查看更多>>摘要：? 2021 Elsevier B.V.This paper presents two new types of dynamically tunable THz nonreciprocal power dividers based on graphene. These components fulfill division of the input power between three output ports. Besides, they possess isolation properties, i.e., a source connected to the input port is protected against unwanted reflections from the output ports. The components consist of a disk-shaped graphene resonator and six graphene waveguides connected to it. The graphene elements are deposited on a dielectric substrate. The resonator is subject to an external electric and DC magnetic field. The dipole mode resonance in the magnetized resonator is excited by surface plasmon–polariton waves propagating in the graphene waveguides. Numerical simulations and the analytical temporal coupled-mode theory are used in the analysis. The division level of the input power between the three output ports of dividers is about ?8 dB and the isolation of the source in relation to the output ports is higher than ?15 dB with the central frequency 7.45 THz and the bandwidth 3%. DC magnetic field is 0.29 T and Fermi energy is 0.15 eV. The central frequency of the dividers can be dynamically adjusted by an external electric field.
查看更多>>摘要：? 2021 Elsevier B.V.A distributed optical fiber sensing (DOFS) system based on a bidirectional sensing structure and filtering effect of an unbalanced Mach–Zehnder (M-Z) interferometer is proposed and demonstrated. Unlike the M-Z interferometric DOFS system, in this system, the sensing fiber is not in the interferometer, but in series with the interferometer. The M-Z interferometer is employed to only convert the optical phase fluctuation caused by an external disturbance acting on the sensing fiber into the optical intensity change due to its comb filtering effect. Disturbance location can be realized by cross-correlating the two output signals of the sensing system and obtaining their time difference. Experimental results show that the location error is less than 5 m for the 5.7 km-long sensing fiber. The system is feasible in disturbance detection, accurate and repeatable in location results. The detection sensitivity is adjustable by changing the arm length difference of the M-Z interferometer and the frequency response is wideband.
查看更多>>摘要：? 2021 Elsevier B.V.A novel temperature and strain sensor based on fiber Sagnac interferometer (FSI) with Vernier effect is proposed and demonstrated experimentally. The FSI fabricated by fusion splicing a segment of thin polarization maintaining fiber (TPMF) with a coupler works as sensing. The Vernier effect is generated by cascading a polarization mode interferometer (PMI). The PMI is fabricated by fusion splicing a polarization maintaining fiber (PMF) with a polarizer. The suitable free spectral range (FSR) of the sensor is fabricated based on numerical simulation. The experimental results indicate that with the Vernier effect, the temperature sensitivity can be improved from ?1.924 nm/°C to ?32.830 nm/°C and the strain sensitivity can be improved from 25.9 pm/με to 407.32 pm/με. The measured amplification factors are about 17 and 16. The good performance of the FSI with the Vernier effect provides a new option in the temperature and strain sensing fields.
查看更多>>摘要：? 2021 Elsevier B.V.In the traditional image encryption methods based on Fresnel diffraction (FRT), the Fresnel diffraction distance and the wavelength of light are considered as two essential secret keys, named distance key and wavelength key, respectively. However, these two keys are both vulnerable to be attacked due to their low sensitivities, resulting in an inadequate intensity of secret key for several high-security applications. This is a common and serious problem in image encryption methods based on FRT. In this paper, a security-enhanced image encryption method based on Fresnel diffraction with chaotic phase (CPFRT) is proposed and verified, where CPFRT is an alternative algorithm of diffraction calculation that combines ordinary FRT with special phase generated by chaotic system. In such a way, the sensitivities of distance key and wavelength key could be substantially improved thanks to the nonlinear, pseudo-random, high sensitivity of initial value of chaotic phase, which effectively solves the previous common problem, contributing to an extremely high security of image encryption method meanwhile the number of secret keys is not increased. Moreover, computational simulations are performed to demonstrate the validity of the proposed security-enhanced image encryption method, and the results confirm that the proposed method has a substantially enhanced security in distance key and wavelength key, and the high quality of correct decrypted image could be successfully maintained. Furthermore, the proposed method based on CPFRT can also be applied to double-phase image encryption by replacing FRT with CPFRT. Analogously, the sensitivities of distance key and wavelength key in double-phase image encryption are also greatly improved, which leads to an outstanding anti-attack capability. In addition, the effectiveness of the proposed double-phase image encryption method based on CPFRT is also verified by rigorous computational simulation, and the proposed method based on CPFRT shows a promising potential for other image encryption methods in order to substantially enhance their securities.
查看更多>>摘要：? 2021 Elsevier B.V.We propose and numerically investigate a dual-band D-shaped surface plasmonic resonance (SPR) fiber sensor, which consists of a simple D-shaped fiber with metallic nanowire grating on the polished flat surface. The correlation between resonance coupling and degenerate mode properties are fully investigated based on birefringence analysis. To the best of our knowledge, it is the first report that plasmon resonances occurs in both directions which are parallel and perpendicular to the polished surface in this simple structure. The sensing figure of merit (FOM) of the two orthogonally SPR modes reach up to 200 and 190 RIU?1. The proposed structure would be a good candidate for dual-channel biosensing, medical diagnosis and environmental monitoring in a wide refractive index range of 1.31–1.37.
