查看更多>>摘要：This paper proposes a physical layer security strategy of space division multiplex orthogonal frequency division multiplexing system based on memristive neural network (MNN). Among them, the two-level chaotic control, the application of the memristive chaotic model with conditional symmetry and the encryption of the MNN are all proposed for the first time in the access network. The memristive chaotic model with conditional symmetry generates 8 multiple masking factors and achieves high-security encryption. In the encryption process, the key space is selected by Logistic mapping, and the MNN encryption mechanism disturbances and compresses bits. The proposed encryption strategy has been proved on the experimental platform. The generated OFDM signals are modulated, which transmit data rate of 124.4 Gb/s over the 2 km multi-core fiber (MCF). Bit error rate (BER) and different anti-interference are analyzed in experiments. The key space of this proposed scheme is 2 x 10(399), which is large enough number to effectively resist any brute force or illegal optical network units (ONUs) intrusion in passive optical network (PON). The results show that the proposed SDM-OFDM scheme has high sensitivity and security. This scheme has very reliable safety performance and benefit improvement, and is very promising in the future SDM-OFDM system.
查看更多>>摘要：We report the design and performance of a 2.1 mu m tunable holmium-doped fiber laser (HDFL) in both continuous-wave (CW) and mode-locked regime. Nonlinear polarization rotation (NPR) mechanism is used as a saturable absorber to realize the evolution from CW to mode-locked state. Another function of NPR mechanism is to realize wavelength tuning as a wavelength selection device. The tunable single-wavelength CW laser can be tuned from 2039 nm to 2118 nm. In addition, conventional soliton operation is obtained with the wavelength range of 2049.2-2087.3 nm.
查看更多>>摘要：Transmissive terahertz (THz) metasurfaces have potential applications in imaging, biosensing, and optical communications. Traditional THz metasurfaces have a lot of problems such as non-adjustable spectra and low transmission efficiency. In this paper, discrete transmission phase principle and temperature-tunable material indium antimonide (InSb) are used to complete full phase coverage in the frequency range from 0.75 THz to 0.95 THz. Then a series of broadband temperature-controlled transmissive functional metasurfaces, such as anomalous refractors, focusing metalens, and orbital angular momentum (OAM) beam generators, are realized. The working states of these proposed metasurfaces can be switched from "ON"to "OFF'' by adjusting the temperature of InSb from 220 K to 360 K. In addition, by adding vertical copper plates to form a Fabry-Perot (FP) cavity-like structure, the transmission efficiencies of these metasurfaces are improved drastically to 60%. This work provides a method for designing tunable high performance multifunctional metasurface, which is expected to broaden the practical application of metasurfaces in the THz band.
查看更多>>摘要：Vortex beams carrying fractional orbital angular momentum (FOAM) mode show great potential in optical encryption, anisotropic edge enhancement, optical communication, etc. However, the small mode interval of FOAM makes it sensitive to transmission scattering, which distorts the helical phase-front and leads to substantial mode crosstalk. Herein we propose a transmission matrix method to recognize FOAM modes under scattering. By constructing the linear relationship between the input and scattering field, a transmission matrix retrieval algorithm was used to recover the helical phase-front of the vortex beam from its intensity distribution, and FOAM mode was recognized via the phase period. We showed that FOAM modes under scattering are successfully recognized with an accuracy of > 99.8%. As a proof-of-concept, a 100 x 100 pixel image was transmitted and recovered in a high scattering. Our results indicate that the transmission matrix method presents a new approach to recognize FOAM modes under scattering, which may benefit various FOAM applications.
查看更多>>摘要：A refractive index (RI) sensor with a high sensitivity based on an integrated double-layer resonant meta-grating (DRMG) is theoretically demonstrated. The presented DRMG consists of two top/ bottom grating layers with meta-grooves, and a slab waveguide layer. The physical mechanism of the device is elucidated by combining a theoretical model with the eigenmode information of the grating structures. The reflection spectra of DRMG-based sensor are then investigated by the theoretical model, which is simultaneously verified by rigorous coupled-wave analysis (RCWA). It is found that the sensitivity is improved by controlling the key structural parameters of the DRMG for enhancing the interaction between its leaky mode and analyte. Meanwhile, the optical field is trapped around the region of DRMG. Consequently, the sensitivity of the RI sensor is further increased without decreasing its quality factor (Q-factor). The simulation results show that the sensitivity of the sensor is achieved up to 930 nm/RIU with its Q-factor as high as 11575. Such an all-dielectric meta-grating sensor is expected to hold great promise for realizing high-performance sensor.
