOR]