Inter-satellite Laser Communication Research Articles

Parameter-free fiber coupling method for inter-satellite laser communications based on Gaussian approximation

Satellite constellation networks play a pivotal role in making Beyond 5G services universally accessible to people worldwide. Inter-satellite laser communication is a crucial component of satellite communication networks. To utilize fiber amplifiers, it is imperative to couple the free-space optical beam into a single-mode fiber. However, vibrations and non-coaxial errors significantly impact coupling efficiency, thereby affecting communication quality. We have developed a parameter-free fiber coupling method based on Gaussian approximation to rectify non-coaxial errors and mitigate incident angle deviations induced by vibrations, ensuring consistently high coupling efficiency. This method is fast, stable, and parameter-free, eliminating the need for manual intervention. Furthermore, experimental results demonstrate the method’s effectiveness across the entire correction range of the correction device. In environments with strong vibrations, the communication link experiences a bit error rate of 6.734 E-2, which is reduced to zero bit errors after implementing this method in a closed loop.

Study on Linewidth and Phase Noise Characteristics of a Narrow Linewidth External Cavity Diode Laser.

In the field of inter-satellite laser communication, achieving high-quality communication and compensating for the Doppler frequency shift caused by relative motion necessitate lasers with narrow linewidths, low phase noise, and the ability to achieve mode-hop-free tuning within a specific range. To this end, this paper investigates a novel external cavity diode laser (ECDL) with a frequency-selective F-P etalon structure, leveraging the external cavity F-P etalon structure in conjunction with an auxiliary filter to achieve single longitudinal mode selection. The laser undergoes linewidth testing using a delayed self-heterodyne beating method, followed by the testing of its phase noise and frequency noise characteristics using a noise analyzer, yielding beat spectra and noise power spectral density profiles. Furthermore, the paper introduces an innovative bidirectional temperature-scanning laser method to achieve optimal laser-operating point selection and mode-hop-free tuning. The experimental results showcase that the single longitudinal mode spectral side-mode suppression ratio (SMSR) is around 70 dB, and the output power exceeds 10 mW. Enhancing the precision of the F-P etalon leads to a more pronounced suppression of low-frequency phase noise, reducing the Lorentzian linewidth from the initial 10 kHz level to a remarkable 5 kHz level. The bidirectional temperature-scanning laser method not only allows for the selection of the optimal operating point but also enables mode-hop-free tuning within 160 pm.

Accurate Real-Time Laser Spot Locating Based on Template Correlation in Intersatellite Laser Communications

Accurate Real-Time Laser Spot Locating Based on Template Correlation in Intersatellite Laser Communications

Research on Inter-Satellite Laser Communication Link Transmission and Routing Switching Technology of Satellite Internet (Invited)

Research on Inter-Satellite Laser Communication Link Transmission and Routing Switching Technology of Satellite Internet (Invited)

Design and Simulation Analysis of Piezoelectric Ceramic Tube-Based Fiber Optic Nutator Applied to an Intersatellite Laser Communication System

The signal-receiving end of acquisition, pointing, and tracking (APT) systems applied to intersatellite laser communication terminals usually uses a fast-steering mirror (FSM) to control the fiber-coupling process, has a complex structural design, and induces large errors in the nonideal coaxial optical path. Herein, we propose a fiber-optic nutator using a piezoelectric ceramic tube (PCT) as the driving unit that allows scanning in the focal plane of the light signal to achieve active fiber coupling in the APT system. Specifically, this article describes the structural design principle of a PCT-based fiber optic nutator, establishes a simulation model of the mechanism, and proves the correctness of the simulation model by measuring the deflection angle of a PCT based on a parallel light collimator. The minimum accuracy of the designed nutator was 0.145 μm, the maximum nutation radius R was 20.09 μm, and the maximum nutation bandwidth was 20 kHz, as determined through simulation. Finally, the design parameters of the nutator were evaluated. The PCT-based fiber optic nutator, which met the design parameters, structurally replaced the fiber optic coupling component FSM and fine tracking camera in conventional APT systems successfully. Therefore, the PCT-based fiber optic nutator allows the active coupling control of signal light to a single-mode fiber (SMF) based on energy feedback on a theoretical basis and promotes the lightweight design of relay optical paths in APT systems. In addition, with future work in optimization of the nutation control algorithm, the scanning range and accuracy of the nutator can be improved.

