Laser Communication System Research Articles

地球静止轨道激光通信系统的遮光罩优化设计

地球静止轨道激光通信系统的遮光罩优化设计

Investigation of isolation for free space laser communication in the mono-wavelength optical T/R channels

Favorable isolation performance in optical transmitting-receiving (T/R) channels is key to establishing full-duplex bi-directional laser communication links when designing mono-wavelength laser communication terminals. This study proposes a novel design scheme for T/R channels that use a single laser wavelength. The system includes measures such as the deflection at λ/4 wave plate angles, light trap installation at a polarization beam splitter, and removal of polarization analyzers in optimized systems. A mathematical model describing isolation in the optical T/R channels is presented. Three types of factors that affect the isolation of optical T/R channels were analyzed and simulated in detail. The present simulation results show that the roughness of the λ/4 wave plate and the solid angle of the detector field of view are exponentially related to T/R isolation, which can thus be regarded as the most significant factors affecting isolation. According to the present design scheme, the prototype T/R channel was developed for measuring isolation. The measured value (77.8 dB) agrees well with the result determined using the established mathmatical model (77.3 dB). Conclusively, the present theoretical model can provideguidance for optimizing isolation in a mono-wavelength optical channel. Thus, it can be applied extensively in laser communication systems, including inter-satellite networks.

Ground equivalent verification for atmospheric turbulence effects on fiber-coupling efficiency of satellite-to-ground downlink

The fiber-coupling efficiency of a satellite-to-ground (STG) downlink laser communication system is particularly influenced by atmospheric turbulence, thereby reducing the communication performance significantly. We report on a highly reliable ground equivalent verification to demonstrate the atmospheric turbulence effects on the fiber-coupling efficiency of an STG downlink. Statistical distribution of fiber-coupling efficiency obtained by experiment contributes a high consistency with the numerical simulation, with a correlation coefficient of above 0.97. The average deviation of fiber-coupling efficiency is only around 10% according to different trials on the course of ground equivalent verification. Ground equivalent verification provides a way to study the actual STG downlink with a smaller diameter of aperture, which might also potentially be a great help to serve on an experimental basis and reference for the optimal design of the STG downlink laser communication systems.

The Optimization Design of Sub-Regions Scanning and Vibration Analysis for Beaconless Spatial Acquisition in the Inter-Satellite Laser Communication System

We mainly report an efficient approach to optimize the design of beaconless spatial laser acquisition. In order to analyze the influence of vibration environment on the beaconless spatial acquisition, a demonstration of an efficient method will be presented to simulate the level of satellite platform vibration. An important power spectral density (PSD) model of satellite platform vibration used by the European Space Agency is introduced. Then through LABVIEW, a simulated vibration waveform is generated, and the simulated PSD has a basic consistency with the in-orbit measurement spectral line, referring to the design of the filter, generating the Gaussian white noise sequence, filtering the pseudorandom signal and the whole processing procedure of parameter matching. The variation interval of the vibration amplitude is from −100 to 40 μrad. Then, the model of beaconless acquisition is proposed, which uses the spiral scan pattern, laser with a divergence angle of 80 μrad scan in an uncertainty cone (UC), and also the scan coupled with the simulated vibration is given. Through the beaconless acquisition program, the vibration is suppressed between −10.7 and 5.9 μrad; when the overlap factor is selected as 10%, it does not produce any leakage scanning area. It can be used for a wide variety of platforms. This new method will be good for analysis and test of beaconless spatial acquisition algorithm. Finally, the description of acquisition duration, which mainly includes the fast steering mirror spiral scan and the mechanical rotary table step scan, is introduced. And the corresponding optimal selection method of sub-region is proposed; the design of 1.6-mrad scanning pattern with 7 × 7 sub-regions in the 8-mrad acquisition UC is given. Through the actual measurement, the acquisition time is 18.21 s, the tracking error is better than 0.4 μrad.

