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A free-space optical communication system using non-mode-selective photonic lantern (PL) based coherent receiver is studied. Based on the simulation of photon distribution, the power distribution at the single-mode fiber end of the PL is quantitatively described as a truncated Gaussian distribution over a simplex. The signal-to-noise ratios (SNRs), bit-error rate (BER), and outage probability of PL based receiver using selection combining (SC), equal-gain combining (EGC), and maximal-ratio combining (MRC) over Gamma-Gamma turbulence channels are analyzed and compared with the single-mode fiber (SMF) receiver and multimode fiber (MMF) receiver. We demonstrate that EGC is the most suitable combining for PL based receiver because the PL power distribution has limited influence on the BER and outage probability when EGC is used and can greatly affect the BER and outage probability when SC is used. Numerical results show that PL based receiver with EGC requires 6 dB less SNR than the SMF receiver and 5.5 dB less SNR than the MMF receiver at BER of 10−6 in moderate turbulence.

As an efficient power technology, a triboelectric nanogenerator (TENG) has made a huge impact on many research fields and exhibits exciting prospects in the future world. There are two output modes of the TENG, the alternating current (AC) mode based on the coupling of triboelectrification and electrostatic induction, and the direct current (DC) mode based on the coupling of triboelectrification and air-breakdown. AC can be converted into DC by rectification. However, from the symmetry argument, there should be an inverting mode to convert the DC into AC. Herein, an inverting TENG to realize the AC mode based on the coupling of triboelectrification and air-breakdown is proposed for the first time by setting alternating polarity distribution areas. Different from the fixed waveforms of a traditional AC-TENG, the inverting TENG exhibits such unique characteristics that both the width ratio and amplitude ratio of AC signals can be changed by tuning the distributed width and electronegativity difference of two opposite materials. This work paves the way for the development of a symmetry output mechanism for the TENG, from AC to DC, and from DC to AC, or even hybrid DC and AC in one TENG, and these powerful functions endow the inverting TENG with more advantages for future self-powered intelligent systems.

For a coherent Doppler lidar system, it is extremely important to accurately extract the heterodyne signal while improving the system’s speed measurement accuracy. In this paper, All-fiber coherent...

This letter focuses on the diffusive molecular communication system with a reversible reactive receiver. By assuming the receiver is uniformly covered by finite receptors, a new boundary condition is proposed to model the reception reaction. In addition, an iterative approximation method is developed to model the expected received signal in this system. The simulation results verify the effectiveness of the approximation method.

Bi-directional reflection distribution function (BRDF) is a common method to study the laser scattering characteristics of targets, and it is an important parameter for the theoretical demonstration of laser active detection, target recognition and classification. Scholars at home and abroad have proposed many mature BRDF models to describe the scattering characteristics of different targets. However, almost all of these models do not take into account the effect of incident wavelength on scattering characteristics. In addition, limited by the frequency modulation range of the laser, the existing BRDF measurement devices cannot obtain the BRDF data of the target at any wavelength, which restricts the application of the existing BRDF model. In view of this limitation, a method is proposed to calculate the unknown wavelength BRDF data using the BRDF measurement data of known wavelengths. Firstly, based on the Kirchhoff approximation theory, the spatial distribution of the scattered light field of the metal aluminum target at any wavelength was simulated and analyzed. Secondly, the error of the theoretical simulation model was analyzed through the experimental data. Finally, the BRDF data at any wavelength were calculated using the simulation data and the experimental data with known wavelengths. The final results showed that at the 1064nm wavelength, the RMSE value of the calculated data obtained by this method is 0.3553, which is 0.2233 smaller than the RMSE value of the simulation data.This method is effective in calculating the BRDF of metal aluminum targets at different wavelengths.

