Electro-optical (EO) tracking systems, while exhibiting strong nonlinear characteristics, are difficult to accurately model. Nonlinear resistance torque is proposed to describe the system’s nonlinear phenomenon and the genetic algorithm is used to identify model parameters. The model’s root-mean-square error (RMSE) was reduced using nonlinear resistance torque by 2.5 times compared to the Stribeck friction model and by 12 times compared to the linear model. Under the identified model, the system’s nonlinearity was effectively compensated. The results demonstrate the feasibility of the proposed method for the identification of EO tracking systems.
An experimental platform was built for the atmospheric turbulence simulation upon a liquid crystal spatial light modulator. The phase-only spatial light modulator is driven by a series of dynamic phase screens that have non-Kolmogorov turbulence characteristics. After passing through the simulated non-Kolmogorov turbulence, the angle-of-arrival (AoA) and light intensity of the laser beam were analyzed statistically. The variation trends of the AoA fluctuation and the scintillation index exhibited by the experimental results are in accordance with the theoretical predictions. Although there was some divergence between the experiments and theory that needs to be improved further, the experimental data demonstrate the feasibility of using the platform to simulate the turbulence influence on laser communication in the lab.
In the spatial coherent optical communication, after long distance transmission, the receiver needs to detect and extract the weak signal, and the balanced detection technology is the key technology in the coherent detection. Using balanced detection technology, not only can further improve the receiving sensitivity of coherent optical detection, but also can suppress the common mode intensity noise, so as to improve the output signal to noise ratio of the system. But the problems are (1) due to external causes (polarization control, input optical power loss error, atmospheric turbulence caused by decay, and so on),at both ends of the balanced detector’s input signal can not keep consistent (2) two arms of balanced detector electronic devices (including photodiode and amplifier electronic components) are difficult to achieve perfectly matched. The paper is based on the principle of coherent optical detection, firstly, the advantage of balanced detection compared to single detection is detailed analysis, it is shown that balanced detector used in coherent optical communication system and with the improvement of communication rate, this advantage will more obvious. The theoretical and experimental results have verified several important factors that affect the efficiency of coherent balanced detection, namely, the consistency of the detection response, the atmospheric turbulence and the tracking jitter. It is of reference significant to study the balance detection technology in the spatial coherent optical communication.
In order to improve the dynamic tracking performance of coarse tracking system in space optical communication, a new control method based on active disturbance rejection controller (ADRC) is proposed. Firstly, based on the structure analysis of coarse tracking system, the simplified system model was obtained, and then the extended state observer was designed to calculate state variables and spot disturbance from the input and output signals. Finally, the ADRC controller of coarse tracking system is realized with the combination of nonlinear PID controller. The simulation experimental results show that compared with the PID method, this method can significantly reduce the step response overshoot and settling time. When the target angular velocity is120mrad/s, tracking error with ADRC method is 30μrad, which decreases 85% compared with the PID method. Meanwhile the disturbance rejection bandwidth is increased by 3 times with ADRC. This method can effectively improve the dynamic tracking performance of coarse tracking and disturbance rejection degree, with no need of hardware upgrade, and is of certain reference value to the wide range and high dynamic precision photoelectric tracking system.
Increasingly importance has been taken seriously for high frame rate CMOS camera to optical communication acquisition pointing and tacking (APT) system, with its compact structure, easy to developed and adapted to beacon light spot detection in atmospheric channel. As spot position accuracy directly determines the performance of space optical communication, it is very important to design a high precision spot center algorithm. Usually spot location algorithm uses gravity algorithm, shape center capturing algorithm or self-adaption threshold algorithm. In experiments we analyzed the characteristics of the spots which transmitted through atmospheric turbulence and studied light transmission characteristics in turbulent channel. We carried out a beacon light detection experiments in a distance of 3.4km, collected the beacon spots on CMOS camera and signal light power. We calculated spot position with two different algorithm and compared the calculation accuracy between field dispersive spot and ideal Gaussian laser spot. Experiment research show that, gravity center algorithm should be more suitable for beacon beam spot which accuracy can be improved about 1.3 pixels for a Gaussian spot. But the shape center algorithm has higher precision. The reasons were analyzed which made an important preparation for subsequent testing.