On-orbit demonstration of inter-satellite free-space optical stable communication enabled by integrated optical amplification of HPA and LNA.

Satellite free-space optical (FSO) communication is very promising in improving the bandwidth and capacity of space information networks in the future. However, the inter-satellite transmission distance of over 1000km leads to unstable optical beam pointing, acquisition, and tracking and then generates optical power jitter by a large margin before detection-demodulation. Therefore, it is difficult to realize high-stability and long-time FSO communication between satellites due to the generated bit error rate (BER) by jitter. In this paper, we report an autonomously self-designed and high-integration laser communication payload (LCP) and on-orbit-demonstrated inter-satellite 145min, zero-BER FSO stable communication with a line rate of 2.8Gbps. Moreover, based on the inter-satellite laser communication link, a video phone was clearly implemented for more than 10min, and authentic user data transmitted 459,149 packets, achieving results of zero-packet loss. Summarily, this on-orbit experiment demonstrated an excellent performance of the LCP owing to the distinctive design of integrating a high-power amplifier and low-noise amplifier optical amplification function. Our space mission was successfully completed, and the on-orbit demonstration results may offer a significant reference for the field of satellite laser communication and space information networks.

Multi-parameter influenced acquisition model with an in-orbit jitter for inter-satellite laser communication of the LCES system.

With the development of large low earth orbit (LEO) communication constellations, the efficiency of laser inter-satellite link (ISL) establishing become the bottleneck for subsequent large-scale launch and rapid networking applications of LEO communication constellations. Hence, we establish the pointing jitter error structure of LEO communication experiment satellites (LCES) system. The error structure can be used to trace the source of errors and evaluate the in-orbit jitter. And we derive an analytical expression of the acquisition probability density function (PDF) which comprehensively considering the influence of the scanning region, the pointing jitter error, the overlap factor and the in-orbit jitter error. The multi-parameter influenced acquisition model is validated by Monte Carlo (MC) simulations and semi-physical tests. The results reveals that the multi-parameter influenced acquisition model can be used to guide the in-orbit ISL establishing.

Polarization‐independent dual narrow‐band perfect metamaterial absorber for optical communication

AbstractIn this paper, we demonstrate a dual narrow‐band perfect metamaterial absorber for optical communication, where a silicon nano‐disk array is placed on a thin gold film separated by a dielectric layer. Numerical results reveal that dual narrow‐band perfect absorption peaks, which are attributed to waveguide mode and magnetic dipole resonance mode, are achieved at 1310 and 1550 nm, respectively. The results show that the absorption of the two peaks is higher than 99% under the normal incidence of both TE and TM polarizations and the bandwidth is 5 and 15 nm, respectively. Moreover, the absorption peak at 1310 nm can be independently tuned by varying the thickness of the dielectric layer. These findings imply that the proposed absorber can be used in the field of optical communication, frequency‐selective photodetection, and intersatellite laser communication.

Cooperation in Space: HAPS-Aided Optical Inter-Satellite Connectivity With Opportunistic Scheduling

Issues with tracking, precision pointing, and Doppler shift are the major sources of performance loss in laser inter-satellite communication that can severely decrease the coverage and overall performance of satellite constellations. As a solution to these problems, we propose a cooperation strategy in which a high altitude platform station (HAPS) staying at a quasi-stationary position contributes to the inter-satellite connectivity. In this setup, the HAPS node uses two different scheduling approaches: one that relies on the zenith angle; the other on instantaneous signal-to-noise ratio. To quantify the performance of the proposed scheme, overall outage probabilities for the two scheduling methods are obtained. In addition, guidelines for the design of practical inter-satellite networks are provided.

Dynamic Network Formation for FSO Satellite Communication

Satellite network optimization is essential, particularly since the cost of manufacturing, launching and maintaining each satellite is significant. Moreover, classical communication optimization methods, such as Minimal Spanning Tree, cannot be applied directly in dynamic scenarios where the satellite constellation is constantly changing. Motivated by the rapid growth of the Star-Link constellation that, as of Q4 2021, consists of over 1600 operational LEO satellites with thousands more expected in the coming years, this paper focuses on the problem of constructing an optimal inter-satellite (laser) communication network. More formally, given a large set of LEO satellites, each equipped with a fixed number of laser links, we direct each laser module on each satellite such that the underlying laser network will be optimal with respect to a given objective function and communication demand. In this work, we present a novel heuristic to create an optimal dynamic optical network communication using an Ant Colony algorithm. This method takes into account both the time it takes to establish an optical link (acquisition time) and the bounded number of communication links, as each satellite has a fixed amount of optical communication modules installed. Based on a large number of simulations, we conclude that, although the underlying problem of bounded-degree-spanning-tree is NP-hard (even for static cases), the suggested ant-colony heuristic is able to compute cost-efficient solutions in semi-real-time.

