This article introduces the basic concepts and related technologies of wireless communications and vortex electromagnetic waves. After that, we give a comprehensive overview of the experimental research progress of wireless communication technologies based on vortex electromagnetic waves in recent years, including microwave vortex electromagnetic wave communications, terahertz vortex electromagnetic wave communications, mid-infrared vortex electromagnetic wave communications, near-infrared vortex electromagnetic wave communications, and visible light vortex electromagnetic wave communications. At the same time, the theoretical research progress of satellite vortex laser communication technology and the experimental research progress of extended structured light (such as Bessel light beam and vector light beam) communications against obstacles and turbulence effects are introduced. At the end of the article, the challenges and prospects of vortex electromagnetic wave wireless communication technologies are also discussed. Multi-band and multi-dimensional fusion of vortex electromagnetic waves and extended structured electromagnetic waves, may provide potential solutions for multi-scenario, cross-scale, large-capacity, long-distance, and high-robustness wireless communications, and are also expected to be applied in future satellite communications.

Visible light communication (VLC) has gradually become a research hotspot in satellite communication owing to its significant advantages. For example, VLC provides rich and unlicensed spectrum resources, high transmission rates, strong security, and resistance to electromagnetic interference. Laser VLC devices exhibit high emission power, strong resistance to radiation, and a narrow laser beam divergence angle and are expected to be used in high-capacity long-distance satellite communication link transmissions. In this study, we implement an integrated 40-channel wavelength division multiplexing (WDM) laser VLC system, utilizing 29 visible light wavelengths. The system employs discrete multi-tone (DMT) bit loading modulation and the Levin-Campello (LC) algorithm, achieving a total transmission data rate of 418.3 Gbit/s. To the best of our knowledge, this represents the highest data rate achieved in laser VLC using WDM to date. To address the issues of bandwidth limitation and high-frequency fading in laser VLC systems, this system employs digital pre-equalization technology. Based on the signal characteristics of the system, a corresponding Zobel network is designed to enhance and reduce the energy of high-frequency and low-frequency signals, respectively, thereby improving the overall communication performance. The experimental results reveal that digital pre-equalization significantly enhances the laser VLC performance. This system demonstrates the significant potential of laser VLC in high-capacity satellite communication.

Optical phased array technology can achieve high-speed and flexible directional radiation by adjusting and controlling the relative phase of optical antenna elements, gradually developing into the mainstream solution of non-mechanical beam control. It has the advantages of low power consumption, high integration, small size, and light weight, and can simultaneously control the transmission and reception of multiple beams, meeting the urgent development needs of future one-to-many laser communication. This article mainly elaborates on the current application status of three mainstream technical solutions of optical phased array technology in the field of laser communication, and compares the technical characteristics and application advantages and disadvantages based on liquid crystal, micro-electro-mechanical systems, and integrated optical waveguide platforms. Finally, some thoughts and suggestions are provided for the future development of optical phased array technology in the field of laser communication.

Optical phased array (OPA), a nonmechanical beam steering technology, greatly improves the efficiency and stability of optical systems. It offers various advantages, including lightweight construction, compact size, fast beamforming, and low power consumption, rendering it extensively applicable across various fields. This paper provides an overview of the research advancements in liquid crystal-based OPAs employed for beam steering in laser communication. Furthermore, we outline the current challenges and prospects of liquid crystal OPAs in laser communication.

With the rapid development of LEO satellite internet, inter-satellite laser communication link, which is the key to connect each other for the LEO large constellations, and is the base for users to access satellite internet and achieve the end- to-end communication around the world. It is different between the ring network of the GEO satellites and the LEO satellite internet, due to the high dynamics of the LEO satellite motion, the inter-satellite laser links need to reestablish continually, so it brings a lot of technology problems in the addressing mode and switching routing method along the space nodes, and is the hotspot in this field. In addition, user terminals access the satellite internet by microwave link, which needs aggregation and distribution between microwave link and laser link. It is also a key and difficult problem that needs to be urgently studied and solved.

Satellite Elastic Optical Network (SEON) is an important development direction of satellite Internet with the advantages of large capacity, strong anti-interference ability and flexible resource management and control. The routing and spectrum allocation (RSA) problem is a critical issue in SEON. To solve the RSA problem in SEON, a dynamic routing and spectrum allocation algorithm based on the path state-aware (PIV-SSA) is proposed. The PIV-SSA consists of a segmented spectrum allocation (SSA) algorithm and a path influence value routing selection (PIV) algorithm. In the SSA, the location of the spectrum resources allocated to the services are determined by the transmission rate it requires. In the PIV, the optimal transmission path is selected by comprehensively considering factors such as spectrum resource consumption, link spectrum status, and path survival time based on the pre-allocation results of the SSA algorithm. The results of the simulation experiments show that, compared to the classical KSP-FF algorithm, the PIV-SSA algorithm reduces the network blocking rate by an average of 4.60% and achieves an average improvement of 4.78% in network spectrum utilization under different load intensities.