KEYWORDS: Unmanned aerial vehicles, Solar cells, Solar energy, Laser applications, Lithium, Wireless energy transfer, Control systems, Gallium arsenide, Laser systems engineering, Laser processing
Currently various types of aircraft booming and maturing, however, their long-time navigational capability should be improved urgently. This paper aims at studying laser power beaming, which includes the technology of high-efficient photoelectric conversion and APT(acquiring, pointing and tracking) technology, to provide power for flying UAV(unmanned aerial vehicles) and improve their flight endurance.
The experiment of testing different types of solar cells under various conditions has been done to choose the solar cell which has the highest photoelectric conversion rate and find its most sensitive wavelength. In addition, the charge management module has been chose on the base of the characteristics of lithium batteries. Besides, a laser APT system was designed and set up, at the same time FSM (Fast Scan Mirror) control program and digital image processing program were used to control the system. The success of the indoor experiment of scan-tracking and charging for the moving UAV model via laser proves that this system is workable. And in this experiment, the photoelectric conversion rate of the whole system is up to 17.55%.
Communication experiments of free space optical system which was laid on the vibration platform were implemented. In the communication experiments, the transmitting terminal is static and the receiving terminal is laid on the vibration platform. The vibration platform was used to validate the performance of the APT (acquiring, pointing and tracking) system. The analytic results of experimental data prove that the APT system can effective restrain the destabilization of laser beam caused by the vibration platform. The communication distance is 3.4 km and the communication rate is 2.5 Gbps.
A simulation platform is established for target motion using a liquid crystal (LC) spatial light modulator as a nonmechanical beam steering control device. By controlling the period and orientation of the phase grating generated by the spatial light modulator, the platform realizes two-dimensional (2-D) beam steering using a single LC device. The zenith and azimuth angle range from 0 deg to 2.89 deg and from 0 deg to 360 deg, respectively, with control resolution of 0.0226 deg and 0.0300 deg, respectively. The response time of the beam steering is always less than 0.04 s, irrespective of steering angle. Three typical aircraft tracks are imitated to evaluate the performance of the simulation platform. The correlation coefficients between the theoretical and simulated motions are larger than 0.9822. Results show that it is highly feasible to realize 2-D target motion simulation using the LC spatial light modulator.
A simulation platform is set up to study the ship-borne laser communications. A liquid crystal spatial light modulator is used as a non-mechanical beam steering control device to simulate beam motions of moving platforms. Because of its light weight, low power and compact form, the liquid crystal spatial light modulator is superior to its mechanical counterparts. The theory of beam steering is introduced firstly. And then the performance of the simulation platform is researched and evaluated. The steering angles range from -2.89° to 2.89° with the control precision of ~ 0.02°. The simulated sinusoidal frequency of the spatial light modulator can reach 4 Hz in maximum. Finally, the beam motions of the ship platforms with different natural rolling periods are simulated based on the platform. The correlation coefficients between the theoretical and the simulated motion curves are greater than 0.9805. Results show that it is feasible to realize the beam motions simulation of specific moving platforms using the liquid crystal spatial light modulator.
The dynamic atmospheric turbulence is simulated in the laboratory upon the phase-only liquid crystal spatial light
modulator. Dynamic phase screens are generated by the spline function method and the frozen turbulence method. The
average cross-correlation coefficient of the Zernike coefficients between these two methods is 0.6608. Moreover, the
laser atmospheric transition experiment is carried on under different turbulence intensities. The logarithmic light
intensity probability density distribution is close to the normal distribution, and the fitting determine coefficient is above
0.9. In the weak turbulence (r0=0.1m), the standard deviation of the arrival angle fluctuation is approximately 30 to 40
μrad, while it is 40 to 50 μrad in the moderate turbulence (r0=0.01m). The simulation result is compliant with the
turbulence theory. Compared with the frozen method, the arrival angle fluctuation spectrum in high frequency upon the
spline method is smoother. It reveals that although two methods have a good consistency with each other, the dynamic
simulation of the spline method is supposed to be more favorable in the researching of the free-space laser propagation.
Based on DWDM and EDFA technology, 7.5 Gbps free space laser communication demonstration system was developed.
Experiments of 7.5 Gbps 40km free space laser communication were completed in Qinghai lake Qinghai province on the
Aug. 2010.Error free communication was achieved at the speed of 155 Mbps, and 10-7 BER was achieved at the speed of
2.5Gbps, 5Gbps and 7.5Gbps test with BER of
10-8~10-7 and 10-7 ~ 10-6 were completed. Laser atmospheric propagation
experiments were conducted and analyzed.