Coarse-refined combined scanning method of beaconless inter-satellite laser communication system

Coarse-refined combined scanning method of beaconless inter-satellite laser communication system

Beacon correction method for inter-satellite laser communication

Positioning errors are caused by wavefront distortion owing to the thermal deformation of optical systems. To address this issue, we propose a restoration algorithm to correct for the beacon centroid, which is important for inter-satellite laser communication links. Based on the far-field distribution model of the distortion facula, the objective function is established to solve the wavefront distortion for reducing the pointing error of the optical communication system. The feasibility of this approach is verified by theoretical analysis and experimental verification, including in-orbit experimental verification. The proposed approach is likely to be useful for designing inter-satellite laser communication systems.

Research on simulation methods for Doppler frequency shift of a coherent inter-satellite laser link in a ground test system

Spatial Doppler frequency shift occurs during ground integration testing of an inter-satellite laser communication link and is difficult to simulate it consistently; therefore, to address this problem, a Doppler frequency shift simulation method for an optical communication carrier in space based on a dual-parallel Mach–Zehnder modulator (DP-MZM) is proposed. By adjusting the RF signal added to the MZM and the bias voltage of the DP-MZM, the output carrier suppresses single-sideband modulation to achieve a controllable optical carrier frequency shift. In this study, theoretical analysis and experimental research on this simulation method was performed, and a Doppler frequency shift simulator for ground testing of two laser communication terminals was developed, which can realize controllable optical carrier frequency shift adjustment in the range of at least ±18 GHz and a sideband rejection ratio that is greater than 40 dB. The research results have been applied to an inter-satellite laser link ground test, obtaining good simulation and test results.

Denoising and predictive tracking method for the beacon in intersatellite laser communications

In the satellite-to-satellite optical communications, two optical terminals need to keep precise tracking for the beacon each other. However, in the tracking stage, the beacon recognition can be influenced by the star background light, which will result in the decline of tracking precision or the loss of the beacon. To solve these problems, we propose the centroid coordinate sequence fast Fourier transform algorithm, which distinguishes the beacon and the star background light by the frequency feature. Furthermore, for improving the tracking precision, the prediction method for the beacon is also introduced. To study the tracking precision of the algorithm, we simulated the satellite platform vibration. Finally, an experiment was performed to verify the practicability of the algorithm. Compared with the tradition method, the results showed that the prediction accuracy was improved more obviously. When the vibration frequencies are 50 and 80 Hz, the tracking precisions were improved by 58% and 33%, respectively. The new algorithm will be beneficial for improving the tracking precision and maintaining the steady laser link.

Received power attenuation due to the wave-front aberrations induced by the receiving optical antenna in an inter-satellite laser communication link

Due to the design defect, adjustment error, thermal deformation and gravity deformation in orbit, the bad optical antenna of the communication receiving terminal in an inter-satellite laser communication link will induce aberrations to the received wave-front. This will cause attenuation of the received power and deterioration of the bit error rate (BER) performance. The calculation model of the compensated and normalized received power is established when the received light spot is pointed at the center of the focal plane and the limited receiving radius is considered. The numerical results show that, for receivers with a single first-order Zernike mode wave-front aberration, the primary spherical aberration has the most influence on the received power attenuation, and for receivers with the same type aberrations, higher-order Zernike aberrations have more influence on that. The universality of the result for different antennas is verified by introducing a characteristic radius and a relative receiving radius. Some suggestions are proposed to lower the requirements of the wave aberrations control, thus reducing the difficulty of optical design, structural design, thermal design, assembly and adjustment. The outcome of this study would contribute to the design of the inter-satellite laser communication systems.

