Optically operated microwave phased-array antenna system
United States Patent Wright et al.  
 OPTICALLY OPERATED MICROWAVE PHASED-ARRAY ANTENNA SYSTEM Apr. 15, 1975 3,293,438 12/!966 Davis 250/199 169L483 9/l972 Klein 332/7.5l 3,73l,l03 5/l973 O&#39;Meara 356/5 Primary Examiner-Eli Lieberman Attorney, Agent, or Firm-Lindenberg, Freilich, Wasserman, Rosen, and Fernandez [57] ABSTRACT An optical processing technique is employed to optically generate a set of properly phase-controlled signals that are appropriate to forming and steering, in  
 space, a beam from a two-dimensional phased array antenna.  
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 SEEU 2 8f 7 PATTERN AT &#39;nME 15 PATTERN AT T\ME t PATTERN AT TIME ()PTICALLY OPERATED ICROWAYF. PHASED-ARRAT ANTENNA SYSTF.  
 BACKGROUND OF THE INVENTION This invention relates to a novel optical system for generating signals which can be used for exciting a twodimensional phased array antenna.  
  A system for generating signals such as microwave signals for the purpose of exciting a phased array antenna system to radiate and steer electromagnetic beams. is known to be complicated. bulky. and espen sive.  
 OBJECTS AND SL&#39;MMARY OF THE INVENTION It is an object of this invention to provide a novel optical structural arrangement for generating signals which can be used for exciting a phased array antenna system.  
  It is another object of this invention to provide structure for generating signals for exciting a phased array antenna system. which is more compact than structures provided for that purpose heretofore.  
  It is still another object of this invention to provide a relatively inexpensive arrangement for generating signals which are applied to excite a phased array antenna.  
  These and other objects of the invention may be achieved by first generating two optical beams with a difference frequency equal to the desired microwave frequency to be transmitted.  
  These two beams are combined. in a controlled manner. to produce a two-dimensional optical pattern that contains the correct microwave phase and amplitude information to form and steer the linal antenna beam in space. This two dimensional optical pattern. at any instant. is an optical analog ol&#39;the microwave excitation applied to the antenna radiating elements. and is converted to a two-dimensiomil microwave pattern in a transducer system called an optical-to-microwave con- \erter. The output of this optical-to-microwave converter is a two-dimensional array of microwave signals. each of which signals is connected to a single radiating element of a phased array antenna. These elements then cooperate to radiate a beam in space in well known manner.  
  The novel features of the invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in conjunction with the accompanying drawings.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a block schematic diagram of the components required for this invention.  
  FIG. 2 is a representation ofthe instantaneous microwave field strength present across the two-dimensional aperture of a phased array antenna.  
  FIGS. 3A. 3B. and 3C are representations ofthe time behavior at times I. I. and of a two-dimensional escitation pattern across the two-dimensional aperture of a phased array antenna.  
  FIGS. 4. 5 and 6 are three representations of a twodimensional excitation pattern applied to a phased array antenna. illustrating resulting antenna beam positions.  
  FIG. 7 is a drawing representing the Fourier trans form basis of this invention.  
  FIG. 8 is a block diagram illustrating a method for modulating a laser light beam with microwave frequency modulation.  
  FIG. 9 is a schematic diagram. representative of an embodiment of the two-dimensional phase processor used in this invention.  
  FIG. 10 is a schematic diagram reprcsentathe of an embodiment of an optical-to-microwavc converter which may be employed with this invention.  
  FIG. II is a two dimensional phased array antenna employing photodiodes.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. I is a block schematic diagram illustrating the major sub-systems required in an embodiment of this invention. By way of illustration. a laser light source 8. provides two light beams. One ofthese beams is applied to a microwave modulator I0. comprising an arrangement for modulating the laser light beam in an appropriate microwave frequency from a microwa\e frequency source II. The microwave modulated laser source produces an optical beam whose frequency differs from that of the other light beam provided by the source 8 by the desired microwave frequency to be transmitted. The two optical beams are directed at a second sub-system called a two-dimensional optical phase processor 12. This sub-system combines the two optical beams in a controlled manner. according to electrical beam positioning signals. received from a beam position input signal source I4. The controlled combination of the two optical beams produces a twodimensional optical pattern that contains the correct microwave phase and amplitude information to form and steer the final antenna beam in space. This twodimensional optical pattern is then applied to an optical-to-microwave com erter [6. which converts its input to a plurality of signals comprising a twodimensional microwave signal array. The output of the optical-to-microwave converter is applied to a twodimensional phased array antenna system 20. This contains a plurality of radiating elements 18. each of which is individually excited by an output signal from the array system 20.  