With the deepening exploration of space by humans, traditional radio frequency (RF) communication increasingly fails to meet the demands of high-speed deep space communication. Various spacefaring nations have undertaken research on deep space optical communication technology. Optical communication is a crucial means of achieving space communication. Compared to traditional RF communication, optical communication offers advantages such as high communication speed, small terminal payload, strong anti-interference capabilities and so on, making it more suitable for ultra-long-distance deep space communication. Currently, the United States leads in research and has successfully conducted two deep space optical communication demonstrations. This paper provides a comprehensive review of existing research plans and demonstration results in deep space optical communication. It analyzes the key technologies for implementing deep space optical communication, reflects on the developmental history, and contemplates the future prospects of deep space optical communication.

Photoelectric interconnection technology has distinct advantages in military fields, such as photoelectric communication and navigation, high-performance processors, and civil communication. The demand for higher bandwidth continues to increase for the current high-performance computing and high-rate communication systems, whether it is board-to-board or the link between modules on the board. Photoelectric interconnection can be used to alleviate this issue, and reduce the system costs and power consumption, allowing the system to be downsized and high-performing. Optical interconnection technology is classified into three types based on the optical transmission medium: free-space optical interconnection technology, polymer optical waveguide optical interconnection technology, and optical fiber optical interconnection technology. The definition and three types of photoelectric interconnection are introduced briefly, and the development trends of polymer optical guide photoelectric interconnection and optical fiber photoelectric interconnection at home and abroad are expounded. After weighing the benefits and drawbacks of the three types of photoelectric interconnection, the key technology and development trends in this field are identified. It serves as a reference for future research directions in this field.

Laser coherent synthesis is an effective technique for enhancing laser power and maintaining beam quality in fields such as remote sensing and communication. The filling factor affects laser coherent synthesis and plays a critical role in assessing coherent synthesis arrays. However, it is not a comprehensive factor. Accordingly, this study introduces plane wavefront distortion (PWD) as an integrated parameter for evaluating the performance of laser coherent synthesis. PWD simultaneously considers factors such as beam quality, array alignment, and component manufacturing errors. Based on the theoretical derivation of the expression for PWD and analysis of its effect on the efficiency of the system, simulations and experimental measurements demonstrate that PWD can be used to reflect the overall efficiency of laser coherent synthesis, exhibiting a negative correlation with it. The findings offer scientific value for the practical application of wavefront modulation techniques in coherent beam combinations in multi-aperture laser array coherent combination systems.

Wavelength division multiplexing (WDM) technology has been shown to effectively increase the capacity of optical communications with minimal channel crosstalk in the near-infrared light band. However, to date, WDM systems in the blue-green band support a limited number of wavelength channels with coarse channel spacing. This is because no multi/demultiplexers (MUX/DeMUX) exist that use fine wavelength spacing for visible light. To clarify this issue, this study reviews the development of WDM in the visible band and discusses the recent development of dense blue-green WDM based on integrated optical phased array DeMUX. Finally, the study summarizes and prospects the development trends of blue-green WDM.

The composition and control mode of space optical communication and tracking systems are analyzed, and the source of time lag in the fine tracking control process is summarized. Furthermore, the influence of the time lag link of a fine tracking system on the final tracking accuracy and stability is analyzed, and the precision of fine tracking is improved via system optimization. Initially, the time lag of the fine tracking system is streamlined by optimizing the program processing logic without losing the system function. This in turn eliminates the variable-length time lag and reduces the fixed-length time lag to shorten the time lag of the fine tracking system. Based on this, a robust prognostic control algorithm is proposed to reduce the adverse effect of the fixed-length time lag on the fine tracking system. The final results show that the tracking error of the fine tracking system is reduced from 4.1 μrad to 2.3 μrad after streamlining the time lag link when compared with the original system, and the tracking error is reduced from 4.1 μrad to 2.6 μrad after using the robust prediction control algorithm in the presence of the matching delay error. Additionally, the tracking accuracy of the system is improved by 43.9% and 36.6%, respectively. The tracking accuracy of fine tracking is as high as 1.9 μrad when the robust prognostic control algorithm is tested in the fine tracking system.

