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Photonic crystal fibers (PCFs) have brought tremendous advancements due to their predominant feature of a peculiar air-hole arrangement in the 2D direction. Functional materials like metals, magnetic fluids, nematic liquid crystals, graphene and so on, are being extensively adopted for integration with PCFs to obtain extraordinary transmission properties. This review takes the development stages of photonic devices based on functional material-infiltrated PCFs into consideration, covering the overview of common materials and their photoelectric characteristics, state-of-art infiltrating/coating techniques, and the corresponding applications involving polarization filtering and splitting devices in optical communication and sensing elements related to multiple-parameter measurement. The cladding airhole of PCFs provides a natural optofluidic channel for materials to be introduced, lightâ€"matter interaction to beenhanced, and transmission properties to be extended, in this case, a lab on a fiber are able to be implemented. This paves the way for the development of photonic devices in the aspects of compact, multi-functional integration as well as electromagnetic resistance. When the PCFs are integrated with the phemomenon of surface plasmon resonance, the property of tunable refractive indices, and the flexible geometry structures, it comes up to some representative researches on polarization filters, multiplexer-demultiplexers, splitters, couplers and sensors. It makes a candidate for widespread fields of telecommunication, signal-capacity, and high-performance sensing. [ABSTRACT FROM AUTHOR]

A polarization filter is designed at two communication windows of 1310 and 1550 nm based on microstructured optical fiber. The model has four large diameter air holes and two gold-coated air holes. The influence of the geometrical parameters of the photonic crystal fiber on the performance of the polarization filter is analyzed by the finite element method. The numerical simulation shows that when the fiber length is 300 μm, the corresponding extinction ratio is 209.7 dB and 179.8 dB, the bandwidth of extinction ratio (ER) better than 20 dB is 150 nm and 350 nm at the communication wavelength of 1310 nm and 1550 nm. [ABSTRACT FROM AUTHOR]

A novel photonic crystal fiber polarization filter with Au-coated and liquid-filled air holes is designed in this paper. The coupling theory is introduced to explain the complete coupling and incomplete coupling. We can adjust the resonance point to the communication band by optimizing the parameters of the fiber structure. Numerical simulation results demonstrate that the resonance strength can reach 434 dB cm −1 at the communication wavelength of 1.31 μm in x-polarized direction. By filling liquid analyte can help adjust the resonance peak to the proper place. Furthermore, when the fiber length of L equals to 0.1 cm, the crosstalk peak value can reach 353.74 dB at the wavelength of 1.31 μm, and the bandwidth of the crosstalk better than 20 dB can reach 80 nm when the length of the fiber L is 400 μm. These characteristics make it a promising candidate for designing new types of polarization filter devices. [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]

A polarization filter based on photonic crystal fiber selectively coated with gold film is investigated. The loss spectra and the dispersion characteristics of the designed polarization filter are studied and analyzed by the finite element method. Simulation results show that the loss of core mode is 433.65 dB/cm in y polarization and 2.64 dB/cm in x polarization at the wavelength of 1.55 µm, respectively. The bandwidth of extinction ratio greater than −20 dB is 150 nm. The corresponding wavelength range is from 1.48 µm to 1.63 µm. The position and size of the little air holes along with the thickness of gold film showed influence on the polarization filter properties. And the proposed polarization filter has stable properties of with difference size and position of the little air holes. The designed polarization filter showing excellent properties can be used in polarizations filter in communication transmission. [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]

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]

We propose a modified design for a photonic crystal fiber (PCF) filter based on surface plasmon resonance(SPR). The air holes are arrayed in rectangular lattices, while the size and the pitches of holes around the gold-coated holes are different. That can separate the x-polarization and y-polarization of second-order surface plasmon polariton (SPP). The resonance strength of the surface plasmon mode and import of structural parameters of the PCF on the filter characteristics are studied through using the finite element method (FEM). Numerical simulations demonstrate that the thickness of the gold layer, the gold-coated or gold-filled, and the asymmetry around the gold-coated holes have a great effect on the filter characteristics. It is certain to obtain a resonance strength as high as 873 and 771.5 dB/cm at the communication wavelength of 1050 and 1310 nm in x-polarization by adjusting the size and the place of the gold-coated holes, while the loss is extremely low in y-polarization. [ABSTRACT FROM AUTHOR]

