Patent Publication Number: US-2023161172-A1

Title: System and method for correcting for atmospheric jitter and high energy laser broadband interference using fast steering mirrors

Description:
CROSS-REFERENCE TO RELATED APPLICATION AND PRIORITY CLAIM 
     This application claims priority under 35 U.S.C. § 120 as a continuation of U.S. patent application Ser. No. 16/559,136 filed on Sep. 3, 2019 (now U.S. Pat. No. 11,567,341), which is hereby incorporated by reference in its entirety. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with U.S. government support under contract number W9113M-17-D-0006-0002 awarded by the Department of Defense. The U.S. government may have certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     This disclosure is directed in general to laser pointing correction. More specifically, this disclosure relates to a system and method for correcting for atmospheric jitter and high energy laser broadband interference in laser beam pointing systems using fast steering mirrors. Here, laser beam pointing systems can include high-energy laser weapons, laser communications, beacons for laser guided weapons, laser imaging systems, and any other system application that requires a laser beam to propagate through the atmosphere, where the observed laser return motion for atmospheric compensation on the downlink is different than the laser disturbance on transmission or uplink 
     BACKGROUND 
     For high energy laser (HEL) tactical ground-to-air engagements with elevation angles greater than the horizon, HEL beam quality loss from atmospheric disturbances on the laser beam propagation is due to atmospherically induced beam tip-tilt or jitter, in addition to optical transmission losses. Uncompensated HEL beam jitter decreases the HEL power on the intended target, which increases target kill times and reduces target kill probability. Compensation for the atmospheric jitter of the HEL is important to maximizing HEL power-on-target. Similarly, for any laser beam pointing systems, such as laser communications, compensating for atmospheric jitter of the laser beam maximizes signal-to-noise-ratios, as in communications and laser imaging and laser spot location for remote imaging and laser weapon guidance. 
     SUMMARY 
     This disclosure provides a system and method for correcting for atmospheric jitter and high energy laser broadband interference for high energy weapon systems and any other system that requires a laser beam to be pointed accurately in the atmosphere at a target or object. 
     In a first embodiment, a system includes a high energy laser (HEL) configured to transmit a HEL beam and a beacon illumination laser (BIL) configured to transmit a BIL beam. The system also includes at least one fast steering mirror (FSM) configured to steer the BIL beam to be offset from the HEL beam. The system further includes at least one Coudé path FSM configured to correct for atmospheric jitter of the HEL beam and the BIL beam while maintaining the offset of the BIL beam from the HEL beam. 
     In a second embodiment, a jitter correction system includes at least one FSM configured to receive a BIL beam and steer the BIL beam to be offset from a HEL beam. The jitter correction system also includes at least one Coudé path FSM configured to steer the HEL beam and the BIL beam to correct for atmospheric jitter of the HEL beam and the BIL beam while maintaining the offset of the BIL beam from the HEL beam. 
     In a third embodiment, a method includes transmitting a HEL beam and transmitting a BIL beam. The method also includes steering, by at least one FSM, the BIL beam to be offset from the HEL beam. The method further includes steering, by at least one Coudé path FSM, the HEL beam and the BIL beam to correct for atmospheric jitter of the HEL beam and the BIL beam while maintaining the offset of the BIL beam from the HEL beam. 
     Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    illustrates examples of different kinds of atmospheric jitter; 
         FIG.  2    illustrates an example system for correcting for atmospheric jitter and high energy laser broadband interference according to this disclosure; 
         FIG.  3    illustrates additional details of a jitter correction system shown in  FIG.  2    according to this disclosure; and 
         FIG.  4    illustrates an example method for correcting for atmospheric jitter and high energy laser broadband interference according to this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The figures described below and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any type of suitably arranged device or system. 
     For simplicity and clarity, some features and components are not explicitly shown in every figure, including those illustrated in connection with other figures. It will be understood that all features illustrated in the figures may be employed in any of the embodiments described. Omission of a feature or component from a particular figure is for purposes of simplicity and clarity and is not meant to imply that the feature or component cannot be employed in the embodiments described in connection with that figure. 
     As discussed above, compensation for atmospherically induced jitter in high energy laser (HEL) systems is critical to maximizing HEL power-on-target. Atmospheric jitter includes both downlink jitter and uplink jitter.  FIG.  1    illustrates examples of both kinds of atmospheric jitter. As shown in  FIG.  1   , a focusing Gaussian HEL uplink beam  101  is transmitted from a laser source  110  to a target  120 . The diffuse surface of the target  120  spreads the beam  101 , which results in a diverging downlink beam  102 , which is a spherical wave or plane wave. Both the uplink beam  101  and the downlink beam  102  pass through cells  103  of optical turbulence, which act as small lenses that distort the beams  101 - 102 . 
     As indicated by the arrows of the beam paths, the uplink beam  101  and the downlink beam  102  interact with different cells  103  of optical turbulence. Thus, the resulting jitter is different on the uplink than on the downlink. Typically, the uplink HEL beam jitter is the dominant atmospheric jitter effect contributing to atmospheric induced loss of power on target. For tactical ranges, the uplink jitter is different than the downlink jitter as seen by traditional tracking systems, since the uplink beam  101  is a focusing Gaussian beam, and the downlink beam  102  is a spherical wave or plane wave. 
     The optical turbulence in the atmosphere degrades the effect of the HEL beam  101  by distorting its wavefront profile, which in effect reduces the focused power on target. Target dynamics also introduce tracking errors in HEL beam pointing. The wavefront errors introduced by optical turbulence are composed of multiple orders. The primary wavefront distortions are tip-tilt of beam jitter, and the other wavefront errors can be grouped in a category of higher order terms. As discussed above, the jitter of the beam  101  going up in the atmosphere is different than the jitter of the downlink beam  102 . In addition, as the target  120  heats up from the HEL beam  101 , significant disturbances from broadband interference from the generated heat interferes with tracking and atmospheric correction systems, thereby causing dropped track. 