查看更多>>摘要：? 2021 Elsevier B.V.In this paper the performance of Discrete Multi-Tone (DMT) optical links using nonlinear companding schemes for peak-to-average power ratio (PAPR) reduction is investigated. First, it is developed an analytical expression to evaluate the total distortion at the demodulation stage introduced by the expansion process, and also to estimate the BER vs. SNR performance curves for companded DMT systems. The analytical expression and procedure for BER estimation is tested with three compressing functions: μ-Law, Erf and Trapezoidal. The obtained results show a very good agreement between simulations and predictions, validating this simple, quick and accurate BER estimation tool for the considered system. Next, the effects produced by a nonlinear laser diode (modeled as a lower hard limiter) that is directly modulated, are investigated in terms of its input back-off (IBO). A trade-off between the distance that can be reached in an optical fiber link and the allowed BER degradation is numerically demonstrated. These results are compared against a nonlinear amplifier model (soft limiter), which results in a slight improvement in performance. Finally, it is shown that even though companding schemes degrade the BER for nearly linear operation, they can improve the BER performance of a DMT link if a moderate back-off value is allowed (particularly for an IBO=7 dB when the SNR>20 dB for a system with an Erf companding).
查看更多>>摘要：? 2021 Elsevier B.V.In this paper, we firstly applied label propagation algorithm (LPA) to coherent optical communication system for nonlinear impairment compensation (NLC), which could exploit the relationship among constellation points to segment the community with only very small amount of label data to predict the label situation on most test data. The effectiveness of the proposed scheme was verified through 28 GBaud polarization division multiplexing (PDM)-16 quadrature amplitude modulation (QAM), PDM-32QAM and PDM-64QAM simulation system. By analyzing the simulation results, we obtained the best propagation coefficient α, which demonstrated that the proposed scheme could effectively make nonlinear decision on received symbols and compensate signal impairment caused by Kerr nonlinear effect and amplified spontaneous emission (ASE) noise, showing a strong robustness and generalization to variations of the transmission environment under different modulation formats, transmission distances, launch power and channel numbers. Then, the proposed scheme was further studied by 20 GBaud PDM-16QAM long-haul transmission experiments. The results demonstrated that the proposed scheme required no training process and exhibited optimistic compensation performance to the coherent optical communication system with nonlinear impairments. Finally, we analyzed computational complexity of the LPA-assisted NLC scheme in detail, which was O(m*N).
查看更多>>摘要：? 2021 Elsevier B.V.Biochemical sensors have drawn extensive attention for cell/molecular detection, health care, and food safety. Biochemical sensing devices conventionally built with single or double functions show a lack of functionality, which impedes integrated sensing systems but also limits various practical applications. Here, we report a switchable multifunctional gold nanopillar array biochemical sensor that is based on localized surface plasmon resonance (LSPR). Biochemical sensors can be used to measure a wide variety of spectral signals such as absorption spectra, fluorescence, surface plasmon resonance (SPR) performance and surface-enhanced Raman scattering (SERS). Specifically, based on the absorption spectroscopy results, the refractive index (RI) sensitivity of ethylene glycol (EG, concentration is 20%) is 489.6 nm/RIU. Furthermore, the limit detection of SERS performance for rhodamine 6G (R6G) dye is 10?8 M, and the analytical enhancement factor (AEF) is 2.5 × 108. Our theoretical and experimental data demonstrate that this nanoscale-based metasurface presents promise potential for in situ biomedical sensing, immunoassay and cell/molecular spectroscopy applications.
查看更多>>摘要：? 2021In this work, optical properties of a tuned quantum dot/ring system with considering the electron–phonon (e-p) interaction are theoretically investigated. In particular, the second harmonic generation (SHG) and third harmonic generation (THG) of the system under e-p effect are determined. Here, we consider three phonon modes, longitudinal optical (LO), the surface optical (SO), and LO+SO phonons. First, the energy levels and the wave functions of the system are evaluated without e-p effect. Then, the SHG and THG are investigated with considering the LO e-p, SO e-p, and LO+SO e-p interactions. We have noticed that the e-p interaction has a great effect on the SHG and THG of our system. The effect of SO phonons on the obtained optical properties is more than the LO phonons. With considering the LO+SO-phonons, we obtain the largest influence on the optical properties.
查看更多>>摘要：? 2021 Elsevier B.V.Analytical expression is derived for the mode-coupling coefficient between non-identical step-index (SI) cores, which enables a quick estimate of the crosstalk in the heterogeneous SI multi-core fibers (Hetero-SI-MCFs). The analytical results are in good agreement with the results obtained by numerical simulations by finite-element method (FEM). Using the derived analytical expression, the feasibility of Hetero-SI-MCF design within standard 125-μm cladding diameter is discussed. The designed MCFs have potential applications in space division multiplexing (SDM) systems.
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