Laser Communication System Research Articles

Scintillation index and outage probability of vortex Gaussian beams for horizontal links in weak atmospheric turbulence

Abstract By using Rytov method, for the Gaussian vortex beam, off-axis scintillation index of is examined in horizontal laser communication links operating in weakly turbulent atmosphere. Performances of laser communication systems, defined by the outage probability in this study, is evaluated by the use of lognormal distributed intensity in finding the scintillation index. Off-axis scintillation index of vortex Gaussian beams is analytically derived and evaluated in horizontal atmospheric link. The scintillation index obtained from the figures drawn versus the source size and propagation length is used to calculate the outage probability. It is found that turbulence affects vortex Gaussian beam less when compared with Gaussian beams that are non-vortex. Our important finding is that the scintillation index reduces when the topological charge increases.

Dynamic response characteristics of optical beam deflection in liquid crystal optical phased array

The beam deflector based on liquid crystal optical phased array (LCOPA) is a crucial component of space laser communication systems. Understanding and mastering the beam deflection characteristics under the dynamic response of LCOPA is essential for achieving real-time acquisition and tracking in space laser communication. This paper thoroughly explores the beam deflection characteristics during the dynamic response process by analyzing the dynamic response and far-field diffraction models of LCOPA. It presents the far-field diffraction patterns under the dynamic response of LCOPA and validates the analysis through experiments. This study not only enhances the understanding of the dynamic performance of LCOPA but also provides a theoretical basis for its control in space laser communication systems.

Research on the performance of Reed-Solomon frequency-phase keying in the modulating retro-reflector-assisted terrestrial-high-altitude-platform-satellite uplink laser communication system

Research on the performance of Reed-Solomon frequency-phase keying in the modulating retro-reflector-assisted terrestrial-high-altitude-platform-satellite uplink laser communication system

Effects of solar radiation on the performance of long-distance atmosphere-ocean laser communication links

We investigated the effect of solar radiation noise on the performance of a long-range atmosphere-ocean laser communication system. To model an inhomogeneous channel in the atmosphere-ocean system, a Monte Carlo (MC) simulation method was utilized in combination with measured underwater chlorophyll profiles. We conducted field measurements to accurately assess the effects of background noise, specifically solar radiation at different underwater depths and times. The influence of different scenarios on the system's signal-to-noise ratio (SNR) was then scrutinized based on the detector type. Additionally, a comprehensive analytical model was proposed for diverse background light noises, enabling the exploration of the effects of solar noise and Photomultiplier Tube (PMT) on the communication link's performance in terms of Pulse Position Modulation (PPM) demodulation symbol error rate (SER). The results indicate that expanding the receiver's field of view (FOV) from 1° to 15° can enhance the system's communication depth by 11 m, while increasing the receiver's radius from 1 cm to 20 cm can elevate the communication depth by 37 m. Moreover, the study investigates the impact of transmit power and the multi-aperture receiving technique on communication depth, providing valuable insights for designing atmosphere-ocean laser communication links and offering robust support for the practical implementation of the Free-Space Optical (FSO) - Underwater Optical Communication (UWOC) system.

Research on angular displacement detection technology of inter-satellite laser communication based on coherent system

With the rapid development of spatial information networks technologies, inter-satellite laser communication links have played an important role in the satellite Internet. High-precision relative angular displacement detection is the foundation for achieving real-time tracking of laser communication links, and its accuracy directly impacts the tracking performance and communication quality of the laser communication link. Traditional laser communication systems utilize intensity-based position detection methods to calculate angular displacement, requiring setting separate Charge Coupled Device (CCD) or Four-quadrant Detector (QD) as well as independent optical tracking branches. This setup makes it difficult to integrate with high-speed coherent communication systems. Additionally, the method employing intensity detection to determine angular displacement based on spot position has lower accuracy in theory. A angular displacement detection of coherent-based inter-satellite laser communication is proposed in this paper. Firstly, real-time calculation of phase differences at different positions of the beam is achieved through heterodyne coherent array detection and dual high-precision phase-locked loops. Then, a model correlating wave front phase difference with relative angular displacement is established, phase noise during beam transmission and detection processes is analyzed, and algorithm for detecting angular displacement in coherent systems is studied. Finally, a laser communication link is constructed under laboratory condition, and experimental validation work is carried out. When the relative angle deviation is 1°, the phase difference changes by 0.089049795 rad, and the angular displacement detection error of 0.66697μrad(σ). When the relative angle error is 0.603873μrad(σ), the detection sensitivity is −43.93dBm. The experimental results show that in the coherent system, the relative angular displacement detection with high precision can be realized by using the wavefront phase difference of beam, which can support the laser communication tracking system to achieve higher precision beam tracking.