Aiming at the problem that the background noise mixed in the target echo will affect the calculation of the target polarization degree when the traditional polarization detection system obtains the target polarization degree, based on the polarization Gm-APD detection model, a set of target echo polarization correction method is proposed. The target is imaged in a xenon lamp environment, the influence of target attitude and polarization angle on detection is explored, and the polarization imaging results are analyzed. The results show that the polarization system has a significant effect on metal materials with low surface roughness. When circularly polarized light is incident, the echo trigger probability of the metal material reaches a peak at the polarization angle of 135°. The greater the incident angle, the greater the echo depolarization and the lower the trigger probability. By inverting the distribution of echo photons, the number of background noise photons in the echo and the number of target echo photons can be obtained respectively, and a more accurate correction of the polarization degree of the target echo can be obtained. For metal materials, when the target attitude angle is 30°, the target polarization before and after correction are 0.47 and 0.57 respectively, and the target echo polarization after correction is 7% higher than that without correction. This research work provides experimental support for the effective detection and target detection of GM-APD lidar in the daytime.

This article mainly shows that coherent accumulation of multi-aperture receiver array based on frequency modulation continuous wave (FMCW) coherent lidar has an excellent performance for the weak signal detection of target which is far distance or moving with a high velocity. This method can improve the signal and noise ratio (SNR) and detection range accuracy by multi-aperture receiver array. In addition, the analysis done by simulation shows that phase fluctuation of atmospheric turbulence has a significant influence on the performance of coherent accumulation of multi-aperture receiver array. Stimulation result shows that while μx is equal to - σ2x , the amplitude fluctuation of signal could degrade the quality of coherent accumulation based on multi-aperture receiver array and its existence leads to the worse performance before non-amplitude fluctuation. Phase fluctuation of signal deteriorates the performance of coherent accumulation while its size is big or small.

We study the power distributions at single-mode fiber end of the non-mode-selective photonic lantern for the coherent optical receiver using multivariate Gaussian distribution and uniform distribution. Equal-gain combining is used to combine the electrical signals at the outputs of the balanced photodetectors. The signal-to-noise ratio is studied for the coherent optical receiver. It is found that, when the number of single-mode fibers of the photonic lantern is equal to the number of guided modes at multimode end of the photonic lantern, the average signal-to-noise ratio attains its maximum value.

When the Geiger-mode avalanche photodiode (Gm-APD) laser detection and ranging (LADAR) carries out the air-to-ground detection, it can detect the echo that is successively reflected by targets and the ground, which causes the false target in the image obtained by the LADAR. In this paper, the echo which is successively reflected by two surfaces is called the secondary reflected echo, and the echo which is just reflected once is called the single reflected echo. Considering the Gm-APD’s superior ability to detect photon, we investigate and analyze the triggering characteristics of the Gm-APD for detecting the secondary reflected laser echo from the targets, also, we carry out the experiments on detecting the secondary reflected echo in near field and far field by using array Gm-APD to verify that the array Gm-APD can effectively detect the secondary reflected echo, and this paper lays the foundation for the research of the secondary reflection’s influence on the images obtained by Gm-APD. In this paper, firstly, we use the echo triggering model, which contains LADAR ranging equation and Poisson probability model, and the Monte Carlo simulating method to completely verify the possibility of detecting the secondary reflected echo by using Gm-APD, in theory. The theoretical results show that Gm-APD can detect the secondary reflected echo, and the wider the field of view(FOV) is, the longer the detecting range is, besides, the higher the LADAR is located at, the lower the target’s secondary reflected echo’s intensity is and the higher the ground’s single reflected echo’s intensity is; then, using the Gm-APD with an array of 64 × 64 and 32 × 32 to detect the indoor scene and the moving van in far field, respectively, the results show that the indoor experiment obtains the complete first and second reflected image and the single pixel can detect the secondary reflected echo from multiple targets, also, for the far-field experiment, the single reflected echo and second reflected echo can be obtained in a single frame image, and under this experimental conditions, two pixels which are studied, realize the range extension of 25.6 m and 41.9 m respectively, which shows that the Gm-APD can likely detect the false target in the far field; finally, the methods that finding the secondary reflected surface by using the multiple-frame statistics for static targets and the image of triggering position in chronological order for moving targets, respectively, are proposed to eliminate the secondary reflected image. This article can provide some theoretical references for eliminating the secondary reflection before target recognition.

