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• 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 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]

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]

In this paper, a photonic crystal fiber with the gold layer is simulated by finite element method. Due to birefringence, surface plasmon resonance only occurs in the y-polarization direction. The loss of y-polarization mode is much larger than that of x-polarization mode. The photonic crystal fiber is suitable for the polarization filter. The polarization filter has the short length and high extinction ratio. Furthermore, the insertion loss of the x-polarization mode is only 0.07 dB, which is the smallest as far as we know. [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]

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 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 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]

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]

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]

• 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]

Abstract: A kind of sandwich structure photonic crystal fiber with diameter-increasing air holes is proposed. The sandwich structure is composed of the rectangular lattice in the center and triangular lattice in both sides, which is used to gain high birefringence. The air holes in the cladding, whose diameter in outer ring is always larger than the next inner ring, is used to reduce the confinement loss. The model field, dispersion, birefringence and confinement loss of the fiber fundamental mode are simulated by full-vector finite element method. It was shown from numerical results that it is possible to design a PCF with birefringence B =3.8×10−3 and the fast axis confinement loss less than 10−3 dB/km at wavelength of 1550nm. [Copyright &y& Elsevier]

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]

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

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]

• A novel SPR sensor based on D-shaped PCF with offset-core is proposed and demonstrated for the liquid RI measurement. • The structural design of the D-shaped fiber makes the placement of the liquid to be tested more convenient. • The SPR sensor with high sensitivity can be realized by introducing the offset-core. • Ultra-high wavelength sensitivity and amplitude sensitivity of 42,000 nm/RIU and 2204.49 RIU−1 are obtained. We propose a novel D-shaped offset-core photonic crystal fiber (PCF) sensor based on surface plasmon resonance (SPR) for the liquid refractive index (RI) measurement. The D-shaped structural design makes the placement of the liquid to be tested more convenient. The offset-core is designed to improve the sensitivity of the SPR sensor. The optical propagation characteristics of the proposed sensor were investigated by the finite element method (FEM). Numerical results show that more energy of the core mode transfers to the gold surface as the analyte RI increases. The wavelength sensitivity increases with the increase of RI, and the maximum wavelength sensitivity can reach to 42,000 nm/RIU. What's more, the maximum amplitude sensitivity is up to 2204.49 RIU−1. The proposed sensor shows excellent performances in terms of wavelength sensitivity and amplitude sensitivity than some previous reported sensors. We believe that it can become a favorable competitor in the liquid RI measurement fields. [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]

• 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]

• The D-shaped microstructure fiber sensor is suitable for external plasmonic sensing. • The fiber sensor performs sensing detection by means of double resonance peaks. • The fiber sensor has high sensitivity, high resolution and high figure of merit at the near-infrared wavelength. • Double-peaks detection is simultaneously self-calibrated as both detection and verification. A miniature D-shaped microstructure fiber sensor is proposed for external plasmonic sensing. The hollow core not only effectively modulates the core field, but also efficiently facilitates the excitation of surface plasmon resonance near the micro-opening. The refractive indices of analyte ranging from 1.37 to 1.41 can be detected by double-loss peaks tracing in the near-infrared wavelength. And the more reliable method of double-peaks detection achieves detection and verification simultaneously, which improves largely the accuracy of sensing detection. In addition, the transmission properties and sensing characteristics for the designed sensor is explored as well corresponding to various geometrical parameters. An average wavelength sensitivity is 26,927 nm/RIU with the resolution of 3.71 × 10−6 RIU, and the maximum amplitude sensitivity of 5059 RIU−1 is obtained with the resolution of 1.97 × 10−6 RIU, moreover, the maximum figure of merit can reach to 1282, which can be found a wide application prospect in the field of chemical, biological and medical. [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 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]