  Two inputs are required for the system shown in FIG. I. The first input is the microwave signal to be transmitted. This is supplied by a microwave frequency source II. which provides a microwave signal similar to those used in communication or radar systems. The frequency. phase. and amplitude of the microwave signal can be varied or modulated as required for communication. radar or other applications. The second signal input consists of electrical steering signals required to steer an antenna beam in two dimensions in space. This constitutes the output from the beam position input signal source [4.  
  If an instantaneous snapshot were taken of the escitation pattern across a two-dimensional antenna aperture. it would have the appearance of the representation shown in FIG. 2. This figure shows the instantaneous microwave field strength that is present across a two-dimensional aperture for the transmission of a pencil beam to some distant point in space. In this figure. the dark shaded portions respectively. 22. 24. by way of example. represent the spatial positions of high instantaneous field strength. The lighter portions. respectively 26. 28. by way of example. represent lower field strength. This sinusoidal amplitude pattern propagates at a velocity v,. across the two-dimensional antenna surface in a certain direction. 0. with respect to the reference direction as shown in the drawing.  
  The two variables that are required to form and point the beam in various directions in space are the spatial wavelength A... which is the distance between the two dark shaded portions. and the angular positioning of the direction of propagation. H. of this wave across the two-dimensional aperture. If these t\\ o quantities are varied. the antenna beam can he pointed throughout the entire range of normal beam positions available for the particular array configuration of interest.  
  FlGS. 3A. 3B. and 3C illustrate the time behavior of a two-dimensional excitation pattern. These effectively may he considered as snapshots&#34; at successive times t... it. and r. The direction of propagation is along the Y axis of the two-dimensional aperture.  
  Three examples of how this two-dimensional pattern which varies with varying spatial antenna beam posi tions are respectively represented in FIGS. 4. 5 and 6. The lower shaded portions. respectively 30. 32. and 34. respectively shown in FIGS. 4. Sand 6. illustrate the ex citation pattern which would appear on the optical-tomicrowave converter surface to secure beam steering. as wili he described. The relative spatial locations of the antenna elements 18 of the microwave phased array antenna. are shown above the optical excitation pattern. These drawings are meant to illustrate electrically corresponding points only. The microwave array will be many orders of magnitude larger in size than the optical pattern; thus. the drawings should not be considered to scale.  
  Appearing above the microwave phased array in each one of these drawings are insets showing the spatial position that the microwave antenna beam assumes in response to the optical excitation pattern. represented by the shaded areas. respectively 30. 32 and 34. The center inset in FIGS. 4. 5 and 6 respectively designated by reference numerals 36. 38. and 40. each shows a perspective view of a cone 36A. 38A and 40A. which defines the maximum desired angular excursion of the antenna beam in three dimensions. An antenna beam respectively 36B. 38B and 40B is shown within each cone at the position determined by the excitation. The inset to the left ofthe center inset in FIGS. 4. Sand 6. respectivelydesignated by reference numerals 42. 44 and 46 each shows a top view of the respective cones 36A. 38A and 40A. in the adjacent insets. Each also shows by the position of the shaded circle. respectively. 36B. 38B and 408. the two-dimensional position of the antenna beam which is attained as a result ofthe excitation pattern shown in that figure of the drawings.  
  The inset on the right of the central inset in the respective F105. 4. 5 and 6.. represent the X. Y coordinate system in the microwave phased array plane.  