To solve the problem of traditional beacon laser spot detection algorithms being susceptible to complex background interference during the initial capture stage of satellite laser communication. YOLOv5s neural network is used to optimize and improve the initial pointing scene of satellite platforms. Selecting the original loss function with the smoothed intersection over union (SIoU) loss function and replacing the original upsampling structure with a lightweight content aware feature recombination (CARAFE) upsampling structure, adding convolutional block attention module (CBAM) attention mechanism to C3 layer, using SimSPPF to replace the original structure, and adding Coordconv structure that is conducive to perceiving position information. The improved neural network has better accuracy than traditional coarse tracking beacon spot detection algorithms, and can accurately detect the position of the spot in complex backgrounds. It is suitable for beacon spot detection in the initial capture stage and coarse tracking stage. The optimized YOLOv5s neural network achieves a precision rate of 99.7%, a recall rate of 99.3%, and exceeds the average accuracy (mAP) @0.5 by 99.7% and mAP@0.5∶0.95 by 74%.

In deep space exploration, microwave-based communication and ranging payloads face problems such as high link loss and tight spectrum resources. Compared with microwave, laser beam has a small divergence angle and more concentrated energy, which can reach a longer transmission distance, and the laser-based communication and ranging payloads have the advantages of small size and light weight. In this paper, an integrated deep space exploration system based on laser coherent heterodyne spread spectrum communication and ranging is constructed, an interpolation resampling method based on a curve model is proposed, and theoretical simulation and experimental verification of the curve model are carried out. The model is constructed from a priori information of the system and is a linear function of the pseudo-random code phase difference. The experimental results show that for static targets, the ranging deviation is no more than 0.55 mm and the ranging precision does not exceed 0.42 mm, and for dynamic targets, the ranging deviation is no more than 0.59 mm. Communication with zero bit rate is also realized in ranging for static and dynamic target. In addition, the integrated design of spread-spectrum communication and ranging is applied to deep space navigation and deep space time-frequency synchronization, which can improve the real-time performance.

In this study, to achieve high-speed, high-sensitivity, and low-cost laser communication, we optimized and improved a new InGaAs/InP single-photon avalanche diode (SPAD) to better apply to the near-infrared laser communication system detected using a single single-photon detector (SPD). Compared with the previous generation, we added a dielectric-metal reflective layer and improved the double Zn diffusion process while optimizing the structure of each layer. The fabricated InGaAs/InP SPAD achieved a photon detection efficiency (PDE) of 30%, a dark count rate (DCR) of 3 kHz, and an afterpulsing probability (Pap) of 2.4% under a high-frequency sine-wave gate (SWG) operating mode with a frequency of 1.25 GHz, temperature of 225 K, and bias of 6 V. A free-running negative feedback avalanche diode (NFAD) prepared based on the high-performance SPAD was used as a receiver in the real-time spatial laser communication system. The performance parameters of the laser communication system with the NFAD were experimentally obtained. The experimental results show that the InGaAs/InP NFAD with a bit rate of 1 Mbit/s using the 4-pulse position modulation (4PPM) scheme has a bit error rate of 1.1×10-5 and sensitivity of -69.6 dBm.

Deep-space laser communication systems commonly use pulse-position modulation (PPM) to improve the energy efficiency of communication and use single-photon detectors for efficient signal reception. In particular, superconducting nanowire single-photon detectors (SNSPD) are one of the most suitable detector choices. In this study, we investigated the performance of high-speed PPM-SNSPD-based deep-space laser communications in terms of the dead time and jitter of SNSPD as well as the trailing phenomenon of high-speed pulsed signals. The photocount characteristics of SNSPD were analyzed. Based on this analysis, we proposed a bias-compensated guard-time PPM symbol synchronization algorithm. Compared with the conventional guard-time symbol synchronization algorithm, the proposed algorithm effectively reduces synchronization errors and improves system error performance.

A space optical communication system with intensity modulation direct detection based on four quadrant detector tracking and communication multiplexing has been reported, which uses the dual optical wedge driven by ultrasonic motor as the beam deflection execution unit to form a closed-loop system for beam position tracking. The rotation period of the driving motor is 15 ms, and the position resolution is 0.83 μrad. Through theoretical analysis and experimental verification, the position closed-loop tracking -3 dB bandwidth of this system is about 4 Hz. When the position detection error is less than 10%, that is, the beam detection accuracy is less than 12 μrad, and the corresponding detection sensitivity is -45.2 dBm. At a communication rate of 10 Mbit/s and without signal encoding, the corresponding communication sensitivity is -44 dBm when the bit error rate is 1×10-3. It has been verified that it is feasible to use the four-quadrant detector as a tracking and communication multiplexing detector, which can be applied to small and lightweight interstellar laser communication terminals.

To solve the problem of aero-optical effect of the aeronautical platform on the performance of the laser communication system of the aeronautical platform, the influence of the aero-optics effect on the optical transmission performance of the laser communication system is examined under the condition of different flight speeds and altitudes. The large eddy simulation method is used to simulate the aerodynamic flow field around the optical terminal of laser communication at different flight speeds and altitudes, the density change of the flow field is calculated, and the refractive index field is established according to the density data. Furthermore, the Runge-Kutta method is used to solve the beam equation, trace the beam, and calculate and analyze the optical path difference distribution and Strehl ratio of the whole field of view of the communication beam passing through the flow field. The simulation results show that as the flight speed increases, the change in flow field density becomes more intense, optical path difference increases gradually, and Strehl ratio decreases. In the case of the same speed, as the flight altitude increases, the flow field density fluctuation decreases, optical path difference decreases, and Strehl ratio increases. The results of this study are of guiding significance for compensating the aero-optical effect in the process of airborne laser communication and obtaining better communication quality.

