System and method for coherent phased array beam transmission and imaging

A Coherent Phased Array Beam Transmission and Imaging System is disclosed for end-to-end compensation of a plurality of laser beams through a turbulent medium to a non-cooperative target where the optical device used to transmit and receive laser energy is a plurality of optical transceivers (typically a telescope, but often referred to as a subaperture telescope or transceiver). The Coherent Phased Array Beam Transmission and Imaging System controls the plurality of laser beams (that originate from a single master oscillator laser and are amplified and/or transported using separate beam paths) to coherently combine the outgoing beams from each subaperture to form a single phased beam at the target. The preferred embodiment for the Coherent Phased Array Beam Transmission and Imaging System includes a method to maintain the beam hit spot on the target aim point at the full resolution of the array.

FIELD OF INVENTION

The present invention relates to a method and system implementation for projection of laser beams through a turbulent medium where the optical device used to transmit and receive laser energy is a plurality of optical transceivers (typically a telescope, but often referred to as a subaperture telescope or transceiver) that are controlled to coherently combine the outgoing beams from each subaperture to form a single phased beam at a target. The target is assumed to be non-cooperative. A non-cooperative target refers to a target in which no laser beacon is provided directly by the target for wavefront sensing.

BACKGROUND OF THE INVENTION

Projection of laser beams requires an aperture diameter, or effective aperture diameter, large enough such that the diffraction limit of the optical system, given by λL/D (where λ is the wavelength of light, L is the propagation distance, and D is the aperture diameter), is sufficiently small to provide the required beam size on the target of interest. By the same token, imaging requires that the aperture diameter be large enough such that the diffraction limit is smaller than the object features to be resolved by the imaging system. The size and cost of conventional telescope systems, which utilize a large monolithic mirror, or several large mirror segments, to achieve these requirements can be exorbitant. An alternative technology that has been pursued for many years is that of phased array beam combining and phased array imaging. In a phased array system, a plurality of smaller telescopes are used in a coherent fashion so that the plurality of smaller telescopes, when combined coherently, have the same diffraction limit as a larger conventional telescope.

While this appears on the surface to offer a creative and viable solution to the size and weight problems associated with large aperture optical systems, there is a significant lack of insight and methods for means to actually achieve coherent phasing of the array. While much of the prior art focuses on creative devices for phased array beam control and/or imaging (see references: 1, 2, 3, 4, 5), only a small fraction of prior art focuses on the sensing and control required for active phasing of the array (see references 6, 7, 8, 9). The published methods for beam phasing and imaging utilize hill climbing metric optimization based methods whose performance is heavily dependent on the choice of metric and are usually inherently limited in the speed at which compensation can be achieved.

This is particularly true if the phased array must operate in a real time manner for compensation of the effects of turbulence on laser propagation. In the case of laser propagation, the prior art relies on a closed loop feedback signal that operates over the round trip from the transmitter to the target and back, inherently limiting the characteristic compensation time constant to be at least roughly 20 times slower than the round trip time of flight to the target and back. While this characteristic compensation time may be acceptable for some applications, it is inadequate for the majority of applications and is certainly not adequate for compensation of phasing errors resulting from on board vibration sources. On board vibration sources can be the most significant source of phasing errors—particularly if fiber laser and amplifier systems are used, for which phase aberrations are inherently highly susceptible to both mechanical and acoustic vibration sources. The present invention is relevant to general phased array systems, but in particular focuses on the challenging problems posed by phased arrays of fiber laser systems.

What is needed is a method for coherently combining a plurality of subapertures to phase a plurality of beams at a target. The method must not require a monolithic beam director (which would eliminate the size and weight advantages of the phased array). The method must be capable of being effective with moving targets and platforms. The method must have a means to accomplish isolation of the transmitted beam from a return beam used for measurement of aberrations in the path. The method must not rely on measurements from one wavelength of light to compensate through fiber optical transport at another wavelength of light (this is due to the fact that measurements through fiber can only be measured within a single wavelength—unless an ultra-precise fiber length measurement system is incorporated into the system, in which case such a system must be integrated without a non-common path that passes through fiber optic beam transport). The method must utilize a common reference plane to serve as the “zero phase reference” for the phased array. The method must not have any non-common path that is in fiber optical transport. The method must accommodate a means for on-platform stabilization of high speed, large amplitude aberrations that occur in either a fiber amplifier or due to mechanical/acoustical vibrations. The method must have a means for measurement of the tilt error on each subaperture and must have a means for measurement of the global tilt error of the entire array.

The present invention provides for a means to meet these requirements in an innovative fashion that enables the potential size and weight advantages of coherent phased array laser and imaging systems to be realized.

REFERENCES

SUMMARY OF THE INVENTION

The primary aspect of the present invention is to provide a System and Method for Coherent Phased Array Beam Transmission and Imaging to measure and correct for aberrations in a laser beam projection system formed by a plurality of optical transceivers (typically a telescope, but often referred to as a subaperture telescope or transceiver).

