查看更多>>摘要：Since its first experimental demonstration,"ghost imaging"has attracted much attention,perhaps not only because of its interesting physics but also because of its attractive application.This review article discusses the physics and application of ghost imaging:[1]emphasizes the nonlocal two-photon interference nature of ghost imaging,including detailed analysis and calculations;[2]introduces three types of applications with unique advantages of ghost imaging,including a light detection and ranging device with imaging ability,namely,an Imaging Lidar or ILidar system;a turbulence-resistant,or turbulence-free,imaging technology;and a vibration-resistant X-ray microscope of high resolving capability.This article is prepared for a Special Issue of Chinese Optics Letters,intended for general audiences,especially young researchers and students who are interested in ghost imaging.
Petr SkakunenkoDarya BykovaAnton AfanasievAlexey Kalmykov...
103-108页
查看更多>>摘要：We developed a new single-layer atom chip with an additional U-shaped current-carrying structure.The new U-shaped microwire creates optimized magnetic field distribution,which increases the trapping volume of a magneto-optical trap[MOT]near the chip.Our approach allows one to localize more atoms,while a setup remains relatively simple[single-layer approach]and consumes low current[up to 10 A].The total number of trapped 87Rb atoms in our setup is 5 × 107.
查看更多>>摘要：In this study,we propose a superposed probabilistically shaped[PS]quadrature amplitude modulation[QAM]constellation scheme for multiple-input multiple-output visible light communication systems.PS QAM signals are generated from a non-linear coding equation that converts uniformly distributed 8-level signals into PS 9-or 10-level signals,which are then mapped into PS 9QAM or 10QAM signals.Square-shaped 9QAM and trapezoid-shaped 10QAM constellations are introduced to maximize the minimum Euclidean distance[MED]of the superposed constellation.Finally,the PS 9QAM and 10QAM signals are superposed with the 4QAM signals in a flipped manner to obtain PS 36QAM or 40QAM signals at the receiver,respectively.To exploit the temporal correlation of the resulting signal from nonlinear coding,we developed a detection algorithm based on Viterbi decoding.Experimental results confirmed the superiority of the proposed schemes by achieving a higher MED and stronger ability to resist nonlinearity.Compared with the traditional scheme,the peak-to-peak voltage dynamic ranges of the superposed 36QAM and 40QAM constellation schemes were improved by 52％and 48％,respectively.
查看更多>>摘要：This Letter proposes a post-equalizer for underwater visible light communication[UVLC]systems that combines channel estimation and joint time-frequency analysis,named channel-estimation-based bandpass variable-order time-frequency network[CBV-TFNet].By utilizing a bandpass variable-order loss function with communication prior knowledge,CBV-TFNet enhances communication performance and training stability.It enables lightweight implementation and faster con-vergence through a channel estimation-based mask.The superior performance of the proposed equalization method over Volterra and deep neural network[DNN]-based methods has been studied experimentally.Using bit-power loading discrete multitone[DMT]modulation,the proposed method achieves a transmission bitrate of 4.956 Gbps through a 1.2 m underwater channel utilizing only 38.15％of real multiplication calculations compared to the DNN equalizer and achieving a bitrate gain of 440 Mbps and a significantly larger dynamic range over the LMS-Volterra equalizer.Results highlight CBV-TFNet's potential for future post-equalization in UVLC systems.
查看更多>>摘要：We characterize the current crowding effect for microwave radiation on a chip surface based on a quantum wide-field microscope combining a wide-field reconstruction technique.A swept microwave signal with the power of 0-30 dBm is supplied to a dumbbell-shaped microstrip antenna,and the significant differences in microwave magnetic-field amplitudes attributed to the current crowding effect are experimentally observed in a 2.20 mm × 1.22 mm imaging area.The normalized microwave magnetic-field amplitude along the horizontal geometrical center of the image area further demonstrates the feasibility of the characterization of the current crowding effect.The experiments indicate the proposal can be qualified for the characterization of the anomalous area of the radio-frequency chip surface.
