Search Results (43)

Free-space optical (FSO) communication systems face significant challenges from atmospheric turbulence, which induces time-correlated fading and burst errors that critically affect link reliability. This paper presents a comprehensive end-to-end CCSDS O3K simulation platform with detailed atmospheric channel and pointing error modeling, enabling realistic performance evaluation. The atmospheric channel model follows ITU-R P.1622-1 recommendations and incorporates amplitude scintillation with temporal correlation using Ornstein–Uhlenbeck processes, while the pointing error model captures beam misalignment effects inherent in satellite optical links. Through extensive Monte Carlo simulations, we investigate the impact of coherence time, and interleaving depth on system performance. Results show that deeper interleaving significantly improves reliability under realistic channel conditions, providing valuable design guidance for CCSDS-compliant optical communication systems. This study does not propose new algorithms or protocols; rather, it delivers the first end-to-end CCSDS-compliant simulation framework under realistically modeled turbulence and pointing errors. Accordingly, the results offer meaningful reference value and practical benchmarks for inter-satellite optical communication research and system design. Full article

The optical communication performance of cylindrical vector partially coherent Laguerre–Gaussian (PCLG) beams in different atmospheric turbulence models are investigated. Based on the unified theory of coherence and polarization and turbulence theory, analytical formulas for the signal-to-noise ratio (SNR), crosstalk equivalent intensity and bit error rate (BER) of cylindrical vector PCLG beams are derived in Kolmogorov turbulence, non-Kolmogorov turbulence and strong turbulence, respectively. Numerical analyses indicate that selecting a smaller azimuthal index l₀ or a larger radial index p₀ of beams can effectively enhance the SNR. In addition, selecting appropriate beam width, coherence length, wavelength of the beam, propagation distance and receiving aperture diameter enables the acquisition of the optimal signal detection position. Our results are conducive to the application of cylindrical vector PCLG beams in FSO communication. Full article

Free-space optical communication has emerged as a pivotal technology for space-to-ground downlinks; however, signal degradation caused by atmospheric turbulence continues to pose a significant challenge. In this study, a model for the polarization transmission characteristics of a Gaussian-Schell model (GSM) beam in downlink was established, and conditions sufficient for maintaining the polarization transmission characteristics were derived. The impact of the source spatial coherence on the performance of optical communication systems using circular polarization shift keying (CPolSK) modulation was investigated. Additionally, models for the probability density distribution and scintillation index of the optical intensity under atmospheric turbulence were developed along with a bit error rate model for the optical communication system. The effects of the laser spatial coherence on these models were also analyzed. The results indicate that the optimal performance in the turbulent downlink is achieved with fully coherent light, where the GSM-beam-based CPolSK-modulated system demonstrates a reduction of 1.51 dB in the required power compared to that of an on–off keying system. The implications of this study suggest that optimizing spatial coherence could significantly enhance the reliability of space-to-ground communication systems under atmospheric disturbances. Full article

Propagation properties represent a critical aspect of laser beams utilized in free space optical (FSO) communications. We examined the evolution characteristics of the electric field associated with partially coherent flat-topped beam rectangular arrays propagating bidirectionally through the turbulent atmosphere and plasma links. Utilizing the optical transmission matrix, alongside the second moment theory and Wigner distribution functions, we derived analytical expressions for both the intensity distribution and propagation factors of the partially coherent flat-topped beam rectangular arrays affected by the atmospheric turbulence and plasma disturbances. The numerical results indicate that appropriately selecting parameters such as beam order, transverse spatial coherence width, and beam width can effectively mitigate the adverse effects on propagation properties caused by the turbulent atmosphere and plasma. Our results have significant implications for FSO communications within specific environmental contexts. Full article

Coherent free-space optical communication offers significant advantages in terms of communication capacity, making it particularly suitable for high-speed inter-satellite transmission within satellite communication networks. Nonetheless, the presence of Doppler frequency offset (FO) and phase noise (PN) associated with lasers adversely affects the bit error rate (BER) performance of these communication systems. Conventional methods for FO and phase estimation are usually hindered by high computational demands and phase cycle slips, especially in environments characterized by elevated channel noise. To address these challenges, a noise-tolerant method is proposed to facilitate accurate carrier phase recovery (CPR) with reduced complexity. This method merges a second-order feedback loop and a feedforward stage to achieve accurate estimation. The simulation results indicate that the proposed method surpasses traditional methods in terms of noise tolerance and resource efficiency. Particularly, the BER of the proposed method can be decreased to $6.7\times {10}^{-3}$ at a signal-to-noise ratio (SNR) of 4.5 dB, in contrast to a BER of 0.25 for the traditional method. Additionally, the resource consumption of the proposed method can be decreased by 64% under equivalent conditions. Furthermore, the experimental results reveal that the phase estimation error and BER for the proposed method are $2.1\times {10}^{-4}$ and $7.5\times {10}^{-4}$, respectively, when the received power is −41 dBm. These values are significantly lower than those achieved with traditional methods, which obtain errors of $1.85\times {10}^{-3}$ and a BER of 0.48, respectively. Full article