查看更多>>摘要：In order to achieve high-efficiency and independent control of the amplitude and phase of light transmitted from the coded metasurface, we propose a double-sided all-dielectric unit structure. The all-dielectric metasur-face unit structure is composed of a silicon dioxide substrate with silicon elliptical cylinder and silicon cylinder on both sides. The phase of the transmitted light can be adjusted by rotating the elliptical cylinder, and the amplitude of the transmitted light can be adjusted by changing the radius of the cylinder. In order to obtain the ideal amplitude and phase control, we set the initial rotation angle of the elliptical cylinder to eliminate the additional phase, which is caused by the radius change of the nanocylinder. We can digitally encode the unit structure of the elliptical cylinder with different rotation angles to construct different sequences of encoded metasurfaces. By adjusting the structural parameters of the encoded particles in the encoding metasurface, the amplitude and phase of the transmitted beam can be independently adjusted. The independent modulation of amplitude and phase of coding particles is verified by the far-field scattering characteristics of different coding metasurfaces.
查看更多>>摘要：We experimentally study the superfocusing performance of the Terahertz super-oscillation lens (TSOL). The TSOL which generates Terahertz super-oscillation light field in the focal plane is composed of a Terahertz aspheric lens and an optimized binary 0-pi. phase device. For a proofed TSOL (numerical aperture NA = 0.4614) @ working wavelength of lambda = 119 mu m, the FWHM of the super-oscillation spot similar to 0.80 lambda. is about 0.70 times of the Airy spot. Theoretical simulation is performed to validate the confocal scanning imaging resolution enhancement of the TSOL and the imaging resolution of the TSOL is about 0.85 times of the Rayleigh criterion. This study has potential applications in the terahertz super-resolution imaging.
查看更多>>摘要：A simple dielectric corona pre-ionized TEA CO2 laser, which is suitable for SHG applications, has been developed with the help of high concentration of carbon dioxide. The intense, very uniform UV pre-ionization and low-inductance discharge circuit permit the best laser performance with a gas mixture rich in CO2. In order to achieve the most acceptable combination of the peak power, output energy and pulse duration, not only N-2:CO2 ratio varies in favor of CO2 but also helium concentration is significantly reduced. When CO2 concentration varied from 20 to 53% in the experiment, a more symmetrical, shorter output pulse (a FWHM of ~60 ns) with a significantly lower tail and a lower energy (~700 mJ) was obtained with a rich gas mixture of CO2:N-2:He = 10:1:8 than a lean gas mixture of 1:1:3 while the peak output power was virtually kept constant (~5.8 MW). The effect of the laser gas mixture on optimizing the energy and time parameters of SH waveform has been studied numerically. The calculations show that for the rich gas mixture the residual tail of the SH pulse is completely eliminated by the nonlinear conversion and the corresponding waveform has a lower total duration (~600 ns). Moreover, the energy conversion efficiency can be increased by 29% with the rich mixture compared to the lean mixture.
查看更多>>摘要：We report broadband supercontinuum (SC) spectra generated from low-loss single-mode fibers (SMF) based on novel Ge-As-S-Se (GASS) chalcogenide (ChGs) glasses. The step-index fibers (SIF) possess an ultra-low background loss of 0.27 dB/m at wavelength of 2.5 & nbsp;similar to 7.5 mu m were fabricated by an optimized peeled-off extrusion method. SC generation was studied by pumping the fiber with 15 cm and 100 cm at same condition (5 mu m, similar to 150 fs, 1 kHz, 25 mW), respectively. Output SC covering 1.46 similar to 9.32 mu m was obtained in this 15-cm fiber, while a relatively wide output SC spanning from 1.68 to 8.28 mu m was observed in 100-cm fiber. Due to simulated numerical model does not account for higher-order modes at shorter wavelengths, cladding modes, polarization effects etc. Experimental SC spectral profile in the short and long S/Se fibers is slightly different with the simulation analysis. To the best of our knowledge, this is the first report on the performance of GASS fiber.
查看更多>>摘要：Using a simple design method cutting away some parts of a circular metallic patch resonator, we propose an ultra-broadband terahertz metamaterial absorber (MA) with bandwidth of 4.11 THz. Simulation results demonstrate that the simple design method greatly improves the absorption performance compared with previously reported absorbers. The proposed absorber exhibits the co-polarization ultra-broadband absorption performance with absorptivity above 90% in the range of 2.65-6.76 THz and the relative absorption bandwidth (RAB) of 87.35%. Moreover, the proposed absorber can maintain high absorptivity above 80% for incident angles up to 40 degrees under both transverse electric (TE) and transverse magnetic (TM) polarizations. The ultrabroadband absorption originates from the combination of dipolar and LC resonances. The simple design method can also be applied to some other structures such as square metallic patch resonator. Furthermore, by adjusting the dimensions of proposed absorber, a broadband microwave MA can be easily obtained. Therefore, it can become an effective design method to achieve broadband absorption in various frequency ranges from microwave to optical frequencies.
NSTL
Elsevier