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In order to enhance the OAM mode purity and quality, we propose a graded-index ring-core photonic crystal fiber (GRC-PCF) and its general design strategy by the finite element method (FEM). In addition, the numerical results turn out that the GRC-PCF is better than conventional step-index ring-core-photonic crystal fiber (SRC-PCF) in terms of mode purity and mode quality, when the two fibers support the same numbers of OAM modes at 1.55 μ m , respectively. After optimizing structural parameters, a GRC-PCF that can support 14 OAM modes without phase distortion and high-radial modes in C+L bands is designed. Particularly, the mode purity and mode quality of all the OAM modes are higher than 97.05% and 98.42%, respectively. The confinement losses are maintained at the order of 1 0 − 11 and flat dispersion. The proposed fiber can be used in the MDM system and realize the stable transmission of the OAM modes. • We propose a GRC-PCF to improve the OAM mode purity and quality. • The proposed GRC-PCF is better than SRC-PCF in terms of mode purity and quality. • The OAM mode purity and quality are higher than 97.05% and 98.42% in C+L bands, respectively. [ABSTRACT FROM AUTHOR]

The propagation of Pearcey–Gaussian (PeG) pulses in optical fibers close to the zero-dispersion wavelength point is investigated. It is found an interesting phenomenon that the PeG pulses maintain the envelopes for a sufficient long propagation distance under the co-action of the second- and third-order dispersions of the fiber. This phenomenon is due to the asymmetric distribution of the pulse spectrum, which allows the contributions from the second- and third-order dispersions having opposite signs within the entire spectrum and canceling each other during propagation. This finding may become new candidates for optical fiber telecommunication. [ABSTRACT FROM AUTHOR]

Cadmium ion (Cd2+) is one of the most crucial industrial pollutants that cause serious harm to the human body. For rapidly and sensitively determining the concentration of Cd2+ in a solution in real time, a highly selective Cd2+ sensor with high sensitivity is proposed based on a multimode interference effect in an air-hole-assisted multicore microstructured optical fiber (AA-MCF). Seven large air holes in an AA-MCF isolate of six fiber cores; one of them is cut open to expose two adjacent cores and form a microfluidic channel. It can be used as the region where the analyte liquid interacts with light. When the refractive index in this region varies, the multimode interference effect is influenced, so that the refractive index of the analyte can be quickly observed. Quantification is achieved by calculating the Cd2+ concentration through the refractive index. Theoretical and experimental results show that the sensitivity of this sensor made of an AA-MCF can reach 7.443 × 109 nm/(mol/L) and the experimental limit of detection is 6.0 × 10–12 mol/L. The proposed sensor exhibits high stability, specificity, and a compact structure, offering promising potential for biochemical sensing applications, such as environmental monitoring and disease diagnosis. • The first time an air-hole-assisted multicore microstructured optical fiber suitable for sensing is proposed. • The sensor based on AA-MCF can achieve trace detection of Cd2+. • The Cd2+ sensor has good specificity and practical application ability. [ABSTRACT FROM AUTHOR]