Free Space Optical Communication (FSOC) is a wireless communication method that utilizes laser beams for high speed and secure data transmission. Its performance is affected by various factors, among which atmospheric turbulence causes random fluctuations in the atmospheric refractive index, significantly impacting the reliability of communication links. The atmospheric coherence length is a key parameter describing the coherence properties of a laser signal as it propagates through the atmosphere, and accurately measuring it is crucial for assessing the quality of FSOC links. This paper proposes a novel strategy that utilizes extended sources directly as the information sources, combining the wavefront phase variance method with the extended source offset algorithm based on Shack–Hartmann wavefront sensors to directly measure atmospheric coherence length. Existing methods in extended scenarios typically rely on deploying laser beacons to aid in the calibration of atmospheric coherence length but setting up suitable beacons on horizontal communication links is challenging. Additionally, these approaches can be costly in terms of equipment and measurement expenses. Compared to traditional measurement methods, the algorithm proposed in this paper can measure directly based on extended scenarios in horizontal links, thereby effectively reducing system complexity and equipment costs. To verify the feasibility and effectiveness of this method, targeted simulations and experiments were conducted, and the results show that the coherence length measured by the algorithm is highly consistent with that measured by the Differential Image Motion Monitor (DIMM), with a deviation of less than 2% from actual values, effectively demonstrating the algorithm’s feasibility in coherence length assessment. Full article

The bidirectional propagation properties of partially coherent Laguerre–Gaussian (PCLG) beams under atmospheric turbulence and plasma were numerically investigated. The corresponding analytical formulas for the intensity distribution, effective beam width, and M² factor of PCLG beams were derived by utilizing the generalized Collins integral formula, atmospheric turbulence theory, and second-order moments theory of the Wigner distribution function. The intensity distribution of the PCLG beams ultimately evolved into a Gaussian-like intensity distribution. Additionally, the effective beam width and M² factor could be less affected by selecting appropriate parameter values for the beam order, transverse coherence width, and wavelength of the PCLG beam. The impact of parameters such as the beam order, transverse coherence width, and wavelength for reverse transmission on the PCLG beam propagation properties was greater than that for forward transmission. These results are beneficial for applications in free-space optical communications. Full article

(1) Background: The significant progress observed over the last two decades in coherent beam combining (CBC) technology has mainly focused on its applications in high-energy physics and laser weapons. This work provides insight into the basic principles of CBC and the search for an alternative, namely optical angular momentum (OAM) generation using CBC. (2) Methods: A semi-analytical model based on the paraxial wave equation was explored, generating OAM-CBC beams by manipulating the tilts and phases of the CBC (T&P-CBC) of hexagonal architecture. (3) Results: The specially arranged T&P-CBC shows typical properties of OAM, such as annular profiles for the zero diffraction order and 1st-order replicas in the far field and correlation coefficients of 1% between different OAM-CBC fields. (4) Conclusions: The differences between classical OAM beams and OAM-CBC are substantial due to hexagonal lattice properties. Moreover, applications in free space optical communications are feasible as T&P CBC fulfills the main conditions and requirements for OAM generation. Full article

To effectively mitigate the effects of atmospheric turbulence in free space optical (FSO) communication, we propose a parallel carrier frequency offset estimation (FOE) scheme based on time-tagged QPSK partitioning (TTQP). This scheme can be applied to spatial diversity polarization multiplexing (PM) coherent FSO communication systems. Specifically, the TTQP scheme performs QPSK partitioning by time-tagging signal points, accurately recording the time intervals between signals, and significantly reducing implementation complexity through a modified Mth power algorithm. The simulation results for the PM 16-quadrature amplitude modulation (QAM) validate the effectiveness of the proposed scheme. Compared to traditional QPSK partitioning algorithms, the TTQP algorithm achieves high accuracy, low complexity, and multi-format versatility in high-speed coherent FSO communication. Full article

The fast and accurate reconstruction of the turbulence phase is crucial for compensating atmospheric disturbances in free-space coherent optical communication. Traditional methods suffer from slow convergence and inadequate phase reconstruction accuracy. This paper introduces a deep learning-based approach for atmospheric turbulence phase reconstruction, utilizing light intensity images affected by turbulence as the basis for feature extraction. The method employs extensive light intensity-phase samples across varying turbulence intensities for training, enabling phase reconstruction from light intensity images. The trained U-Net model reconstructs phases for strong, medium, and weak turbulence with an average processing time of 0.14 s. Simulation outcomes indicate an average loss function value of 0.00027 post-convergence, with a mean squared error of 0.0003 for individual turbulence reconstructions. Experimental validation yields a mean square error of 0.0007 for single turbulence reconstruction. The proposed method demonstrates rapid convergence, robust performance, and strong generalization, offering a novel solution for atmospheric disturbance correction in free-space coherent optical communication. Full article