  In FlG. 4. the sinusoidal spatial period is infinite and therefore there is no sinusoidal spatial pattern present in the phase processor. This is represented by the uniformly shaded surface 30 in the lower portion of HO. 4. However. this uniform optical excitation varies in amplitude at a microwave frequency. Therefore. each element [8 of the phased array is excited by an identical signal. thus producing a broadside antenna beam. This broadside beam position. as represented by the insets at the top of FIG. 4 is centrally located within the cone 36A.  
 S wavelength in the two-dimensional processor pattern.  
 This is represented by the long-wavelength shaded sinusoidal pattern 32.  
  In FIG. 6. the angle from broadside is increased to the maximum value. thus increasing the required fre- Ill quency of the optical excitation pattern. This is represented by the pattern 34 in FIG. 6. Also. the azimuth angle has been changed. resulting in a different angle of propagation throughout the two-dimensional pattern. This is shown by a propagation direction not par- IS allel to the Y axis in the optical excitation pattern. and  
 corresponds to the antenna beam position displaced from the Y axis as represented by 408. shown in the inset 46.  
 The overall operation of the processor subsystem 3U can also be described in Fourier transform terms. as  
 35 transform plane 54. The microwave modulated portion ofthis beam is exactly the pattern described in connection with the foregoing figures of the drawing. which is required to form and point a microwave beam in space. However. this optical pattern can be generated in a 30 space a few millimeter on a side. rather than many meters. Thus. an optical-to-microwave conversion process performed by an optical-to-microwave converter 56. will produce a set of microwave signals that is appropriate for driving the full sized two-dimensional phased 35 array antenna 58. The antenna array aperture. in turn.  
 produces a transform at infinity that is a single point 60 in space. Thus. the overall system accomplishes two transforms: The first. an optical one. produces a pattern as described above; the second is performed naturally by the antenna itself. In this way. a single point within the system is transformed to a single point at infinity in space. lfthe optical point source within the system is moved over a two-dimensional spatial surface. the final microwave beam position will move over some solid angle in the far field with a one-to-one correspondence between points.  
  It will be seen from the foregoing that the first problem is to generate two optical signals displaced by a particular microwave frequency for the purpose of generating a two-dimensional excitation pattern. The laser source 8 provides a reference beam and a beam which will be displaced by a microwave frequency by the microwave modulator 10. The most obvious method for producing microwave frequency displacement is to use a modulator in one ofthe light paths of two optical signals. An example is the Serrodyne&#34; frequency displacement technique. ln this technique. one of the optical paths is linearly phase modulated in time with an optical phase modulator. Because a continuous linearly increasing phase cannot be generated indefinitely by the modulator. this type of modulation is normally reset at multiples of 360 phase modulation. This gives a savvtoothed shape to the phase variation as a function of time. hence. the name. &#34;Serrodyne.&#34; The Serrodyne technique is described in a U.S. Pat. No. 2.927.280. Another suitable electro-optic modulation technique. is described in LES. Pat. No. 3.393.955. or in an article by S. E. Harris and A. E. Seigman. \&#39;ol. 3. No. 9. Pages 1089 of the September I964 issue of Applied Optics. and entitled A Technique for Optical Frequency Translation L&#39;tilizing the Quadratic Electro-optic Effect in Cubic Crystals.  
  It is advantageous to use a single laser source to derive the two optical signals required in the phase processor. The use of a single laser in approximately equal optical path lengths of the two signals will minimize the coherence requirements that are placed on the light source. An alternative arrangement in accordance with this invention for performing this modulation operation is schematically represented in FIG. 8. The laser source 8 in FIG. I is here represented by a laser 62 which provides a vertically polarized light beam. This is split into two beams 65. 67. by the half-silvercd mirror 63. The vertically polarized laser beam 67. is circularly polarized by means of a quarter \vaveplate 64. which is a well known optical circular polarization device. and the output from the quarter wave plate is applied to a polarization analyzer 66. The polarization analyzer is a well known element. such as a Wallaston prism. which splits the circularly polarized light beam into a vertically polarized light component 68. and a horizontally polarized light component 70. Each of these respective light beams are passed through respective light gates. 72. 74. These may be a Kerr cell or a Pocltcls cell.  
