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This paper combines molybdenum dioxide (MoO2) instead of the gold film with photonic crystal fiber (PCF) for the first time. A novel C-type micro-opening surface plasmon resonance (SPR) optical fiber sensor coated with MoO2 nanofilm is proposed. The polarization and sensing characteristics of the sensor coated with plasma film on the outer surface of the polished optical fiber are numerically analyzed by the finite element method (FEM). The simulation results show that when the plasma material deposited by the optical fiber micro-opening is noble metal gold, the sensor can realize the sensing detection of the refractive index of the analyte from 1.33 to 1.39, and the average wavelength sensitivity in the x-polarization direction reaches 1107 nm/RIU. It is most noteworthy that when the plasma coating material of the optical fiber is replaced by metal oxide MoO2, the sensor can realize the sensing detection of the refractive index of the analyte from 1.31 to 1.37, and the average sensitivity of the x-polarization direction can reach 4821 nm/RIU. By comparison, we can know that the sensor coated with plasma material MoO2 has excellent sensing characteristics. This research is of great scientific significance for expanding the application range of MoO2 materials and promoting the development of photonic crystal fiber sensors coated with new materials. [ABSTRACT FROM AUTHOR]

Fiber optic sensing technology has extensive applications in many fields and demands high sensitivity and cost-effectiveness for sensors. In this study, we propose an ultra-high sensitivity D-type photonic crystal fiber sensor based on MoO 2 nano-film coating to meet this requirement. Firstly, MoO 2 is selected as the plasmonic excitation material. Compared to expensive metals such as Au and Ag, MoO 2 has the characteristics of low cost and charge carrier density, which is suitable for use in optical fiber sensing modules. The sensitivity of the sensor to the external environment can be increased by coating the surface of the optical fiber with MoO 2 nano-film. Then, we use the finite element method to evaluate the performance of the sensor. Numerical simulation results show that the average wavelength sensitivity of the sensor in the x-polarization direction is as high as 24000 nm/RIU when analyzing substances with refractive indices ranging from 1.38 to 1.43. This means that the sensor has a very sensitive response to refractive index changes and can be widely applied to sensing needs in different fields. Overall, the D-type photonic crystal fiber sensor based on MoO 2 nanofilm coating has enormous potential for applications. This sensor can achieve high-sensitivity detection within a specified range of refractive indices, providing new ideas and methods for research and applications in the field of fiber sensing. • MoO 2 was combined with D-type PCF as a plasma exciton material for the first time to form a sensor. • An ultra-high-sensitivity D-type PCF sensor based on MoO 2 nanofilm coating is proposed. • Numerical simulation results show that the refractive index (RI) detection range of the sensor is between 1.38 and 1.43. And the average wavelength sensitivity of the sensor in the x-polarization direction is 24000nm/RIU. • The results demonstrated that MoO 2 can be utilized as an excellent plasma sensing material in optical fiber sensing applications. • This sensor can achieve high-sensitivity detection within a specified range of refractive indices, providing new ideas and methods for research and applications in the field of fiber sensing. [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]

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 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 D-shaped photonic crystal fiber sensor based on surface plasmon resonance effect is proposed in this paper. Experimental results show that the D-shaped sensor can perform sensing detection in high or low refractive index regions. In addition, the refractive index of different analytes can be double calibrated by peaks and troughs. For high refractive index region from 1.3621 to 1.4010, the average sensitivity of the peaks and troughs is only 30.0 nm/RIU and 72.7 nm/RIU, respectively. However, in the low refractive index region of 1.3415–1.3468, the average wavelength sensitivity reaches 842.6 nm/RIU. The research in this paper has important reference value and positive scientific significance for exploring novel D-shaped photonic crystal fiber sensors. [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]

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 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]

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 three-core photonic crystal fiber refractive index sensor based on the surface plasmon resonance effect is proposed. The three cores of this sensor are symmetrical about the center. The gold film deposited outside the fiber structure makes this sensor contact with the analyte directly, which not only reduces the complexity of the fabrication but also makes the limiting loss very low. The sensing performance of the proposed sensor is investigated by using the finite element method. Numerical results show that the loss spectra of the x-polarized core mode are consistent with the y-polarized core mode. The sensor realizes the sensing detection of the analyte refractive index ranging from 1.37 to 1.42. When the analyte's refractive index is 1.42, the maximum wavelength sensitivity of 2427 nm/RIU for x and y polarization, the corresponding maximum resolution can reach 4.12 × 10−5 RIU. [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 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]