Free space optical technology promises to revolutionize point-to-point communications systems. By taking advantage of their vastly higher frequencies, coherent optical systems outperform their radio counterparts by orders of magnitude in achievable data throughput, while simultaneously lowering the required size, weight, and power (SWaP), making them ideal for mobile applications. However, the widespread implementation of this technology has been largely hindered by the effects of atmospheric turbulence, often necessitating complex higher-order adaptive optics systems that are largely unsuitable for deployment on mobile platforms. By employing tip/tilt beam-stabilization, we present the results of a bespoke low-SWaP optical terminal that demonstrated single-mode fiber (SMF) coupling. This was achieved by autonomously acquiring and tracking the targets using a combination of aircraft transponder and machine vision feedback to a root-mean-square (RMS) tracking error of 29.4 µrad and at angular rates of up to 0.83 deg/s. To the authors’ knowledge, these works constitute the first published SMF coupled optical link to a full-sized helicopter, and we describe derived quantities relevant to the future refinement of such links. The ability to achieve SMF coupling without the constraints of complex adaptive optics systems positions this technology as a versatile quantum-capable communications solution for land-, air-, and sea-based platforms ranging across commercial, scientific, and military operators. Full article

We derive the analytical expressions for root-mean-square (rms) beam wander (BW) and relative BW of a twisted electromagnetic elliptical vortex (TEEV) beam propagating through non-Kolmogorov atmospheric turbulence with the help of the extended Huygens–Fresnel principle and the second-order moments of the Wigner distribution function (WDF). Our numerical findings demonstrate that the BW of a TEEV beam with a small ellipticity, a large topological charge as well as a small waist width and initial coherent length is less affected by the turbulence. It can be also found that the effect of turbulence with a larger outer scale of turbulence, a generalized exponent parameter, and a generalized structure parameter on BW is more obvious. It is interesting to find that the effect of atmospheric turbulence on BW for a TEEV beam can be effectively reduced by regulating jointly the symbols and sizes of the twisted factor and topological charge. Therefore, modulation of the structure parameters of a TEEV beam provides a new way to mitigate turbulence-induced beam wander. Our work will be useful for free-space optical communications, remote sensing, and lidar distance measurement. Full article

Perfect vortex beams (PVBs) possess the advantage of a stable light field distribution regardless of their topological charges, and thus they are extensively utilized in various applications, such as free-space optical communication, optical tweezers and laser processing. Herein, we report a new strategy to generate and modulate PVBs using coherent beam combining (CBC) technology. Both piston phase and tilting phase controlling methods have been successfully employed, and the corresponding properties of the generated PVBs have been fully investigated. Moreover, the number and position of the gaps in fractional perfect vortex beams (FPVBs) could be precisely controlled, and the relationships between these modulated parameters and the performance of FPVBs are uncovered. These simulation analysis results demonstrate the potential for flexible modulation of PVBs or FPVBs in the CBC system, indicating promising prospects for coherent beam arrays (CBAs) in laser beam shaping and achieving high-power structured light. Full article

Currently, there are three types of optical communication networks based on the communication channel between the transmitter and receiver: the optical fiber channel, visible light channel, and optical wireless channel networks. The last type has several advantages for underwater communication, wireless sensors, and military communication networks. However, this type of optical network suffers from weather conditions in free-space communications and attenuation owing to the scattering and absorption mechanisms for underwater communication. In this study, we present a new transceiver architecture of a coherent optical code-division multiple-access (OCDMA) system based on a hybrid M-ary differential pulse position modulation scheme and a spreading code sequence called weighted modified prime code for underwater communication to minimize channel dispersion, increase the transmission rate per second, enhance the network bit error rate (BER) performance, and improve network security. Using an OCDMA system, we can simultaneously expand the network coverage area and increase the number of users sharing the network over the same channel bandwidth. The simulation results in this study proved that the proposed system can accommodate 1310 active users and a network throughput of 180 Gbps*user over a transmission distance of 930 m without any repeater at a 10⁻⁹ BER performance, compared to the 45 Gbps*user network throughput and 100 m transmission distance reported in the literature. Full article

Adaptive free-space optical (FSO) communication links have been extensively studied in order to adapt to variable atmospheric channel environments due to factors such as atmospheric turbulence. As a supporting technology, an all-in-one BPSK/QPSK switchable transmission and reception method is proposed and experimentally demonstrated for adaptive modulation format switching in FSO links. The transmission and reception of both modulation formats are realized based on the same IQ modulator and single-photodetector coherent receiver. Simulation and experimental results show that the QPSK signal has a power penalty of about 3–4 dB compared to the BPSK signal with a BER of about 1 × 10⁻³. The basis for format switching is given according to the various atmospheric channel conditions. The proposed method provides a flexible and efficient solution for variable FSO communication environments to improve their performance. Full article