Abstract This paper investigates the modulation characteristics and drive circuit design of LiNbO 3 phase modulators in coherent free-space optical communication systems. The goal is to enhance modulation accuracy and reduce bit error rate (BER). A high-gain broadband drive circuit based on the OP27G chip is proposed to improve system stability. Theoretical and experimental results show that the optimal performance occurs when the drive voltage is slightly below the half-wave voltage, minimizing signal distortion and BER. This approach outperforms traditional methods, improving both signal quality and system stability. Future research may explore advanced modulation formats and machine learning techniques.

Light propagation is inherently vectorial. However, conventional adaptive optics compensates only the wavefront and thus falls short of the practical requirements of vectorial optical communication. To address this limitation, we propose a polarization-order-multiplexed vectorial vortex light communication technique tailored for propagation through turbulent atmospheric conditions. A convolutional neural network (CNN) fuses intensity and phase information to enable high-precision estimation of the polarization state, and this estimator is integrated with a genetic-algorithm-based vectorial adaptive optics module for polarization correction. Results indicate that atmospheric turbulence causes inter-channel crosstalk in polarization-multiplexed links. After correction, the polarization coherence improves from 0.1308 to 0.9831. In image-transmission tests employing coherent demultiplexing, the peak signal-to-noise ratio increases from 16.28 dB to 34.26 dB. These results confirm the feasibility of polarization-order-multiplexed transmission and demonstrate the effectiveness of the proposed algorithm, providing a sound basis for its prospective practical application.

This article proposes a laser-based, non-contact vibration signal transmission system for rotating machinery. The non-coaxial misalignment between the optical and rotational axes is modeled, revealing that the detector's effective photosensitive area and received optical power vary periodically with shaft speed, thereby introducing harmonics at integer multiples of the rotational frequency. Beam non-uniformity and wavefront aberration manifest as additive noise, which jointly degrade the signal quality. A compact joint time-frequency analysis is achieved by projecting the signal with the fractional Fourier transform. Experiments show that the optimal transform order p=0.92 is identified using the criterion that maximizes the rate of spectral entropy change, demonstrating high-resolution feature extraction. A C#-MATLAB mixed-programming platform completes the system by providing data storage, offline processing, and web-based publishing.

In underwater blue-green optical communication, blue-green light is attenuated by seawater absorption, scattering, and ocean turbulence, and the scattering coefficients of blue-green light are different for different depths of seawater; that is, the transmission attenuation of blue-green light varies in different depths of seawater. We calculated the scattering coefficients and albedo of seawater at different depths and analyzed the received optical power of the detector and bit error rate when the blue-green light was line-of-sight (LOS) transmitted in different ways (horizontal, vertical, and oblique range) with the same distance in the same seawater depth range under the effect of absorption and scattering of seawater. Numerical results show that under the effect of absorption and scattering of seawater, the transmission attenuation of blue-green light is large in seawater with large albedo. Therefore, blue-green light in different depths of seawater in the same way LOS transmission of the same distance, the power of light required in the albedo of large seawater is large.

Integrated sensing and communication (ISAC) has emerged as a transformative technology for intelligent transportation systems. Index modulation (IM), recognized for its high robustness and energy efficiency (EE), has been successfully incorporated into ISAC systems. However, most existing IM-based ISAC schemes overlook the spatial multiplexing potential of millimeter-wave channels and remain confined to single-hop vehicle-to-vehicle (V2V) setups, failing to address the challenges of energy consumption and noise accumulation in real-world multi-hop V2V networks with complex road topologies. To bridge this gap, we propose a spatial scattering modulation-based ISAC (ISAC-SSM) scheme and introduce it to multi-hop V2V networks. The proposed scheme leverages the sensed positioning information to select maximum signal-to-noise ratio relay vehicles and employs a detect-amplify-and-forward (DAF) protocol to mitigate noise propagation, while utilizing sensed angle data for Doppler compensation to enhance communication reliability. At each hop, the transmitter modulates index bits on the angular-domain spatial directions of scattering clusters, achieving higher EE. We initially derive a closed-form bit error rate expression and Chernoff upper bound for the proposed DAF ISAC-SSM under multi-hop V2V networks. Both theoretical analyses and Monte Carlo simulations have been made and demonstrate the superiority of DAF ISAC-SSM over existing alternatives in terms of EE and error performance. Specifically, in a two-hop network with 12 scattering clusters, compared with DAF ISAC-conventional spatial multiplexing, DAF ISAC-maximum beamforming, and DAF ISAC-random beamforming, the proposed DAF ISAC-SSM scheme can achieve a coding gain of 1.5 dB, 2 dB, and 4 dB, respectively. Moreover, it shows robust performance with less than a 1.5 dB error degradation under 0.018 Doppler shifts, thereby verifying its superiority in practical vehicular environments.

