查看更多>>摘要：? 2021 Elsevier B.V.In this paper, a novel scheme which utilizes cyclic delay combined with peak shrinking and interpolating (PSI), named as CD-PSI, is proposed to reduce the PAPR of multi-intermediate- frequency-over-fiber (Multi-IFoF) signals in mobile fronthaul. The key idea of the proposed scheme is to generate multi-IF OFDM signals through once combination of cyclic delay search so as to reduce the probability of the peak values. Then the PAPR is further reduced by peak shrinking and interpolating. The combination strategy can greatly reduce the sideband information overhead compared to PSI by reducing the recorded peak information. The simulation results demonstrate that the proposed scheme outperforms the previously proposed schemes in terms of both PAPR reduction performance and computational complexity.
查看更多>>摘要：? 2021 Elsevier B.V.Generally, only binary modulation formats, such as on–off keying (OOK) and pulse position modulation (PPM), can be detected by a single photon avalanche diode (SPAD). In this paper, we propose to use a single SPAD receiver to achieve underwater wireless optical communication (UWOC) with high-order modulation formats. The feasibility is theoretically analyzed, and firstly demonstrated in a proof-of-concept orthogonal frequency division multiplexing (OFDM) experiment. The proposed OFDM system using quadrature phase shift keying (QPSK) and 16 quadrature amplitude modulation (16-QAM) can work with less than 68 and 390 photons per symbol, and with receiver sensitivities below the forward error correction (FEC) limit as high as ?84.5 dBm and ?77.8 dBm, respectively. These extraordinarily sensitive results indicate that the proposed scheme is a promising solution to long-distance UWOC systems or internet of underwater things (IoUT).
查看更多>>摘要：? 2021 Elsevier B.V.A selectively ethanol-filled twin-core photonic crystal fiber (TC-PCF) sensing scheme is proposed and theoretically investigated for highly sensitive temperature sensing. Different filling cases and fiber structure were simulated to investigate the mode coupling of two solid fiber cores of the TC-PCF. Our calculation for the sensing performance shows that with properly designed fiber structure and filling configuration, a high temperature sensitivity of ?16.7 nm/°C could be achieved in a range from 20 to 50 °C.
查看更多>>摘要：? 2021 Elsevier B.V.Surface-enhanced Raman scattering (SERS) is commonly associated with metal nanostructures. However, recently large Raman enhancement has also been reported for two-dimensional (2D) semiconductors. In the present work, we exam systematically the influence of thin 2D molybdenum disulfide (MoS2) films on the SERS properties of a typical silver (Ag) nanocavity formed by a single Ag nanoparticle (NP) and a flat Ag surface. The dependence of the localized electric field, Raman enhancement and spatial resolution on the NP-MoS2 film distance, the thickness of MoS2 film, as well as the NP radius were quantitatively calculated and analyzed. The results show that the presence of MoS2 film could efficiently enhance the plasmonic coupling between the NP and the substrate, leading to more significant SERS enhancement. Thinner MoS2 film and shorter NP-film distance will further improve the localized plasmonic enhancement. The calculated Raman enhancement factor reaches 9.1 ×1010 with monolayer MoS2 film under the 660-nm excitation. We also show that the spatial resolution for Raman spectroscopy could be significantly improved by reducing the NP radius, and the maximum resolutions is 4.5 nm at the radius size of 20 nm. Our results offer a theoretical guidance for better understanding the SERS properties of the novel 2D material and its further applications.
查看更多>>摘要：? 2021 Elsevier B.V.Optical dynamic range compression (ODRC) is an analog operation that enhances the signal to noise ratio by nonlinear reshaping of the signal. This operation can be implemented using nonlinear optics in electrically biased nanoscale silicon waveguides with saturated Raman amplification. Here we describe how the optical dynamic range compression can improve coherent optical orthogonal frequency division multiplexing (OFDM) communication. This occurs because the optical compression reduces the high peak-to-average-power ratio leading to a reduction of the phase noise arising from fiber nonlinearity. It also reduces the impact of the quantization noise of the analog to digital converter. We further describe how the optical energy which is removed from the signal in the compression process can be harvested and recycled as useful electrical energy via an intriguing optical physics phenomenon known as the two-photon photovoltaic effect.
查看更多>>摘要：? 2021 Elsevier B.V.The structure of disordered gain media for a two-dimensional random laser was designed to obtain the specified spectrum of the emitted light. A scattering medium with a checkerboard structure was used to produce random lasing. Structural optimization was performed using a direct binary search method. The simulation results show that the emitted light can be concentrated at a single wavelength or within a specific wavelength range. The effect of the pump rate and manufacturing errors on the optimized spectrum of the laser emission was also examined. In this paper, we propose a new design scheme for achieving laser emission with any specified spectral shape by considering a multiple scattering medium as deterministic cavities.
查看更多>>摘要：? 2021 Elsevier B.V.The single photon lidar (SPL) system can collect data within a wide range, which is widely used in multispectral depth imaging and underwater depth imaging. However, SPL systems are subject to solar noise and dark count returns, resulting in poor quality of image construction. Therefore, a high-efficiency 3D imaging technology is needed to improve the performance of the SPL system. To address this problem, this paper develops a new short-range single-photon three-dimensional imaging method with both high quality and low complexity. In this paper, we use a 400 × 100 SPAD detector and an 850 nm laser source composed of VCSEL array to reconstruct the 3D image of a target within 5 m. By applying principal component analysis (PCA), the complexity of the proposed method can be reduced by 20 times compared with existing algorithms without compromising imaging quality, making the proposed method a promising candidate to be applied to a wide range of fields such as consumer electronics.
查看更多>>摘要：? 2021 Elsevier B.V.Bessel light beam formation from annular fields is important for many applications related to laser remote sensing and also for the free-space laser communication. Annular beams have a few advantages, such as optimal combination with optical projectors based on mirror telescopes, self-healing property after passing around obstacles including stability in the atmospheric turbulence in some cases, and also large focal range. In the paper a process of annular beam transformation into a Bessel-type field by the free-space propagation diffraction is investigated. The transverse Bessel beam profile at large propagation distances is calculated. It is shown that the profile contains small number of rings, the on-axis maximum and the first ring contain more than 90% of light power.
查看更多>>摘要：? 2021 Elsevier B.V.We demonstrate a multi-curved liquid lens with movable liquid–liquid(L–L)? interface, which has a wide range of focal length. Due to movable L–L interface, the proposed lens can also be used as a single lens to achieve zoom function. The movement of the L–L interface can be achieved by controlling the flow of the liquid, which in turn changes the object distance and image distance to achieve the desired magnification. We fabricated a prototype based on this concept. The experimental result shows that when the movement distance of the L–L interface is ～10 mm, the shortest positive and negative focal lengths of the lens are 9.3 mm and ?8 mm, respectively, and the zoom ratio is 1.3×. The proposed multi-curved variable-focus lens simplifies the zoom imaging system, and its widespread applications in imaging systems are predictable.
查看更多>>摘要：? 2021 Elsevier B.V.Spiral shaped transparent electrode is used to replace the weakly conductive layer traditionally used to build an electrically tunable liquid crystal lens (TLCL) of modal control geometry. This replacement simplifies the manufacturing process and the operation control of the TLCL thanks to its lower temperature dependence. We present a theoretical analysis of its optical performance and propose an optimized structure that helps to decrease wavefront distortions. TLCLs with 2.2 mm clear apertures were fabricated and characterized in view of their use in miniature cameras. These lenses demonstrate relatively low RMS spherical aberrations (less than 0.25 um) for optical powers ranging from 0 D to 10 D.
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