Lunar laser ranging (LLR) has the highest precision in measuring the earth-moon distance. The lunar laser ranging retro-reflector (LRRR) is an essential piece of equipment for realizing lunar laser ranging. China has plans to deploy a manually emplaced new generation LRRR. For the LRRR to work effectively, the azimuth and altitude angle of the LRRR must be adjusted such that the LRRR is aligned with the mean Earth direction. We established a method for calculating the adjustment angle of the LRRR and analyzed the misalignment caused by the biases of emplacement time and emplacement site. Results show that the aiming of error of the LRRR can be estimated at approximately 2.7°, and the maximum value is less than 5.0°, which satisfies the requirements for an aiming accuracy of 5.5°. The calculation method and analysis presented in this study can act as guidelines for future China LRRR emplacement missions.

In order to improve the satellite laser ranging data accuracy and stability of Changchun Station, analysis was performed with a data assessment system using precise satellite ephemerides. The assessment system was established from physical mechanism and widely recognized evaluation standard of satellite laser ranging, using precise orbit ephemerides as input. With the help of above assessment system, a new method known as the leading edge half maximum (LEHM) algorithm is used to improve the data pre-processing method and reduce the impact of satellite signature effect on data accuracy and stability. Analysis showed that the output of the data assessment system is consistent with those of International Laser Ranging Service (ILRS) analysis centers. After data pre-processing with the LEHM algorithm, normal point accuracy was improved from 4.9 mm to 3.9 mm, short-term stability was improved from 19.8 mm to 18.1 mm, long-term stability was improved from 6.2 mm to 5.4 mm and range accuracy was improved from 79.6 mm to 68.2 mm. The LEHM algorithm can effectively improve the data accuracy and stability, which points out the way to further improve the accuracy and stability of Changchun laser ranging data.

To adapt to the networking technology of space laser communication, the multi-system multiplexed communication mode was developed. In the transmitter, an intensity modulator is used to realize on-off keying (OOK) and binary phase shift keying (BPSK) modulation compatibility; while in the receiver, demodulation of these two modulation signals is realized via intradyne detection. In practical engineering applications, shot noise and thermal noise of electronic devices in laser communication system will inevitably affect the communication performance. Accordingly, the influences of bias point error of intensity modulator and optical filter bandwidth on the signal-to-noise ratio of incoherent OOK and coherent BPSK communication are further analyzed herein. Simulation results show that the optical filter bandwidth significantly influences the incoherent OOK communication performance: the incoherent OOK SNR overhead of 1 nm filter bandwidth is approximately 1.26 when the received optical power is -46 dBm; the coherent BPSK SNR overhead of 20 nm filter bandwidth is less than 0.02 when the received optical power is below -50 dBm. If the codeless communication sensitivity of incoherent OOK is required to meet -46 dBm@10-6 at the rate of 1.25 Gbit/s, the optical filter bandwidth should not be greater than 0.8 nm, the bias error of the intensity modulator should be controlled within 1% of the half-wave voltage. Under the condition of common-mode noise elimination and optimized optical filter bandwidth, the codeless sensitivity of BPSK communication at 1.25 Gbit/s rate is satisfactory to -55 dBm@10-6.

In order to enhance the sensitivity and resolution of the ground receiver for satellite-based laser communication, as well as simplify the acquisition process for beacon light, a 500 mm aperture ground receiver system is developed. This system is designed based on the link and design scheme of satellite-based laser communication, incorporating adaptive optics (AO) technology. It consists of four units: Cassegrain antenna with common aperture spectral detection, precision tracking tilt mirror, ultra-precision tracking AO, and AO wavefront detection. The antenna objective utilizes a coaxial Cassegrain structure combined with a refractor group to form a Kepler telescopic structure that considers volume and pupil distance requirements. To address optical axis correction issues causing pupil plane drift, a 4f system is implemented between the precise tracking tilt mirror and AO tilt mirror. Additionally, a double telecentric system is employed between the wavefront detector and deformable mirror to establish conjugate relationship and minimize axial error in wavefront detection. Optical passive methods are utilized in designing these four units to improve temperature adaptability of the system. Experimental results demonstrate that each element's wave aberration falls within 1/10λ (λ=632.8 nm) range at temperatures ranging from 10 ℃ to 30 ℃, meeting design specifications while offering valuable references for engineering applications.