  These light gates are alternately gated by signals from a microwave gating signal source 76. which corresponds to the microwave frequency source II. shown in FIG. 1. Thus. the vertical and horizontal light components are alternately passed at the desired microwave frequency. The output of the gate 74. which passes the horizontally polarized light is applied to a quarter wave retardation plate 78 so that it is rr/Z radians out of phase with the vertical component. The output of the quarter wave retardation plate 78. is then applied to mirrors 80. and 82. which rotate the polarization 90. so that the horizontally polarized light beam is now a vertically polarized beam. The half-silvered mirror 82 also serves to combine the output of the gate 72 and the gate 74 which is now vertically polarized. so that the two microwave modulated light beams are now a composite microwave modulated beam which is all vertically polarized and which is modulated at the desired microwave frequency.  
  As indicated in connection with FIG. 2. the two variables that are required to form and point the beam into various directions in space are the spatial wavelength and the angular positioning of the direction of propagation of this wave across the two-dimensiomtl aperture. By varying these two quantities. the antenna beam can be pointed throughout the entire range of normal beam positions available for the particular array configuration of interest. Since effectively. the optical excitation pattern of a two-dimensional plane is the analog of the microwave excitation pattern shown in FIG. 2. and since. the spatial wavelength is determined by the arrangement shown in FIG. 9. the control provided by the beam position input signal source I4 (shown in FIG. I l is achieved by controlling the direction of propagation of the wave across the two dimensional aperture. The two-dimensional aperture comprises the area of interface. or the region of the optical-to-microwave converter that is illuminated by the two superimposed light beams. FIG. 10 illustrates how the angular positioning of the direction of propagation of the optical wave may be determined across the two-dimensional aperture.  
  FIG. 9 is a schematic view of one type of the optical phase processor. In FIG. 9. the light beam from FIG. 8. illuminates an array of light pipes tthere being as many light pipes as there are desired beam positions). The number of light pipes shown is only illustrati\ c and is a side view. It will be understood that the array comprises light pipes arrangcd in columns and rows. Each light pipe terminates in a light valve 92. The light valves 92 are controlled by a light valve control 94 so that light is. or is not passed in response to signals from the light valve control 94. A light valve may be a diaphragm that is solenoid operated. or any of the more sophisticated electro-optical control devices. if desired. The function of the light valve control is to sequence the opening and closing of the light valves so that a move-able optical point source is effectively provided. which. as described in connection with FIG. 7. will result in the antenna beam being correspondingly moved.  
  The light output of all of the light pipes is directed at a lens 95. whereby the light is formed into a planar wave for illuminating the interface area of the opticalto-microwave converter. The microwave modulated light beam 86 is received from FIG. 8 is also applied to a lens 96. to be converted to a planar wave. This light wave is reflected by mirror 98 onto half-silvered mirror I00. to be combined with the light wave from the light pipes. The combined waves are directed by mirror I00 onto the interface area of the optical-to-microwave converter. These combined waves from the light or optical analog of the microwave signals applied to the phased array antenna elements.  
  In FIG. 9. the reference light beam is applied to the light pipes and shutters whereby it may be moved to appear to emanate from dilferent spatial positions. and the modulated light beam is thereafter combined with the reference beam. It should be understood that this is by way of illustration and is not exclusive. The two light beams may be interchanged without affecting the operation of the device.  
  As a matter of fact. it can be advantageous to have a plurality of modulated light beams. each of which is applied to a different light pipe. and a single reference beam. This permits the formation of a plurality of antenna beams at different frequencies and in different directions. This may be accomplised by the arrangement shown in FIG. 10. There. a laser light source I02 applies its light output to a beam splitter 104. which splits the input light beam into as many light beams as are required. The beam splitter may. for example. be a plurality of half-silvered mirrors each of which splits an input light beam into two parts. Each of these parts are then split into two parts by another half-silvered mirror.  
  One of the beam outputs of the beam splitter is used as a reference beam 106. Each of the other light beam outputs from the beam splitter is applied to a different modulator I08A. 1088. and 108C. for example. There are as many modulators as the desired number of different antenna beams. Each modulator may be of the type previously described in connection with FIG. 8.  
