查看更多>>摘要：? 2021An intensity-modulated side mode-based fiber Bragg grating (FBG) refractive index (RI) sensor is demonstrated. The feasibility is based on Fresnel reflection from the end-face of fiber-tip. Theoretically, the forming of asymmetrical irregular side mode fringes is firstly explained and simulated, the feasibility of this sensor for RI measurement is proved. As a result, the proposed intensity-modulated side mode-based FBG RI sensor exhibits a high sensitivity of ?590.625 dB/RIU at the RI of 1.466, which is the highest RI sensitivity among the previous reported fiber-tip based RI sensors. The intensity fluctuations of this sensor are less than 0.824 dB. Moreover, remote RI monitoring using this sensor is studied. As a result, maximum RI sensitivity of the remote sensing system is ?134.174 dB/RIU at the RI of 1.466, with the intensity fluctuations less than 0.201 dB.
查看更多>>摘要：? 2021 Elsevier B.V.The design and experimental demonstration of a large, ultrahigh-quality (Q) factor, and single-mode operation silicon ring resonator based on a strip silicon photonic waveguide are presented. The ring resonator comprises multimode straight strip waveguides (MMWG), single-mode strip waveguides (SMWG), 90° Bezier bends, and linear tapers. The SMWG are used in the coupling region to achieve mode coupling for the fundamental mode without exciting the higher-order mode. The MMWG, 1.6 μm in width, are used to decrease the propagation loss. The taper smoothens the straight-to-bend transition and reduces the intermode crosstalk at the interface between the straight and bent waveguides. The design of the 90° Bezier bend ensures that only the maintained fundamental mode propagates at the bend with ultralow loss and low intermode interference. The resonator fabricated in this study had an ultrahigh-Q factor of 1.54 × 106, with a free spectrum range of 0.036 nm, and achieved an actual loss of unit transmission of approximately 0.25 dB/cm.
查看更多>>摘要：? 2021 Elsevier B.V.One of the most exotic structure in liquid crystal is the cholesteric structure. Due to their periodic helical structure, cholesteric liquid crystals (CLCs) promising candidates for a myriad of different photonic applications. CLCs can exhibit bubbles domain, defects arranged in two dimensions, which have been used as an array of micro-lenses with electrically tunable focal length. The focal length of the micro-lenses is limited in micro-meter scale, moreover each micro-lens has it is own focal point. The purpose of this work is how to construct a system of lenses with just one electrically tunable focal length. The system is formed by the bubble domain (micro-lens) accompanied by additional droplet of glycerol (big-lens). Hence, by tuning the focal length of the micro-lenses, the focal length of the system can be varied. This work covers the methodology that was used for making the bubbles domain based on CLCs
查看更多>>摘要：? 2021 Elsevier B.V.In this paper, we study the ergodic capacity (EC) and average bit error rate (BER) of spatial diversity underwater wireless optical communications (UWOC) over the generalized gamma (GG) fading channels using quadrature amplitude modulation (QAM) direct current-biased optical orthogonal frequency division multiplexing (DCO-OFDM). We derive closed-form expressions of the EC and BER for the spatial diversity UWOC with the equal gain combining (EGC) at receivers based on the approximation of the sum of independent identical distributed (i.i.d) GG random variables (RVs). Numerical results of EC and BER for QAM DCO-OFDM spatial diversity systems over GG fading channels are presented. The numerical results are shown to be closely matched by the Monte Carlo simulations, verifying the analysis. The study clearly shows the adverse effect of turbulence on the EC & BER and advantage of EGC to overcome the turbulence effect.
查看更多>>摘要：? 2021 Elsevier B.V.In this paper a performance analysis is presented for an asymmetric two-hop amplify-and-forward (AF) relaying system with multiple relays in an underlay cognitive radio network (CRN), where a partial relay selection (PRS) is used to select the best relay for the signal transmission. In the considered system, the two hops employed are hybrid and asymmetric, which include the source-to-relay (SR) and the relay-to-destination (RD) links. The first hop (SR link) is a radio frequency (RF) channel and the second hop (RD link) is a free space optical (FSO) channel. The RF and FSO channels follow Rayleigh, and Gamma–Gamma distribution, respectively, where some practical concerns such as atmospheric turbulence and pointing errors are taken into consideration. To start the performance evaluation and analysis, the cumulative-distribution-function (CDF) of the received signal-to-noise-ratio (SNR) at the destination side is mathematically derived. Then, by means of the derived CDF, closed-form expressions are derived for the outage performance, average BER and the average channel capacity of the underlying system. Finally, the numerically evaluated results, validated by computer simulations, provide insights about system performance under various parameters and practical situations.
