When the airborne laser transmitter is located above or in the center of the cloud, the cloud will reduce the laser communication performance. In order to solve this problem, the effects of different types of clouds on laser energy attenuation, signal-to-noise ratio (SNR), maximum symbol transmission rate and bit error rate are simulated and analyzed. It is concluded that the cloud mainly causes laser energy attenuation, which affects maximum transmission rate and bit error rate, but has little effect on SNR. For communication systems with link margin greater than 18.9 dB, 4 km cloud cover is allowed on the link. The effect of cloud on the maximum communication rate and bit error rate is mainly caused by inter-symbol crosstalk caused by time extension. Cirrus has little effect on communication performance, cumulus has a great impact on communication performance, and stratus, stratocumulus, and cumulonimbus have a greater influence on the communication performance, but the differences between the three types of clouds are small and could be not be distinguished. Altostratus cloud and nimbostratus have greatest influence on communication performance, of which nimbostratus has greater influence than altostratus cloud.

Based on the theory of angular spectrum representation, the non-Gaussian rough surface is simulated by Johnson transfer system, and the field distribution characteristics of Laguerre-Gaussian vortex beams passing through the random non-Gaussian rough surface are studied. The effects of the direction correlation length, kurtosis, skewness and root mean square roughness of non-Gaussian rough surface on the field distribution of the vortex beam are analyzed. The range of the root mean square roughness is studied when the intensity distribution of the vortex beam passing through the random rough surface changes, and the corresponding experiment is carried out. The experimental data are compared with the simulation results. The results show that when the direction correlation length of non-Gaussian rough surface is 20 mm, the skewness is 0.001, the kurtosis is 6, and the root mean square roughness is more than 0.12 mm, the intensity distribution of the Laguerre-Gaussian beam passing through the random surface no longer keeps the hollow distribution, and the corresponding phase singularity disappears.

For complex battlefield environments, it is especially important to ensure the safety of flight between uninhabited aerial vehicles (UAV) formations and reliable communication within the formation. This paper proposes an algorithm for collaborative avoidance using wireless ultraviolet light between drones in a bee colony drone formation. Combined with the above algorithm and using the characteristics of wireless ultraviolet light coverage, the avoidance strategy of ultraviolet virtual fence is designed. And by enhancing the vector field histogram method to improve the cost function of the state of motion of the drone when performing mutual avoidance. In addition, the algorithm uses the unscented Kalman filter to predict the flight status of nearby uninhabited aerial vehicles. The simulation results show that in the avoidance simulations of the two prediction scenarios, the overall motion trajectory of this algorithm is smoother than that of the enhance vector field histogram method. At the same time, there is no obvious jitter when local avoidance occurs, the total length of the avoidance path is reduced by 3.46% on average, and the total time consumption is reduced by 18.94%. This verifies that the wireless ultraviolet cooperative avoidance algorithm in a bee colony drone formation is effective.

Aiming at the space laser communication system requirement of miniaturization, this paper proposes a scheme of using the four-quadrant detector to complete the acquisition and tracking. By analyzing the position distribution of the spots on the four-quadrant detector (4QD), the acquisition models are derived for the three kinds of spots distribution. Through the calculation of the position of the spots and the accurate calibration of the tracking mechanism, the high precision tracking is realized. Then besed on the laserteco and system test platform the capture and tracking scheme and detection performance of the 4QD are experimentally verified. The test results show that under the laboratory dynamic conditions, using 4QD as the capture and tracking detector, the capture probability is up to 100%, and tracking accuracy is better than 3 μrad, which verifies the feasibility of the scheme and lays a foundation for the miniaturization design of the laser terminal.

The orbital angular momentum (OAM) multiplexing and encoding technologies can effectively increase the channel capacity of the optical communication systems. In recent years, some researchers focus on using machine learning (ML) technology to detect OAM modes to improve the performance of OAM optical communication system. In this paper, the OAM modes detecting schemes based on ML technology are reviewed, including error back-propagating (BP) neural networks, self-organizing feature map (SOM), support vector machine (SVM), convolutional neural network (CNN), mode recognition techniques base on beam transformations and all-optics diffractive deep neural networks (D2NN). The performance, advantages and obstacles of each kind of the neural networks in atmosphere and underwater channels are analyzed.