Research on APT Spot Detection Algorithm in Space Optical Communication

In space laser communication system, the coarse tracking accuracy of the APT system directly affects the field of view selection and accuracy of fine tracking system, and the accuracy of the selection of the spot center position directly affects the accuracy of the coarse tracking system. Therefore, the selection of reasonable spot detection algorithm is an important guarantee for the stable establishment of APT system. Centroid method, hough transform method, and circle fitting method are often used in the analysis of spot center and the realization of spot tracking. In order to improve the detection accuracy and anti-jamming performance, this paper compares and analyzes several commonly used spot detection methods, and obtains the applicable conditions of each algorithm.

Effects of atmospheric turbulence on fiber-coupled DPSK system in Satellite-to-Ground downlink

Fiber-coupled differential phase-shift keying (DPSK) modulation is potentially an effective approach for further high-speed satellite lasercom, due to the high detection sensitivity, low linewidth requirements, and uncalled-for complex phase-locked loop techniques. Nevertheless, the phase distortion caused by atmospheric turbulence will destroy the spatial coherence of the incident light field, thereby limiting the coupling efficiency of spatial light to single-mode fiber. Here, we report on the effects of atmospheric turbulence on fiber-coupled DPSK system in Satellite-to-Ground (STG) downlink. Fading characteristics of receiving signals and average BER performances have been also studied to optimize DPSK system performances by using wavefront modal compensation method based on orthonormal polynomials. An optimal receiving aperture could be obtained to achieve lowest BER in finite items of mode compensation. The analysis results can provide a reference for optimization of the receiving aperture in the STG downlink laser communication system and the reasonable system margin design.

Pointing and tracking errors due to low-frequency deformation in inter-satellite laser communication

ABSTRACTA Gaussian random-phase screen (GRPS) model is proposed to represent the low-frequency wave-front deformation in inter-satellite laser-communication links. The model is used to investigate mutual-alignment errors comprising pointing and tracking errors caused by the low-frequency deformation. Theoretical analysis and numerical simulation show that the GRPS root-mean-square (RMS) value, pointing error, and tracking error increase with the amplitude factor and the scale length in the model. The expectation of RMS increases in the form of a linear function with and increases in the form of a cubic function with . The expectations of the pointing and tracking errors increase with increases in the expected values for and . For a specific pointing error or tracking error, the 99.5% confidence interval of the RMS of the low-frequency wave-front deformation is provided. From this confidence interval, the lower limit of the processing precision of optical antennas corresponding to certain mutual-alignment errors can be predicted. These conclusions are expected to be instrumental in designing inter-satellite laser-communication systems.

Radiation-induced mismatch effect on performances of space chaos laser communication systems.

Chaotic modulation serves as an excellent scheme to enhance the confidentiality of space laser communication systems. Considering radiation and intensity scintillation, we establish bit error rate (BER) computation models for both an inter-satellite chaos laser communication system and a satellite-to-ground chaos laser communication system. Based on such models, numerical simulations are conducted to investigate the mismatch effect on BER performance of these two typical systems. For an inter-satellite system, radiation can induce great parameter mismatches. For a satellite-to-ground system, the BER only slightly rises due to intensity scintillation being a more dominant deterioration effect than radiation. Our work has good reference value for the practical design of space chaos laser communication systems.

Approach for recognizing and tracking beacon in inter-satellite optical communication based on optical flow method.

In inter-satellite laser communication systems, accurate positioning of the beacon is essential for establishing a steady laser communication link. For inter-satellite optical communication, the main factor affecting the acquisition and tracking of the beacon is the background noise, such as stellar background light. In this study, we considered the effect of the background noise on a beacon in inter-satellite optical communication and proposed a new recognition algorithm for the beacon, which uses the optical flow vector obtained from the image data. We verified the feasibility of this method by performing simulation analysis and experiments. Both simulation and experiments showed that the new algorithm could accurately obtain the position of the centroid of the beacon under the effect of the background light. Furthermore, considering the identification probability of a light spot through the background light, the locating accuracy of the new algorithm was higher than that of the conventional gray centroid algorithm. Therefore, this new approach would be beneficial for the design of satellite-to-ground optical communication systems.

M-pulse amplitude modulation of a flat-topped beam for aeronautical laser communications.