Aiming at the problem that the Geiger-mode Avalanche Photodiode (GM-APD) is susceptible to background noise and the detection effect decreases during the day, based on the polarization GM-APD detection model, a set of GM-APDbased polarized lidar imaging experimental equipment is proposed. Using this equipment to image the target in the simulated sunlight environment, the effect of polarization detection on the echo triggering performance was studied. The results show that, compared with the non-polarized system, the polarized system reduces the impact of noise on target detection, improves the image quality, and reduces the false alarm probability of the image. When the laser single pulse energy is 400nJ, and the polarization angle is 135° , the trigger probability of the metal plate is increased by 10.5%, and its false alarm probability is reduced by 4.6%; the trigger probability of the rough wood board is reduced by 15.2%, and its false alarm probability is reduced 8.9%.Due to the deterioration of imaging results caused by background noise, it is proposed to use polarization degree images for further background filtering to extract a more complete target contour. This research work provides experimental support for the effective detection of GM-APD lidar during the day.

We propose a laser nutation tracking sensor for beaconless laser communication, which uses a micro-electro-mechanical system (MEMS) mirror to achieve high-efficiency and large-amplitude nutation at its resonant frequency. We derive a new formula for the case of incompletely detectable optical power in the nutation cycle. In the experiment, we measure the performance of the sensor in calculating boresight error under three different nutation radii. Combining with the proposed algorithm for the new scene, we complete the accurate boresight calculation in the range of ± 200 µ r a d , at the nutation radius of 4.9 µm. We trust that the receiving field of view (FOV) of this tracking sensor can be further expanded by increasing the nutation radius. The sensor, as proposed in this paper, will be of constructive help to simplify tracking systems in the future.

The coil spring is an important element in the suspension system of railway vehicles, and its structural vibration caused by the mass distribution can deteriorate the dynamic performance of the vehicle. However, the coil spring is usually modelled as a simple linear force element without considering the dynamic characteristics in multibody dynamic simulations of railway vehicles. To integrate the dynamic characteristics of the coil spring into the simulation, three equivalent dynamic models of the coil spring are established by treating the coil spring as multimass spring series, Timoshenko beam, and flexible spring, respectively. The frequency-sweep method is applied to obtain the dynamic response of the proposed models of coil spring, and the accuracy of the models’ results has been compared and verified by the laboratory test. Results show that all of these three equivalent models can reflect the influence of the spring mass distribution on its dynamic responses. Compared with the mass-spring series and beam element equivalent models, the flexible spring model can better reflect the dynamic stiffness and stress of the coil spring changing with the exciting frequency. Thus, the flexible spring model proposed in this paper is more applicable to railway vehicle system dynamics and the fatigue analysis.

The enhanced self-heterodyne synthetic aperture imaging ladar (SAIL) self-heterodynes two polarization-orthogonal echo signals to eliminate the phase disturbance caused by atmospheric turbulence and mechanical trembling, uses heterodyne receiver instead of self-heterodyne receiver to improve signal-to-noise ratio. The principle and structure of the enhanced self-heterodyne SAIL are presented. The imaging process of enhanced self-heterodyne SAIL for distributed target is also analyzed. In enhanced self-heterodyne SAIL, the phases of two orthogonal-polarization beams are modulated by four cylindrical lenses in transmitter to improve resolutions in orthogonal direction and travel direction, which will generate ghost image. The generation process of ghost image in enhanced self-heterodyne SAIL is mathematically detailed, and a method of eliminating ghost image is also presented, which is significant for far-distance imaging. A number of experiments of enhanced self-heterodyne SAIL for distributed target are presented, these experimental results verify the theoretical analysis of enhanced self-heterodyne SAIL. The enhanced self-heterodyne SAIL has the capability to eliminate the influence from the atmospheric turbulence and mechanical trembling, has high advantage in detecting weak signals, and has promising application for far-distance ladar imaging.

