查看更多>>摘要：? 2021 Elsevier B.V.In this paper, a novel fully-connected deep neural network (FC-DNN)-based receiver is proposed and experimentally demonstrated in an underwater wireless optical communication (UWOC) system, with the objective of direct decoding orthogonal frequency division multiplexing (OFDM) signals via deep learning methods. Unlike existing schemes, which treat deep neural networks as a single equalizer or a symbol classifier that requires pre-processed labels, the receiver we proposed can achieve direct output from received OFDM signals to the desired bit stream without any redundant pre-processing and post-processing. The feasibility of the proposed receiver has been validated by changing the transmission data rate and the received optical power of experimental setups. Furthermore, our experiments indicate that the proposed direct decoding FC-DNN-based receiver can perform as well as the typical conventional bulky multi-structure receiver without using pilots and cyclic prefixes (CPs) in OFDM-UWOC systems.
查看更多>>摘要：? 2021 Elsevier B.V.We report a microwave photonic link to transmit dual-chirp microwave signals over fiber based on stimulated Brillouin scattering (SBS) using a dual-polarization Mach–Zehnder modulator (DPol-MZM). The DPol-MZM is driven by a radio frequency (RF) carrier and a baseband single-chirp waveform to realize carrier-suppressed single-sideband (CS-SSB) modulation based on SBS. The dual-chirp waveforms centered at 12.4 GHz and 15.5 GHz with bandwidths of 0.4 GHz have been successfully generated. In our link, the CS-SSB modulation based on SBS is performed to overcome fiber chromatic-dispersion-induced power fading (CDIP). The proposed scheme is theoretically analyzed and experimentally verified.
查看更多>>摘要：? 2021 Elsevier B.V.A new scheme for two-dimensional (2D) electromagnetically induced grating is proposed based on spontaneously generated coherence in a Λ-type atomic system, in which the atom interacts with two orthogonal standing-wave fields and an incoherent pumping field. Due to spatial periodical modulation, the zero absorption or even gain accompanied with large dispersion is obtained in atomic medium, and electromagnetically induced grating is formed. Thus the weak probe field could be diffracted into first-order or high-order directions with high efficiency. The diffraction feature and efficiency of the gratings could be adjusted by the probe field detuning, the interaction length, the incoherent pumping field intensity, and the relative phase as well. The scheme of 2D grating we presented may have some potential application in beam splitting and all-optical switching.
查看更多>>摘要：? 2021 Elsevier B.V.Stimulated Raman scattering (SRS) in barium nitrate powder was studied in two temporal modes: excitation by laser pulses of 11 ns and 30 ps from the room temperature to the temperature of liquid nitrogen. For the first time, an increase in the efficiency of SRS conversion by two orders of magnitude was observed when the sample temperature was lowered from room temperature to the temperature of liquid nitrogen. The maximum achieved value of the conversion efficiency was 30 percent. Temporal characteristics of all the SRS components registered in transient mode were measured. The detected effect of the pump energy influence on the delay time of the first Stokes component can be used for measuring the nonlinear parameters of the medium.
查看更多>>摘要：? 2021 Elsevier B.V.In this study, a composite Surface-Enhanced Raman Scattering (SERS) substrate, consisting of an Au 2D sinusoidal grating and Ag colloidal nanoparticles (AgNPs-Au2DSG), was designed. The impact of the coupling of surface plasmon polariton (SPP) and localized surface plasmon (LSP) on electric field enhancement of the composite substrate, insensitive to the polarization of incident light, is thoroughly discussed and studied. FDTD simulations showed that the maximum enhancement factor (EF) of the substrate appeared at the LSP–SPP coupling resonance frequencies. For the incident light of TM at 785 nm, the substrate had a larger hot spot area and EF when the AgNP radius was 50～60 nm. By adapting the size of the AgNPs, the SPP–LSP coupling can be optimized to offer greater electric enhancement at the wavelengths of interest, contributing to the practical improvement of SERS detection.
查看更多>>摘要：? 2021 Elsevier B.V.Optical patterns with the orbital angular momentum (OAM) can serve as “optical spanners”, and have attracted much attention for their applications in biophysics, micromechanics or microfluidics. In this work, we theoretically demonstrate the propagation characteristics of elliptic vortex beams carrying the OAM in nonlocal nonlinear media. The elliptic vortex beams cannot evolve as solitons due to their non-circular-symmetry, but can evolve as breathers in the nonlinear media without anisotropy. During breathing evolutions, the rotating phenomenon is observed. The rotations take place in both the vortex cores and the beam envelopes, which exhibit opposite rotating directions. The breathing and rotating evolutions both have close relations with the input power and the beam size. The analytical results are well confirmed by the numerical simulations. The results obtained in the paper might find potential applications in beam shaping, and in optical manipulations of rod-shaped micro-particles.