Another aspect of the present invention is to provide for phase correction from a master oscillator, through a plurality of laser amplifiers, through a plurality of subapertures, and through a turbulent path to a target, strictly by use of phase correction devices in low power beam paths.

Another aspect of the present invention is to maintain the phased beam on the target aimpoint, enabling maximum energy deposition on the target aimpoint. Other aspects of this invention will appear from the following description and appended claims, reference being made to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.

The present invention provides a method for end-to-end compensation of a plurality of laser beams through a turbulent medium to a non-cooperative target where the optical device used to transmit and receive laser energy is a plurality of optical transceivers (typically a telescope, but often referred to as a subaperture telescope or transceiver). The Coherent Phased Array Beam Transmission and Imaging System controls the plurality of laser beams (that originate from a single master oscillator laser and are amplified and/or transported using separate beam paths) to coherently combine the outgoing beams from each subaperture to form a single phased beam at the target. The preferred embodiment for the Coherent Phased Array Beam Transmission and Imaging System includes a method to maintain the beam hit spot on the target aimpoint at the full resolution of the array

A summary of the present invention can also be described with reference to the Figures below.

1. A Coherent Phased Array Beam Transmission and Imaging System for end-to-end compensation of a plurality of subaperture transceivers that form a plurality of laser beams through a turbulent medium to a non-cooperative target, the system comprising:

A. A first control loop comprising:a master oscillator producing a master oscillator beam;a beam splitter to divide said master oscillator beam into a reference beam and a plurality of outgoing beams;a plurality of first phase modulator control devices functioning to control a phase of each of the plurality of outgoing beams;a plurality of amplifiers functioning to amplify each of the plurality of outgoing beams;a plurality of subaperture transceivers functioning to direct the plurality of outgoing beams through a path to a target;a plurality of return beams reflected from the target back to the plurality of subaperture transceivers functioning to direct the return beams to a plurality of second phase modulator control devices, each functioning to control a phase of the plurality of return beams;the plurality of second phase modulator control devices passing the plurality of return beams into a sensing assembly which functions to focus a coherent sum of the plurality of return beams onto an internal high speed detector;said internal high speed detector functioning to produce an electronic signal and pass said electronic signal to a high speed computer;said high speed computer functioning to control said plurality of second phase modulator control devices in a closed loop fashion to maximize the coherent sum of the plurality of return beams onto the high speed detector;

B. A second control loop comprising:a third phase modulator receiving said reference beam and directing said reference beam into the sensing assembly;said sensing assembly then passing the reference beam to the plurality of second phase modulator control devices and into the plurality of subaperture transceivers functioning to form a plurality of interference patterns between the reference beam and the plurality of outgoing beams producing a plurality of detector signals;said detector signals passed to the high speed computer functioning to control the plurality of first phase modulator control devices such that the phase of an outgoing beam will be locked to a phase of the reference beam at an aperture sharing element within each subaperture transceiver;

C. A third loop comprising:

a return image from the target;said return image passing through a path to the plurality of subaperture transceivers and focused to a plurality of cameras or detector arrays internal to each subaperture transceiver;said plurality of subaperture transceivers forming an electronic image signal and passing said electronic image signal to an image processor; andsaid image processor controlling a plurality of beam steering devices internal to the plurality of subaperture transceivers functioning to keep the plurality of outgoing beams locked onto the target and to maintain an outgoing beam hit spot on a target aimpoint with a coordinated imaging resolution of the plurality of the subaperture transceiver.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1is a schematic illustrating the elements of a subaperture assembly160equipped with the necessary optical components to implement the phasing and imaging method of the present invention; where the subaperture is depicted as having a transmissive beam steering device at the entrance aperture. Illustrated are the necessary elements of a subaperture assembly160to effect the beam phasing and imaging system of the present invention.FIG. 1is shown by way of example and not of limitation. There are alternate means of displaying the laser path(s) as those skilled in the art would recognizeFIG. 1illustrates subaperture assembly160integrated with a transmissive beam steering device108. The subaperture assembly160consists of an interface for injection of an outgoing beam in fiber100in single mode polarization maintaining fiber which is connected to a fiber 2-axis or 3-axis steering and/or focus control assembly101. Fiber100, containing outgoing beam102, terminates at focus control assembly101allowing the outgoing beam102to expand in free space until it is collimated by collimating lens103. Outgoing beam102is noted to be linearly polarized. Outgoing beam102propagates through collimating lens103to an aperture sharing element104, which is highly reflective at the outgoing beam polarization (nominally assumed to be “S” polarized with respect to the aperture sharing element104) and highly transmissive at the orthogonal polarization (nominally assumed to be “P” polarization with respect to the aperture sharing element104). Aperture sharing element104is also noted to be highly transmissive at the wavelength or wavelength band of the imaging beam150(which is discussed in further detail below). The majority outgoing beam102A then reflects from the aperture sharing element104and propagates to a quarter wave plate105, which converts the polarization of majority outgoing beam102A from linear to circular polarization. Majority outgoing beam102A then propagates to a lens106(which the reader skilled in the art will note could equally be an appropriately specified mirror) which in conjunction with the lens107expands outgoing majority beam102A to maximize the fill factor of the subaperture assembly160, where fill factor is the ratio of the size of the majority outgoing beam102A to the subaperture output diameter. The majority outgoing beam102A then propagates through a beam steering device108, which has the ability to control the pointing of the outgoing majority beam102A. Beam steering control signal111includes the x and y tilt commands that control beam steering device108. There are numerous methods for such a device with multiple trades associated with the operation of such devices. In some cases, the operation of the device will preclude use of circularly polarized light. In these cases, the ordering of the quarter wave plate105and the beam steering device108can sometimes be reversed in a manner that enables beam steering with the device—such methods are well known to those skilled in the art and are dependent on the specific behavior of the beam steering device108selected. The majority outgoing beam102A then propagates to a target (not shown) where it reflects from the target to form the return beam120.