We report a coherent tracking system based on fiber nutation for inter-satellite beaconless laser communication, which uses a piezo-electric ceramic tube (PCT) to drive the end face of single mode fiber (SMF) for nutation, and uses coherent demodulation method to directly calculate the boresight error from the intensity envelope fluctuation of signal light. The method is given theoretically and verified experimentally. Under the condition of fiber nutation frequency is 2000Hz and nutation radius is 1.1um, the experimental verification results in our interested range of signal light power (1nW-10nW) meet our design requirements. The receiving field of view (FOV) of tracking system is more than 300urad, and the closed-loop tracking bandwidth (-3dB) is about 115 Hz. When the boresight error is fixed at 80urad, the real calculation error is less than 10%. The closed-loop performance of tracking system is insensitive to the change of signal light power. Our coherent tracking system is of great significance to the inter-satellite beaconless laser communication.

Optical fiber links have proven their powerful ability to transfer the ultrastable clock signals over a national and continental metrological optical network. In this Letter, we propose and demonstrate a new scheme that time and frequency signals are modulated on the same laser wavelength with phase and intensity modulation by an independent electro-optic modulator, respectively. Different wavelengths are used in two opposite directions to prevent Rayleigh scattering. The noise changes caused by the fluctuations of the transfer links are actively canceled at the local site by optical delay lines. Radio frequency with 1 GHz and time signal with one pulse per second (1 PPS) are transmitted over 110 km fiber spools. The experimental results demonstrate that frequency instabilities of 1.7 E − 14 at 1 s and 5.9 E − 17 at 10 4 s and time interval transfer of 1 PPS signal with sub-ps stability after 1000 s are obtained. This novel scheme offers advantages with respect to reduce the channel in the fiber network, and can keep the time and frequency signal free from interference with each other.

Optical fiber links have demonstrated their ability to transfer the ultra-stable clock signals. In this paper we propose a new method to transfer both time and radio frequency via the same wavelength. Time signal is encoded as a binary phase-shift keying(BPSK) to the optical carrier using electro optic modulator(EOM) and the frequency signal is modulated onto the optical carrier by an EOM amplitude modulation. We have demonstrated the experiment over 50 km fiber spools and the result shows that relative stabilities 7E-15 at 1s. Moreover, longterm timing stability is lower than 2ps. This technique may have application in Very Long Baseline Interferometry, et al.

A high-precision inter-satellite velocity measurement method based on two-one-way laser Doppler is presented in this paper. This method's working principle and signal-to-noise ratio's effect under different measurement times of signal on velocity precision are analyzed theoretically. This method is also tested by laboratory experiments and 1 mm/s velocity precision is achieved in 1 ms integrating time. The proposed method potentially contributes to inter-satellite velocity measurement, especially for the relative velocity measurement between two satellites in high dynamic motion and a long distance apart.

We propose a pointing, acquisition and tracking (PAT) system without independent fine beacon for coherent receiver in inter-satellite optical communication, which is compatible with two modes: no beacon mode and no fine beacon mode.

A static-mode synthetic aperture imaging ladar (SAIL) in which the target and carrying platform are kept still during the collection process is proposed and demonstrated. A target point of 0.5 mm \times 0.5 mm and a two-dimensional (2D) object are reconstructed in the experiments, in which an optical collimator with a focal length of 10 m is used to simulate the far-field condition. The achieved imaging resolution is in agreement with the theoretical design. The static-mode down-looking SAIL has the capability to eliminate the influence from the atmospheric turbulence and can be conveniently operated outdoors.

Design the electric energy meter constant magnetic field verification device system, carry out the constant magnetic field verification, and realize the electric energy meter constant magnetic field automatic verification. To ensure the accuracy and traceability of the magnetic field, a magnetic field sensor is installed at the bottom of the tray, and the test magnetic field is read in real time through the magnetic field sensor. The device is fixedly connected with an infrared sensor to detect the rotation angle of the magnetic steel. The operation of the three-axis motor of the magnetic field detection device changes the position of the magnetic steel around the bottom and top of the electric energy meter to complete the verification of the entire electric meter. The electric energy meter constant magnetic field verification device has high accuracy, strong safety, and can complete the automatic detection function.

