Laser Communication Research Articles

Loss on space-ground laser communication downlinks due to optical turbulence: results from wave-optics simulations

Loss on space-ground laser communication downlinks due to optical turbulence: results from wave-optics simulations

Enhanced frame synchronization and carrier recovery in coherent FSO communication: a pseudo-random and cyclic QPSK approach.

To alleviate the impact of atmospheric turbulence on receiver carrier recovery in free-space optical communication, this study introduces a novel frame synchronization and carrier recovery method. This method employs a blend of pseudorandom sequences and specific cyclic quadrature phase shift keying (QPSK) training sequences, facilitating frame synchronization and frequency offset (FO) estimation. The research designs a time-series metric curve to counteract the effect of side peaks, enhancing the accuracy of frequency offset estimation via QPSK spectrum features. Furthermore, the method incorporates a two-stage frequency offset estimation process, utilizing the inherent sign characteristics of the x and y polarization states. Applied to the quadrature amplitude modulation system under atmospheric turbulence conditions, the proposed algorithm demonstrates high precision under both strong and weak turbulence conditions. Furthermore, we confirmed the universality of the algorithm across multiple modulation formats.

Demonstration of a Low-SWaP Terminal for Ground-to-Air Single-Mode Fiber Coupled Laser Links

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.

Direct observation of atmospheric turbulence with a video-rate wide-field wavefront sensor

Turbulence is a complex and chaotic state of fluid motion. Atmospheric turbulence within the Earth’s atmosphere poses fundamental challenges for applications such as remote sensing, free-space optical communications and astronomical observation due to its rapid evolution across temporal and spatial scales. Conventional methods for studying atmospheric turbulence face hurdles in capturing the wide-field distribution of turbulence due to its transparency and anisoplanatism. Here we develop a light-field-based plug-and-play wide-field wavefront sensor (WWS), facilitating the direct observation of atmospheric turbulence over 1,100 arcsec at 30 Hz. The experimental measurements agreed with the von Kármán turbulence model, further verified using a differential image motion monitor. Attached to an 80 cm telescope, our WWS enables clear turbulence profiling of three layers below an altitude of 750 m and high-resolution aberration-corrected imaging without additional deformable mirrors. The WWS also enables prediction of the evolution of turbulence dynamics within 33 ms using a convolutional recurrent neural network with wide-field measurements, leading to more accurate pre-compensation of turbulence-induced errors during free-space optical communication. Wide-field sensing of dynamic turbulence wavefronts provides new opportunities for studying the evolution of turbulence in the broad field of atmospheric optics.

Cloud Shape and Attenuation Based UAV Trajectory Optimization for FSO Communication

Cloud Shape and Attenuation Based UAV Trajectory Optimization for FSO Communication

Research on laser communication ranging Integration and VLC nonlinear compensation technology

Abstract Visible light communication (VLC) is a new spectrum communication technology that provides a new direction to solve spectrum resource problems. The nonlinearity problem in VLC will lead to a significant reduction in the performance of the communication system. To solve this problem, this paper studies the integration of laser communication ranging and VLC nonlinear compensation technology based on analyzing the principles of ranging methods and factors affecting ranging performance. Aiming at the problem that the complexity of the Random Fourier Features Kernel adaptive filter (RFF-KAF) method will increase significantly as the dimension of the mapping space increases, we introduce some second-order Volterra series square terms into the input vector of RFF-KAF and propose an improved prediction method. Experimental simulation results show that the method proposed in this article can greatly improve the performance of the RFF-KAF method while maintaining the same mapping space dimension. Compared with obtaining similar performance by increasing the mapping space dimension, the cost of this method is greatly reduced.

Performance Analysis of Modulation Techniques for Indoor Free Space Optics

This paper deals with designing and analyzing the performance of indoor Free Space Optics (FSO) link for various modulation techniques in diffused channel conditions. At the present time, FSO is widely adopted because of its features; high speed, security, low power consumption and unlicensed spectrum. LEDs and LASERs are optimum choice to cater the dynamic range of requirements for optical communication system. The indoor FSO system has been simulated in Optisystem 14 software using various modulation schemes under different attenuation factors which may influence the performance of the system. In this paper, we have analyzed the performance of different modulation schemes such as Return to Zero, Non-Return to Zero, Phase Shift Keying, Pulse Amplitude Modulation, Quadrature Amplitude Modulation and Minimum Shift Keying. This work mainly focuses on the parameters; Bit Error Rate, Q-factor and power corresponding to transmission distance achieve minimum attenuation.

