查看更多>>摘要：? 2021 Elsevier B.V.Output spectral and spatial parameters of the two-channel fiber laser operating in an intracavity spectral beam combining mode were investigated. We showed the possibility of the occurrence of a collective lasing mode when various channels participate in the creation of common radiation. We measured spectral and spatial parameters of collective lasing. Conditions of its occurrence and possibility of negative influence on the result of intracavity spectral beam combining were studied both theoretically and experimentally.
查看更多>>摘要：? 2021 Elsevier B.V.We report an Aubrey–André–Harper (AAH) model based quasi-periodic lossless evanescently coupled waveguide lattice to study the unconventional physics of light localization. We find an exclusively new way to establish localization phase transition in a 1D-AAH lattice by observing the slope variation of the band and accordingly study the modal characteristics to reveal the fact that a higher value of quasi-periodic modulation strength is imperative for observing a signature of fully localized light states having higher eigenenergy. This analytical concept has numerically been implemented in the proposed quasi-periodic lattice to achieve light localization, where we have shown that the supported states not only depend on localization phase transition parameter, but also on the specific location of excitation which is due to quasi-periodic nature of lattice. Furthermore, we have investigated a unique effect of the presence of disorder on light localization phenomenon, where it has been reported that the presence of off-diagonal disorder, which is otherwise detrimental, favours light localization in the proposed structure due to its quasi-periodic nature. The findings indeed have the potential to open up a fertile platform to manipulate light in passive photonic devices.
查看更多>>摘要：? 2021 Elsevier B.V.There is a minimum measurable thickness when gauging micron-level coating films with time-of-flight based terahertz thickness measurement technologies. To improve this minimum measurable limit, a time-domain model of terahertz signals is derived by taking multiple reflected pulses into consideration, using the Differential Evolution algorithm to calibrate parameters automatically. Empirical Mode Decomposition is applied to remove noises and non-flat baselines introduced by water vapor and other structure-borne noise resources. Combining methods mentioned above, thicknesses of micron-level coating layers could be measured even if the Signal to Noise Ratio is not large enough, and the minimum measurable limit is also improved to about 40μm. Experiments also demonstrate that, during drying processes, the time gaps between pulses reflected from different interfaces shrink and the thicknesses change in an exponential trend, which reveals the potential of terahertz technologies in terms of monitoring coating processes.
查看更多>>摘要：? 2021 Elsevier B.V.In this study, a novel liquid crystal (LC) stackable optical phased array (SOPA) for fine tracking with fast and precise beam steering is proposed. The SOPA was composed of multiple layers of sliced phase shifters (SPSs). Each SPS had two parts for loading different voltages to form programmable phase steps. All LC materials were compatible with this architecture, including blue-phase, polymer network, and ferroelectric LCs. To verify and evaluate the properties of this SOPA scheme, a three-layered SOPA prototype was designed and fabricated according to the standard processing of LC devices. A couple of steering experiments, with steering angles ranging from ?100 to 100 μrad in a step of 10 μrad, were performed. The steering efficiency recorded in all the experiments exceeded 90%. All the measured switching time of angle-to-angle were less than 10 ms. Herein, the fastest switching time reported was only 1.52 ms.
查看更多>>摘要：? 2021 Elsevier B.V.In this paper, a kind of light-emitting tunnel junction composed of three layers of metal–insulator–semiconductor (MIS, Au-Al2O3-Si) is introduced and this structure mainly relies on electron tunneling to excite SPPs. The finite element method is used to calculate the electric field distribution and extinction spectrum of the silver nanorod (AgNR) placed on the metal–insulator–semiconductor tunnel junction (MIS-AgNR). In the simulation, the influence of tunneling current on the strength of SPPs propagating along the metal surface is quantitatively studied by changing the thickness of the insulator layer of the MIS tunnel junction and the bias voltage across it. The AgNR is placed on the tunnel junction, and the gap distance between the gold surface of the MIS structure and the AgNR is changed to qualitatively study the relationship between the extinction characteristics of the AgNR and the gap distance. The study found that the amount of the tunneling current is proportional to the intensity of the SPPs on the metal surface. In the range of 4 nm–20 nm gap distance, the adjustment of the resonance peak position in the 500 nm–700 nm range of the extinction spectrum is realized. As the gap distance increases, the position of the resonant peak in the AgNR extinction spectrum will undergo blue-shift, and at the same time the intensity of the extinction peak will decrease. It is worth noting that the change of the blue-shift displacement is exponentially related to the change of the gap distance.
查看更多>>摘要：? 2021 Elsevier B.V.The sampling frequency offset (SFO) will induce phase rotation, inter-symbol interference and inter-carrier interference, which may seriously deteriorate the receiver's performance in asynchronous optical discrete multi-tone (DMT) systems. In this work, a high-accuracy training symbols-based SFO estimation (SFOE) method is studied in both the additive white Gaussian noise channel and short-reach directly-modulated laser-based DMT transmission system. Two general SFO compensation (SFOC) methods, i.e., digital interpolation (DI) and phase compensation (PC), are comparatively investigated through both simulations and experiments. The results indicate that the accuracy of the SFOE method can be within ±0.2 ppm under the SFO range of ±500 ppm and different received optical powers. With the high-accuracy SFOE method, the 4th order DI has the best compensation performance compared to PC, 2nd and 3rd order DI methods. Moreover, the negligible EVM performance penalty compared with the case of SFO-free can also be achieved in an optical DMT transmission system. Besides, the PC-based SFOC method has similar EVM performance to the 4th order DI in the case of small SFO within ±300 ppm.
查看更多>>摘要：? 2021 Elsevier B.V.In order to improve the scanning efficiency of lidar, a set of high precision scanning system with 40 × 40 sub-beams is established, in which a liquid crystal optical phased array is used to load the phase calculated by Gerchberg–Saxton(GS) algorithm to split the incident plane wave. Technically, the imperfections of GS algorithm often lead to problems such as sidelobes and uneven intensity of output spots, which eventually result in poor pointing accuracy of sub-beams and low scanning resolution of lidar. To solve those problems, we propose an optical method rather than the classical algorithm optimization methods, which restricts the preset sub-beams to the diffraction limit to unify the actual sub-spot morphology and ensure high pointing accuracy. In this way, the standard deviation of pointing error eventually decreases from 42 μrad to 16 μrad. In the aspect of eliminating zero-order leakage, the zero-order light leakage cannot be separated by the conventional tilt method in the case that the spatial spectrum span of the generated beam array exceeds the cut-off frequency. Concerning this issue, a novel zero-order leakage elimination method is proposed to avoid zero-order leakage by splicing two half beam array of adjacent diffraction orders on both sides of the cut-off frequency. Meanwhile, the intensity correction is carried out to achieve leakage avoidance without damaging the intensity uniformity of the beam array.
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