A photonic crystal fiber (PCF) with high relative sensitivity was designed and investigated for the detection of chemical analytes in the terahertz (THz) regime. To ease the complexity, an extremely simple cladding employing four struts is adopted, which forms a rectangular shaped core area for filling with analytes. Results of enormous simulations indicate that a minimum 87.8% relative chemical sensitivity with low confinement and effective material absorption losses can be obtained for any kind of analyte, e.g., HCN (1.26), water (1.33), ethanol (1.35), KCN (1.41), or cocaine (1.50), whose refractive index falls in the range of 1.2 to 1.5. Besides, the PCF can also achieve high birefringence (∼0.01), low and flat dispersion, a large effective modal area, and a large numerical aperture within the investigated frequency range from 0.5 to 1.5 THz. We believe that the proposed PCF can be applied to chemical sensing of liquid and THz systems requiring wide-band polarization-maintaining transmission and low attenuation.

A broadband single-polarization single-mode (SPSM) hollow core anti-resonant photonic crystal fiber (HC-ARPCF) is proposed and analyzed by the finite element method in this paper. The HC-ARPCF design consisted of outer semicircular cladding tubes and inner circular cladding tubes. The SPSM behavior is achieved through controlling the effective material absorption loss (EML) by loading epsilon negative (ENG) material in the selected semicircular cladding tubes. Optimization of the configuration parameters is conducted to yield a large loss difference (LD) between one of the two orthogonally polarized fundamental modes and all the other unwanted modes. Therefore, only one desired mode will exist after a proper propagation distance, i.e., SPSM guidance. Specially, the optimal design provides a 288 nm (from 1408 nm to 1676 nm and from 1680 nm to 1700 nm) bandwidth in terms of 40 dB/m minimum LD (MLD) and 168 nm (from 1452 nm to 1620 nm) bandwidth in terms of 100 dB/m MLD. Furthermore, this fiber also exhibits a large effective mode area and near-zero dispersion properties over the entire operation bandwidth. The proposed HC-ARPCF may find its applications in polarization maintaining and high-power laser systems.

A robust and simple mid-infrared hollow-core anti-resonant fiber (ARF) based refractive index (RI) sensor with an intensity demodulation method is presented and analyzed for monitoring liquid analytes. The ARF allows liquid analytes to flow through its hollow area for detection. To obtain ideal sensing performance, an epsilon negative (ENG) material is introduced into the selected anti-resonant tube. With the high absorption of the ENG material, only one fundamental mode is available for detection and is sensitive to the RI variation of analytes. Moreover, the effects of structural parameters on the sensing performances are discussed and analyzed to further understand the mechanism and optimization. The final result shows that the ARF sensor can exhibit a high sensitivity of −372.58 dB RIU⁻¹ at a fixed wavelength within a broad RI range from 1.33 to 1.45, which covers most liquid analytes. It is a promising candidate for chemical and environmental analysis. Additionally, it has the potential for deep research to feed diverse applications.

This letter describes a synthesis method for wideband bandpass filters (BPFs) that uses an optimized matrix scaling technique for direct circuit implementation. The method commences with constructing filtering polynomials in the bandpass domain tailored for bandwidth, passband response, and the locations of transmission zeros (TZs). Then, the in-line lumped-element prototype is realized through a simple implementation. Subsequently, the proposed scaling matrix with solvable restraints and a tuning process is employed to adjust practical component values while maintaining the predetermined responses flexibly. The complexity of the circuit can be reduced by selecting critical values of the scaling factors. Finally, the proposed method is validated by several synthesis examples and measured results.

An accuracy evaluation method for optical fiber shape sensing is proposed, which allows us to compare the performance of sensors with different specifications. To achieve a more accurate assessment of the curve reconstruction, we fully consider the spatial information of the reconstructed curve and perform flexible segmental weighting on the shape sensor. Subsequently, by normalizing the average sensing length, the performance of sensors with different specifications can be compared. During numerical simulation, we reconstruct spatial curves using four sensors with different specifications, then, evaluate the accuracy of the reconstruction results by using the traditional accuracy evaluation method and the proposed method, respectively. The results show that the sensor reconstruction accuracy from the global and local scales of a single sensor can be evaluated, meanwhile, the normalized errors of sensors with different specifications are not always optimized with the increase of the number of sensing points, which can be used as a reference to make a balance between the increase in sensor cost and the improvement in reconstruction accuracy. In addition, we demonstrate through experiments that the proposed shape reconstruction accuracy evaluation method has higher robustness than traditional methods.

We report the strong coupling between plasmonic surface lattice resonances (SLRs) and photonic Fabry-Pérot (F-P) resonances in a microcavity embedded with two-dimensional periodic array of metal-insulator-metal nanopillars. For such a plasmonic-photonic system, we show that the SLR can be strongly coupled to the F-P resonances of both the odd- and even orders, and that the splitting energy reaches as high as 153 meV in the visible regime. Taking advantage of the strong coupling, the resulted high-energy upper polariton has similar characteristics as the plasmonic SLR, but the quality factor is almost twice of that of the SLR. We expect that this work will provide a new scheme for strong coupling between plasmonic and photonic modes, and will point to a new direction to improve the quality factor of SLRs.