  The remaining part of FIG. I0 is the same as is shown in FIG. 9 and therefore. the same reference numerals as are used in FIG. 9 is applied to these structures. Each modulator beam from each modulator is applied to a different one of the light pipes 90. Each light pipe terminates in a shutter 92. These are controlled from the light valve control 94. The reference light beam is directed at lens 96 which forms it into a planar wave. Mirror 98 directs this planar ave at half-silvered mirror I00 upon which there is also directed a planar wave from lens 95. The latter planar wave is a modulated wave. which is obtained when one of the shutters 92 is opened to enable light from its associated light pipe to illuminate the lens 95. The mirror lllll now directs the superimposed modulated and reference planar light wa\ es at the interface area ofthe optical-to-microw ave converter.  
  Referring now to FIG. ll. there is shown a schematic diagram of an opticafto-micronave converter. The on tical-to-microwave converter must preserve the phase of the microwave signal throughout the conversion process. Photodetector diodes with a microwave fre quency response are available. as are low and high power microwme amplifiers. L&#39;sing these individual components. a system which can presen e the phase of the signal detected can be built. In FIG. II. a twodimensional array of photodiodes. (arranged in columns and rows). exemplified by photodiodes H0. H2. l H and H6. is provided. there being one photodiode for each element 18 in the two-dimensional phased antenna array 20. which is to be excited. It is most convenient to allow the laser beams to illuminate light pipes. there being one light pipe respectively 120. I22. 124. and I26. for each one of the photodiodes 110 through I16. fhe ends of the light pipes which are illuminated provide a two-dimensional surface which is the inter face between the optical-to-microwave converter and the two-dimensiomil phase processor.  
  As pointed out. there should be one diode for each radiating element and the packing density should be such that each diode should be able to identify or be affected by one or less fringes in the optical image. That is one reason why the light pipes may be preferable since they are small enough to be responsive to a single fringe.  
  The photodiodes respectively [[0 through [[6 drive correspondingly amplifiers l20 through 126. The outputs from these amplifiers are then applied. in well known fashion. to drive the two-dimensional phased array antenna 20.  
  Besides the advantages of compactness. simplicity and being relatively inexpensive. another key advantage to the optically excited device is that the normally complex. expensive. and bulky microwave distribution systems that exist in many phased array antennas may be replaced by thin. lightweight. inexpensive. fiber optic bundles that are used to relay the amplitude modulated optical signal from the two-dimensional plane in the phased processor to each individual optical-to microwave converter. Each of these converters would be located directly at the antenna element that it is to drive. It appears that a considerable savings and size. weight. and cost may be possible by converting from the usual microwav e distribution system to the fiber op tics distribution system.  
  The final antenna aperture and antenna element ar rangement are conventional. and thus. need not be redescrihed here. While this invention has been described in connection with generating microwave frequencies for a phased array antenna. this should be considered as exemplary and not as limiting. It will be obvious to those skilled in the art how to use this light analog technique for generating other radio frequency ill or acoustic frequency signals without departing from the scope and spirit of this invention.  
  There has accordingly been described and shown herein-above. a novel. useful and improved system for driving a phased antenna array.  
 What is claimed is:  
  1. In a phased-array antenna system of the type having an array of antenna elements to which excitation is applied. the method of exciting said elements comprismg illuminating a t\vodimensional area with an optical analog of a two-dimensional excitation pattern for said antenna elements. comprising generating a reference light beam.  
 generating a plurality of separately operable. normally closed light shutters in a two-dimensional ar ray.  
 directing a different one of said plurality of different radio frequency. modulated light beams at one side of a different one of said normally closed light shutters.  
 opening desired ones of said light shutters to enable light to emanate from different positions in said two4limensional array.  
 directing light emanating from said different posi tions at said two-dimensional area. and  
 directing said reference light beam at said twodimensional area. generating a plurality of electrical signals responsive to said optical pattern at a plurality of locations over said two-dimensional area. equal in number to the number of antenna elements to be excited. and  
 applying said plurality ofelectrical signals to said plurality of antenna elements.  
