Abstract:
A terrestrial light beam communication system is described for compatible utilization with existing radio transmission systems. The transmitter (FIG. 4) utilizes aiming means (41 and 42) to control the aim of the transmitted light beam. The receiver employs an array of detectors (FIG. 5) from which a circuit (FIG. 6) determines the position of the received light beam. The position of the received light beam is used to control the position of the transmitted light beam from which control signals are developed to maintain the aim of the transmitted light beam to combat the occurrence of ongoing fluctuations in the vertical deflection experienced by the transmitted light beam.

Description:
This invention relates to communication systems and, more particularly, to terrestrial communication systems which utilize a transmission medium subject to the variations of atmospheric conditions. 
     Radio systems provide one form of terrestrial communication systems. Such systems often operate at microwave frequencies employing highly directional antennas on towers separated by spans to complete numerous voice and data circuits linking one location to another, such as between distant cities. The burgeoning demands of the present (digital) information age are being borne by communication systems. In order to accommodate such demands, digital techniques are frequently being employed in terrestrial radio systems to increase their informational carrying capacity. Unfortunately, multipath propagation or clear air fading disrupts the operation of the systems. Diversity antenna, protection switching of spare channels, equalizers and various other sophisticated techniques have been employed with varying degrees of success to combat the problem of clear air fading. Numerous technical articles addressing this problem clearly illustrate its extent and severity. 
     When such a problem is severe, it would be highly desirable to be able to resort to some form of alternative communication link to avoid the problem and its attendant effects. Since such systems have a rather high informational capacity, an existing alternative may not be readily available without overloading its capability. 
     One of the primary purposes of this invention is to provide an alternative adaptable to existing radio systems to the extent of being piggyback thereon and yet is immune to the clear air fading phenomenon. When multipath fading disrupted radio transmission at frequencies of 6 and 11 GHz in an experimental radio span, optical communication over the same span exhibited immunity to fading and was demonstrated to be a reliable alternative with high informational carrying capacity. 
     Although highly focused light beam communication is not susceptible to multipath fading, the aim of the light beam is susceptible to ongoing fluctuations in the vertical direction probably due to the changing nature of the temperature and pressure gradients in the atmosphere along the transmission path. Horizontal deflections are small relative to the vertical deflections since the atmosphere is relatively homogenous in the horizontal direction. Accordingly, it would be necessary for a reliable light beam communication system to provide automatic compensation for maintaining aim of the light beam at the receiver location. 
     SUMMARY OF THE INVENTION 
     Broadly, the invention takes the form of an arrangement for automatically controlling the aim of a well focused light beam directed at a receiver to maintain reliable communication over spans corresponding to those used in terrestrial radio communication systems. 
     The invention in one of its aspects utilizes the position of a received light beam to control the aim of a transmitted light beam to compensate for ongoing fluctuations in vertical deflections encountered by the transmitted light beam. An attendant advantage of such a technique is that the channel capacity of the optical light beam is fully realized since the form of the control information does not occupy channel capacity. 
     More specifically, the invention compensates for ongoing vertical fluctuations in the deflection of the transmitted light beam while capitalizing on the presence of relatively small horizontal fluctuations. Accordingly, the lateral position of a received light beam indicates the vertical position of a transmitted light beam at its receiver. The inventive technique controls vertical deflection by using lateral positioning each on light beams transmitted in opposite directions to complete an adaptive control loop employing a form of negative feedback. Accordingly, effective automatic aiming control is established between two points between which reliable two-way light beam transmission is maintained. 
     A further aspect of the invention provides a technique and arrangement for reestablishment of the two-way transmission after total disruption of transmission. This aspect capitalizes on the recognition that vertical deflections of light beams transmitted in opposite directions transverse the same medium and track each other. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Features of the invention and additional objects of the invention will be more readily appreciated and better understood by reference to the following detailed description which should be considered in conjunction with the drawing. 
     FIG. 1 represents a transmission span over which the invention may suitably provide high capacity communications with reliability. 
     FIG. 2 is a radio transmission tower on which the inventive arrangement is deployed. 
     FIG. 3 is a block diagram of an illustrative transmitter employing inventive principles. 
     FIG. 4 illustrates the mechanical arrangement for controlling the direction of the transmitted light beam. 
     FIG. 5 illustrates an array for receiving the transmitted light beam. 