A Routing and Wavelength Assignment Algorithm Based on Two Types of LEO Constellations in Optical Satellite Networks

Research on LEO inter-satellite laser communication based on wavelength division multiplexing inter-satellite links and its routing and wavelength assignment (RWA) technology is an important way to solve the problem of low-latency, high-capacity, high-speed transmission, and low-cost on-orbit real-time routing of free-space optical communications. Since both the transmitting and receiving sides of the inter-satellite communication are high-speed moving satellites, the network topology in the whole inter-satellite communication process is time-varying. Therefore, based on this particularity, we propose the SpacePro constellation and combined the classical NeLS constellation for mathematical modeling, and the network topology and inter-satellite links are predicted and analyzed in their entire cycles. The RWA under the super satellite nodes with computation and storage functions algorithm (SCSA) is proposed and its effect is compared and analyzed in the wavelength resource requirement and connectivity. The results showed that the SCSA algorithm can save up to 50% in wavelength requirements, and the SpacePro constellation can achieve the same network connectivity effect with fewer on-board processors and wavelengths than NeLS. Satellite optical network node connectivity and wavelength demand are a trade-off problem. Studying the RWA problem in this scenario has a very critical reference for how the future satellite optical network performs wavelength usage configuration to help improve the performance of networks.

Point-ahead analysis and pre-pointing link stability study of intersatellite laser communication

The static bias error angle obviously affects pre-pointing links’ stability in the presence of vibration in intersatellite laser communication. The 2nd order point-ahead angle is a source of misalignment which was ignored in most solutions, and this is the concern of our paper. In this study, we present a further analytical investigation into the point-ahead angle in complex satellite maneuvering environment. Static bias error angle induced by the 2nd order point-ahead angle has been studied under different intersatellite links. The probability density function of the pre-pointing links’ outage has been derived in the presence of pointing jitter taking consideration of the static bias angle, and the link budget has also been analyzed. Simulation model of link stability has been established to verify the numerical results by the Monte Carlo method in Matlab-Simulink environment. The results have shown that the 2nd order point-ahead angle has a significant detrimental impact on link stability in long distance links. It is a neglectable factor. This work is dedicated to intersatellite laser communication system design.

Construction and maintenance of satellite-to-ground and inter-satellite laser communication network

Construction and maintenance of satellite-to-ground and inter-satellite laser communication network

Inter-satellite laser communication system based on double Risley prisms beam steering.

We designed an intensity modulation direct detection free-space laser communication terminal. In this system, the output power of the signal-modulated semiconductor laser amplified by two-stage fiber amplifier is 2W, and the modulation contrast is 15dB. Tracking of the incident beam and demodulation of the communication signal are realized by using a quadrant avalanche photodiode (APD). By means of rotating double Risley prisms, the ranges of the pitching angle and azimuth angle are ±35 and ±180 deg, respectively. The focal length of the transceiver lens is 125mm, and the final communication field of view is 4mrad. Without error correction coding, the sensitivity of -44.5 dBm was achieved, while the bit-error rate was below 10-3 at 70Mbps. We also established a laser communication experiment at the laboratory; the platform jitter below 5Hz can be effectively suppressed. The laser communication system has a compact structure, low power consumption, and effective suppression of low-frequency and high-amplitude angle jitter of the platform. The experiments demonstrate that the laser communication terminal can be used in an inter-satellite laser communication with a distance of thousands of kilometers. The theoretical analysis and experimental results are presented in the paper.

A compact and lightweight two-dimensional gimbal for inter-satellite laser communication applications.

To achieve precise alignment, tracking and pointing, inter-satellite laser communication generally requires a two-dimensional gimbal. To satisfy the requirements of space applications, the gimbal needs to be high-precision and as small as possible. To meet this need, we designed a small and lightweight gimbal assembly. We adopted piezoelectric ceramic motors instead of a torque motor to ensure driving precision and effectively reduce the mass of the drive unit. The yoke was made from low-volume fraction SiCp/Al with excellent specific stiffness, which ensured rigidity and reduced the mass of the structure. This allowed us to reduce the mass of the gimbal to just 5.8 kg. With a laser communication head mass of 5.55 kg, this gave a load-to-gimbal mass ratio of 95.7%. We carried out experiments on our small and lightweight gimbal, followed by a finite element analysis. The results of vibration test and shaking test showed that the structure has adequate stiffness and high precision.