  2. In a phased-array antenna system of the type ha\- ing an array of antenna elements to which excitation is applied. the method ofexciting said elements comprising:  
 illuminating a two-dimensional area with an optical analog of a two-dimensional excitation pattern for said antenna elements. including generating a reference light beam.  
 generating a modulated light beam.  
 moving one of said reference light beam or said modulated light beam to different locations over a twodimensional region.  
 directing the other one of said reference light beam or said modulated light beam at said twodimensional area. and  
 directing light emanating from one of said different locations at said two-dimensional area.  
 generating a plurality of electrical signals responsive to said optical pattern at a plurality of locations over said two-dimensional area equal in number to the number of antenna elements to be excited. and applying said plurality of electrical signals to said plurality of antenna elements.  
 3. A method as recited in claim 2 wherein said method of generating a modulated beam comprises:  
 generating a vertically polarized laser beam.  
 circularly polarizing said laser beam.  
 separating said circularly polarized beam into vertically polarized light components and horizontally polarized light components.  
 gating said vertically polarized and horizontally polarized light components at a desired frequency to provide a horizontally polarized. gated. light beam and a vertically polarized. gated. light beam. phase delaying said horizontally polarized modulated light beam at predetermined amount.  
 rotating said phase delayed horizontally polarized gated light beam until it is vertically polarized. and  
 combining both said vertically polarized gated light beams to provide a composite modulated light beam.  
 4. The method of exciting a two-dimensimtal phasedarray antenna comprising:  
 generating a reference light beam.  
 moving said reference light beam to desired positions in a two-dimensiomtl plane.  
 generating a microwave modulated light beam.  
 establishing a two-dimensional region.  
 directing said reference light beam from said desired positions at said two-dimensional region.  
 directing said modulated light beam at said twodimensional region to provide a light pattern which propagates across said two-dimensional region in a direction determined by the spatial position of said reference light beam relative to said modulated light beam and with a predetermined velocity.  
 generating a plurality of radio frequency signals responsive to said light pattern at a plurality of locations over said two-dimensiomil region. equal in number to the number of antenna elements to be excited in said two-dimensional phased-array. and  
 driving said two-dimensional phased-array antenna with said plurality of radio frequency signals.  
  5. A system for exciting elements of a phased-array antenna of the type having an array of antenna ele ments which are excited by radio frequency waves comprising:  
 a plurality of light to radio frequency signal transducer means for converter light into radio frequency signals. said plurality of light to radio frequency signal transducer means being positioned in an array to establish a two-dimensional region with their inputs.  
 means for illuminating said two-dimensional region with an optical analog of a two-dimensional excitation pattern for the elements of said phased-array antenna. comprising means for generating a reference light beam.  
 means for generating a plurality of different radio frequency. modulated light beams.  
 a plurality of separately operable normally closed light shutters positioned over a two-dimensional array.  
 means for directing each of said plurality of different radio frequency. modulated light beams at a differ ent one of said normally closed light shutters.  
 means for opening desired ones of said normally closed light shutters to enable light to emanate from different positions in said two-dimensional array.  
 means for illuminating said two-dimensional region with light passing through said normally closed shutters. and  
 means illuminating said two-dimensional region with said reference light beam. and  
 means for applying the outputs from said light to radio frequency signal transducer means to the elements of said antenna.  
 6. A system as recited in claim 3 wherein said light to radio frequency transducers includes:  
 It) for generating a radio frequency modulated light beam comprises:  
 means for deriving a second light beam from said reference light beam.  
 means for circularly polarizing said second light beam.  
 means for separating the output of said means for circularly polarizing into vertical and horizontal light components.  
 a source of microwave signals.  
 means for alternately gating said vertical and horizontal light components with signals from said source of microwave signals.  
 means for phase delaying by a predetermined amount said gated. horizontal light components.  
 means for rotating said phase delayed gated. horizontal light components until they become vertical light components. and  
 means for combining both said vertical components to provide a radio frequency modulated light beam.  