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.

Multi-Gb/s free-space laser communication at 4.6-μm wavelength using a high-speed, room-temperature, resonant-cavity infrared detector (RCID) and a quantum-cascade laser.

Research has shown that free-space laser communication systems may experience fewer outages due to atmospheric impairments such as haze, fog, clouds, and turbulence by operating at a longer wavelength in the mid-wave or long-wave infrared, if disadvantages such as lower-performance transceiver components may be overcome. Here we report a resonant cavity infrared detector (RCID) with 4.6-µm resonance wavelength that enables 20-dB larger link budget than has been reported previously for ∼ 5 Gb/s operation. The device combines high responsivity, 1.97 A/W, with a low noise equivalent power (NEP) of 0.7 pW/H z at room temperature, and a high bandwidth of 6.7 GHz at 3-dB. The relatively large surface-normal-incidence device with 30-µm diameter simplifies the coupling relative to intra-subband quantum cascade detectors. Although the RCID NEP is expected to increase with frequency to ∼ 1.5 pW/H z, we estimate that the total equivalent noise power in a 2.5-GHz bandwidth is less than 200 nW. When combined with a relatively high power (∼100-mW) distributed-feedback quantum cascade laser, the difference of > 50 dB between modulated laser power and RCID noise significantly outpaces that of existing devices.

Design and Outage Analysis for VLP-Assisted Indoor Laser Communication Systems

Design and Outage Analysis for VLP-Assisted Indoor Laser Communication Systems

Transmission characteristics of cascaded four-wave mixing laser communication system in mid-infrared band

Transmission characteristics of cascaded four-wave mixing laser communication system in mid-infrared band

Characterization of Doppler shift in inter-satellite laser link between LEO, MEO, and GEO orbits

Over the past few decades, the development of optical wireless communications (OWC) has led to its application in a wide range of environments, including indoor, outdoor, and space. Optical carriers offer advantages such as high bandwidth, security, and directivity, making it a suitable technology for applications in radio astronomy, remote sensing, cellular networks, high-speed train communications, last-mile access, underwater/terrestrial communications, and satellite communications employing laser links. However, laser links between satellites experience Doppler shift due to their relative motion, and this effect should be taken into account from a system design perspective. This paper presents a study of the Doppler shift characterization in laser links for circular Keplerian orbits. An analytical derivation establishes the relationship between the Keplerian orbital elements and the Doppler shift in laser links. The derived equations provide a more comprehensive understanding of the impact of these orbital elements on the Doppler shift in laser links. Additionally, equations have been derived for the Doppler shift rate and the visibility window condition during satellite communication. Furthermore, we conducted a link failure analysis using simulations to evaluate the effects of the Doppler shift, also identifying the worst-case scenarios for orbits most significantly impacted by this shift. The results showed that among the six Keplerian orbital elements, the semi-major axis, inclination, and right ascension of ascending node contribute to the Doppler shift in circular orbits. While the Doppler shift impact is less significant for laser communication systems employing intensity modulation/direct detection, it can degrade the communication system performance for coherent detection. This work contributes to the effective estimation of Doppler shift from a system design perspective.

Optimized neural-network-based calibration method for quadrant detector array

The quadrant detector (QD) faces the limitation of a restricted field of view, which is effectively circumvented by implementing a QD array. Despite the potential advantages, there has been a scarcity of research on positioning methods for QD arrays. In this letter, we present a novel method for QD arrays utilizing a neural network approach. This method eliminates the need for intricate preprocessing of the signal in accordance with the array's architecture and demonstrates the capability to predict the radius of the incident beam with high accuracy. This technique holds promising potential for addressing contemporary challenges in space laser communication systems.