This study aims to address the issues of wavefront aberration and polarization distortion caused by atmospheric turbulence in free-space optical communication. We conduct research on the transmission characteristics of vectorial vortex beams based on the multi-phase screen transmission method in an anisotropic atmospheric turbulence channel. A joint modulation strategy leveraging vectorial adaptive optics for wavefront correction and polarization compensation is proposed and implemented. Specifically, wavefront correction is accomplished through polarization multiplexing and demultiplexing transmission. On this basis, polarization compensation is realized using a Gauss-Newton iterative algorithm based on two serial liquid crystal spatial light modulators. Our results demonstrate that after wavefront correction, the relative power increases from 0.9927, 0.8869, 0.7751 to 0.9962, 0.9964, 0.9944 at transmission distances of 100 m, 1 km, and 5 km, respectively. Then, after polarization compensation, the polarization correlation rises from 0.6444, 0.5395, 0.4393 to 0.8585, 0.9491, 0.9408. The optimization of the optical field's focusing characteristics, along with the enhanced stability of the interferometric optical field, offers a theoretical foundation for the wavefront-polarization composite correction technology in next-generation free-space optical communication systems.

Atmospheric and oceanic turbulence can severely degrade the orbital angular momentum (OAM) mode purity of vortex beams in cross-media optical links. Here, we propose a hybrid correction framework that fuses multiscale phase-screen modeling with a lightweight U-Net predictor for phase-distortion—driven solely by measured optical intensity—and augments it with a feed-forward, Gaussian-reference subtraction scheme for iterative compensation. In our experiments, this approach boosts the l = 3 mode purity from 38.4% to 98.1%. Compared to the Gerchberg–Saxton algorithm, the Gaussian-reference feed-forward method achieves far lower computational complexity and greater robustness, making real-time phase recovery feasible for OAM-based communications over heterogeneous channels.

The development of new optical field regulation is a key trend in free-space optical communication to expand channel capacity and mitigate the negative effects of atmospheric turbulence. A coherent detection model for a partially coherent vector vortex beam with 4-channel multiplexing under atmospheric turbulence transmission is constructed, and multiplexing transmission and wavefront correction of radially polarized partially coherent vortex signal light and angularly polarized partially coherent beacon light are performed. The impact of polarization crosstalk on wavefront correction performance and the correction effect on communication performance improvement are analysed. The results indicate that the proposed radial angular polarization multiplexing transmission can realize wavefront correction with a smaller relative power disturbance than the other distribution polarizations. A partially coherent vortex beam can suppress the fluctuation in the mixing efficiency with a certain degree of coherence. These results provide insights for subsequent research on fibre coupling and the application of partially coherent vortex beams in the field of free-space optical communication.

Download This Paper Open PDF in Browser Add Paper to My Library Share: Permalink Using these links will ensure access to this page indefinitely Copy URL Copy DOI

