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An easy-to-implement side-polished micro-structured fiber MSF)based on surface plasmon resonance (SPR) is proposed. The influence of fiber structure parameters on transmission performance is investigated by finite element method (FEM). The chemical stable gold is deposited on the side polished plane to provide the plasmonic material required for SPR. Ten cladding air-holes are used to confine light transmission and improve the coupling strength between the core mode and surface plasmon polaritons (SPP) mode. The optimization results show that the MSF integrates the characteristics of high refractive index (RI) sensing and broadband tunable polarization filtering. In addition, the RI detection range of the sensor can be further extended by depositing a $Si_{3}N_{4}$ dielectric layer on the gold film. The RI detection range of the sensor with gold coating is 1.20--1.325 with the maximum sensitivity of 12,000 nm/RIU. The RI measurement range of the sensor with gold- $Si_{3}N_{4}$ bilayer film can be extended to 1.20--1.37, and the maximum sensitivity is 10,000 nm/RIU. Furthermore, by changing the ambient RI of gold-coated fiber or the polishing depth of fiber, a broadband tunable filter with excellent performance can be obtained. Therefore, the proposed MSF not only has application value in the detection of biochemical liquid samples, but also has broad application potential in broadband communication networks, which is helpful to promote the development of miniaturization and integration of optical devices. [ABSTRACT FROM AUTHOR]

A single-mode single-polarization photonic crystal fiber (SMSP-PCF) filter at the wavelengths of 1.125 and 1.31 μm based on surface plasmon resonance is proposed and studied by the finite element method. Numerical results indicate that compared with the filter with the same size of holes near the core, the filter with four small holes near the core makes its extinction ratio (ER) at the wavelengths of 1.125 and 1.31 μm twice as large. That is, when the fiber length is 500 μm, the ER at 1.125 μm increases from 33.43 to 58.62 dB, and the ER at 1.31 μm increases from 129.41 to 244.85 dB. Furthermore, the designed SMSP-PCF filter also has a good structural tolerance. The proposed optical fiber filter has excellent performance of compact structure, high ER and good structural tolerance, and can be used in an integrated communication transmission system. [ABSTRACT FROM AUTHOR]

A novel ultra-wide bandwidth photonic crystal fiber (PCF) polarization filter based on surface plasmon resonance (SPR) is proposed. The PCF has two large cladding air holes in the direction of y-axis that are selectively coated with gold films to create resonance. There are two small holes in the direction of x-axis, which provides high birefringence. The polarization characteristics of PCF are studied by finite element method (FEM). The numerical simulation results indicate that the loss of y-polarized core mode is 718.87 dB/cm, while the loss of x-polarized core mode is only 0.95 dB/cm at the resonant wavelength of 1.55 µm. The extinction ratio is better than 20 dB at the wavelength ranging from 1.43 to 1.79 µm, the corresponding bandwidth is 360 nm. [ABSTRACT FROM AUTHOR]

An elliptical photonic crystal fiber (PCF) polarization filter based on surface plasmon resonance is proposed. Two smaller elliptical air-holes are selectively filled with gold wires to improve the filter characteristics by the surface plasmon effect. The transmission characteristics and influence of structural parameters are analyzed by the finite-element method. Research shows that the loss of y-polarization is much greater than that of x-polarization. The air holes filled with gold have the greatest influence on the loss of PCF, and the air holes around the fiber core have the greatest effect on the dispersion. In the optimized model, the loss of y-polarization is 333.84 dB/cm, but the loss of x-polarization is only 1.5 dB/cm at 1.55 $\mu \text{m}$. Besides, when the length fiber is $L= 3$ mm, the extinction ratio is 866 dB, and the bandwidth can reach 500 nm, which embodies high extinction ratio and wide bandwidth of this filter. [ABSTRACT FROM AUTHOR]

In this study, the silver mirror reaction was used to coat the silver film on the surface of self-made microstructured fiber (MSF) to stimulate the surface plasmon resonance effect, and Polydimethylsiloxane (PDMS) with a high thermal-optical coefficient was coated on the silver film as temperature-sensitive material. The MSF with silver and PDMS films was coupled with multi-mode fiber on both sides to form the temperature sensor. In this sensor system, the energy is coupled into the cladding of the microstructure fiber by multi-mode fiber, and the surface plasmon resonance can be further excitated in the MSF. When the temperature of the external environment changes, the refractive index of PDMS will also change. At this time, combined with the surface plasmon resonance effect, a resonant absorption peak corresponding to the temperature appears in the transmission spectrum so that the temperature can be measured quickly and accurately. We found that, in the temperature range of 35 °C to 95 °C, the average temperature sensitivity of the sensor during heating and cooling was −0.83 nm/°C and −0.84 nm/°C, respectively. The advantages of this sensor are the simple structure, convenient operation and good reversibility. The relative sensitivity deviation value (RSD = 0.0059) showed that the sensor has high stability. The temperature sensor based on MSF has favorable prospects for use in fields such as medical treatment, biochemical detection and intelligent monitoring. [ABSTRACT FROM AUTHOR]