The polarization filter characters of a square lattice photonic crystal fiber with a large diameter gold-coated air hole are studied by finite element method. Results show that the resonance strength of y-pol direction is far stronger than x-pol direction. It can achieve 517 and 375 dB/cm, respectively, in y-pol direction at the communication wavelengths of 1310 and 1550 nm by adjusting structural parameters. If a second large hole is added in the opposite corner of the lattice, the resonance strength of y-pol direction can increases to 701 dB/cm at 1310 nm. At the same time, the influence for polarization filter characters is studied by changing the fiber length, the thickness of gold layer, the diameter of large air hole, and the diameters of small air holes. [ABSTRACT FROM AUTHOR]

An ultrabroad bandwidth polarization filter based on D-shaped photonic crystal fibers with gold film is proposed. The design of gold film deposited on the polished plane of the D-shaped photonic crystal fibers is benefit to generate multiple resonances between surface plasmon polaritons and photons. In our designed polarization filter, the resonant intensity in y-polarization direction is much stronger than in x-polarization direction. An ultrabroad bandwidth of exceeding 1 μm with extinction ratio better than -20 dB is obtained. The performance characteristics of the filter can be improved further by optimizing the structure parameters of the D-shaped photonic crystal fibers. [ABSTRACT FROM AUTHOR]

A broadband single-polarization photonic crystal fiber polarized filter based on surface plasmon resonance is proposed based on finite element method. Numerical simulations show the confinement loss of y-PCM (y-polarized core mode) is much higher than that of x-PCM (x-polarized core mode) in the wavelength range 1.20-1.63 μm. The confinement loss of y-polarized mode is 45,240 and 10,200 dB/m at the communication wavelength 1.31 and 1.55 um, respectively, and the corresponding loss of x-polarized mode is just 90 and 80 dB/m. When the fiber length is 3 mm, the bandwidth of extinction ratio better than -20 dB is greater than 430 nm covering almost all the communication wavelength. To our best knowledge, the bandwidth is the widest. The impacts of structural parameters on the resonance characteristics are also discussed. The structure could be further optimized for better result. [ABSTRACT FROM AUTHOR]

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

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

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]

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]

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

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 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]

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]

Photonic crystal fiber-based polarization filter has important application prospects in optical fiber communications, nonlinear optics, and sensors. In order to achieve polarization filters with optimal performance, some numerical methods have been used to search for optimal results under different geometric structure parameters of fibers. However, it is unrealistic to calculate solutions under every parameter. In addition, the search direction and scope of adjusting fiber geometric structure parameters still depend on the designer's intuition and experience. Therefore, we propose a novel design method for photonic crystal fiber polarization filters to overcome these questions. The optimization design problem of polarization filters is transformed into the single objective function optimization problem. The combination of artificial neural networks and intelligent optimization algorithms realizes the optimal value-solving problem in two-dimensional parameter space. The method efficiently and automatically searches for optimal structures in the parameter space by fine-tuning both variables simultaneously, improving the bandwidth of the polarization filter and achieving higher crosstalk at 1550 nm. The method eliminates computationally expensive wavelength parameters and greatly reduces computation time. This novel design method realizes the reverse design of photonic crystal fiber-based polarization filters. In addition, the new method can also be used to design polarization filters at other objective wavelengths, and it is potentially applicable to the design of other photonic crystal fiber devices. • Wavelength parameters are removed, drastically cutting down on computation time. • The polarization filters optimization design is transformed into function optimization. • Machine learning techniques provided optimal design parameters of polarization filters. [ABSTRACT FROM AUTHOR]

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

A 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]