Objective Vortex beams, distinguished by their unique spiral wavefront structure, phase singularity, and orbital angular momentum, offer possibilities for enhancing system performance. Notably, vortex beams with different modes are spatially orthogonal, enabling their use in orbital angular momentum multiplexing for augmenting the channel capacity and spectral efficiency. Moreover, demultiplexing these beams at the receiving end provides an additional boost to system capabilities. This paper focuses on the application of coherent detection technology to a bi-directional slant path optical wireless communication system, employing orbital angular momentum multiplexing. This approach eliminates the need for image recognition steps, such as diffraction interference, thereby reducing errors introduced during the process. It analyzes the impact of the topological charge, altitude, and transmission distance on the performance of a vortex optical multiplex communication system. By incorporating an adaptive optics system for uplink and downlink correction, we can minimize crosstalk between modes, leading to improved detection sensitivity and channel capacity. Methods The architecture of the orbital angular momentum multiplexing coherent detection system for optical wireless communication using bi-directional slant transmission is illustrated in Fig. 1. Fig. 2 presents a schematic view of the system's transmitting and receiving ends. At the transmitting end, the source signal laser is split into four beams using a 1x4 coupler. The signal for each channel undergoes external modulation after series-to-parallel conversion and is then transformed into a vortex beam, with topological charges of 1, 2, 3, and 4, through spiral phase plates. These channels are then combined with a 4x1 coupler for coaxial transmission. At the receiving end, a 1x4 coupler divides the beam into four channels, each mixed with local oscillator vortex beams with corresponding topological charges. Following balanced detection, the electric signal is recovered, demodulated, and then converted back into signals through parallel-to-series conversion, enabling signal transmission from the source to the end. The uplink and downlink employ a single adaptive optics system, located near the downlink receiver, for correction. This system's working principle is depicted in Fig. 3. Given the reversibility of the transmission link and the reverse superposition of the wavefront, the distortion of the uplink signal wavefront at the transmitting antenna is conjugate with the wavefront distortion of the downlink received by the receiving antenna. This feature allows for post-correction of the downlink and pre-correction of the uplink. Results and Discussions Fig. 4 illustrates the coherent gains of signal and local oscillator light with varying topological charges after mixing. These gains include uplink, downlink, corrected, and uncorrected scenarios. When the topological charge of the signal light (l(s1)) is 1, and the topological charges of the local oscillator (LO) light (l(lo)) are 1 and 2, the corresponding coherent gains stand at 0.867 and 0.156, respectively. Atmospheric turbulence, which is most potent near the surface, induces wavefront distortion in vortex light, reducing coherence between signal and LO light, and affecting their orthogonality. As a result, the downlink' s correction effect is superior to that of the uplink. Fig. 8 reveals the bit error rate of each channel and the system's bit error rate under varying transmission distances. With increasing transmission distances, the wavefront distortion caused by strong turbulence in the uplink exceeds the adaptive optics' correction capability. Insets in Fig. 8 show the uncorrected and corrected light intensity and phase distribution after uplink and downlink transmission. The uplink utilizes pre-correction processing, leading to a larger corrected spot diameter compared with the uncorrected one. Wavefront correction does not influence the light intensity distribution, so no differences are observed in the light intensity distribution before and after downlink correction. Conclusions The study concludes that atmospheric turbulence can trigger mode crosstalk during vortex optical multiplexing transmission, and extending the transmission distance heightens the system's bit error rate. At the same transmission distance, mode crosstalk becomes more pronounced as the topological load increases. Adaptive optics is typically apt for phase compensation in weak turbulence conditions, with the correction effect of the downlink more obvious than that of the uplink. The vortex beam orbital angular momentum multiplexing coherent detection significantly enhances the system' s detection sensitivity and channel capacity. These findings apply to coherent detection communication involving multiple orbital angular momentum multiplexing with an expanded multiplexing interval.

Atmospheric turbulence causes signal beam wavefront distortion at the receiving end of a coherent detection system, which decreases the system mixing efficiency. Based on the coherent detection theory, this study establishes a mathematical model of wavefront distortion with mixing efficiency and mixing gain. It also analyzes the improvement limits of wavefront correction on mixing efficiency and mixing gain under different atmospheric turbulence intensities and experimentally measures them. Simulation results show that the mixing efficiency can be improved to 51%, 55%, and 60% after correcting for tilt, defocus, and astigmatism terms, respectively, when turbulence intensity D/r0 is 2. The mixing gain with homodyne detection is 3 dB higher than heterodyne detection. Meanwhile, the wavefront correction orders required for optimal mixing efficiency are higher than the heterodyne correction order. In the experiment, Haso4 NIR + DM 40 was used, and the turbulence intensity D/r0 was 2. After the closed-loop control algorithm corrects the tilt, defocus, and astigmatism terms, the indoor experimental results showed that the mixing efficiency is improved to 36%, 47%, and 62%, respectively. The outdoor experimental results showed that the mixing efficiency improved to 36%, 51%, and 68%, respectively.