• A High-sensitivity plasmonic RI sensor based on MMF-MOF-MMF structure is proposed. • The RI sensitivity of the sensor is 3223 nm/RIU in the range of 1.3328–1.3990. • The linearity of the sensor is 0.96912. • In the range of 25–46 °C, it has excellent temperature insensitivity. • This sensor is compact, easy manufacture, and has good reproducibility. A surface plasmon resonance (SPR) refractive index (RI) sensor composed of a multimode fiber-microstructure fiber-multimode fiber (MMF-MOF-MMF) structure is studied. The experimental results show that the sensor has good linearity (R-square = 0.96912) in the refractive index detection range of 1.3328–1.3990, and the sensitivity is as high as 3223 nm/RIU. Moreover, the sensor shows excellent temperature insensitivity within the temperature fluctuation range of 25–46 °C. In addition to good linearity and high sensitivity, the proposed SPR sensor with MMF-MOF-MMF structure also has the advantages of compact structure, easy manufacturing and good reproducibility. Therefore, it can be used to detect chemical and biological analytes. [ABSTRACT FROM AUTHOR]

• A compact, ultrabroadband, and single-mode single-polarization photonic crystal fiber (PCF) filter based on surface plasmon resonance (SPR) is proposed. • Compared with some reported SPR-based PCF polarization filters, this filter can operate in the 1.31 and 1.55 μm communication windows, and achieve a wider bandwidth over a shorter fiber transmission distance. • When the fiber length is only 700 μm, the bandwidth can reach 730 nm, from 0.97 to 1.70 μm, covering all communication bands from 0 to U. • With the increase of wavelength, the birefringence of the PCF filter and the confinement loss difference of x - polarized and y -polarized core modes increase, which is of great importance for the study of a broadband and single-mode single-polarization PCF filter. • The designed PCF is deformed from a square lattice, which is easy to stack and fabricate. • The fiber exhibits excellent tolerance to the changes of structural parameters and thickness of the gold film, which further reduces the difficulty of fabrication and also facilitates its application in experiments and practice. An ultrabroadband and compact square-lattice photonic crystal fiber polarization filter based on surface plasmon resonance is proposed. Four large air holes are introduced near the core to destroy the symmetry of the original fiber structure so that the resonance wavelengths of x - and y -polarized core modes can be well separated. The inner walls of two large air holes along the x polarization direction are coated with gold film to generate the surface plasmon mode. In addition, to further enhance the coupling resonance strength between the core mode and the surface plasmon polariton mode, the six air holes near the gold-coated air holes are replaced by solid glass rods. Then the dispersion characteristics and loss spectrum of the proposed polarization filter are studied by the finite-element method. The results show that when the fiber length of the proposed filter is only 700 μm, ultrawideband y -polarized light transmission with a bandwidth of up to 730 nm in the wavelength range from 0.97 to 1.70 μm can be achieved. The effective filtering range covers all optical fiber communication bands from O to U. Moreover, the filter has excellent structural tolerances. These excellent properties are expected to make it a good candidate for a broadband polarization filter. [ABSTRACT FROM AUTHOR]

A tunable single polarizing filter is proposed by selectively coating gold film on the air holes of photonic crystal fiber (PCF). The polarization properties of the PCF filter are evaluated by the finite-element method. Simulation results show that the loss of y-polarized core mode at 1250 and 1550 nm is 136.23 and 839.73 dB/cm, respectively. Furthermore, we innovatively combine stable modulation with flexible modulation. To be specific, the resonance wavelengths are slowly controlled in a small wavelength range by altering the diameter of the air-hole-coated gold film, while the resonance wavelengths are flexibly controlled in a wide wavelength range by altering the thickness of the gold film or the diameter of the small air holes. When the length of the PCF is 500 µm, the bandwidth of extinction ratio greater than − 20 dB is only 60 nm at the communication window of 1550 nm. It is beneficial to fabricate a narrow-band polarization filter. [ABSTRACT FROM AUTHOR]

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 groove micro-structure optical fiber refractive index sensor with nanoscale gold film based on surface plasmon resonance (SPR) is proposed and analyzed by the finite element method (FEM). Numerical results show that the average sensitivity is 15,933 nm/refractive index unit (RIU) with the refractive index of analyte ranging from 1.40 to 1.43 and the maximum sensitivity is 28,600 nm/RIU and the resolution of the sensor is 3.50 × 10 −8 RIU. The groove micro-structure optical fiber refractive index sensor do some changes on the D-shaped fiber sensor, compared with conventional D-shaped fiber sensor, it has a higher sensitivity and it is easier to produce than the traditional SPR sensor. [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]

A novel D-shaped photonic crystal fiber sensor based on surface plasmon resonance has been designed in this paper. The refractive index sensing with double loss peaks are studied in detail. Numerical simulation shows that the average wavelength sensitivities of the primary peak and the secondary peak can reach 18900 and 17500 nm/RIU in the refractive index range of 1.35-1.38, and the corresponding resolutions are 5.291 × 10-6 and 5.714 × 10-6 RIU, respectively. In addition, it is found that the resonance wavelengths do not shift as the sensing length increases. Moreover, the coupling characteristics between fundamental mode and the surface plasmon polarization (SPP) mode are investigated. It worth noting that the sketch diagram about specific operation is presented. It can except that the research results can be applied into the fields of double-loss-peak detection, which improves the practicability of the sensor. [ABSTRACT FROM AUTHOR]