Simulation Method for the Impact of Atmospheric Wind Speed on Optical Signals in Satellite–Ground Laser Communication Links

To analyze the intensity of atmospheric turbulence in a satellite–ground laser communication link, it is important to consider the effect of increased atmospheric turbulence caused by wind speed. Atmospheric turbulence causes a change in the refractive index, which negatively impacts the quality and focusing ability of the laser beam by altering its phase front. To simulate the changes in amplitude and phase characteristics of laser beam propagation in atmospheric turbulence caused by wind speed, a transverse translation phase screen is used. To better understand and address the influence of atmospheric wind speed on the phase of optical signals in satellite–ground laser communication links, this paper proposes a Monte Carlo simulation method. This method utilizes the spatial and temporal variations in the refractive index in the atmosphere and integrates the principles of optical signal propagation in the atmosphere to simulate changes in the phase of optical signals under different wind speed conditions. By analyzing the variations in the received optical signal’s power, the Monte Carlo method is employed to simulate phase screens and logarithmic amplitude screens. Additionally, it models the probability density of the statistical behavior of received optical signal’s fluctuations, as well as the time autocorrelation coefficient of optical signals. This paper, under the coupling condition in satellite–ground laser communication links, conducted a Monte Carlo simulation experiment to analyze the characteristics of the optical signal’s fluctuations in the link and discovered that atmospheric wind speed affects the shape of the power spectral density model of the received optical signal. Increasing wind speed leads to a decrease in the time autocorrelation coefficient of the received optical signal and affects the coupling efficiency. The paper then used a cubic spline interpolation fitting method to verify the models of the power spectral density and the autocorrelation time coefficient of the optical signal. This provides a theoretical foundation and practical guidance for the optimization of satellite–ground laser communication systems.

Spot position scheme on a quadrant detector for a spaceborne laser communication system

The paper proposes a spot positioning method based on a four-quadrant detector for the limited computing power and memory of spaceborne laser communication, in which the adaptive interpolation segmentation (AIS) algorithm is used to fit the theoretical position curve. The algorithm uses linear operations though the fitting process and the simulated result indicates that it has higher positioning accuracy in the center area of the quadrant detector. A spot receiving and positioning system was built for experimentation and the final location of the spot was calculated. The positioning error is analyzed to evaluate the performance of the whole system. It is shown that the positioning accuracy is highest in the stable communication area of the system. In result, the scheme achieves high accuracy with simple operations, which is more suitable for spaceborne laser communication systems to release more performance for communication.

A Bidirectional Multi-Format/Rate-Adjustable Integrated Laser Communication System for Satellite Communication

A Bidirectional Multi-Format/Rate-Adjustable Integrated Laser Communication System for Satellite Communication

Time transfer over 113 km free space laser communication channel.

The space time frequency transfer plays a crucial role in applications such as space optical clock networks, navigation, satellite ranging, and space quantum communication. Here, we propose a high-precision space time frequency transfer and time synchronization scheme based on a simple intensity modulation/direct detection (IM/DD) laser communication system, which occupies a communication bandwidth of approximately 0.2%. Furthermore, utilizing an optical-frequency comb time frequency transfer system as an out-of-loop reference, experimental verification was conducted on a 113 km horizontal atmospheric link, with a long-term stability approximately 8.3 × 10-16 over a duration of 7800 seconds. Over an 11-hour period, the peak-to-peak wander is approximately 100 ps. Our work establishes the foundation of the time frequency transfer, based on the space laser communication channel, for future ground-to-space and inter-satellite links.