Using the Rytov method, bit error rate (BER) performances of M-pulse amplitude modulation (M-PAM) are investigated for a flat-topped beam when such a beam is employed in an aeronautical laser communication system operating in vertical paths having weak atmospheric turbulence. By using the on-axis scintillation index and the log-normal distributed intensity, the average BER (⟨BER⟩) is evaluated for M-PAM when M=2,4,8. The scintillation indices of collimated flat-topped beams of various flatness orders N are compared against propagation lengths, source sizes, and zenith angle for laser communication vertical paths, including uplink and downlink. Also, the ⟨BER⟩ versus the average signal to noise ratio (⟨SNR⟩) is examined for various beam flatness orders. It is shown that as the flatness order increases, the scintillation index decreases. Taking one of the best flatness order values, N=15, for reducing the scintillations, ⟨BER⟩ versus propagation lengths, source sizes, zenith angle, and ⟨SNR⟩ are found for various values of M. When M is increased, the ⟨BER⟩ is found to deteriorate.

Effects on performance of a 90° optical hybrid due to rotation of a 45° reflector scanner in a free space optical communication terminal

The effects on the performance of a 90° optical hybrid due to rotation of a 45° reflector scanner in a free space optical communication terminal were analyzed. A geometrical model of the 45° reflector was first established. Its polarization transmission characteristics were analyzed by three-dimensional polarization calculus, and the effect on the response of the 90° optical hybrid was analyzed. The relationship between the scanning angle in the pitching and azimuth directions and the effects on the beam ratio and phase shift were simulated for both a polarization-maintaining film and Ag. The results showed that the performance of the 90° optical hybrid was hardly affected during scanning of 45 ± 12° in the pitching direction for both the polarization-maintaining film and Ag, but the phase shift of the 90° optical hybrid changed markedly during scanning from −12° to 12° in the azimuth direction, especially for Ag. These results may be useful in the design of coherent laser communication systems.

Probability of fade for airborne laser communication system over exponentiated Weibull distribution under aperture averaging

In this paper, the probability of fade is performed for the airborne laser communication systems considering both the atmospheric and aero-optic effects. The atmospheric fluctuation is characterized by the exponentiated Weibull (EW) fading channel. The novel analytical expression is derived for probability of fade according to Meijer’s G function. The probability of fade is demonstrated to the simulation data with the Gamma–Gamma and log-normal distributions. The probability of fade is obtained for different flight altitudes and propagation distances. The probability of fade is analyzed in the airborne laser communication systems under the effect of aperture averaging in weak-to-strong turbulence regime. Results suggest that the proposed EW model is valid in airborne laser communication with high altitude, especially in the lower values of the irradiance under atmospheric turbulence and aero-optics effect. The fading of outage performance can be effectively mitigated by aperture averaging. Furthermore, this work is helpful for the compensate technique of system performance on airborne optical communication system.

Laser communication based on a multi-channel single-photon detector

We demonstrate a free-space laser communication system with a multi-channel single-photon detector to improve the efficiency of transmission. On account of the photon-number-resolving capability, the discrimination level is adjustable to optimize the extraction of effective signals from the background noise. This method greatly reduces the error bit rate even under the high intensity of the background noise of 6×106 counts/s in daylight. Meanwhile, combined with the pulse position modulation, the bandwidth of the laser communication system can be improved greatly. This study paves the way to the high accuracy and high speed free-space optical communication for 24 h a day.

Design of flexure support of space compact reflector subassembly and dynamic analysis

The mirror assembly of an electro-optical tracking and pointing system for a space borne laser communication system is studied, three flexible supports are contrasted, according to practical applications of space load, the structural stiffness advantage and surface figure of the three flexible support scheme have been evaluated. The analysis results show that the surface figure RMS of neck side grooving flexible support scheme resisting microgravity and thermal environment change can reach 2.05 nm and 8.88 nm, the fundamental frequency mode is 926.1 Hz, in the balance between the surface figure RMS and the higher stiffness resisting hevibration damage, the flexible design is most reasonable. On this basis, the parameterized design of the flexible support structure of the reflector is completed and dynamic analysis have been done. The maximum stress of the frequency response is 96 Mpa, which is less than the material's tensile strength limit. The results of random vibration analysis show that root mean square of acceleration response is 11.14 g RMS, meeting 3σ law. Finally, a 0.2 g sine sweeptest proves that the relative error of the modal analysis is 2%, the experimental result show that the analysis results are basically accurate and reliable, that is, flexible support design is reliable to meet the requirements of use.