Generation of multiwavelength cylindrical vector beams from a random fiber laser assisted with nonlinear amplifying loop mirror

In past decades, vector mode random fiber laser (RFL) has attracted much exploration and rapid development. In this work, we proposed and demonstrated a novel multiwavelength RFL that produces cylindrical vector beams (CVBs). The gain profile is manipulated by a homemade Sagnac loop filter assisted with the nonlinear amplifying loop mirror (NALM) mechanism to improve multi-wavelength oscillation capability. The CVBs are generated with the help of long-period fiber grating and the wavelength switchablity CVBs random fiber laser is obtained. The wideband spectrum multiwavelength CVBs with the maximum channel number of seventeen are realized with NALM. This currently holds the record for the highest number of wavelength channels of vector mode random lasers which holds great promise for free-space optical communication. Coherent-tunable low-coherence high-order modes and cylindrical vector mode random lasers utilizing wavelength tailoring have been realized with high mode purity. The proposed CVBs with modeless behavior have potential applications prospects in optical signal multiplexing, space optical communication, and speckle-free laser active imaging.

Outage-guaranteed transmission for IRS-assisted FSO systems.

Intelligent reflecting surface (IRS)-assisted free space optical (FSO) communication can overcome line-of-sight and bring spatial diversity, which is considered to be a good complement to direct FSO links. This paper is interested in understanding the role of IRS-assisted FSO links when coexisting with direct FSO links. Due to the diverse nature of existing FSO channel models and the reliability requirements of IRS-assisted FSO systems, this paper aims to investigate the transmission problems of link selection and power configuration under the quality of service guarantee for link outage, which can be applicable to different channel models. Despite the inherent complexity of the problems, transmission schemes, i.e., sum rate transmission (SRT) and rate fairness transmission (RFT) are developed, which can achieve the respective optimal performance. Also, some insights for transmission schemes are provided. The results verify the efficacy of the proposed transmission schemes. The performance of the system may be improved by 40% and 41% for SRT and RFT, respectively.

Mid-infrared barium titanate electro-optic modulators based on a germanium-on-silicon platform

Mid-infrared electro-optic (EO) modulation efficiency and high-frequency performance of barium titanate (BTO) modulators on a germanium-on-silicon platform are investigated. Leveraging its exceptional Pockels coefficients, BTO exhibits remarkable EO modulation capabilities in both transverse-electric (TE) mode for a-axis growth and transverse-magnetic (TM) mode for c-axis growth. At the wavelength of 3.8 µm, the V π ⋅L for a-axis oriented BTO (TE polarization) is 1.90 V·cm, and for c-axis oriented BTO (TM polarization), it is 2.32 V·cm, which have better performance than those of the Pockels effect based EO modulators from literature. In addition, the high-frequency EO response of the modulator is simulated, and a 3-dB EO bandwidth of 62.82 GHz with the optimized traveling wave electrodes is achieved, showing promise in the high-speed MIR applications such as free-space optical communications.

Characterizing the fractional coherence vortices through the area of the intensity cross-correlation function

We have generated the fractional coherence vortices using the speckle patterns obtained from the scattering of the fractional vortex beams. In this study, we found the topological charge of the fractional vortex beam with a resolution of 0.01 using the area of the coherence function of scattered fractional optical vortex beams. We have also provided accuracy for the measurement of topological charges of fractional vortex beams using the studies of the area of the coherence function. Our experimental results are well matched with the theoretical results. These fractional coherence functions can be used to generate a security key for data authentication and data encryption. In addition, fractional vortex beams have multiple OAM modes and can be used to address the explosive growth in free-space optical communication.