When imaging through fog using Geiger mode avalanche photon diode (Gm-APD) ladar, strong backscattering photons make it difficult to effectively extract an accurate target signal via the traditional peak-select algorithm. Single-parameter estimation (SPEA) is proposed in this paper to solve this problem. First, the physical properties of the gamma distribution model’s characteristics are analyzed, and the model is improved based on Gm-APD ladar’s counting characteristics. Next, a Monte Carlo simulation for photons propagating in fog is conducted. The SPEA’s backscattering distribution has the highest correlation with the original backscattering photon distribution. Realistic imaging data are processed through fog, and the SPEA has the best recovering effect, achieving 71% recovery using 20,000 frames. This study demonstrates that the SPEA enables imaging through fog with Gm-APD ladar.

In the near-field underwater detection of lidar, the low accuracy of time integration model leads to a lot of noise and a low signal to backscatter ratio during target signal extraction. Aiming at this problem, this paper divides the water into thickness units according to the scale time of the AD acquisition card, and establishes a near-field water echo model of lidar called distance layer summation model(DLSM). According to the underwater optical transmission theory, this paper establishes the formula for the time delay of the echo signal of the water with the system angle. The pulse width error of the calculated and measured results is 0.55ns. The experimental results after normalization show that compared with the time integration model, the pulse width percentage error of the model in this paper is reduced by 18.7 %, the root mean square error is reduced by 0.04, and the signal to backscatter ratio is increased by 8.74 dB. The simulation results of the model in this paper are more consistent with the measured water waveform, which can lay the foundation for the subsequent work of water backscatter suppression and target echo signal extraction.

The bit-error-rate performance of free space optical communication systems with binary phase shift keying modulation and coherent homodyne detection is performed. Besides the turbulence-induced wave-front phase error and the amplitude fluctuation, the servo bandwidth of adaptive optics system is investigated. It is shown that Greenwood frequency is large enough for the servo bandwidth of adaptive optics system when the detected photons per bit are more than 100. However, if the photons per bit are less than 70, the Greenwood frequency is only sufficient for weak scintillation. We should increase the servo bandwidth to almost twice the value of Greenwood frequency at least in order to obtain an acceptable BER performance when the scintillation index is larger than 0.7. In addition, we also investigate the aperture averaging effects when the receiving aperture is larger than the coherent length.

The phase coded (PC) waveform in synthetic aperture ladar (SAL) outperforms linear frequency modulated (LFM) signal in lower side lobe, shorter pulse duration and making the rigid control of the chirp starting point in every pulse unnecessary. Inherited from radar PC waveform and strip map SAL, the backscattered signal of a point target in PC SAL was listed and the two dimensional match filtering algorithm was introduced to focus a point image. As an inherent property of laser, linewidth is always detrimental to coherent ladar imaging. With the widely adopted laser linewidth model, the effect of laser linewidth on SAL image quality was theoretically analyzed and examined via Monte Carlo simulation. The research gives us a clear view of how to select linewidth parameters in the future PC SAL systems.

