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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 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]

Highlight • A high-sensitivity PCF sensor was designed for analyte detection. • This optimized sensor had an excellent linearity and good sensitivity. • The sensitivity within the refractive index range of 1.425–1.45 is 60,611 nm/RIU. • The corresponding minimum detection limit is 1.58 × 10−6 RIU. • The sensitivity within the refractive index range of 1.45–1.46 is 28,000 nm/RIU. • The corresponding minimum detection limit is 2.86 × 10−6 RIU. Abstract A high-sensitivity photonic crystal fiber (PCF) refractive index sensor based on the resonant coupling of core mode and defect mode is studied by the finite element method. The large liquid-filled is placed near to the core for easy coupling with the core mode. The simulation results show that when the refractive index of the analyte is 1.425–1.45 and 1.45–1.46, the average sensitivity is 60,611 and 28,000 nm/RIU respectively, and the corresponding minimum detection limits are 1.58 × 10−6 and 2.86 × 10−6 RIU. [ABSTRACT FROM AUTHOR]

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]

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]