查看更多>>摘要：? 2021 Elsevier B.V.In this paper, a novel scheme to employ one external polarization division multiplexing Mach–Zehnder Modulator (PDM-MZM) and one polarizer to generate millimeter-wave four-level pulse amplitude modulation (PAM4) signal is proposed without the usage of expensive and power hungry digital-to-analog converter (DAC). Firstly, we theoretically derive the generation conditions for the mm-wave PAM4 signal scheme. Based on this cost-effective scheme, 6 Gbaud PAM4 signal carried by 84 GHz millimeter-wave transmission over 15 km single-mode fiber (SMF-28) with only 0.5 dB power penalty without any CD compensation is experimentally demonstrated. Furthermore, VPI simulation software is employed to numerically simulate the mm-wave PAM4 signal generation and explore the influences of polarization rotation of the polarizer and direct current (DC)-Bias on the performance of generated W-band signals. In a word, it would be a promising scheme for future 5G short-reach intensity modulation and direct detection (IM/DD) system.
查看更多>>摘要：? 2021Feed forward equalizer (FFE) has been widely used in high-speed short-haul optical transmission. When system suffers from serious inter-symbol interference (ISI), i.e. low-cost transceiver induced narrow-bandwidth limitation, FFE with large number of taps is necessary, which significantly increases the computational complexity. In this paper, we proposed a hybrid FFE (H-FFE) algorithm with tap delay adjusted between T space and T/2 space. Enabled by the H-FFE and conventional T/2-spaced FFE algorithms, we demonstrated the intensity modulation and direct detection (IM/DD) transmission of 56, 70, and 80 Gbaud PAM4 signals over standard single mode fiber (SSMF) links up to 2, 1 km and 500 m employing the 18 GHz (3 dB) system bandwidth at C band. The experimental results indicate that the proposed H-FFE algorithm has the similar performance with the conventional T/2-spaced FFE algorithm. In addition, using the proposed H-FFE algorithm, the computational complexity is one third of that of the conventional T/2-spaced FFE algorithm.
查看更多>>摘要：? 2021 Elsevier B.V.Detailed picture of continuous shape evolution of femtosecond laser pulse has been recorded directly in the process of filament formation in fused silica using time-resolved polarization microscopy with ～70 fs temporal resolution. The main stages of the pulse transformation (temporal self-compression and splitting, formation of conical wave) have been studied. The maximum temporal compression has been found to be achieved by the leading subpulse after the time splitting event. It was found that the Bessel zone of conical wave formation is shifted backwards from the pulse front. Sub- and superluminal propagation velocities of the pulse maxima after the time splitting have been measured.
查看更多>>摘要：? 2021 Elsevier B.V.The remote excitation of a localized surface plasmons (LSP) by the propagating plasmonic excitations, i.e., propagating surface plasmons (PSP), receives increasing attention in recent years. Here, we study the local near-field optical response of the gold nanohole, which support LSP, remotely excited by the PSP (PSP-LSP) and the results show that this optical response of the gold nanohole relates to the excitation field, the local near-field optical spectra show a redshift phenomenon, but the intrinsic frequency of LSP remains invariant. We attribute this spectral redshift phenomenon of the PSP-LSP to the dominating effects of the PSP. A weak dependence of the resonant wavelength peak on the size of a nanostructure has been observed. Moreover, it is found that the dephasing time of the PSP-LSP is longer than the case directly excited by the laser field, and it presents a decreased trend with the increasing distance from the PSP launching location. The results in this work can be used to improve the efficiency of remote excitations and provide a new path to prolong the dephasing time of the enhanced resonant field.
查看更多>>摘要：? 2021 Elsevier B.V.A polarization-independent highly-efficient transmission beam splitter based on the cross-shaped ridge structure is proposed in this paper. Based on the rigorous coupled-wave analysis (RCWA) and the simulated annealing (SA) algorithm, the efficiencies of 32.33% of 0th order and 32.42% of ±1st order for transverse electric (TE) polarization and 32.53% of 0th order and 32.60% of ±1st order for transverse magnetic (TM) polarization can be achieved. In addition, the simplified modal method (SMM) is applied to analysis the internal energy changes process in detail. What is more, different from the reported references only considering the case of one layer, we also used the SMM to analyze the three-port cross-array beam splitter under normal incidence with the approximation treatment. The results show that the method of which has certain reference value, which can be the guidance of the design of the gratings with similar structures in the future.
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