RZ‐PPM impulse tone‐based CSI estimation for adaptive threshold decision in FSO communications

RZ‐PPM impulse tone‐based CSI estimation for adaptive threshold decision in FSO communications

Deep Learning-Assisted High-Pass-Filter-Based Fixed-Threshold Decision for Free-Space Optical Communications

This paper proposes a deep learning (DL)-assisted high-pass-filter (HPF)-based fixed-threshold decision (FTD) for free-space optical (FSO) communication. HPF is applied to reduce the scintillation effect by filtering out the low-frequency components of the received signal. However, the performance is limited owing to the signal distortion from HPF and remnant scintillation effect due to insufficient filtering. Therefore, the DL model is adopted to improve the performance of HPF-based scintillation effect compensation. The multilayer perceptron (MLP) model is used to adaptively select the peak frequency component of the received signal as the optimized cutoff frequency of HPF. Furthermore, recurrent neural network (RNN) and long short-term memory (LSTM) models are cascaded after HPF to compensate for the remnant scintillation effect and recover the signal distortion without the optimization of HPF cutoff frequency. The simulation was conducted under different turbulence channels and data rates. Simulation results showed that MLP-assisted adaptive optimized cutoff frequency and cascaded LSTM and HPF methods were close to the adaptive-threshold decision with precise channel state information under various turbulence channel degrees.

A new statistical fading model for loss factor induced by angle-of-arrival fluctuations in free space optical communication.

The impact of angle-of-arrival (AOA) fluctuations on the loss factor is investigated, and a new fading model is proposed to describe the associated attenuation. Distinct decay characteristics induced by AOA fluctuations are revealed, which depend on the ratio of detector size to beam width (M). The fading model can be divided into two sub-models based on the value of M0, where M0 is the value of M when the root-mean-square errors of the two sub-models are equal. In the case of M⩾M0, this fading model follows an inverse S-shaped pattern to describe the impact of AOA fluctuations. When M < M0, an exponential function to characterize the attenuation induced by AOA fluctuations. The numerical computation results indicate that the proposed fading model offers higher accuracy compared to traditional methods. Furthermore, the continuous analytical expressions for the probability density function (PDF) and cumulative density function (CDF) of the presented fading model caused by AOA are derived. Based on the proposed PDF and CDF models, the outage probability of inter-satellite laser link is analyzed.

Practical downlink satellite‐FSO/RF cooperative relays: Performance analysis and LSTM prediction

SummaryThe present research work aims to investigate the reliability of the mixed free space optical (FSO)/radio frequency (RF) decode‐and‐forward (DF) relaying system where the satellite intends to communicate with the ground station through the unmanned aerial vehicles (UAV) as the relay node. Moreover, it has been considered that a second UAV interferes with the intended UAV. The operation of the UAVs has been represented considering the small scale fading, path loss, 3‐D location, and probability of maintaining line‐of‐sight (LoS) and non line of sight (NLoS) links, while FSO link undergoes Malaga distributed turbulence. Using the aforementioned model, the closed form expressions for the outage probability and bit error rate (BER) have been derived. The exact expressions have been extended to obtain the high signal‐to‐noise ratio (SNR) results for the outage probability and BER. The analytical expressions have been numerically evaluated and the results obtained through these expressions have been verified using the Monte Carlo simulations. More importantly, long short‐term memory (LSTM)‐based deep learning model has been trained for prediction of outage probability. The model is trained offline and then utilized to predict the values of the outage probability in online mode with mean square error (MSE) and root mean square error (RMSE) of MSE = −45.02 and RMSE = −18.34 dB.

Observation of 2 µm multiple annular structured vortex pulsed beams by cavity-mode tailoring.

In the past few years, annular structured beams have been extensively studied due to their unique "doughnut" structure and characteristics such as phase and polarization vortices. Especially in the 2 µm wavelength range, they have shown promising applications in fields such as novel laser communication, optical processing, and quantum information processing. In this Letter, we observed basis vector patterns with orthogonality and completeness by finely cavity-mode tailoring with end-mirror space position in a Tm:CaYAlO4 laser. Multiple annular structured beams including azimuthally, linearly, and radially polarized beams (APB, LPB, and RPB) operated at a Q-switched mode-locking (QML) state with a typical output power of ∼18 mW around 1962 nm. Further numerical simulation proved that the multiple annular structured beams are the coherent superposition of different Hermitian Gaussian modes. Using a self-made M-Z interferometer, we have demonstrated that the obtained multiple annular beams have a vortex phase with orbital angular momentum (OAM) of l = ±1. To the best of our knowledge, this is the first observation of vector and scalar annular vortex beams in the 2 µm solid-state laser.