Return beam120propagates back through beam steering device108and lens pair107and106. Upon propagation through the quarter wave plate105the polarization is converted from circular back to “P” polarization to enable transmission of return beam120through the aperture sharing element104. Return beam120then propagates to filter121that is highly reflective at the wavelength of return beam120(also highly reflective of outgoing beam102) while being highly transmissive at other wavelengths of light which can be used for the imaging and tracking aspects of the subaperture (i.e. the filter is highly transmissive at the wavelength or wavelength band of the imaging beam150). The return beam120then reflects from the filter121and is directed via the lens122into a fiber 2-axis or 3-axis steering and/or focus control assembly123. The return beam120in polarization maintaining fiber130is then denoted in fiber124. The polarization of polarization maintaining fiber130is keyed to be oriented to the “P” polarization of the return beam120and return beam in fiber124. Counter-propagating to the return beam in fiber124is the reference beam in fiber131. The reference beam132, contained in fiber131, is required to originate from the same laser as the outgoing beam102(in fiber100) so that the two beams are coherent with one another. The reference beam132, contained in fiber131, is assumed to be composed of both polarizations, in approximately equal proportions. The fact that the reference beam132in fiber131and the return beam in fiber124counter-propagate in the same polarization maintaining single mode fiber130is denoted by the index130indicating grouping of the two counter-propagating beams. The reader should recall that polarization maintaining fiber supports propagation of both polarizations, however, the polarization will be randomly scrambled unless the input polarization state is aligned to the polarization maintaining fiber. Thus the return beam in fiber124will maintain its “P” polarization state but the reference beam in fiber131will have a random polarization state with roughly equal parts in the two polarization states.

The reference beam in fiber131is directed by the fiber 2-axis or 3-axis steering and/or focus control assembly123to expand the reference beam132in free space for collimation by the lens122. Reference beam132then is directed by the filter121to the aperture sharing element104. Recalling that the aperture sharing element104is highly reflective at the “S” polarization of the outgoing beam102the aperture sharing element104will reflect roughly half of the reference beam132to form an over-lapping beam133which is composed of the minority sample of the outgoing beam102(at primarily the “P” polarization state—generated by the residual polarization impurity in the outgoing beam102) and a sample of the reference beam132(at the “S polarization state). The pair of beams133is rotated in polarization by 45 degrees by a half wave plate134with respect to the polarizer135which rejects the orthogonal polarization component of both beams, leading to an interference pattern between the minority sample of the outgoing beam102and the sample of the reference beam132. This interference pattern is focused by the lens136onto a detector137. The speed of the detector137is sufficiently high to enable rejection of high speed platform mechanical and acoustic disturbances and phase disturbances due to propagation through a laser amplifier. The detector signal138runs from the subaperture assembly160and carries the output signal of the detector.

The final aspect of importance inFIG. 1is the imaging beam path. The imaging beam150is formed from either passive or active illumination of the target or target scene. In the case of the transmissive beam steering device108there may be sensitivity to the choice of the wavelength or wavelength band of the imaging beam150. The imaging beam150wavelength must be sufficiently different from the outgoing beam102/return beam120wavelength to enable isolation but must still be suitable for beam steering by the beam steering device108. Such trades in the wavelength or wavelength band selection of the imaging beam150are well known to those skilled in the art. The imaging beam150passes through beam steering device108and then focusing pair lens assemblies107and106and quarter wave plate105. Imaging beam150then propagates to aperture sharing element104, which is noted at this point to be highly transmissive at the imaging beam150wavelength, or wavelength band. The imaging beam150then propagates to and passes through the filter121(which is highly transmissive at the imaging beam150wavelength or wavelength band), whereupon the beam passes through a blocking filter151that blocks light at the wavelength of the outgoing beam102and return beam120and transmits light at the wavelength or wavelength band of the imaging beam150. Next the imaging beam150propagates to a lens152, which focuses the imaging beam150onto a detector array153. Detector array153will be used for subaperture beam tracking and pointing and for full aperture array imaging. Detector array153generates the image signal154. Signal enter/exit points A, B, C, D, E, F, G are shown with reference toFIG. 3below.