To reduce of setup time of communication link and increase of acquisition probability for free-space optical
communication, the GPS technology is applied and the unidirectional acquisition experiment system, controlled by user
interface, is established with the control core of TMS320F2812. the coordinate from GPS is input to user interface by
manual, formed into the horizontal- and vertical- Angle to adjust based on the GPS coordinate and sent to F2812 for
driving of two-dimension platform to finish initial acquisition. The acquisition experiment is done many times and the
result show that the quick and high probability acquisition could be achieved based on GPS technology.
A 2.1 km terrestrial free-space optical data link operating at 5 Gbps by using WDM and 1550 nm fiber communication techniques was established. Two 2.5 Gbps pattern generators are used to directly modulate 1530 nm and 1550 nm DFB lasers respectively. The output of each modulated optical signal are combined by a CWDM, and then amplified by a 20dBm EDFA. The amplified optical signal is then applied to a specially designed transmitting telescope. The received optical signal is focused onto the core of a 62.5μm multi-mode optical fiber using coupling optics within the receive telescope. The coupled optical signal is interfaced to a de-multiplexer to separate the two individual wavelengths, where each of two outputs is then connected to a 2.5 Gbps receiver. The BER and optical stability measurements are presented to quantify and demonstrate link quality for a 2.1 km span. BER and received optical coupled power data is presented to illustrate atmospheric effects associated with typical link performance.
As the effects of various weather conditions on laser beams propagating in the atmosphere are hard to mitigate, in order to obtain high quality of the laser propagation, a new method is put forward in this paper. The theory uses feedback compensation technology to mitigate atmospheric scintillation effect, namely collecting the atmospheric turbulence factor in real time, and feedback it to the transmitter of laser real-timely. An experiment and simulation tests to verify the effect of feedback compensation have been taken to analyze the feedback effects and feasibility. The obtained results show the method can mitigate atmospheric scintillation in the atmospheric transmission.
It is general that with the distance increasing the communication quality of Free Space Optical (FSO) drops seriously, and the maximum transmitting distance is usually about 2 kilometers. In order to increase the covering range and transmission quality of the FSO system, a relaying device which is between the nodes is researched in the paper. In the device, the optical signal is transformed into electric signal by phototube. The device analyzes the receiving signal, acquires the effective signals and normalizes the signal waveform. Then, the reshaped signal is sent to the objective node. It can reshape, retime and regenerate signals, and the covering range can be extended. At the same time the transmission quality can also be improved. The approach is able to permit maximum transmitting distance more than 2km without deterioration of signal quality. At present, the experiment in our lab has already realized the 200m and 8M communication with the relaying technology successfully. The details are presented in the paper.
A portable protocol independent free space optical communication terminal was developed, which enables customer to quickly deploy optical bandwidth services for applications such as fiber extension, wild field point to point communication and wireless backhaul while avoiding costly and time-consuming fiber installation. By using specially designed optical components and optical-mechanical structure, the system is very compact and effective, can establish optical link within a few minutes, with total weight 4kg, size 160 x 360 x 155 mm, effective transmitting/receiving aperture 40mm, data rate 100Mbps, maximum communication distance 1500m. The system and experiments are presented in the paper.
In this paper, an adaptive transmission system with a special multiplex core for the optical wireless communication (OWC) is proposed, which can adjust the transmission mode automatically according to the channel status information (CSI). The atmosphere channel is a time-variable channel in which link performance could be affected by weather and scintillation. Adaptive transmission (AT) technique is introduced to solve the problem. Certain CSI that can evaluate the quality of atmosphere channel should be chosen to vary the transmission mode of OWC terminals in order to confront bad weather condition and maximize average throughput of transceiver. Considering that the system can choose suitable communication bit-rate, transmission power and with or without channel coding and some other flexible features, the special multiplex core has been used to guarantee the data to be multiplexed correctly. The data can be packaged into fixed length frame with the channel ID. Data stream is buffered by Dual-port FIFO with multi-clock feature and multiplexed by a smart controller with a certain algorithm. The special system structure makes it possible to replace the data interface without any change of the rest parts.