查看更多>>摘要：? 2021In recent years, it has been a new topic to fabricate fiber optical tweezers probes by using photopolymers. However, this method requires coupling the laser into the fiber through a spatial optical path with strict laser and experimental environment requirements. It significantly limits the application scope of optical tweezers. This paper proposes and demonstrates an improved photopolymerization method to fabricate fiber optical tweezers based on an all-fiber structure, improve the fabrication process, and solve the above problems. We configure the all-fiber structure by coaxial welding a single-mode fiber and a multimode fiber to excite a Bessel-like beam. The power density distribution of the Bessel-like beam is advantageous for forming the photopolymerization tip that makes the output beam highly convergent. The fabrication method of the proposed fiber optical tweezers is economical and straightforward. Moreover, it provides a new way of improving the photopolymerization process.
查看更多>>摘要：? 2021 Elsevier B.V.Flow cytometry is an essential technique in biomedical discovery for cell counting, cell sorting, and biomarker detection. In vivo flow cytometers based on one-photon or two-photon excited fluorescence have been developed for over a decade. One drawback of the laser beam scanning two-photon flow cytometer is that the two-photon excitation volume is limited to the focal spot due to the short Rayleigh range of focused Gaussian beams. Hence, the sampling volume is much smaller than in one-photon flow cytometers, making it challenging to count or detect rare circulating cells in vivo. Non-diffracting light waves like Bessel beams and Airy beams have narrow intensity profiles with an effective spot size as small as several wavelengths, making them comparable to Gaussian beams. More significantly, the theoretical depth of field (propagation distance without diffraction) can be infinite, making them an ideal solution as a light source for scanning beam flow cytometry. The trade-off of using Airy beams rather than Gaussian beams is side lobes Airy beams have, which contribute to background noise. Two-photon excitation can reduce this noise, as the excitation efficiency is proportional to intensity squared. Therefore, we developed a two-photon flow cytometer using 2D Airy beams to form a light-sheet that intersects the blood vessel a microfluidic channel, which was used to model a blood vessel. The setup can successfully detect and count flowing fluorescent microspheres in a microchannel.
查看更多>>摘要：? 2021 Elsevier B.V.We propose an indium antimonide (InSb) THz plasmonic metasurface featuring a surface lattice resonance (SLR) with the quality factor of Q ～ 375 and theoretically investigate the plasmonic response of the ensemble of InSb metamolecules. The localized surface plasmon resonances (LSPRs) of individual metamolecules couple together through Rayleigh Anomaly (RA) diffraction, leading to the appearance of the SLR at 0.75THz, suitable for biophotonic applications. The major advantage of the proposed metasurface over metal-based ones in the THz regime is the tunability of SLR frequency by changing the carrier concentration. Apart from SLR, it is possible to achieve additional resonances by tuning the superstrate thickness exploiting Fabry–Perot (FP)-like cavity effects. Achieving a narrow-linewidth SLR and the resultant remarkable local-field enhancement in the InSb-based metasurface opens the door to a wide range of applications. Conducive to optical manipulation and sensing, the capability of the proposed metasurface is examined in plasmonic tweezing and medium-perturbation sensing fields.
查看更多>>摘要：? 2021 Elsevier B.V.Although it appears that there have been made enough researches as regards the estimation the optical feedback factor C of a self-mixing interferometer, any method of the C estimation cannot ensure its reliability if the operation of the self-mixing displacement sensor covers the strong feedback regime with fringe loss due to large displacement of a target. That is the main reason why the precision of the sensor degenerates for the measurement of large displacement. In the paper we propose a novel C estimation method called region tracking for the C value, which is indispensable for the measurement of long-range displacement, and demonstrate its fidelity experimentally. The experimental results in the paper were given when the target distance was about 60 cm and the target displacement 25 mm. The displacement reconstruction based on the proposed region tracking method ensures a better precision than a half wavelength, depending on the method of displacement retrieval. The approach presented will make a great contribution to further improvement of the accuracy of the self-mixing displacement sensor for large displacement.
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