FIG. 2. is a schematic illustrating the elements of an alternate subaperture assembly160A equipped with the necessary optical components to implement the phasing and imaging method of the present invention; where the subaperture is depicted as having a reflective beam steering device at the entrance aperture. Illustrated are the necessary elements of subaperture assembly160A to effect the beam phasing and imaging system of the present invention.FIG. 2is shown by way of example and not of limitation. There are alternate means of displaying the laser path(s) as those skilled in the art would recognizeFIG. 2illustrates a subaperture assembly integrated with a reflective beam steering device. The difference betweenFIG. 1andFIG. 2is that the transmissive beam steering device108inFIG. 1is replaced with flat mirrors109and110, one or both of which have steering capability to point to the beam. Nominally the flat mirror110would be a steering mirror. Those skilled in the art will immediately recognize numerous other possible configurations of the focusing pair lens assemblies106and107and flat mirrors109and110. For example, the beam expander formed by focusing pair lens assemblies106and107could be composed of reflective elements. One of the flat mirrors109,110could be eliminated and/or placed in “small” beam space before the beam expander formed by lenses106and107. The same inter-changing of components could apply to the use of a transmissive beam steering device108and/or the mixing of reflective and transmissive components. The trades associated with selection of the appropriate beam steering device are well known to those skilled in the art and are driven by a number of factors: field of regard, field of view, optical throughput and optical fill factor over the field of regard, wavelength of the beams utilized in the system, bandwidth requirements for the beam steering device, etc. An important point to note is that a reflective beam steering device will enable greater freedom in the choice of wavelength or wavelength band of imaging beam150. Signal enter/exit points A, B, C, D, E, F, G are shown with reference toFIG. 3below.

FIG. 3is a functional “block” representation of either subaperture assembly160or160A represented inFIG. 1orFIG. 2; this “block” representation is utilized to represent the subaperture interface in more complex full system drawings (i.e.FIG. 4below).FIG. 3is shown by way of example and not of limitation. Subaperture assembly160is used to depict the interfaces to the relatively complex drawings of subaperture assemblies160,160A inFIG. 1andFIG. 2. This is shown in a manner that permits understanding of the relationships of the various interfaces of the subaperture assembly160to the Coherent Phased Array Beam Transmission and Imaging System. The interface point A is the input interface for the imaging beam150. The interface point B is the output interface for the majority outgoing beam102A. The interface point C is the input interface for the return beam120. The interface points A, B, and C are understood by way of reference to like points shown inFIG. 1andFIG. 2to be coincident to one another, even if they are depicted as separate interfaces. The interface point D is the input interface for the outgoing beam in fiber100. The interface point E is the input interface for the polarization maintaining fiber130, which directs counter-propagating return beam in fiber124and reference beam in fiber131. The interface point F is the output interface for the image signal154. The interface point G is the output interface for the detector signal138. The interface point H is the input interface for the beam steering control signal111. The functional block representation of the subaperture assembly will be used inFIG. 4below.

FIG. 4is a schematic of the present invention comprised of a plurality of the subaperture assemblies160illustrated in detail inFIG. 1orFIG. 2(and represented byFIG. 3) and the necessary control interfaces to effect outgoing beam phasing and steering and receive beam or light imaging. Given the supporting description provided byFIGS. 1,2, and3, the full description of the Coherent Phased Array Beam Transmission and Imaging System is provided inFIG. 4is shown by way of example and not of limitation.

Master oscillator laser200transmits master oscillator beam201(nominally in polarization maintaining single mode fiber) to a multi-beam splitter202. Multi beam splitter202samples master oscillator beam201into the master reference beam250and a plurality of outgoing beams100B. The plurality of outgoing beams100B pass through a plurality of transmit path phase modulators204that are used to phase the plurality of majority outgoing beams100A at target212. After correction by the plurality of transmit path phase modulators204the plurality of majority outgoing beams100A propagate (nominally in polarization maintaining fiber) to a plurality of amplifiers210which amplify the plurality of majority outgoing beams100A to form the plurality of amplified outgoing beams100. The plurality of amplified outgoing beams100are then directed via polarization maintaining fiber to the plurality of subaperture assemblies160at the input interface point D. Each subaperture assembly160directs its respective amplified outgoing beam100(ref.FIG. 1orFIG. 2) to the plurality of output interface point B to form a plurality of majority outgoing beams102A. The plurality of majority outgoing beams102A propagate through path211to target212and then reflect back from target212to form the plurality of return beams120.

