System of three-axis stabilization of axis of laser radiation directivity diagram in atmospheric communication line
number of national and foreign companies are involved in the development of atmospheric optical communication lines (AOCL) . Operation principle of such devices is known but the specialists are interested in new variant of the systems providing the reliable signal reception under the conditions of weather and transmission medium fluctuations. In this paper the device of automatic control of the axis of directivity diagram (ADD) of laser radiation in AOCL is considered in details. In case of shift of optical axis of receiving-transmitting module (RTM) under the influence of external factors, the automatic correction will allow applying the laser beam with lower divergence angle and reduce the number of ruptures of optical communication channel, which occur due to the line misalignment.
It is required to provide accurate alignment of RTM ADD for the reliable data transmission through optical channel. In the process of actual operation of AOCL, due to the temporary instability of supporting structures the need of constant automatic three-axis pointing of one receiving-transmitting module on the other one arises. AOCL devices with automatic pointing (automatic tracking) of one RTM on another RTM produced by different companies are well known, for example, [2–3]. However, the practical implementation of automatic tracking is not shown in papers, and the specific characteristics of RTM with automatic tracking devices are not given. Therefore, we cannot compare the characteristics of these devices with automatic pointing.
Change of three-axis position and angular attitude of RTM is stipulated by the influence of such external factors as temperature, mechanical vibrations and oscillations, wind loads and perturbations, variations of support geometry as result of aging, shrinkage, compaction and others. In coordinate system of amplitude-speed (Fig. 1), it is reasonable to divide the impact factors into several categories: slowly changing (area 1); low-amplitude with the amplitude of angular attitude variation Amin less than 10–20% of divergence angle (area 2); fast vibration impacts with the amplitude comparable with divergence angle (area 3); oscillations with the amplitude Amax with the range, which is not greater than dynamic range of three-axis stabilization system (TASS), and speed and Vmax, which is not higher than the set speed (area 4). Shaded area 5 (see Fig. 1) refers to the region of absence of correct compensation of perturbation due to the technical restrictions of TASS (dynamic range by the amplitude (angle) and maximum variation speed of last variable).
Let us consider the range of speeds and amplitude of vibrations, which are subject to correction with the help of stabilization device of ADD. Let us determine the permissible limits of Amax and Vmax for ground fixed equipment.
The support movement is usually classified as low-, mid – and high frequency movement.
Low-frequency movement refers to the movement with oscillation period of minutes to months, which can be determined on the basis of daily and seasonal temperature fluctuations. Temperature gradients cause the bend and twist of buildings. Amplitude of these deformations depends on building dimensions, shape and construction in greater degree. Deviation has tendency to the increase with building height and can have significant impact on the equipment, which is installed on the roof, even in relation to short buildings. Besides, it is noted that these deviations have greater impact on the elevation angles than on the azimuth angles.
Mid-frequency movement has the period of the order of several seconds and it is connected with the movement of buildings under the influence of wind. This type of oscillations can be very significant for tall buildings. Communication disruption for AOCL for such reason usually has short-term form because at the end of wind blast the building goes back to initial position. Receiver-transmitter units with quite broad angle of view and divergence and with quite efficient system of automatic pointing and tracking must have the capability of compensation of these rare and strong deviations without communication disruption.
High-frequency oscillations with the period, which is less than one second, can be caused by the operation of large-sized equipment (for example, large fans), human activities (walking, closing of doors). Their frequency stands at 1 Hz  and it depends significantly on the installation method of RTM of AOCL. Installation on the floor, wall or roof (in other words, on the surface of roofing or parapet wall) can give levels of fluctuations, which significantly differentiate. The value of vibration can greatly vary during certain period of time in relation to one building. It should be noted that almost any integrated movement lies within the limits of frequency band of lower than 10 Hz.
Results of the measurements published in the paper  showed that maximum of angular deviation due to vibration with the frequency higher than 1 Hz rarely exceeds 1 mrad but it more often approximates to half of this value. However, the equipment installation must be carefully planned in such a way that the oscillations experienced by RTM AOCL would not be amplified. Taking into account the need of installation of RTM AOCL on posts, pipes, balconies, towers, antenna posts, which occurs in many cases, it should be emphasized that the specified factors cause more significant deviations by frequency and amplitude.