Gm-APD (Geiger-mode avalanche photodiode) is widely used in the LIDAR (Light Detection and Ranging) for its excellent photon detection capability. However, the low SNR (Signal-to-Noise Ratio) limits the laser echo's triggering probability and enhances the false alarm probability in the daytime, which severely affects the target's detection results, moreover, it increases the difficulty of the signal reconstruction algorithm. So it's an important path to relieve the algorithm's burden and get a better detection results by fundamentally increasing the echo's SNR. Nevertheless, there is a lack of researches discussing such issue, especially for array imaging detection. In this paper, it was proposed that by using the polarization detection could help to enhance the echo's SNR and get good detection results in the daytime. By combining polarization bi-directional reflection distribution function (pBRDF), Mueller matrix, LIDAR equation, and Poisson models, the polarization Gm-APD LIDAR detection model was first established to describe the received Stokes vector as well as the triggering and false alarm probabilities, which could aid in the daytime polarization detection including polarization analysis, depolarization contrast analysis, detection probabilities analysis, and the feasibility of the models was supported by the simulation and experimental results. Based on the established model, the triggering properties of the Gm-APD LIDAR with the polarization system were discussed, and the theoretical analysis showed that the optimal detection angle was 135°, at which the polarization detection could effectively increase the triggering probability and lower the false alarm probability. A series of outdoor experimental verifications were carried out, and the experimental results showed that the proposed Gm-APD polarization detection model and the theoretical analysis were absolutely correct. Besides, compared with the non-polarization detection, the intensity image of optimal polarization detection could completely obtain the targets’ contour, moreover, the triggering probabilities of two targets increased by 4.9% and 7.1%, respectively, and the false alarm probabilities lowered by 26.3% and 21.4%. It made great improvement in the Gm-APD detection research. This work offers the theoretical and experimental supports for enhancing Gm-APD LIDAR's detection results in the daytime by using polarization detection, furthermore, it can lay some theoretical foundations for target recognition of Gm-APD polarization detection.

Multi-aperture receiver with optical combining architecture is an effective approach to overcome the turbulent atmosphere effect on the performance of the free-space optical (FSO) communications. In this paper, we built a 1 km urban horizontal link, and adopted beam combining system with four apertures at receiver. By analogy with aperture smoothing factor, we proposed the concept of fluctuation suppression ratio to evaluate the mitigation ability of the system on atmospheric fading. The experimental results confirmed that the beam combining system can perform best when the aperture interval is larger than the coherent length, and the performance decreases with the increase of the coherent length. Moreover, we found that the change of phase differences between the input beams is more than 40 π rad, which provides reference for future system design.

It is not feasible to use the Geiger-mode avalanche photodiode (Gm-APD) detection model to guide detection quantitatively during daytime, because of the incompletely built echo intensity model. In this study, because the background noise and the signal light are both modulated by the target's optical-scattering characteristics (OSCs), a hybrid light-scattering model (the modified kernel-based bidirectional reflection distribution function (BRDF) model) is substituted in the laser detection and ranging (LADAR) ranging equation. In such a BRDF model, the Li-Liang-BRDF model is taken as the kernel, and the background noise equivalent intensity coefficient is employed to link the background noise part to the laser echo part. According to the results of a Gm-APD outdoor experiment, the proposed LADAR ranging equation is applicable to detection during daytime, and the noise scaling factor d(θs) is not a constant with the variation of solar zenith angle; the signal intensities in the theoretical and experimental results only exhibit a difference of 5%. The effects of various factors (e.g., solar zenith angle, LADAR observation angle, and different target materials) on the echo signal-to-noise ratio and triggering probability are analyzed by the proposed LADAR ranging equation. Moreover, a maximum-likelihood estimation-based theoretical method is proposed to calculate the maximum detection range during daytime. Subsequently, the change law of the detection range in daytime is analyzed, and the calculated detection range of the developed Gm-APD LADAR reaches 3.5 km under an actual daytime background. This study can provide theoretical support and formulates several strategies for Gm-APD detection in daytime. Furthermore, novel ideas can be proposed to calculate theoretically the effective detection range of Gm-APD LADAR.

The influences of nutation trail accuracy, simplification of coupling model, spot position jitter, and power variation of incident light on the detection error are analyzed theoretically. Under the condition of satisfying the requirements, the nutation radius is less than 1.13 μm, the accuracy of the nutation trail is less than 0.04 μm, and the detection range is [−5 μm, +5 μm]. The nutation frequency is 160 times spot position jitter frequency and 100 times intensity jitter frequency of incident light. The analysis is of great significance for determining nutation radius and frequency in the tracking system based on fiber nutation.