The reader should note that the plurality of return beams120in fact is a single beam composed of the return signal from the reflection of the plurality of majority outgoing beams102A and the term plurality refers to the effective plurality of beam paths from the individual subaperture assemblies160to the target212aimpoint. The plurality of return beams120are received at the plurality of each respective subaperture assembly160interface entry points C. Subaperture assemblies160each direct their respective return beams120to form a plurality of return beams in fiber124at the input/output interface points E of each respective subaperture assembly160. The plurality of polarization maintaining fiber130directs the return beams in fiber124to a plurality of return path phase modulators240to form the plurality of modulated (compensated) return beams in fiber124A. The plurality of compensated return beams in fiber124A are directed to a plurality of collimator assemblies241which are precisely aligned with respect to the common reference plane of the reference beam splitter242. The plurality of collimated return beams223after projection from the plurality of collimator assemblies241pass through the beam reference beam splitter242. The reference beam splitter242is a polarizing beam splitter—highly reflective at the interface plane at the “S” polarization and highly transmissive at the “P” polarization. The plurality of collimated return beams223then are focused, via lens243, into a high speed pinhole detector244, whose output signal245is connected to a high speed computer246(which likely is a dedicated digital or analog high speed processor). The high speed computer246generates a plurality of control signals247which are used to control the plurality of return path phase modulators240to maximize the signal measured in the high speed pinhole detector244. Those skilled in the art will note that there are multiple means to measure and compensate the relative phase on the plurality of return beams223. The method illustrated here represents a straightforward approach well suited to control of in-line phase modulators with a single high speed detector and is considered the preferred method. The method of multi-dither control or stochastic parallel gradient descent is likely the best implementation of the controller (ref: 5; 6; 10). The return path phase control loop described here originating at the target to generate the plurality of return beams120and direct them to the high speed pinhole detector244for control of the plurality of return path phase modulators240will lead to the best estimate of coherent phasing of the return path from the target to the common reference plan of the reference beam splitter242.

Returning to master reference beam250, reference beam250is directed to master reference phase modulator251that cycles over a full wave of phase modulation at sufficiently high speed to enable compensation of on board phase disturbances due to mechanical and acoustic vibration as well as the plurality of amplifiers210. This modulation speed will typically be at least 80-100 times the required compensation bandwidth to achieve sufficient rejection of the disturbances. The resultant modulated master reference beam250A then is transmitted components of rigid sensing assembly2000. The first internal component is a fiber transmission assembly comprised of a 2-axis or 3-axis fiber positioner252and lens253that is tightly aligned to the common reference plane of the reference beam splitter242, thus resulting in optimal coupling of the effective plurality of reference beams231to the plurality of collimator assemblies241. Again the term effective plurality refers here to the plurality of beam paths from the plurality of collimator assemblies241from the 2-axis or 3-axis fiber positioner252. The plurality of reference beams231are then coupled into the plurality of collimator assemblies241to form the plurality of uncompensated reference beams in the plurality of fibers131B, which are in turn transmitted external to rigid sensing assembly2000to the plurality of return path phase modulators240. The plurality of return path phase modulators240impart the same phase on the plurality of uncompensated reference beams in the plurality of fibers131B as on the plurality of compensated return path beams124A (note that the beams all originate from the same master oscillator laser200) to form the plurality of reference beams in fiber131. The plurality of reference beams in fiber131are directed to the plurality of subaperture assemblies160at the input interface points E. The subaperture assemblies160each direct their respective reference beams in fiber131and minority sample of the outgoing beams in fiber100according toFIGS. 1,2to generate the plurality of detector signals138at the plurality of output interface points G. The plurality of detector signals138are directed to a controller213which generates (via standard high speed phase lock loop control or any number of standard methods) control signals214to control the transmit path of the plurality of phase modulators210to stabilize the outgoing beam phase path from the master oscillator200to the reference beam phase established by the return path compensation loop. This compensation loop results in the best estimate of stabilization of the phase of the outgoing beams102A from the master oscillator200to the target212. The only non-common path in the phasing system is the difference between the phase path from the 2-axis or 3-axis fiber positioned to the common reference plane of the reference beam splitter242and the phase path from the common reference plane of the reference beam splitter242to the high speed detector244. The entire sensing assembly2000comprised of the plurality of collimator assemblies241, reference beam splitter242, lenses243and253, high speed detector244, and 2-axis or 3-axis fiber positioned252can be made highly compact and rigid and designed and toleranced for minimum possible sensitivity to vibration—thus reducing the non-common path to a magnitude many times smaller than the wavelength of light. An additional remaining potential source of non-common path is Doppler shift between the transmit path and return path due to a moving target and/or platform. This must be analyzed on a case by case basis and depends on many factors. However, in the case of the Coherent Phased Array Beam Transmission and Imaging System, because the only impact of Doppler shift is to shift the wavelength of the return light relative to the wavelength of the transmitted light (rather than Doppler shifting some much lower frequency modulation of light beam), the impact of Doppler shift even for propagation through 100 meters of fiber (much more than would be expected) is less than 0.25% of a wave of aberration for a Mach 4 directly incoming target. This sample calculation indicates that Doppler shift will not cause significant non-common path aberration. A final potential source of non-common path error is non-linear effects on the two different polarizations in the plurality of counter-propagating fiber paths130. The reader will note that this path is all in low power, thus the magnitude of any non-linear effects will be miniscule (much smaller than the example Doppler shift calculation shown above). Thus, under the assumption of a compact and rigid sensing assembly2000comprised of the plurality of collimator assemblies241, reference beam splitter242, lenses243and253, high speed detector244, and 2-axis or 3-axis fiber positioned252, the non-common path in the Coherent Phased Array Beam Transmission and Imaging System can be controlled to very small levels.