We demonstrate an actively mode-locked holmium-doped fiber laser with a central wavelength of 2.07 μm, and the propagation characteristics under weak turbulent condition are analyzed. A segment of 1.5 m holmium-doped fiber is used as gain medium. Actively mode-locked can be realized by introducing periodic intensity modulation into cavity through LiNbO3 intensity modulator. The nonlinear polarization rotation effect is introduced into the cavity to realize the tunable wavelength of 2058.4 nm~2078.6 nm. Stable mode-locked pulses with fundamental frequency and 10th, 24th, 48th order harmonic operations can be obtained. The signal to noise ratio (SNR) of the corresponding radio frequency (RF) spectrum is 66.79 dB, 61.37 dB, 54.82 dB and 49.66 dB. The stable mode-locked pulse modulated by digital signal and is transmitted in a simulated atmospheric turbulence device. The eye patterns can be obtained at the condition of ΔT=70 ℃，140 ℃, 210 ℃ and back-to-back (BTB). The SNR at ΔT=210 ℃ decreased 9.14 dB compared with BTB condition.

Orbital angular momentum (OAM) carried by the vortex beam provides a new dimension resource in the spatial domain of light waves, which attracting more and more researching attentions. Since the vortex beams with different OAM mode values are orthogonal to each other, the OAM mode is introduced into the field of traditional optical communication, and two new application mechanisms are derived: OAM shift keying (OAM-SK) and OAM division multiplexing (OAM-DM), which provides a potential solution for future high-speed, high-capacity and high-spectrum efficiency optical communication technologies. Based on the basic concepts and theories of OAM beam types and their generation methods, this paper will give a brief overview of typical research cases related to the application mechanisms of these two communication systems. Three key technologies have been discussed, including OAM beam multiplexing technology, OAM beam demodulation technology, and turbulence suppression technology of OAM-based optical communication. Finally, the future developing trends and prospects of OAM-based optical communication technology are analyzed and forecasted.

Based on the analysis of the two main problems of atmospheric channel in free space optics (FSO), aiming at the problem of laser atmospheric channel especially the problems of frequency selective fading and multipath effect in complex turbulent environment, the suppression method is proposed based on OFDM turbulence effect, the FSO-OFDM system is built, the baseband model of this system and the signal of multi-carrier modulation and demodulation method are studied. First of all, this article systematically analyzes the mechanism of turbulence effect of atmospheric channel, and discusses the model of plane wave laser communication system under the influence of atmospheric turbulence, the space optical communication system model of Gaussian beam of a logarithmic normal turbulent channel is established under the influence of atmospheric turbulence, the probability density function of light intensity is deduced, the methods for analyzing the effects of various atmospheric turbulence effects on system performance using the signal-to-noise ratio probability density function; the OFDM multi-carrier modulation scheme of FSO-OFDM system is designed, the baseband mode model of FSO-OFDM system is constructed, and the modulation and demodulation principle of its signal is studied using this model. Finally, FSO-OFDM system is realized by using MATLAB, and the FSO communication system under the multipath interference is simulated, experiments on bit error rate characteristics under different guard intervals are performed. It is confirmed that the FSO-OFDM system has a strong ability to resist multipath interference and frequency selective fading, as well as good bit error rate (BER) performance. It can effectively solve the problem of intersymbol interference and the reliable link, and has a very broad application prospect and using value.

The optical wireless communication channel is complex and changeable. Fountain codes as an emerging rateless coding can achieve adaptive transmission in different channel environments without a priori information of the channel. Compared with traditional coding, it can improve the quality of wireless transmission. In this paper, we first summarize the advantages of fountain codes applied to optical wireless communication and the domestic and foreign researches on the development status of fountain codes. Then we deeply study the design of two kinds of fountain codes coding schemes, as well as the degree distribution function which has significant influence on the performance of fountain code. Soon we put forward one kind of fountain codes, namely Luby transform (LT) codes, and introduce the optimization schemes in recent years. Meanwhile, the key difficulties that need to be solved in the fountain codes design are pointed out. Finally, the necessary technologies and explorations direction of fountain codes for wireless optical communication are proposed.