Metalens optical receiver for multi-beam free-space optical communication

A conventional optical receiver with a single detector accepts optical beams from a small range of angles of arrival (AoAs) and hence is not suitable for simultaneous communication with nodes with angular diversity. Here, we demonstrate a novel, to our knowledge, two-metalens-single-detector optical receiver that can collect optical beams coming from a wide range of AoAs. We show that the capability to collect broad AoAs with high efficiency makes the proposed receiver suitable for multi-beam free-space optical communication. An analysis for an exemplary three-beam system shows more than 70% collection efficiency at 2° AoA, which is 35× higher than conventional receivers.

Analysisof Beam Walk in Inter-Satellite Laser Link: Implications for Differential Wavefront Sensing in Gravitational Wave Detection

Achieving space-based gravitational wave detection requires the establishment of an interferometer constellation. It is necessary to establish and maintain stable laser interferometric links using the differential wavefront sensing (DWS) technnique. When the distant measurement beam experiences pointing jitter, it causes beam walk on the surface of the local detector. The reduced overlap between the local reference spot and the distant spot increases the nonlinear errors in the DWS technique, which need to be suppressed. Numerical analysis was conducted on the spatial beam interference signals of the DWS technique when the distant measurement beam experienced pointing jitter. An experimental measurement system was designed, and the beam walk was suppressed using a conjugate imaging system. The results show that within a range of 300 μrad, the optical path with the imaging system can reduce measurement errors by at least 83%. This way also helps to reduce pointing jitter noise in inter-satellite links, thereby improving laser pointing control accuracy.This method would provide a valuable reference for future DWS measurement systems.

47 Gbps 100 m ultra-high-speed free-space visible light tricolor laser communication system utilizing time domain hybrid Huffman coding.

The escalating demand for greater transmission capacities in the forthcoming 6 G communication landscapes necessitates the investigation of upper segments of the electromagnetic spectrum by both academic institutions and the industrial sector. This effort aims to circumvent the impending spectrum resource limitations. Against this backdrop, laser diodes (LDs) emerge as a critical technology for high-speed visible light communication (VLC), owing to their significant modulation bandwidth potential. This paper details what we believe to be a novel visible light laser communication (VLLC) system powered by red/green/blue (RGB) tricolor laser diodes. It highlights a successful 100-meter free-space transmission achieved through a time domain hybrid huffman coding (TDHHC) technique. The system's performance review unveiled impressive data transmission rates for the red, green, and blue laser diodes channels at 16.852 Gbps, 14.442 Gbps, and 15.755 Gbps, respectively, culminating in a cumulative transmission rate of 47.049 Gbps while maintaining a bit error rate (BER) beneath the stringent threshold of 3.8E-3, mandated by 7% hard decision forward error correction (HD-FEC) standards. As far as we known, this marks the highest data rate documented in a long-distance tricolor VLLC system.

Investigated Signal-to-Noise Ratio for Terrestrial Free Space Optical based on Libya Cities Data

Free Space Optical (FSO) communication links offer high-speed data transmission but can be affected by weather conditions like rain, haze, fog, and snow. This study investigates the impact of rain on FSO link availability in the Libyan cities of Garian, Elkhoms, and Tripoli. An empirical model based on the ITU Carb. (France) Model is proposed to predict FSO link availability for ranges up to 5 km. Rain data shows that Garian has the highest rainfall, followed by Elkhoms and Tripoli. The study examines the effects of rainfall on FSO link performance for these three cities over a 5 km link distance. The results show that signal-to-noise ratio (SNR) and received power at the FSO link are influenced by both rainfall and path length. For a 2 km link distance, the SNR ranges from 16 dB in Tripoli to 10 dB in Garian. Under the influence of rain, the SNR varies from 45 dB to 5 dB over a link distance of 1 km to 5 km.