The final aspect of the Coherent Phased Array Beam Transmission and Imaging System as described inFIG. 4is the imaging path which originates at the target212to form a plurality of imaging beams150which propagation through the path to the target211to the plurality of subaperture assemblies160. The plurality of subaperture assemblies direct the imaging beams150to form a plurality of image signals154that are directed to the steering controller (image processor)263. Individually, each of these image signals154can be utilized to generate a tip and tilt measurement for each subaperture via one of many standard processing techniques well known to those skilled in the art. The tip and tilt measurements then provide a plurality of local control signals254for control of the plurality of beam steering devices (108or109/110) in the plurality of subaperture assemblies160. This local control signal254stabilizes the imaging path and thus by reciprocity the plurality of outgoing beams102A to the target. Under the assumption of boresighting within each subaperture using standard methods well known to those skilled in the art, this stabilization is accomplished up to a global steering offset signal that will be common to the beams.

Although the methodology defined byFIG. 4defines one embodiment of the Coherent Phased Array Beam Transmission and Imaging System, an improvement to this embodiment is necessary for optimal performance. The methodology for beam steering as described inFIG. 4has a limitation that there will be an unknown global steering offset signal of the aimpoint of the beam due to the fact that the resolution of the subapertures is limited by the subaperture diameter and the effective imaging resolution of the full array has not yet been considered. Inclusion of a method for calculation of a global steering offset signal to be added to the plurality of local control signals154completes the definition of the preferred embodiment of the Coherent Phased Array Beam Transmission and Imaging System. Calculation of the global steering offset signal can only be performed by evaluation of an image of the target obtained at the resolution of the entire array and evaluation of the coherent combined beam position on the target at the resolution of the entire array. Prior to describing the calculation of the global steering offset signal for the Coherent Phased Array Beam Transmission and Imaging System, a modification of the imaging system must be described that will provide a plurality of subaperture-resolution images of both the target carried by the plurality of imaging beams150and a plurality of subaperture-resolution images of the outgoing beam on the target carried by the plurality of return beams120. Example schematics of this modification are provided inFIG. 5,FIG. 6, andFIG. 7. These example schematics are provided by way of example and not limitation. Practitioners skilled in the art will be familiar with multiple possible alternate implementations that enable simultaneous imaging of the plurality of target imaging beams150and the plurality of return beams120.

FIG. 5is an alternate schematic of a subassembly1600of the imaging subaperture assembly that is included inFIG. 1andFIG. 2comprised of elements121,151,152, and153to produce the imaging signal154. The imaging portion of the subaperture assembly inFIG. 1andFIG. 2, as depicted, only provides for imaging of the target. The imaging subassembly1600depicted inFIG. 5provides for imaging of both the target and for imaging of the return of the outgoing beam from the target by means of a beamsplitter121A that separates the light wavelength associated with the target and the return from the outgoing beam and imaging both beams on separate cameras. The imaging beam150(formed from either passive or active illumination of the target or target scene) transmits through beamsplitter121A which is highly transmissive at the wavelength of the imaging beam150and partially transmissive at the wavelength of the return beam120(the degree of partial transmissivity of the beam splitter121A at the wavelength of the return beam120is to be determined based on the particular application of interest, using calculations and methods well known to those skilled in the art). The imaging beam then passes through a second beamsplitter151A, which is highly transmissive at the wavelength of the imaging beam150and highly reflective at the wavelength of the return beam120. The imaging beam150then is focused by the lens152A to the imaging beam detector array153A to produce the imaging beam image signal155A (target image signal). The return beam120is partially transmitted through the beamsplitter121A (with the reflected component of the return beam120being understood to follow the beam path defined inFIG. 1andFIG. 2). The partially transmitted component of the return beam120then is reflected from the beamsplitter151A and is focused by the lens152A onto the return beam detector array153B to produce the return beam image signal155B. The combination of the imaging beam image signal155A and return beam image signal155B are depicted as the dual image signal154A (in lieu of image signal154at exit point F of the subaperture assembly) for purposes of connectingFIG. 5to the depictions of the subaperture assembly inFIG. 1,FIG. 2,FIG. 3, andFIG. 4.

