激光通信技术

Modeling of Fine Tracking Sensor for Free Space Laser Communication SystemsHu Zhen,Song Zhengxun Tong Shoufeng, Zhao Xin, Song Hongfei, Jiang Huilin School of Electronics and Information Engineering Space Institute of Photo-Electronic TechnologyChangchun University of Science and TechnologyNo. 7089, Weixing Road, Changchun, P. R. China, 130022zhu@Abstract—The optical communication networks comprised of ground stations, aircraft, high altitude platforms, and satellites become an attainable goal, however, some challenges need to be overcome. One of challenges involves the difficulty of acquisition, tracking, and pointing (ATP) a concentrated beam of light arriving from another platform across the far reach of space. To meet the pointing accuracy requirement, the basic method of tracking between the terminals of optical communication systems includes the use of a beacon laser and tracking system with a quadrant detector sensor on each terminal. In some future optical communication networks, it is plausible to assume that tracking system and communication receiv ers will use the same sensor. In this paper, the architecture of the fine tracking assembly of the designing optical communication terminal (OCT) is described, and the fine tracking assembly sensor is modeled based on the correlation coefficient. The simulation and experiment results of the sensor show that the detecting accuracy satisfies the design demand for our developing OCT.Keywords-modeling; quadrant detector; fine tracking sensor; optical communication networksI.I NTRODUCTIONCommunication from one place to another on Earth is an attractive goal. To achieve this aim, the communication net-works that cover the globe are established. Future opticalcommunication network is pictured in Figure 1.Figure 1. Future optical communication network [1].The optical communication networks comprised of ground stations, aircraft, high altitude platforms, and satellites become an attainable goal, however, some challenges need to be overcome. Laser-based communication links between a satellite and another satellite or a high flying aircraft have been investigated for free-space communication systems They include European Space Agency’s (ESA) Artemis, Japan Aerospace Exploration Agency’s (JAXA) OICETS and the Department of Defense’s (DoD) TSAT [2]. Laser communication systems offer greater capabilities than RF systems, such as smaller size and weight of the terminals, less transmitter power, higher immunity to interference, and larger data rate, but present greater challenges in implementation. One of challenges involves the difficulty of acquisition, tracking, and pointing (ATP) a concentrated beam of laser arriving from another platform across the far reach of space [3].To meet the pointing accuracy requirement the optical communication terminals (OCT) mounted on satellite or other platforms use the Ephemeredes data (the position of the satellite according to the orbit equation) or navigation system for rough pointing, and a tracking system for fine pointing to another OCT. The basic method of tracking between OCT includes the use of a beacon laser and tracking system with a quadrant detector sensor on each OCT. In some future optical communication networks, it is plausible to assume that tracking system and communication receivers will use the same sensor. The reason is the possibility to design simple OCT at a reduced cost, mass, and volume in order to implement very compact, lightweight and low-power consumption precision beam-steering technologies. In view of this, a 4-quadrant detector (4QD) will be adapted in our developing OCT. Having a good mathematical description of the sensor is crucial for successful implementation of the tracking system, as it allows testing various control techniques prior to building a hardware prototype. This paper described the architecture of the fine tracking assembly of the designing OCT, proposed an approach to mathematical modeling of the fine tracking assembly sensor, and performed a number of experiments to validate the derived models.The remainder of this paper is organized as follows. Section II described the ATP subsystem architecture, the fine tracking assembly components briefly. The operating principle of 4QD, the operation of the position detecting sensor, the transfer characteristics for the different spot in sizes, and mathematical model of the sensor are presented in Section III. Section IV gives the simulation and experiment results of the sensor. Finally, our work is summarized in Section V.Supported by High-Tech Research and Development Plan of China (863).978-1-4244-4412-0/09/$25.00 ©2009 IEEEII.A RCHITECTURE OF F INE T RACKING A SSEMBLY A.System DescriptionMost acquisition, tracking, and pointing subsystems of free-space