查看更多>>摘要：In three-dimensional imaging employing phase-shifting profilometry[PSP],the nonlinear response of projector and camera makes the fringe gray distribution non-sinusoidal,which further leads to phase error.Although the double 3-step phase-shifting method is simple and effective,it needs to add an additional set of fringe sequences,which reduces the measure-ment efficiency.To this end,this paper introduces a generic and flexible self-correction method for nonlinearity-induced phase error.First,according to the nonlinearity-induced phase error model,we introduce an additional wrapped phase with a phase difference of π/3.The error waveform of the two wrapped phases is opposite but not coincident.Then,we introduce an estimation algorithm for the additional wrapped phase offset.Finally,we fuse the two wrapped phases to correct the phase error.Experiments confirm that the root mean squared error of the proposed method is 64.1％lower than that of the traditional method and 13.3％lower than that of the Hilbert transform method.The proposed method does not require any additional fringes or hardware assistance and can be easily extended to 4-step or 5-step PSP.
查看更多>>摘要：The development of hybrid optics/microwave communication systems puts forward a new requirement for beam splitters to efficiently transmit microwave signals and simultaneously reflect optical signals.Owing to mechanical constraints,the physical thickness of beam splitters is of the order of tens of millimeters.The corresponding electrical thickness has the same order of magnitude as microwave wavelengths,and the resulting multi-beam interference effect significantly reduces the microwave transmittance,impacting the beam splitting quality.This study presents a new optics/microwave beam splitter based on the ability of the frequency selective surface[FSS]to shape the resonant curve.A beam splitter sample,whose physical thickness and substrate material are 20 mm and quartz glass,respectively,is designed,simulated,fab-ricated,and characterized to validate the feasibility of this strategy.The measured results show that the minimum micro-wave transmittance between 35 and 36.5 GHz with an incidence angle of 45° under TE polarization is 86.43％,and the mean value of the reflectance spectra from 450 to 900 nm and that from 7.7 to 10.5 μm both exceed 96％.This FSS-based optics/microwave beam splitter is expected to play a key role in hybrid optics/microwave communication systems.
查看更多>>摘要：The Raman random fiber laser[RRFL]is a typical complex physical system due to the intrinsic random feedback of the fiber,which causes complexity in the RRFL output.So far,the time-domain statistical attributes of the RRFL are still not fully characterized.In this paper,the temporal statistical properties of the RRFL are investigated comprehensively for the first time under the full bandwidth condition.First,the time-domain intensity statistical characteristics of the RRFL under the full bandwidth condition are theoretically studied:the results demonstrate that the intensity probability density function of the RRFL is related to the pump power and observing position and deviates inward from the exponential distribution,indicating that correlation exists between the different frequency components in the spectrum.Afterward,in the validation experi-ment,an elaborate structure is designed to realize a narrow-bandwidth 1053 nm RRFL,and its full bandwidth temporal intensity statistical features manifest an identical variation pattern to the simulation results.This work fills a vacancy in the study of RRFL temporal statistical features and rigorously reveals the different physical mechanisms between RRFL and amplified spontaneous emission light sources,providing instructions for the application of the RRFL.
查看更多>>摘要：We demonstrate an intracavity self-synchronized multi-color Q-switched fiber laser using a parallel-integrated fiber Bragg grating[PI-FBG],fabricated by a femtosecond laser with a point-by-point parallel inscription method.The multi-color Q-switched pulses can be always self-synchronized when the group delay differences between neighboring spectra range from-3.4 to 3.4 ps.The starting and evolution dynamics indicate that the saturable absorption effect of the carbon nano-tube plays a dual role:synchronously triggering the startup of the pulse at successive colors by active Q-switching and spontaneously compensating to some extent the temporal walk-off of the multi-color pulses through the cross saturable absorption modulation.This work unveils the intracavity self-synchronization mechanism of the multi-color Q-switched pulses and also demonstrates the potential of PI-FBGs for the customizable generation of the synchronized multi-color pulse in a single cavity.
查看更多>>摘要：We have experimentally presented a watt-level noise-like[NL]pulse mode-locked all-polarization-maintaining[PM]fiber laser centered at～1995 nm,which can directly generate stable NL pulses with a maximum output power of～1.017 W and pulse energy of～0.61 μJ,representing the highest output power of mode-locked NL pulse at 2 μm from any fiber oscillators,to the best of our knowledge.The mode-locked NL pulse laser exhibits an excellent stability with a power fluctuation of～0.1％in 8 h of monitoring,and a signal-to-noise ratio of～83 dB at a fundamental frequency of～1.662 MHz.Moreover,the pulse envelope and coherence spike width of the NL pulse can be widely tuned from～4.5 ns to～16 ns,and～364 fs to～323 fs,respectively,with the enhancement of the pump power.Such an all-PM fiber oscillator is the ideal seed source for the implementation of a high-power NL pulse laser and has potential valuable applications in mid-infrared spectroscopy and industrial processing.
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