FIG. 6is an alternate schematic of the imaging subassembly1600described inFIG. 5. The imaging subassembly1600A depicted inFIG. 6provides for imaging of both the target and for imaging of the return of the outgoing beam from the target on a single camera by means of a prism156that separates the light wavelength associated with the target and the return from the outgoing beam and imaging both beams on the detector array. The imaging beam150transmits through beamsplitter121A which is highly transmissive at the wavelength of the imaging beam150and partially transmissive at the wavelength of the return beam120(the degree of partial transmissivity of the beam splitter121A at the wavelength of the return beam120is to be determined based on the particular application of interest, using calculations and methods well known to those skilled in the art). The imaging beam150then passes through prism156which leads to an angular deflection of the imaging beam150with respect to the partially transmitted component of the return beam120. It is to be understood by those skilled in the art that the angular separation must be large enough to separate the fields of view of the imaging beam150and the return beam120. This may require consideration of trades of the camera153readout speed and potential introduction of a spatial filter at the system focus between focusing elements106and107(ref.FIGS. 1,2) of each subaperture. Practitioners skilled in the art will be capable of making and understanding these trades. After passing through the prism156imaging beam150is focused by the lens157to the detector array153. The return beam120is partially transmitted through the beamsplitter121A (with the reflected component of the return beam120being understood to follow the beam path defined inFIG. 1andFIG. 2). The partially transmitted component of the return beam120then passes through the prism156and is focused by the lens157to the detector array153. The prism leads to physical displacement of the two images on the detector array (camera)153, effectively producing two image signals: the imaging beam image signal155A and the return beam image signal155B, both contained and depicted as the aggregate signal154B in the same manner as inFIG. 5.

FIG. 7is yet another alternate schematic of the imaging subassembly described inFIG. 5andFIG. 6. The imaging subassembly1600B depicted inFIG. 7provides for imaging of both the target imaging beam150and for imaging of the return beam120from the target on a single camera (detector array)153by means of a diffraction grating158that separates the light wavelength associated with the target and the return from the outgoing beam and imaging both beams on the detector array153. The imaging beam150transmits through beamsplitter121A which is highly transmissive at the wavelength of the imaging beam150and partially transmissive at the wavelength of the return beam120(the degree of partial transmissivity of the beam splitter121A at the wavelength of the return beam120is to be determined based on the particular application of interest, using calculations and methods well known to those skilled in the art). Imaging beam150then passes through a diffraction grating158, which leads to an angular deflection of the imaging beam150with respect to the partially transmitted component of return beam120. It is to be understood by those skilled in the art that the angular separation must be large enough to separated the fields of view of the imaging beam150and the return beam120. This may require consideration of trades of the camera readout speed and potential introduction of a spatial filter at the system focus between focusing elements (ref.106and107ofFIGS. 1,2) of each subaperture. Practitioners skilled in the art will be capable of making and understanding these trades. After passing through the diffraction grating158, imaging beam150is focused by the lens157to the detector array153. Return beam120is partially transmitted through the beamsplitter121A (with the reflected component of the return beam120being understood to follow the beam path defined inFIG. 1andFIG. 2). The partially transmitted component of the return beam120then passes through the diffraction grating158and is focused by the lens157to the detector array153. Diffraction grating158leads to physical displacement of the two images on the camera (detector array153), effectively producing two image signals: the imaging beam image signal155A and the return beam image signal155B, both contained and depicted as the aggregate signal154B as inFIG. 6and in the same manner as dual image signal154shown inFIG. 5.

Prior to proceeding with description of the means to process this data, it is necessary to provide an example description of a means to avoid detector signal saturation for the return beam image signal155B.FIG. 8is an example timing diagram illustrating a means to avoid detector signal saturation for the return beam image signal155B. The technical problem addressed inFIG. 8is that the random scatter from the optics in the subaperture beam train that results from the outgoing beam102can lead to a background signal on the imaging detector array153,153A, or153B. If the scatter signal level is large enough, then the imaging beam150and return beam120may not be detectable within the shot noise associated with the scatter induced background. Practitioners skilled in the art will be familiar with the calculations to be performed to assess the degree of this technical problem.FIG. 8depicts a frame interval time300that corresponds to the frame rate of the return beam imaging detector array153,153A, or153B. The outgoing beam100will be “ON” or allowed to proceed through the system during the outgoing beam100“ON” time illustrated by301. The outgoing beam100“ON” time should be maximized in general but this will be dependent on the specific application of interest. The outgoing beam100should be blocked or turned off by any of standard methods known to those skilled in the art for camera integration time intervals302that allow for imaging of the return beam120from the target. The method selected for de-activating the outgoing beam100must be cognizant of maintaining laser and/or amplifier stability. During camera integration time intervals302the shutter of the return beam imaging detector array153,153A, or153B will be open to enable imaging of the return beam120. The reader should note that the timing of the frame interval300and the camera integration time intervals302must be such that time of flight to the target and back does not correspond to a time in which no return signal from the target will be measured. This is straightforward to ensure for those skilled in the art.