Research Progress of High-Speed Data Transmission Technology for Lightweight and Small-Sized Unmanned Aerial Vehicle Platforms

With the continuous development of science and technology, UAV technology has been widely used in various fields. As an essential part, light and small UAV platforms have the advantages of being small, lightweight, and easy to operate and thus have been widely used in aerial photography, monitoring, rescue, and other fields. However, the traditional data transmission method can no longer meet the demand for the data transmission speed and stability of small and light UAV platforms. Wireless optical communication has the advantages of being high-speed and license-free, as well as having a large bandwidth strong anti-interference ability, and good confidentiality. Therefore, this paper aims to study high-speed data transmission technology for light and small UAV platforms to improve their transmission speed and stability. In this paper, the characteristics and advantages of UAV airborne laser communication are first elaborated to illustrate the advancements and importance of data transmission technology based on light and small UAV platforms. Then, the composition of a UAV airborne laser communication system is clarified to illustrate the feasibility of conducting airborne laser communication research. Then, the current domestic and international research status of people on light and small UAV airborne laser communication high-speed data transmission systems is reviewed, and its key technical features are analyzed. Finally, the prospects for its application and development trend are investigated.

Analytic pointing error evaluation on nano-satellite laser communication system

This study uses comprehensive pointing control analysis to calculate laser pointing errors for nano-satellite laser communication systems. This methodology especially enables the performance evaluation of the pointing system during the satellite pointing budget design phase. The analysis consists of a two-stage pointing approach: a body pointing stage of a satellite and a fine pointing stage within the optical payload. By simplifying each pointing stage to a single-axis model, we derive their error transfer functions, which are converted into power spectral density (PSD). The PSD of the two stages are combined using linear interpolation due to the difference in their sampling frequencies. This process yields the final laser pointing error PSD. Our method is validated against Monte-Carlo simulation results and control experiments of the fine pointing stage. The simulation and the experimental results maintained correspondence with the modeled power spectral density. These comparisons can suggest the proposed approach can predict the pointing performance of the system. Based on these results, a method to determine controller gains is introduced, aiming to minimize final pointing errors.

Laser communications system with drones as relay medium for healthcare applications.

This article introduces a prototype laser communication system integrated with uncrewed aerial vehicles (UAVs), aimed at enhancing data connectivity in remote healthcare applications. Traditional radio frequency systems are limited by their range and reliability, particularly in challenging environments. By leveraging UAVs as relay points, the proposed system seeks to address these limitations, offering a novel solution for real-time, high-speed data transmission. The system has been empirically tested, showcasing its ability to maintain data transmission integrity under various conditions. Results indicate a substantial improvement in connectivity, with high data transmission success rate (DTSR) scores, even amidst environmental disturbances. This study underscores the system's potential for critical applications such as emergency response, public health monitoring, and extending services to remote or underserved areas.

113Gbps rainbow visible light laser communication system based on 10λ laser WDM emitting module in fiber-free space-fiber link.

With the advent of the sixth-generation mobile communication standard (6 G), the visible light communication (VLC) technology based on wavelength division multiplexing (WDM) technology can effectively solve the problem of shortage of spectrum resources and insufficient channel capacity. This paper introduces one of our technical achievements, namely the construction of a near-real-time visible light laser communication (VLLC) system based on WDM, which includes a self-designed 10-λ fully-packaged visible light laser emission module, 1 m multimode fiber - 0.175 m free space - 1 m multimode fiber optical transmission link, and receiver array. In the transmitter system, we adopt adaptive discrete multitone (DMT) modulation technique combined with Quadrature Amplitude Modulation (QAM) modulation scheme to obtain maximum spectral efficiency (SE). In the receiving system, we utilize the sparse-structured reservoir computing post-equalization algorithm to achieve superior equalization performance on the basis of the traditional post-equalization algorithm. The experimental results indicate that this quasi-real-time communication system has achieved a signal transmission rate of 113.175Gbps. To the best of our knowledge, this work has set a record in the field of high-speed visible light laser communication. Therefore, the laser communication system constructed by this work, with its flexibility in deployment and high-speed performance, demonstrates the significant potential application of visible light laser communication in data center interconnection and high-speed indoor access networks.

Ground Simulation and Pattern Recognition for Arbitrary Distance Optical Transmission of a Free-Space Laser Communication System

Ground Simulation and Pattern Recognition for Arbitrary Distance Optical Transmission of a Free-Space Laser Communication System