Objective Optical wireless coherent communication employs optical hybrid to complete the mixing of signal light and local oscillator light, and balance detectors to complete photoelectric conversion. Optical fiber hybrid has been widely applied in optical wireless coherent communication systems due to its advantages such as high integration and compact structure. It is necessary to efficiently couple spatial light into the optical fiber. Adaptive optical technology can improve the coupling efficiency from spatial light into optical fiber by correcting the distorted wavefront, and it is applied to optical wireless coherent communication systems. The existence of non-common optical path aberration between the wavefront sensing branch and the coupling branch leads to the distorted wavefront in the communication branch after the closed loop of the adaptive optical system. The stochastic parallel gradient descent algorithm to correct the non-common optical path aberration is easy to fall into the local optimum, and the phase difference algorithm employed to correct the non-common optical path aberration is only applicable to the field of imaging systems. We propose a reverse transmission calibration algorithm to measure the non-common optical path aberration for the initial calibration of adaptive optical systems in optical wireless coherent communication. Methods The adaptive optical system in wireless optical coherent communication is shown in Fig. 1. After the laser beam is transmitted through the atmospheric turbulence, the beam is fully reflected by the deformable mirror. The reflected beam is divided into two collimated beams with a power ratio of 1: 1 by the beam- splitter. One transmitted beam is to act on the wavefront sensor to monitor the current distorted wavefront. The other reflected beam is directly coupled into a single-mode fiber after being converged by a coupling lens for optical hybrid, coherent detection, and communication. Reverse transmission is sending the same beam from the receiver to the transmitter. A laser beam identical to the transmission source is supposed to be connected to the coupling optical fiber, as shown in Fig. 2. At the same time, the deformable mirror command in this state is cleared so that the deformable mirror is in a completely flat reflection state. The beam output by the coupling fiber is reflected by the beam- splitter and then reflected by the deformable mirror directly into the wavefront sensor to measure the wavefront information. At this time, the measured wavefront information includes both the wavefront phase that can maximize the coupling efficiency and non- common optical path aberration. The coupling efficiency can be improved when the measured wavefront information is converted to the closed-loop control of adaptive optics. Results and Discussions The local oscillator light in the optical wireless coherent communication system is connected to the coupled single-mode optical fiber by the reverse transmission calibration algorithm. The peak-to-valley value of the non-common optical path aberration (Fig. 7) measured by the wavefront sensor is 3. 71 mu m, with the root mean square value of 1. 34 mu m. This error is enough to exert a significant impact on coupling efficiency. When the wavefront information is converted into the adaptive optical closed- loop control, coupling efficiency increases from the initial 9. 04% to the closed-loop 45. 21% (Fig. 8). The self- noise inside the wavefront sensor causes some synaptic data in the wavefront slope measurement and wavefront reconstruction, but does not significantly affect the fluctuation of coupling efficiency (Fig. 8). In turbulent environments, the coupling efficiency increases from 19. 72% under uncorrected state to 36. 93% under closed-loop state (Fig. 13), and that in complex environments increases from 3. 91% in an uncorrected state to 9. 13% in the closed-loop state ( Fig. 16). This shows that with the increase of communication distance, the influence of atmospheric turbulence on adaptive optical correction effect is more significant than non-common optical path aberration. Conclusions Based on the reversibility principle of optical paths, we propose a reverse transmission calibration algorithm to measure and correct non-common optical path aberration for adaptive optical fiber coupling systems in optical wireless coherent communication. This algorithm converts the non- common optical path aberration into closed-loop control and improves coupling efficiency while correcting the distorted wavefront phase. Compared with conventional stochastic parallel gradient descent algorithms, this scheme will not fall into the local optimum, and will not be affected by the external turbulent environments. It can also assist in the position alignment of optical paths, which is simple, feasible, and easy to realize in engineering. Finally, reference significance and practical value are provided for the optical fiber coupling technology of the optical wireless coherent communication system.

Optical receiving systems with single-lens structures have problems such as low receiving efficiency and small field of view when applied to underwater optical wireless communication systems. In this study, a design scheme for a double-layered fly-eye-lens optical system with wide-angle focusing is proposed. Based on the analysis of the LED light source transmission model and seawater channel, the optical-power receiving equation of the fly-eye lens system is deduced. The fly-eye-lens receiving system was designed and simulated using Zemax according to the geometrical optics principle of the lens array. An experimental device for measuring the insertion loss and receiving efficiency of an underwater blue-green LED communication system was built, and the optical power of the receiving optical system was experimentally measured. For the link distances of 1, 3, and 5 m, the received optical power of the double-layered-compound eye system was higher than that of the single-layered system, with a power increase of 72%, 65%, and 60%, respectively. The results show that the double-layered fly-eye-lens receiving antenna can significantly improve the optical power received by the receiving end; therefore, this antenna structure has strong practicability and good development prospects in the field of underwater optical wireless communication.