Two extreme cases of angular instability should be noted here: long-term and short term instability. For long-term instability the permissible limit of deviation range of cellular communication towers relative to normal position is set on the value Amax = 60 mrad. The lower estimation of period of these impacts gives the value Tmin = 1 hour = 3 600 sec. Then, the angular speed can be estimated as follows:
Short-term instability can be equal to Amax = 1 mrad (permissible angular amplitude of oscillations of antenna posts E4.115.010, ZhY4 115 044 and other similar devices with the frequency 1/Tmin=1 Hz). Then, the estimation of angular speed gives the following value:
Form obtained values it becomes apparent that in the process of operation the automatic alignment of RTM AOCL-equipment is needed because the practical angles of view and divergence of RTM are considerably lower than the amplitude of impacts. In order to ensure stable error-free data transmission through optical channel, the TASS must be calculated. The speed of deviation optimization and accuracy of communication direction maintenance should be referred to significant parameters of three-axis stabilization system.
Having obtained the numerical values of limits of impact parameters, we should generate the requirements to TASS. It is obvious that, first of all, the system must meet the requirement of stabilization by short-term instability. And even significantly long-term instability can be represented in the form of series of short-term instabilities with low amplitude.
Three-axis stabilization system is implemented inside the body of RTM and structurally it represents the optical module, which from one side is connected through spring to RTM body and from other side it leans against the rods of stepping motors (Fig. 2). The laser beam entering the receptor gets on photodetector (PD), which is represented by CCD array. The signal from array is read line-by-line and comes in thresholding unit and unit of determination of total image brightness.
During the signal pickup from array, the energy characteristic (Fig. 3) will have the form represented by line (1). The area, on which the laser beam is incident, gives higher signal than boundary zone. At the same time, the signal of boundary zone is also not equal to zero for a number of reasons: "noise" of electronic component, light radiation reflected from other objects, which got into frame, etc. Therefore, for the purpose of elimination of unwanted noise, thresholding unit establishes the signal level, lower which the signal is assumed to be equal to zero. This threshold is calculated on the basis of ratio between maximum value of signal received by PD and average level of signal.
Actual energy curve is quite different in comparison with the curve given below, and it has larger number of peaks and troughs, therefore in order to determine the center of spot described by this curve it is insufficient just to find maximum. For this purpose the derivative of energy characteristic function must be found (curve 2) and the point corresponding to amplitude maximum is assumed to be the spot center.
Further, the spot position is compared with target position and mismatch is determined. If necessary (in case of center deviation due to temporary instability), the correction of position of optical construction is performed using the stepping motors. The structural scheme of ADD stabilization system consists of a number of devices (Fig. 4).
Incoming laser beam getting through the aperture of receiving window comes to focus at photodetector implemented in CCD array with the dimensions 480Ч480 pixels. From array the signal gets on programmable logic integrated circuit (PLIC). A number of operations with incoming information are implemented in PLIC:
• Signal thresholding
• Determination of total brightness
• Calculation of spot center
• Image averaging in 4 frames
• Determination of mismatch with target
• Plotting of discriminator curve
• Control of stepping motors
In order to control the position of optical frame (Tabl.1) two stepping electric motors ShD200 are used, one for Teach axis. Stepping motors used in TASS system are the main means for correction of directivity of ADD RTM. Stepping motors have a number of advantages stipulating their application in this device. The main advantage of stepping motors consists in the accuracy. When supplying the potentials to winding, the stepping motor will turn by strictly determined angle. Also, relatively low cost and small size can be referred to their advantages. Motors are located at the angle of 45° relative to horizon (Fig.5). Due to such location of axes after the compensation of temporary instability, the optical frame can go back to the initial position under the influence of gravity in both axes, whereas when arranging the motors in parallel with axes X and Y it will be necessary to spend operation of electric motor for the compensation of shift by axis X.