Bidirectional space to ground laser communication system for cubesat

Bidirectional space to ground laser communication system for cubesat

Effect of controllable parameter synchronization on the ensemble average bit error rate of space-to-ground downlink chaos laser communication system.

Chaotic modulation is a scheme used to enhance the information security through the configuration parameter synchronization. When chaotic modulation is adopted in the space-to-ground laser communication system, the traditional bit error rate (BER) calculation model for fiber-based chaos communication system is no longer available to depict the long-term communication performance. To solve this problem, we established a new ensemble average BER calculation model under the effects of intensity scintillation and pointing error. Based on this model, we conduct a simulation to research such a system, and our numerical results indicate that space-to-ground chaos laser communication system has a great anti-interference against these two effects when the detector mismatch approaches zero. Our results display the advantages of chaotic modulation and also reflect the characteristics of space-to-ground chaos laser communication system.

Evaluation of the communication quality of free-space laser communication based on the power-in-the-bucket method.

Atmospheric turbulence seriously affects the quality of free-space laser communication. The Strehl ratio (SR) is used to evaluate the effect of atmospheric turbulence on the receiving energy of free-space laser communication systems. However, the SR method does not consider the area of the laser-receiving end face. In this study, the power-in-the-bucket (PIB) method is demonstrated to accurately evaluate the effect of turbulence on the receiving energy. A theoretical equation is first obtained to calculate PIB. Simulated and experimental validations are then performed to verify the effectiveness of the theoretical equation. This work may provide effective guidance for the design and evaluation of free-space laser communication systems.

Mid-infrared emission in InxGa1−xAs/GaAs T-shaped quantum wire lasers and its indium composition dependence

In this work, the emission wavelength has been extended out to 1.3, 1.5, and 2.2 μm for InxGa1−xAs/GaAs T-shaped quantum wire (TQWR) using multi-band k.p model and variational formalism. We have investigated the impact of the indium composition on the performance of a series of TQWR through a calculation of the optical gain and transition energies. It is found that the optical gain and the emission wavelength are more influenced taking into account the effect of the indium concentration and persisted up at room temperature (RT). The results could open the way to the development of laser communication systems operating at long wavelengths and fabricated from TQWRs structure.

Dynamic tracking and pointing accuracy measurement method of space laser communication system

Dynamic tracking and pointing accuracy measurement method of space laser communication system

Noise stability of signal reception algorithms with multipulse pulse-position modulation

An algorithm for estimating time positions and amplitudes of a periodic pulse sequence from a small number of samples was proposed. The number of these samples was determined only by the number of pulses. The performance of this algorithm was considered on the assumption that the spectrum of the original signal is limited with an ideal low-pass filter or the Nyquist filter, and conditions for the conversion from one filter to the other were determined. The efficiency of the proposed algorithm was investigated through analyzing in which way the dispersion of estimates of time positions and amplitudes depends on the signal-to-noise ratio and on the number of pulses in the sequence. It was shown that, from this point of view, the efficiency of the algorithm decreases with increasing number of sequence pulses. Besides, the efficiency of the proposed algorithm decreases with decreasing signal-to-noise ratio. It was found that, unlike the classical maximum likelihood algorithm, the proposed algorithm does not require a search for the maximum of a multivariable function, meanwhile characteristics of the estimates are practically the same for both these methods. Also, it was shown that the estimation accuracy of the proposed algorithm can be increased by an insignificant increase in the number of signal samples. The results obtained may be used in the practical design of laser communication systems, in which the multipulse pulse-position modulation is used for message transmission.