3D coherent imaging ladar based on frequency modulated continuous wave (FMCW) technology is proposed. The triangle wave is used to modulate chirped laser. The distance information is detected by optical heterodyne technique. Meanwhile, the beam scanning is realized by rotation double-prism. And the 3D target image is obtained. The 3D coherent imaging ladar has advantages of anti-background interference, high sensitivity and long-detected distance. Under the condition of the strongly sunlight background, the targets outdoor are scanned and high resolution images are achieved. This technique has a great potential for applications in extensive imaging ladar fields.

The performance of satellite-to-ground downlink optical communications over Gamma-Gamma distributed atmospheric turbulence are discussed for a direct detection differential phase shift keying system with spatial diversity, which combines the beam coherently before demodulation. Bit-error rate (BER) performances for various numbers of apertures are analyzed and compared for different zenith angle. We also consider the effect of the fiber coupling efficiency and loss ratio of beam coupler to the final BER. The results of numerical simulation shows that the advantage increases with the number of aperture. All the numerical results are verified by Monte-Carlo simulations.

The electric field distribution in LiNbO3 crystal under different electrode shape is presented by using the digital holographic interferometry. Three configurations of phase modulator including the rectangular electrode type, single-triangle electrode type, and dual-triangle electrode type are performed in this experiment. The nonuniform electric field distribution in these phase modulators are observed and the electric field increases with voltage increasing. The digital holographic interferometry with high electro-optic effect improves the measurement precision. The digital holographic interferometry provides an effective way for studying the electric field distribution. Such in situ quantitative analysis of electric field distribution is a key to optimizing electrode shape.

Temporally and spatially varying speckle effect arises as a consequence of the frequency modulation chirped laser signal employed in synthetic aperture imaging ladar (SAIL).A variety of reconstructed images degraded by laser speckle effect have been reported. In this paper, space–time speckle effects and their influence on imaging based on the SAIL system in a far-field diffraction region are systematically studied. The first half of this paper provides the theoretical analyses of the 2D data acquisition with speckle effect in SAIL. Numerical simulations of the temporally varying speckle pattern, the integrated speckle field over a receiving antenna and their influence to the image quality for SAIL are obtained in the remaining of the paper. Our results will be valuable for further studies on the suppression of speckle effect in SAILs.

A duplex optical communication link has been verified in orbit between a LEO satellite and a optical ground station (OGS), the down link used Differential Phase Shift Keying (DPSK) modulation format and allowed to transmit data with data rate of 5.12Gbps, without error correction coding, the bit error rate (BER) was better than 10−6 with the link distance exceeded 1500 km, and an average BER of 1.9×10−9 was realized in 306s. The uplink used 16-ary PPM (Pulse Position Modulation) technique with a data rate of 20Mbps, and a BER below 10−6 was also realized.

Synthetic aperture imaging ladar (SAIL) is one of the most possible optical active imaging methods to break the diffraction limit and achieve super-resolution in a long distance. Nevertheless, two-dimensional reconstructed images of the natural targets have not been achieved. Polarization state change of the backscattered light, which is always determined by the interaction of the light and the materials on the target plane, will affect the imaging of SAIL. The Mueller matrices can describe the complex polarization features of the target reflection and treat this interaction. In this paper, a measurement of the Mueller matrices for different target materials will be designed, and the influences of polarization characteristic of targets on resolution element imaging in side-looking and down-looking SAILs will be theoretically analyzed.

Down-looking synthetic aperture imaging ladar have bidirectional (positive and negative direction) modulation in the orthogonal direction of travel during phase modulation. The return signal can be also collected in the two directions. The imaging processing with bidirectional modulation is used and demonstrated. The signal-to-noise ratio can be enhanced in this mode synthetic aperture imaging ladar. Meanwhile, the velocity of carrying-platform can be faster. In the experiment, the return signals with bidirectional modulation are stacked and rebuilt. Compared to the unidirectional modulation imaging, the faster and clearer imaging is realized with bidirectional modulation. This technique has a great potential for applications in extensive synthetic aperture imaging ladar fields.