In order to control the stepping motors, the discriminator curve is plotted; discriminator curve means the dependence of number of the steps, which the motor has to perform for the compensation of displacement by its axis at the displacement level in pixels. Due to the fact that the signal coming from PLIC output is quite weak for the control of stepping motors, it is previously amplified by current and voltage coming on transistor switches. Implemented TASS provides the parameters given in Table 2.
Practical use of this TASS made it possible to:
• apply narrow diagrams of directivity of transmitters and transmitter angles of view, at this expense the acceptable budget of line, high security and immunity of communication channel are ensured;
• simplify significantly the process of initial system pointing; before the pointing process was measured in hours but now with the use of TASS it is measured in minutes;
• use equipment on the towers of communication statements and other instable constructions, and this fact considerably expanded the range of system application without loss of communication channel quality.
The displacement of coordinates of spot center was selected as the parameter, which characterizes the output of stabilization system operation in the most accurate manner.
In order to check the operability of three-axis stabilization system and whole receiving-transmitting module in general the test bench is used (Fig.6). The test bench represents two RTM modules, which are spread at the distance of about 700 meters and fixed on immovable beds using the universal assembly kits. The module of external interface unit (EIU) is connected to each module using the cable with the length of about 50 meters, and the channel testers Ethernet ETEST are connected to EIU modules. Information from the local network LAN is supplied to EIU1 module and from it – to RTM1 module, which transmits it to RTM2 module. RTM2 module is not connected to the local network and it transmits only housekeeping information to RTM1 using the "red" channel. Channel testers Ethernet are used for operational check of the parameters of communication line.
In order to check the operability of stabilization system the power of transmitter of engineering channel was varied and at the same time the displacement of spot center coordinate was observed. High quality of TASS must be reflected in insignificant change of coordinates of spot center with power variation. Radiation with the wavelength of 785 nm (wavelength of engineering channel) is incident on the array. Visualization of signal from CCD array (Fig.7) makes it possible to see that the signal coming from the area outside the spot is low. The black area in spot center corresponds to high level of signal, grey area at the edges of spot – lower level. The numbers indicate the spot coordinates relative to frame edge.
Let us decrease the power of red channel transmitter by 50%. It is seen (Fig.7a) that the spot changed its shape and its brightness decreased, however, the coordinates of spot center practically did not change. Let us decrease the transmitter power to 5% of initial power (Fig. 7b): the spot dimensions decreased and it insignificantly changed its shape, however, the coordinates of its center remained practically unaltered. Now, let us increase the transmitter power by 230% relative to initial power and we will see (Fig. 7c) that the spot dimensions increased and the local "spikes’ occurred at its boundaries at the expense of pronounced speckle-structure, however, the coordinates of its center again remained practically unaltered.
As we see, TASS allows determining the coordinates of laser spot center with sufficient accuracy even at significant variations of power of incoming radiation, whether increase or decrease of this parameter. It points at high level of operation reliability of AOCL with stabilization system.
Experimental results of the studies of TASS operation in actual practice are illustrated by the records of current values of system parameters within two hours, right after the sunset (Fig. 8; plots a and b are given for Y axis of TASS).
As it is seen from the plot of movement of stepping motor rod (Fig. 8a) almost right after sunset the short-term displacement of supporting structure occurred, and after an hour the continuous displacement began, which TASS had to compensate. At the same time, the displacement of coordinate of spot center of engineering channel did not exceed 1.25 pixels as a result of corrective operation of TASS. It is seen from the plot of power variation in engineering channel receptor during the same period of time (Fig. 8c) that displacement of supporting structure did not have great impact on the level of received power due to timely operation of TASS, and power fluctuations, which are present at the plot, are connected with occurred instability of atmospheric parameters, mainly with turbulence.
The system, which completely meets the requirement to correction of deviations of laser beam ADD caused by vibrations and displacements of RTM in case of installation on fixed objects. Its use significantly enhances the quality and stability of operation of atmospheric optical communication channel, reduces the time needed for its arrangement. In particular, the communication range is increased up to 7 km (with the speed of information transmission up to 100 Mbit/s), and this number exceeds the values of parameters  of domestic analogs.