     FIG. 6 is a block diagram of the receiver circuit which develops control commands in addition to the communication signal output. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates the profile of a typical transmission path over which the inventive principles may be used to provide communication. Transmitting and receiving towers are located at points 11 and 12 which are separated a distance of approximately 23 miles. The vertical scale used in FIG. 1 exaggerates the elevation of the profile. The actual transmission path 13 provides line of sight two-way communication between points 11 and 12. The distance between points 15 and 16 represents about three miles of tidal water. Although the transmission path 13 of FIG. 1 is shown individually typically a number of such spans may be arranged in tandem to traverse any distance, for example, providing communication between distant cities. 
     At each of points 11 and 12, a tower such as shown in FIG. 2 or any other suitable structure may be utilized to provide the desired elevation to provide an obstruction free transmission path. It should be pointed out that in FIG. 2 radio antenna 21 provides radio communication between it and a distantly located antenna as indicated in the foregoing. Radio transmission is subject to the phenomena of clear air fading which disrupts the communication between any one of the spans in a microwave communication system. Accordingly, transmitting structure 22 provides an alternate medium of transmission over a light beam while a receiving array 23 receives an incoming light beam which, in turn, includes receiving and transmitting components similar to transmitter 22 and receiver target 23. Of course if the tower of FIG. 2 serves as a relay for passing communication to neighboring towers on either side of it, a duplicate set of equipment is required for full duplex communication in the opposite direction. 
     FIG. 3 is a block diagram of a transmitter suitable for the application shown in FIG. 2. In FIG. 3, the signal path starts with modulated source 31 which includes laser 32 and modulator 33 to produce modulated coherent light beam 34. Coherent light beam 34 is expanded by convex lens 35 to illuminate mirror 36 which deflects the light beam to illuminate mirror 37. Both mirrors 36 and 37 are planar mirrors. Mirror 37 serves as an optical feed for concave circular mirror 38 which produces an output beam approximately one foot in diameter at the transmitter. The expanded beam avoids possible obstruction of the beams by small particles in the transmission path. The direction of launch of the output beam is controlled by motors 41 and 42. Motor 41 controls the left/right (horizontal) movement or deflection of the output light beam while motor 42 controls the up/down (vertical) movement. Each of motors 41 and 42 are responsive to respective control commands. 
     FIG. 4 depicts an illustrative mechanical embodiment for controlling the vertical and horizontal launch or projection of the output light beam from a transmitter. Components common to both FIGS. 3 and 4 are designated with the same reference numerals. The arrangement of FIG. 4 functions to pivot concave circular mirror 38 in a highly controlled manner in a series of predefined increments or steps through an arc indicated by dual directional arrows 50 and 51 about gimbal 52. Up/down motor 42 may be a small DC motor which turns worm 53 to raise and lower lever 54 which is pivoted about fulcrum 56. As lever 54 changes in elevation it changes the elevation of fulcrum 57 which produces an arcuate displacement of mirror 38 about gimbal 52 in accordance with arrow 50. In an experimental transmitter constructed in accordance with FIG. 4 the occurrence of a control signal for one second moved the optical beam 13.5 inches at the receiving site 23 miles away or through an arc of 9.3 microradians per second. Additionally, worm 58 (shown on end) is turned by motor 41 of FIG. 3 to change the horizontal direction of the projected beam through arc 51 also about gimbal 52. These changes in horizontal direction are also performed in a series of controlled increments or steps. 
     Laser 32 is a relatively low-powered device having an optical output of less than 20 milliwatts (mW) within the visible spectrum. With a transmitted beam of one foot in diameter at the transmitter, the optical beam is disbursed at the receiver site to produce a beam 20 feet in diameter with the optical power at the diameter termination being 10 dB down from the center of the beam. With a power differential of 20 dB the diameter of the beam corresponded to approximately 30 feet. 
     FIG. 5 illustrates receiver array 61 and one of the four identical detectors A-D used therein. Each of detectors A-D has a receptive surface of about one foot square. The four detectors have the same construction as that illustrated for detector 62. The receptive surface of detector 62 initially includes filter 63 which serves to attenuate scattering light from reaching Fresnel lens 64 and filter 66 transparent to the frequency of laser 32. This arrangement desirably reduces background optical noise energy from reaching photomultiplier 67 located at the focal point of Fresnel lens 64. 