FIG. 5,FIG. 6, andFIG. 7all provide example descriptions of means to obtain a plurality of subaperture-resolution images of both the target carried by the plurality of imaging beams150and a plurality of subaperture-resolution images of the outgoing beam on the target carried by the plurality of return beams120.

FIG. 8is a schematic block diagram illustrating one method to ensure isolation of scatter within the optics of the subaperture beam train to isolate the outgoing high power beam100from the return beam image signal120, which returns from the target, in order to enable imaging of the return beam from the target.FIG. 8provides an example timing diagram illustrating a means to prevent detector saturation for the return beam image signal150that may be necessary, depending on the application (practitioners skilled in the art can perform the necessary calculations to evaluate the necessity of this additional modification).

FIG. 9is a flow chart illustrating the process steps to obtain an image of the target and an image of the return beam from the target at the resolution of the full array and use this full array resolution image to compute a global offset tilt signal to maintain the focused beam on the target aimpoint. Given the example methods inFIG. 5,FIG. 6,FIG. 7, andFIG. 8, a method to compute the global steering offset signal to be added to the plurality of local control signals254(ref.FIG. 4) is described. This final important detail, defined by the schematic block diagram inFIG. 9, is the last challenge addressed by the Coherent Phased Array Beam Transmission and Imaging System and in conjunction withFIG. 4defines the preferred embodiment for the Coherent Phased Array Beam Transmission and Imaging System.

FIG. 9represents the processing steps of imaging processor263(ref.FIG. 4). In step400A, the plurality of target image signals155A are received and then each image is processed in step401A to compute an estimate of the x and y tilt on each subaperture. The estimate can be computed using any number of methods well known to those skilled in the art, including centroid calculation, threshold centroid calculation, matched filter calculation, edge detection methods, etc. The selected method will be dependent on the application of interest-however, the recommended preferred embodiment is a matched filter calculation utilizing either one of the subaperture images as a reference or the shift- and added averaged of the subapertures as a reference (where the shift and add technique is well known to those skilled in the art). The recommended procedure nominally used in step402is to form an estimate of the piston phase using gradient optimization methods to improve image Strehl ratio or image sharpness using methods well known to those skilled in the art.

Given the piston phase and x and y tilt measurements, an estimate of the full array image can be formed in step403A according to the following:1. Given two dimensional subaperture image data, Ik, where k denotes the subaperture index with piston phase φkand x and y tilt sx,kand Sy,k.2. Interpolate image data, Ik, from the subaperture imaging sensor two dimensional x and y grid of coordinates, xkand yk, to an over-sampled high resolution x and y two dimensional grid of coordinates X and Y (where selection of the grid spacing on X and Y is determined by methods well known to those skilled in the art), to form the over-sampled subaperture image Jk.3. Compute Mk=Jkexp[iφk+i(2π/λ)(Xsx,k+Ysy,k)].4. Form the global image IT=ΣkMk, k=1 to K, where K is the total number of subaperture images utilized.

Once the full array target image estimate, IT, is formed in step403A, the location of the aimpoint on the target can be estimated in step404. This estimate is formed by methods well known to those skilled in the art and is denoted by axand ay.

The same processing described above in steps401A,402A, and403A (and optional step403AA) is repeated in steps401B,402B, and403B (and optional step403BB). These steps are applied to the plurality of return beam images155B, received in step400B, to form a full array return beam image estimate, IR, in step403B. This image IRcan then be used to form an estimate of the hitspot position on the target, in step405, using methods well known to those skilled in the art. The hitspot position on the target is denoted hxand hy.

Given the aimpoint and hitspot estimates, the global offset signals gx=ax−hxand gy=ay−byare computed in block406. These represent the difference between the return beam images and imaging beam image signals. These global offset signals are then added to the subaperture x and y tilt signals in step407prior to temporal filtering in step408and before generating the plurality of beam steering signal254(ref:FIG. 4) that are passed on to each subaperture's beam steering device. This closed loop process stabilizes the beam on the target aimpoint using the full resolution of the Coherent Phased Array Beam Transmission and Imaging System, ensuring maximum energy at the target aimpoint.