Based on the multi-beam propagation theory, a mathematic model of log-intensity fluctuation variance under multi-beam propagation in slant path atmospheric turbulence near the ground for free-space optical communication is established. Taking single beam, double beam, three beam, and four beams as examples, the intensity fluctuation characteristics of single beam and multi-beam are obtained, and the relationship between the log-intensity fluctuation and the bit error rate (BER) of the heterodyne detection coherent optical communications based on binary phase shift keying modulation is deduced. The results show that, with the increase of beam number, the variance of signal log-intensity fluctuation is significantly mitigated, and the BER of the system is significantly reduced. It proves that the multi-beam propagation method can be effectively applied to the long-distance coherent optical communications, and improve the performance of the system.

Atmospheric turbulence creates a light spot on the focal plane of the receiving end scintillation and angle of arrival fluctuation in optical wireless communication. The combination of a spot centroid detection algorithm and Kalman filter (KF) is proposed for spot tracking. The theoretical calculated value of the spot circle fit is filtered to obtain an optimal estimate of the spot center. The root mean square error of the detected spot centroids is used as an evaluation metric to verify the conventional algorithm and KF algorithm for the real-time tracking of the laser spot at transmission distances of 1.3, 4.1, and 10.3 km. The results show that the root mean square error of the spot in the pitch direction decreases from 0.740, 2.537, and 7.256 pixels to 0.683, 1.792, and 4.756 pixels, respectively, whereas the root mean square error of the spot in the horizontal direction decreases from 0.481, 2.131, and 5.932 pixels to 0.463, 1.471, and 4.362 pixels, respectively, compared with the conventional algorithm at three transmission distances. The envelope fluctuation of the baseband signal at the receiving end is significantly improved using KF algorithm compared with the conventional algorithm, which improves the stability of the optical wireless communication link. (c) 2022 Society of Photo-Optical Instrumentation Engineers (SPIE)

The Gaussian beam propagation theory is employed to analyze the effects of alignment errors including radial offset, end face tilt offset, and axial offset on the efficiency of coupling between the space light and single-mode fiber (SMF). The numerical and experimental results show that when the angle Omega between the end face and radial direction is 90 degrees and 270 degrees, the effects of the three alignment errors on the coupling efficiency are independent from each other. When the angle Omega is 180 degrees, the coupling efficiency reaches the maximum value. To better compensate the influence of alignment errors on the SMF coupling efficiency, we design an optical fiber coupler with a 5 degree-of-freedom coupling structure on the basis of piezoelectric ceramics and the stochastic parallel gradient descent algorithm with a variable gain to find the optimal alignment attitude for coupling space light into SMF. The experimental results show that the 5 degree-of-freedom optical fiber coupler can effectively correct different alignment errors and that the SMF coupling efficiency reaches 53.2% when the system is in closed-loop state.