Performance of satellite laser communications between GEO and LEO satellites can be influenced by background light noise appeared in the field of view due to sunlight or planets and some comets. Such influences should be studied on the ground testing platform before the space application. In this paper, we introduce a simulator that can simulate the real case of background light noise in space environment during the data talking via laser beam between two lonely satellites. This simulator can not only simulate the effect of multi-wavelength spectrum, but also the effects of adjustable angles of field-of-view, large range of adjustable optical power and adjustable deflection speeds of light noise in space environment. We integrate these functions into a device with small and compact size for easily mobile use. Software control function is also achieved via personal computer to adjust these functions arbitrarily. Keywords

Active polarization imaging technology has recently become the hot research field all over the world, which has great potential application value in the military and civil area. By introducing active light source, the Mueller matrix of the target can be calculated according to the incident light and the emitted or reflected light. Compared with conventional direct detection technology, optical heterodyne detection technology have higher receiving sensitivities, which can obtain the whole amplitude, frequency and phase information of the signal light. In this paper, an active polarization imaging system will be designed. Based on optical heterodyne balanced receiver, the system can acquire the horizontal and vertical polarization of reflected optical field simultaneously, which contain the polarization characteristic of the target. Besides, signal to noise ratio and imaging distance can be greatly improved.

A high sensitive coherent receiver for free space laser communication was demonstrated. Based on a digital Costas phase-locked-loop, a sensitivity of -58dbm was achieved with bit error rate below 10-3 at bit rate of 1Gbps.

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 2 W, and the modulation contrast is 15 dB. 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 125 mm, and the final communication field of view is 4 mrad. Without error correction coding, the sensitivity of −44.5 dBm was achieved, while the bit-error rate was below 10−3 at 70 Mbps. We also established a laser communication experiment at the laboratory; the platform jitter below 5 Hz 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.

Owing to the weak signals produced by all-fiber coherent lidar systems, when the instability of the local oscillator laser power is greater than the target echo signal, it is difficult to extract a target's intensity image. In this study, an intensity imaging method for weak signal all-fiber lidars is proposed. First, a phase compensation method is used to correct the position of the heterodyne signal in the time domain to reduce the impact of noise on the positioning heterodyne signal. In addition, an algorithm is proposed to extract the weak echo signal from the corrected heterodyne signal in the time domain to obtain the relative intensity of the echo signal of a single pixel point. Finally, we analyze and verify the proposed imaging method by using false alarm rates, range, intensity accuracy, and the speckle characteristics of the target. The method proposed in this study only requires that the phase of the heterodyne signal be corrected by the proposed numerical method without the need for other optical equipment, thus simplifying the entire system. It is very important to improve the detection sensitivity of coherent lidar remote imaging system.

The availability and reliability of the high data rate space-to-ground optical coherent communication links are critically challenged because the wave front of the signal beam is distorted and impaired when propagating through atmospheric turbulence. Based on the free-space interference of two successive data bits delayed by an unequal-arm-length Mach–Zehnder interferometer (MZI), a pupil-matching time-delay self-homodyne interference differential phase-shift keying (DPSK) optical receiver with 2 × 4 90-deg optical hybrid is designed for the high data rate space-to-ground optical communication links due to its immunity from wave front impairment. The delayed optical path difference (OPD) in the unequal-arm-length MZI corresponds to the duration of one bit and can be stabilized to below 1000th of the wavelength by the closed-loop feedback control. The maximum system insertion loss is

A scanning three-dimensional coherent laser radar (ladar) based on the frequency modulated continuous wave (FMCW) is proposed and demonstrated, which can obtain many three-dimensional high-quality images. The system utilizes an electro-optic modulator and an optical filter to output a linear FMCW with a bandwidth of 2 GHz. The flexible and variable rotating double prism is used for beam scanning. The images of flight demonstration are formed by attitude compensation correction. The experiment result validates the performance of our system for airborne three-dimensional scanning imaging.