 8. A system for exciting elements of a phased-array antenna of a type having an array of antenna elements which are excited by radio frequency waves comprising:  
 means for generating a reference light beam.  
 means for generating a radio frequency modulated light beam.  
 means for moving one of said reference light beams or said radio frequency modulated light beams to assume different angles of incidence on a twodimensional space including an array of light pipes defining a two-dimensional space with one of their ends.  
 means for directing one of said reference light beams or said radio frequency modulated light beams at the other end of said light pipes.  
 shutter means for blocking all of said one ends of said light pipes. and  
 means for operating said shutter means to unblock said light pipes in a manner to cause light to emanate from different positions over said twodimensional space. and  
 means for illuminating said two-dimensional region with said one of said reference light beam or of said radio frequency modulated light beam. not moved by said means for moving. whereby said twodimensional region is illuminated with said optical analog.  
 a plurality of light to radio frequency signal transducer means for converting light into radio frequency signals. said plurality of light to radio frequency signal transducer means being positioned in an array with their inputs forming said twodimensional region which is illuminated by said means for illuminating. and  
 means for applying the outputs from said light to radio frequency transducer means to the elements of said phased-array antenna.  
  9. Apparatus for generating microwave signals for mu ing a two-dimensionally phased antenna array comprising.  
 laser means for generating a reference light beam and a second light beam ha ing the same frequency.  
 a microwave frequency signal source.  
 means for modulating said second light beam with signals from said nticrowzoe frequency signal source to produce a microwave frequency modulated light beam.  
 a two-dimensional array of a plurality of light to microwa e frequency converting transducers. there being one of said transducers for each one of the antenna array elements to be excited.  
 means for moving one of said reference light beam or said microwme frequency modulated light beam to a desired position within a two-dimensional plane to produce a positioned light beam which can ha\e different positions.  
 means for directing said positioned light beam at said two-dimensional array.  
 means for directing said other of said microwave frequency modulated light beam or said reference light beam at said two-dimensional array whereby there is produced an amplitude modulated light pattern propagating at a predetermined velocity across the two-dimensional area and in a direction determined by the position of said positioned light beam. and  
 means for exciting the elements of said twodimensional phased array antenna with the output of said light to microwave frequency transducers 10. Apparatus as recited in claim 9 wherein said means for modulating said second light beam with signals from said microwave frequency source comprises:  
 means for dividing said second light beam into ertically and horizontally polarized light components.  
 means for gating said horizontally and vertically polttl&#39;llCLl light components with signals from said microwave frequency signal source.  
 means for phase delaying said gated horizontally polarized light components a predetermined amount.  
 means for converting said gated horizontally polarized and phase delayed light components to a ertically polarized light components. and  
 means for combining said vertically polarized light components to produce a microwave frequency modulated light beam.  
  l I. A system as recited in claim 9 wherein said means for moving one of said reference light beam or said microwave frequency modulated light beam to a desired position in a two-dimensional plane to produce a positioned light beam comprises:  
 an array of light pipes defining a two-dimensional plane with one of their ends.  
 ill  
 (Ill  
 means for directing one of said reference light beam or said microwave frequency modulated light beam at the other ends of said light pipes.  
 shutter means for blocking all of the one ends of said light pipes. and  
 means for operating said shutter means to unblock said light pipes in a manner to cause light to emanate from different positions over said twodimensional plane.  
 12. In a phased-array antenna system of the type hav ing an array of antenna elements to which excitation is applied. the method of exciting said elements comprisin generating an optical analog of a desired twodimensional excitation pattern of said antenna elements including generating a beam of light.  
 modulating said beam oflight at a predetermined frequency.  
 splitting said modulated light beam into a plurality of modulated light beams which equal the number of antenna elements in said array. and  
 amplitude modulating each of said plurality of light beams at a predetermined frequency and in a predetermined sequence to obtain said optical analog of said desired two-dimensional excitation pattern.  
 generating a plurality of electrical signals responsive to each of said plurality of frequency and amplitude modulated light beams. and  
 exciting said array of antenna elements with said plurality of electrical signals.  
 [3. In a phased-array antenna system of the type having an array of antenna elements to which excitation is applied. means for exciting said elements comprising means for generating the optical analog of a desired two-dimensional excitation pattern of said antenna elements. including means for generating a light beam of light.  
 means for modulating said beam of light at a prede