laser communication platforms consist of two structures, a coarse pointing assembly (CPA) and a fine pointing assembly (FPA). The CPA is loaded with the tasks of the initial acquisition and to change the orientation of the communication transceiver in bigger, but lower bandwidth higher amplitude movements. The FPA needs to be extremely precise and with fast response system in order to compensate for the fast changes in beam orientation and suppresses the disturbances such as the base vibration of the platform.Developed ATP subsystem for space-based laser communication system also comprises of CPA and FPA. The CPA, which consists of a coarse tracking sensor using a charge coupled device (CCD), a 2-axis gimbal mechanism, and a controller for gimbal mechanism. The FPA, which consists of a fine tracking sensor using a 4-quadrant detector (4QD), a fast steering mirror (FSM), a controller for the FSM. The 4QD, which is an important component in the FPA, requires the characteristics of fine resolution and high speed response for this reason. A block diagram of the experiment setup of theFPA is shown in Figure 2.Figure 2. The experiment setup of the FPA.B.The Experiment Setup of FPAThe laser source is the incoming collimated beam incident on the fast steering mirror at 45º, which is with a wavelength of 532nm. The beam splitter is used at 45º angles to split the beam in two directions: one beam is focused on a photodiode of laser communication receiver and the other is focused on the 4QD. The fast steering mirror with a 25.4 mm diameter glass mirror surface is operated open-loop, containing no internal sensor or feedback mechanism. Its angular range is ±2.5 mrad with a 0.05 μrad resolution and its resonant frequency is in the range of 2KHz [4]. The 4QD is a model QP50-6-18 produced by Pacific Silicon Sensor, Inc., which has a diameter of 7.98 mm active area and 18μm gap width [5]. The 4QD-measured position error signals are conditioned and fed to the analog-to-digital converters (ADCs) of the DSP controller. The actuator control signals from the digital-to-analog converters (DACs) of the DSP controller are fed back to the FSM driver, which directly drives the FSM actuator.III.M ODELING OF F INE T RACKING S ENSORA.Operating principle of the 4QDThe 4QD is the fine tracking sensor used in this work, which detects the position of the incoming laser beam with a very high accuracy. It is consists of four separate silicon photodiodes, or quadrants, arranged in a quadrant geometry, as shown in Figure 3. The photodiodes A, B, C, D, where A, B, C, D are the four quadrants respectively, are equal and are separated by small gaps. Its operation principle is based on conversion of optical energy into electrical energy. The photodiodes A, B, C, D convert incoming light into currents I A ,I B , I C , I D , and then the currents are transformed into relative voltage levels V A , V B , V C , V D , by the operational amplifiercircuits. Voltage generated by each quadrant is proportional tooptical energy illuminating its surface. Figure 3. Position detector circuit of fine tracking sensor.B.Operation of the position detecting sensorTo illustration the position sensing operation of the sensor, we assume that the shape of the laser beam or the spot can be represented as a circle with uniform distribution of power onto the 4QD detector. In general, the spot can appear on the four quadrant detector active area as suggested in Figure 4. If the spot is in the perfect centre of the 4QD, which is the cross point of the two gap lines, then currents I A , I B , I C , I D , from all the four photodiodes will be the same. The spot displacement along the x- and y-axes of the detector will be detected as a relative change between these four current outputs, and then removed in the fine tracking control loop.These currents or voltages are added and subtracted in the following manner to calculate the E X and E Y , so-called the pointing error, relative to the centre of the detector,D C B A C B DA X D CB AC BD A X X V V V V V V V V K I I I I I I I I KE ++++−+=++++−+=)()()()( (1)DC B AD C BA Y D CB A DC B A YY V V V V V V V V K I I I I I I I I K E ++++−+=++++−+=)()()()( (2)where K X and K Y are the correlation coefficient of the x-axisand y-axis directions respectively.Figure 4. Relative position of the spot and the 4QD centre.C.Transfer characteristics for the sensor in sizeThere are some constraints to be considered when using position sensor. First, incident laser spot must be smaller than the detector’s total active area, but larger than the gap between separated active areas. Second, the total positional detection range is limited to the incident laser spot size or the detector’s active area size, whichever is smaller. Another consequence of geometry is that