ObjectiveWhenthelasersignalistransmittedthroughtheatmosphericchannelthefluctuationofatmosphericrefractiveindexcausedbyatmosphericturbulencecausesbeamexpansionbeamdriftandwavefrontdistortionwhichharmsthereceptionoftheopticalsignalandevenleadstotheinterruptionofcommunicationinseverecasesInawirelessopticalcommunicationsystemthegaze-gazegaze-scanandskip-scanareusedtoachievethecoaxialalignmentofthebeambetweenthetransmittingandreceivingantennaswhichincreasesthepreparationtimeofsystemcommunicationInpracticeafastacquisitiontrackingandalignmentmechanismmustestablishlinksThewirelessopticalcommunicationsystem????scoaxialalignmentdemandsthattheopticalaxisofthetransmittingandreceivingantennascompletelycoincideinspaceThedetectormustreturnthemeasuredparameterdatatoadjustthetransmittingantennatokeepthebeamstablecoarsealignmentforalongtimeandfinealignmentonthisbasisAtmosphericturbulenceaffectslong-distancedatareturnandpositionadjustmentmakingthetraditionallong-axisbeamalignmentprocessuncertainInthispaperanacquisitiontrackingandpointingsystemwithindependenttransceivercontrolisdevelopedThetransmittercalibratesandtracksthetargetpositionbythecalibrationcameratorealizethecoursealignmentofthebeamatthereceivingendatwo-dimensionalmirrorisusedtocontrolthepositionasthespotcenterfeedbacksuppresstheatmosphericturbulenceandrealizethefinealignmentofthebeamThenon-commonsightaxiscontrolavoidstheinconvenienceoftransmittingcontrolinstructionsfromthereceivingendtothetransmittingendanditisnotnecessarytouseaspacestableplatformforthemovingbasewhichgreatlyfacilitatesthepromotionofwirelessopticalcommunicationMethodsFig1bshowsthewirelessopticalcommunicationusingatwo-dimensionalmirror-assistedalignmentThesystemcomprisesthecalibrationcoarsealignmentfromthetransmittingantennatothetwo-dimensionalmirrorandtheminoraxisfinealignmentfromthetwo-dimensionalmirrortothereceivingantennaThecoarsealignmentfromthetransmittingantennatothetwo-dimensionalmirrorisdirectlypositionedandcalibratedbythecalibrationcameraatthetransmittingendandthetransmittingantennaisadjustedwiththeimageasfeedbackfinealignmentfromthetwo-dimensionalmirrortothereceivingantennaisadjustedbythetwo-dimensionalmirrorbasedonthedetectorfeedbackinformationatthebackendofthereceivingantennaAsthecalibrationcoarsealignmentandminoraxisfinealignmentcanbeoperatedatasingleendthecalibrationalignmentatthetransmittingenddoesnotrequiredatareturnFig4depictstheconstructionofawirelessopticalcommunicationIMDDsystemthatusesatwo-dimensionalmirrortoachieverapidalignmentThetransmittingendloadsthesourcebyintensitymodulationandtheoutputiscollimatedbythetransmittingantennaTocompletetheinitialcalibrationtheaimingplatformatthetransmittingendandthecalibrationcameraconnectwiththetwo-dimensionalmirroratthereceivingendsothatthebeamcompletelycoversthetwo-dimensionalmirroratthereceivingendthusadjustingthetwo-dimensionalmirrortoalignthereflectedbeamcoaxialwiththereceivingantennaTheparallellightemittedbythereceivingantennapassesthroughtheopticalprismisdividedintotwobeamsThefocusinglensconvergestooneofthebeamswhichiscoupledintothephotosensitivesurfaceofthephotodetectorforsystemcommunicationtheotherbeamisfocusedandtheinfraredcameradetectsthespotpositioninreal-timetocompletethebeamtracking ResultsandDiscussionsAftercoursealignmentthebeamistrackedbasedontherelativedeviationbetweenthecalibrationpointandtheimagingcoordinatepositionofthetwo-dimensionalmirrorFig7showsthebeamcoursetrackingcurveofthepositioncoordinateofthetwo-dimensionalmirrorunderthe13kmexperimentallinkinwhichthepitchandazimuthanglesareadjustedtwiceandonceinfourhoursrespectivelyThebeamdriftiscausedbyat mosphericturbulencethegravitysubsidenceandmec hanicalvibrationoftheoptical-mechanicalstructuremakesfrequentlyadjustingthecoarsealignmentinashorttimeunnecessaryFig10showsthe13kmbeam-trackingcurvethecorrespondingtime-domainwaveformandpowerspectraldensityestimationThestepangleofthetwo-dimensionalmirrorisadjustedto1092 mu radWiththeincreaseoftheiterationnumbersthecenterofthespotisgraduallyadjustedfromthefirstquadrantofthedetectionsurfacetothecenterAftertrackingthevariationsofthespotcentroidatthecenterofthecamerainthexandydirectionsare22770and13697pixels2respectivelyThetimerequiredtoperformoneclosedloopis005swhichissufficienttocompensateforthedriftrateofthespotFig14showsthebidirectionalalignmentexperimentfora103kmwirelessopticalcommunicationlinkAfterthebeamemittedbythelaserreachesthereceivingendthroughatmosphericturbulencethetwo-dimensionalmirroratthereceivingendisadjustedtoreflectthebeamtothereceivingantennaThedetectoratthereceivingendcandetectthesourceinformationfromthetransmittingendafterconvergingbyanantennaandfocusinglensLaserBemitsalightbeamatthefocuspositionofthereceivingantennawhichisreflectedbythetwo-dimensionalmirrorandreachesthetransmittingendthroughatmosphericturbulenceToachievethetwo-wayalignmentofthelightbeamtheantennaandthefocusinglensatthetransmittingendconvergethelightbeamanddetectorBcandetectthesourceinformationtransmittedbythereceivingendByadjustingthetwo-dimensionalmirroratthereceivingendthebeamalignmentfromthereceivingtothetransmittingendcanbeaccomplishedwhilecompletingtheentirealignmentfromthetransmittingtothereceivingendbecausetheopticalpathisreversibleConclusionsThisstudypresentsamethodforachievingrapidbeamalignmentusingimagecalibrationatthetransmittingendandatwo-dimensionalmirrorcontrolatthereceivingendthusaddressingtheproblemoflongtime-consumingbeamalignmentintraditionalwirelessopticalcommunicationTheimagecalibrationcoarsetrackingatthetransmittingendcanensurethatthespoteffectivelycoversthetwo-dimensionalmirrorandthecouplingspotcoarsetrackingatthereceivingendcaneffectivelysuppressthebeamdriftcausedbyatmosphericturbulencetheuplinkcanbeestablishedsimultaneouslywiththedownlinkThenon-commonsightaxiscontrolavoidstheinconvenienceoftransmittingcontrolinstructionsfromthereceivingendtothetransmittingendanditisnotnecessarytouseaspacestableplatformforthemovingbasewhichgreatlyfacilitatesthepromotionofwirelessopticalcommunication