Channelized receiving technology is an important part of modern electronic warfare and radar systems. It is a core enabling technology that meets the needs of its high frequency band, large bandwidth, and simultaneous multi-channel reception. In this paper, a zero-IF receiver based on microwave photonic is proposed. The center frequency of the optical frequency comb can be adjusted to correspond to the center frequency of the wideband RF signal group. Therefore, the channelized reception of the 3 GHz wideband RF signal is achieved, and each subchannel has a bandwidth of 600 MHz and is directly demodulated into I/Q baseband signal.

In order to solve the problem of multi-user interference and the subchannel strength generated by the block diagonalization (BD) algorithm in multi-user MIMO (MU-MIMO) indoor visible light communication, the bit error rate of the indoor MU-MIMO visible light communication system is optimized by using the substream selected BD algorithm. This paper establishes the channel model for MU-MIMO indoor visible light communication and compares the channel spatial correlation between the 4×4 MIMO and 8×8 MIMO in different indoor system layout modes by using the control variable method and taking different parameters of LED and PD distance, the system capacity and bit error rate performance of substream selected BD algorithm and BD algorithm are compared and analyzed. The results show that with the continuous enhancement of spatial correlation, the bit error rate performance decreases, and the substream selected BD algorithm can bring a gain of more than 4 dB compared with BD algorithm.

Visible light communication (VLC) is a promising technology that complements existing wireless communication networks to provide high-speed, low-latency, and multi-device access. With the high-performance code modulation technology of traditional wireless communication, various physical layer communication technologies adapted to VLC systems have been designed and implemented. Different from traditional radio frequency (RF) communication, VLC uses LED as the signal source. The modulation of LED is easy to produce nonlinear distortion and the modulation bandwidth is limited. It has become the technical bottleneck of VLC high-speed communication. In view of the challenges of these two aspects, taking white LED as the starting point, this paper expounds that white LED can effectively balance the characteristics of illumination and communication, summarizes and classifies various techniques of nonlinear distortion compensation and extended modulation bandwidth. Finally, this paper proposes open research issues such as LED package materials and processes, new Micro-LED device research, light source layout design, and intercode interference cancellation technology are expected to improve the performance of visible light communication systems.

Based on visible light communication technology which has been a research hotspot in the field of communication, this paper reviews the background of visible light communication, illustrates the basic system architecture and explores the research progress of visible light communication around five frontier directions: material chips, high-speed systems, multiplexing networks, underwater visible light communication and machine learning. The challenges faced by visible light communication are analyzed. Finally, looking forward to the prospect of visible light communication: in the intelligent era of future, visible light communication will become an indispensable part of communication networks with its advantages of high-speed transmission, and cooperate with other communication technologies to complement human life.

In the modulating retro-reflector (MRR) communication system, the influence of the atmospheric turbulence on the system is more serious than that of the traditional FSO communication system. In this paper, in order to reduce the impact of the atmospheric turbulence, an improved scheme based on the diversity of the retro-reflector is proposed. Using the power spectrum inversion method, the atmospheric propagation model of MRR under weak turbulence is established. The influence of the atmospheric turbulence on the retro-reflector diversity system and the traditional MRR communication system is compared and analyzed. The results of the numerical analysis show that under the same condition, the retro-reflector diversity of modulating retro-reflector (MRR) communication system can reduce the scintillation index caused by the atmospheric turbulence.

Optical spatial modulation (OSM), a new optical multiple input multiple output (OMIMO) technique, effectively improves the transmission rate and energy efficiency by using the spatial domain laser index to carry additional information. Meantime, since only one laser is activated per symbol duration to transmit information, the problems of channel interference and synchronization are solved in traditional OMIMO system. This paper firstly introduces several OSM technologies and summarizes their research status both at home and abroad. Nextly, the OSM, optical space shift keying (OSSK), enhanced optical spatial modulation (EOSM) and differential optical spatial modulation (DOSM) schemes are compared and analyzed in terms of transmission rate, spectral efficiency, bit error rate (BER) and complexity. Finally, the key problems and future development direction are pointed out in OSM system.