     FIG. 6 is a receiver control circuit responsive to the outputs of photodetectors A-D to provide control over the aim of the transmitter (control commands) in addition to a received output. Each of multipliers 62 outputs is first gain stabilized by automatic gain circuits (AGC) 71. The output of each of AGC&#39;s is then amplified and homodyned detected in amplifier 72 to produce the outputs designated as A, B, C and D in FIG. 6. For the receiver output, combiner 73 sums the signals A-D to drive receiver 74 which produces transmission output 76. The sum output of combiner 73 is also supplied to integrator 77. The output of integrator 77 is fed to both threshold detectors 78 and 79. Detector 78 provides an input for AGC control 81 which controls AGC&#39;s 71 in unison. This latter arrangement provides immunity from scintillations or momentary fluctuations in the strength of the received optical signal. Threshold detector 79 indicates the occurrence of a system dropout to logic and memory 83. In the event of a dropout of significance, searcher 84 cooperates with logic and memory 83 and operates to reestablish system operation. 
     Summers 86-89 are utilized to determine the position of the light beam on the target array of FIG. 5. For up/down position determination, summer 86 combines the outputs of the two upper detectors (A &amp; B) while summer 87 combines the output of the two lower detectors (C &amp; D). Then comparator 91 compares the outputs of summer 86 to summer 87 and issues either an up/down signal or no signal to indicate the position of the light beam on the target array. The location of the target array will be called the local site. The output of comparator 91 is converted by move left/right 92 to a pulsed voltage level (control command) for incrementally operating a DC motor such as motor 41 to move the light beam transmitted back to the target array located at the original transmitter site which will be called the remote site. The relative lateral position of the light beam on the target array located at the remote site is used to control the up/down light beam positioning at the local site. This arrangement conveniently eliminates the need for a separate control signal to be sent back from the receiver site to the transmitter since the lateral position of the beam at each receiver location is used to control the vertical aim of the transmitter. 
     Similarly, the relative strength of the outputs of summers 88 and 89 at the local site is used by comparator 93. Then, move up/down converter 94 drives the motor, similar to motor 42, to move the position of the light beam up or down at the remote site. Since horizontal deflections are relatively small, a change in horizontal light beam position at a receiver may be used by its transmitter to compensate for the ongoing vertical fluctuations due to diurnal temperature changes in the atmosphere. 
     Directional control of the light beam position may be now summarized. When the received beam at a station moves downward, the beam transmitted from that station is directed to the left. When the received beam moves up, the beam transmitted from that station is directed to the right. When the received beam moves left, the transmitted beam is directed up. When the received beam moves right, the transmitted beam is directed down. The arrangement for controlling the light beam aim in accordance with this summary provides negative feedback that tends to correct the spurious diurnal deflections which would ordinarily disrupt light beam communication over spans compatible with existing radio systems. The reversal of the relationships of left with up and right with down may be readily changed as long as the controlling positional axis is orthogonal to the response axis. 
     In order that large vertical deflections can be corrected by signaling with small horizontal deflections there is an advantage to designing the motor drives so that detection of a horizontal displacement causes the vertical motor to run continuously at a speed proportional to the deflection. Detection of a vertical displacement will then cause the horizontal motor to move by a proportionate angle. Such an arrangement will provide stabilized aim in the transmission of light beams through the atmosphere. 
     When atmospheric conditions are unfavorable, for example, in heavy rain or fog, the power of the received light beam can become so low that it is not capable of being detected by the receiver and control is lost. To regain control, the following method is proposed. The method relies on the observed fact that horizontal drift in the light beam is slow and small, and that drift in the vertical deflections of the two beams track one another since they traverse the same medium. 
     When the received energy at either station falls below a set threshold predetermined in the design of threshold detector 79, the position control switches off and the local transmitter is locked to its horizontal direction and moved to a reference point at one (down) extreme in the vertical directions. This action kills the beam received at the other end and its transmitter is then set to the reference position. Clocks internal to logic and memory 83 and searcher 84 which are located at each station then start to produce regular timing signals in synchronism at periods of approximately 3 seconds. At these times, the beams step up by a fixed amount that is equivalent to a fraction of the receiving aperture. 
     Because vertical deflections introduced in the atmosphere in opposite directions track, both receivers should locate their beams at the same time interval. When they do, control is re-established. If the receivers do not find their beams after a set number of steps the procedure is repeated. The clock intervals are made large compared with the response time of the positioning mechanisms and with expected errors in the clocks. The step size in the vertical aim of the light beam is made small enough to accommodate possible error or backlash in the mechanisms used to aim the light beam. 
     It should be understood that the foregoing describes illustrative embodiments of the invention. For example, possible modifications of the electrical, optical, mechanical, and electromechanical arrangements may readily occur to those skilled in the art while practicing the invention. Although such modifications are numerous and varied, some typical changes may alter the optical arrangement for producing the highly focused optical beam or change the electrical or mechanical aspects of the servo control system. Other possible alterations may include utilization of a different type of receiver array which may employ other types of optical detectors than the photomultipliers disclosed herein.