We give the definition of USB wireless laser communication system (WLCS) and the brief introduction to the protocol of USB, the standard of hardware is also given. The paper analyses the hardware and software of USB WLCS. Wireless laser communication part and USB interface circuit part are discussed in detail. We also give the periphery design of the chip AN2131Q, the control circuit to realize the transformation from parallel port to serial bus, and the circuit of laser sending and receiving of laser communication part, which are simply, cheap and workable. And then the four part of software are analyzed as followed. We have consummated the ISR in the firmware frame to develop the periphery device of USB. We have debugged and consummated the 'ezload,' and the GPD of the drivers. Windows application performs functions and schedules the corresponding API functions to let the interface practical and beautiful. The system can realize USB wireless laser communication between computers, which distance is farther than 50 meters, and top speed can be bigger than 8 Mbps. The system is of great practical sense to resolve the issues of high-speed communication among increasing districts without fiber trunk network.
KEYWORDS: Digital signal processing, Infrared imaging, Field programmable gate arrays, Telecommunications, Image processing, Wavelet transforms, Detection and tracking algorithms, Free space optics, CMOS sensors, Infrared search and track
The paper gives a scheme of the algorithm and the architecture of the information processing system of IR beacon image in the APT system. The CMOS image sensor gets the IR beacon's image in the optical antenna. The FPGA implements the IR image preprocessing and the DSP offers the possibilities of processing large high-resolution images. The wavelet transform is used in the image preprocessing, which can decompose the image in multi-scale to select the different frequency components. The IR beacon's position is detected based on gravity center, and the precision servo system finishes the tracking of the IR beacons. Thus the system can implement the acquisition, pointing, tracking and establish the FSO communication link in real time.
The paper presents a scheme of the miniaturization of APT system and the design of the system based on the investigation of status in quo. It deals with the infrared image of the other terminal's beacon from the Charge Coupled Device (CCD) by the Complex Programmable Logic Device (CPLD). The result of the transaction is delivered to Single Chip Microcomputer (SCM), which controls the micro-servomotor. Subsequently, the precision drive system drives the optical system that uses only one light axis for signal beam and beacon to finish the acquisition, pointing, and tracking of the communication terminals. The anlayses of the APT system's error indicate that the tracking error limits in 70uRad with the weight of the system lighter than 8-kilogram.
This paper gives an expression that describes correlation between sinusoidal —modulated signal and average of interference signal in sinusoidal phase-modulated heterodyne interference. The expression can be used for estimating frequency-modulated coefficient of LD(laser diode)and the influence caused by sinusoidal modulation for the intensity of output LD. Experiments are carried out. The correlation is also used for the distance, the displace and the surface profile measurement.
Vibration of the interferometer signal is amplified and fed back to the laser diode after photo-electronic conversion, which controls the injection current of the laser diode. By using the character that the output frequency of the laser diode changes linearly with the injection current, the output frequency can be modulated by the injection current. Through this, phase vibration is compensated and interference pattern is stabilized. This paper describes the formula of the stability. The stabilization factor is measured and the main factors that affect the stability is described. These can supply references to the adjustment of the interferometer, the feedback module and the stability of the interferometer.
There is a trend towards satellite laser communication using advanced laser technology including developing a network based on microsatellite laser communication. But the advantage of small laser beam divergence creates a complex problem for acquisition, pointing and tracking. Microsatellite laser communication networks require research in systems and algorithms for acquisition. The purpose of the acquisition system is to compensate for large pointing errors at the beginning of the process and to achieve the minimal desired errors that will provide the beginning of the tracking and optical communication phase. In this process the requirement is to detect the optical signal with a given probability within a given time. The paper will describe several acquisition methods and compare their probability of acquisition and the acquisition time. These acquisition methods include Stare/Stare Method Scan/Stare Methoth Scan/Scan MethodS Stare/Scan Method. In the process we mainly discuss two kinds of scanning patterns. They are raster scanning and spiral scanning. Furthermore, we will examine influences of satellite vibrations on the acquisition scanning because this is the major difficulty in this process. We will give the integrate stimulation and suggest a correction algorithm.
Using the character that the emitting optical frequency of the laser diode is controlled by the injected current, the ability of eliminating environmental disturbance of the sinusoidal modulation laser diode active interferometer will be raised by more than one hundred times through putting the disturbed interference signal produced by the environment into the interferometer. When vibrating frequency of objects is different from that of the sinusoidol modulation, 'beat- frequency' will be produced in the interfere signal, which can be analyzed to get the vibrating frequency of objects. This paper described the operation principle and theoretical delusion of the 'beat-frequency' method.
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