detection range increases with spot size, while positional resolution decreases, as shown in Figure 5 [6]. This is because a given movement in a small spot creates a much bigger differential signal than the same movement in the larger beam, as is indicated in Figure 5 with dashed lines. In our effort, the laser spot size is appropriately limited to half thedetector’s active area size.Figure 5. Transfer characteristics for the diffenent spot in size.D.Mathimatical model of the sensorThe x-axis and y-axis outputs of the fine tracking sensor are directly related to the energy of the laser beam that falls in each quadrant. In order to make a mathematical model of the quadrant detector sensor, two main parts are considered: the first part represents the calculation of the illuminated energy of the four photodiodes by incoming laser beam, and another important issue is the shape of the incoming beam. In reality the laser energy is not uniformly distributed over the whole profile, but has a certain shape. Assuming that the laser beam used as the fine tracking has a uniform intensity distribution(see Figure 4.), the energy in each of the four quadrants is given using each illuminated area by the following equations:»¼º«¬ª++−+−++=)arcsin()arcsin(2224222222r y r x r y r y x r x xy r S A π(3)»¼º«¬ª+−−+−−−=arcsin()arcsin(2224222222r y r x r y r y x r x xy r S B π(4)»¼º«¬ª+−−−−−+=)arcsin()arcsin(2224222222r y r x r y r y x r x xy r S C π (5)»¼º«¬ª++−−−+−=)arcsin()arcsin(2224222222r y r x r y r y x r x xy r S D π(6)Where S A , S B , S C , S D are the illuminated area of the four quadrants respectively, x and y are x-displacement and y-displacement or the relative positions of the spot centre and the 4QD centre, r is the radius of the incoming laser beam spot.The positing errors Ex, Ey (see (1) and (2)) in both x-axis and y-axis directions of the laser beam is also calculated using the each illuminated area of the four quadrants by following formulas.DC B A C BD AX D C B A C B D A XX S S S S S S S S K I I I I I I I I K E ++++−+=++++−+=)()()()( (7)DC B AD C B A YD C B A D C B A YY S S S S S S S S K I I I I I I I I K E ++++−+=++++−+=)()()()( (8)If we use (3) - (6) and substitute into (7) and (8), we canobtain the formulas of the positing errors and the displacements of the laser spot, which are the mathematical model of the 4QD sensor, as follows:»¼º«¬ª+−=++++−+=arcsin(221)()(2222r x r x r x r K S S S S S S S S K E X D C B A C B D A XX π (9) »¼º«¬ª+−=++++−+=)arcsin(221)()(2222r y r y r y rK S S S S S S S S K E Y D C B A D C B A YY π(10) Figure 6. Simulation results in the x-axis.IV.S IMULATION AND E XPERIMENT RESULTSIn order to develop a model of the quadrant detector a series of simulations and measurements has been performed. The laser beam has been steered across one quadrant in both x-axis and y-axis directions to obtain the complete characteristics. Figure 6 presents a summary of this simulation. The data have been recorded while moving the beam across quadrants B and A (see Figure 4). For the model described above the (9) and (10), the three different laser spot sizes (in mm) are shown in Figure 6, showing a saturation effect due to the finite beam size, which determines the tradeoff between angle dynamic range and null position sensitivity. As a result, we set the spot size to approximately half the 4QD diameter. Then we measured the 4QD response for various incoming laser power levels, as shown in Figure 7. The experimental results validate the derived models based on the correlationFigure 7. Experimental response in the x-axis.V.C ONCLUSIONThis paper presents an approach to modeling the quadrant detector sensor based on the correlation coefficient. The correlation coefficient of the sensor model is based on a series of measurements. Performance of the model has been assessed using the coefficient of determination. The simulation and experiment results of the sensor show that the detecting accuracy satisfies the design demand for our developing OCT.The obtained model of the fine tracking sensor has been used as the experimental setup for development of a model reference fine tracking control system for the free-space laser communications.R EFERENCES[1]Brandon L. Wilkerson, Dirk Giggenbach, Bernhard Epple, “Conceptsfor fast acquisition in optical communications systems”, SPIE Vol.6304, 2006[2] C. Hindman, S. Lacy, and N. Hatten, “Image Based Acquisition andTracking for Multi-Access Laser Communications”, IEEE Aerospace Conference, March 4-11, 2006, pp. 1-10.[3]John Maksyomwicz and Kenneth Conner, “Research Horizons, LaserBeam Acquisition and Tracking,” Crosslink, Ground Systems, Vol.7, Number 1 (Spring 2006), pp. 43.[4]Newport, FSM Datasheet, .[5]Pacific Silicon Sensor Quadrant Series Datasheet, http://www.pacific-.[6]APT technologies, 。