Abstract. Conventional adaptive optics algorithms are studied through calibration or closed-loop control point of view. A Hadamard matrix algorithm was used to calculate the interaction matrix from the wavefront slope to the deformable mirror voltage. A fuzzy proportional integral differential (PID) control algorithm was used to adjust the control parameters to complete the closed-loop control of the adaptive optics. The simulation shows that when the atmospheric turbulence intensities D  /  r0 are 1, 10, and 20, respectively, the mean wavefront peak to valley (PV) values are are 0.38, 0.93, and 3.53  μm, respectively, after wavefront correction by Hadamard + PID algorithm; the wavefront PV mean values are 0.38, 0.93, and 3.48  μm, after wavefront correction by PushPull + FuzzyPID algorithm; and the wavefront PV mean values are 0.36, 0.92, and 3.30  μm after wavefront correction by Hadamard + FuzzyPID algorithm. The experimental results show that: the wavefront PV mean values are 3.84, 3.56, and 3.36  μm for Hadamard + PID algorithm, PushPull + FuzzyPID algorithm, and Hadamard + FuzzyPID algorithm, respectively. The Hadamard algorithm significantly improves the interaction matrix measurement accuracy, and the FuzzyPID control algorithm improves the adaptive ability under strong turbulence conditions. The wavefront correction using two advanced algorithms is better than that of the combination of advanced and conventional algorithms.

车联网可见光通信不仅受大气湍流影响,同时受交通灯、路灯、汽车前后灯和高亮度广告牌等背景光干扰,传统的光强闪烁模型不能准确描述.文中针对车联网可见光通信系统信道特性进行了分析,提出了一种采用双高斯函数叠加表示的夜间车联网可见光通信系统的背景光噪声模型,通过不同地区和不同天气下的实测数据采用该模型进行数值拟合,以验证该模型的有效性.结果 表明,雨天的概率密度分布与晴天的概率密度分布没有明显的区别,且路灯光强起伏要小于其他光源产生的光强起伏.该模型对于车联网可见光信道模型建立以及噪声去除具有一定的研究意义.

Abstract. The wavefront distortion caused by atmospheric turbulence leads to the decrease of the coupling efficiency of spatial light to optical fiber. Adaptive optics technology was employed to improve the coupling efficiency of free-space coherent optical communication. Accordingly, the effect of the discrete distorted wavefront on the coupling efficiency could be determined. The experimental results showed that the average coupling power was −31.97, −31.42, −30.79, −32.05, and −35.31  dBm when turbulence strength D  /  r0 was 0.2, 0.5, 1, 2, and 5, respectively. After eliminating the noncommon path aberration, the fiber coupling optical power increased from −35 to −27  dBm. After the optical wavefront was transmitted at indoor (0.5 m), 600 m, 1, 5, 10, and 100 km, the coupling efficiency increased from 10.1%, 1.3%, 11.4%, 2.8%, 7.2%, and 8.2% to 95.2%, 83.4%, 93.5%, 56.7%, and 42.4%, respectively, after wavefront correction. This will improve the performance of free-space coherent optical communication system.