     Different compensation systems are sometimes used to address at least some portion of optically induced atmospheric wavefront error. One system is a target illumination laser (TIL) and imaging tracker with a fast steering mirror (FSM) for tip-tilt correction. Another system is a wavefront sensor and deformable mirror for higher order wavefront correction. The TIL approach to compensation is a more basic form of atmospheric jitter correction, while the deformable mirror with adaptive optics (AO) compensation is employed on more advanced HEL systems. The TIL is used to illuminate the target in the short wave infrared (SWIR) band at an offset optical frequency for jitter correction, since looking at any return from the HEL beam will quickly saturate an optical receiver. The received target return (downlink) from the TIL illumination is imaged by a SWIR camera that is aligned optically with the HEL beam. The jitter in the image seen in the TIL SWIR image is the downlink jitter from the atmosphere, target dynamics, and any residual opto-mechanical jitter. The jitter in the SWIR image is estimated with an imaging tracker that estimates the target position error on each frame relative to boresight as well as the targets aimpoint. The error estimates are then provided to a FSM that applies an opposite command of the HEL of the estimated boresight error from jitter. 
     In some systems, the beacon illuminator laser (BIL) transmits using a separate optically aligned transmitting aperture from the HEL beam. Use of a separate transmitting aperture for the BIL introduces errors in estimation of atmospherics since the BIL angle to the target is slightly different from the HEL angle to the target and goes through different atmospheric paths. These errors reduce the effectiveness of any correction applied to the HEL beam at the target. 
     To address these issues, the embodiments described in this disclosure provide a system and method for correcting for atmospheric jitter and HEL broadband interference. The disclosed embodiments provide for accurate compensation of atmospherically induced jitter of a HEL beam on the target. The disclosed embodiments include multiple optical elements controlled by a track and atmospheric compensation algorithms that spatially offset the BIL beam from the HEL beam and perform tip-tilt correction of the HEL beam uplink jitter. By offsetting the BIL beam, the disclosed embodiments provide for a correction of the atmospheric errors while maintaining track in the presence of the HEL beam interference over the course of the engagement. 
     It will be understood that embodiments of this disclosure may include any one, more than one, or all of the features described here. Also, embodiments of this disclosure may additionally or alternatively include other features not listed here. While the disclosed embodiments may be described with respect to laser systems in military applications, these embodiments are also applicable in any other suitable systems or applications. 
       FIG.  2    illustrates an example system  200  for correcting for atmospheric jitter and high energy laser broadband interference according to this disclosure. As shown in  FIG.  2   , the system  200  includes a HEL  205 , a TIL  210 , a BIL  215 , a camera  220 , a jitter correction system  225 , and a controller  230 . 
     The HEL  205  is configured to generate a high energy laser beam that is aimed toward a particular location on a target  240 . The TIL  210  is configured to illuminate the target  240  with an illumination beam  235 , and can be used to measure the distance and angle of the target  240  relative to the HEL  205 . In some embodiments, the TIL  210  generates an illumination light at a wavelength of approximately 1575 nm. However, this wavelength is merely one example, and in other embodiments, the illumination light could be at a longer or shorter wavelength. 
     The BIL  215  is configured to generate a more focused illumination spot  245  on the target  240 . A particular intended location on the target  240  is selected to be illuminated by the BIL spot  245 . For example, it may be predetermined to illuminate a particular feature on the nose of the target  240 , and to offset the position of the BIL beam and the HEL beam to avoid broadband interference from the HEL heating of the target. As shown in  FIG.  2   , the BIL spot  245  is subject to optical turbulence  238  in the atmosphere, which results in uplink jitter of the BIL spot  245 . The actual location of the BIL spot  245  on the target  240  relative to the intended or expected location of the BIL spot  245  on the target  240  is used to determine the uplink jitter. In some embodiments, the BIL spot  245  is at a wavelength of approximately 1005 nm. However, this wavelength is merely one example, and in other embodiments, the BIL spot  245  could be at a longer or shorter wavelength. The wavelength of the BIL spot  245  is close to the wavelength of the HEL  205 ; thus, the two experience approximately the same uplink jitter. 
     The camera  220  is a high-speed SWIR camera co-boresighted with the HEL  205 . The camera  220  is configured to receive and process images from the target  240 . In particular, the camera  220  receives images that illustrate motion of the BIL spot  245  caused by atmospheric jitter. In some embodiments, one camera  220  is used for both TIL tracking of the target  240  and tracking of the BIL spot  245 . In other embodiments, these functions can be performed by separate cameras  220 . 
     The jitter correction system  225  is disposed in the optical path of the HEL  205  and the BIL  215 , and includes multiple optical elements configured to spatially offset the BIL beam from the HEL beam and control atmospheric jitter of both beams. The jitter correction system  225  operates to ensure that both beams reach the target  240  in the right location and are spatial offset from each other so that the two beams can be distinguished in the return signal, and so that the broadband interference generated by the HEL beam on the target is spatially separated from the return of the BIL. Thus, the jitter correction system  225  allows precise, independent pointing of multiple beams. Further details regarding the jitter correction system  225  are provided below with respect to  FIG.  3   . 
     The controller  230  performs multiple algorithms and control operations to correct atmospheric jitter and compensate for HEL broadband interference. The controller  230  can be programmable, and can include any suitable combination of hardware, firmware, and software for image tracking and control of other components, including the jitter correction system  225 . For example, the controller  230  could denote at least one processor  231  configured to execute instructions obtained from at least one memory  232 . The controller  230  may include any suitable number(s) and type(s) of processors or other computing or control devices in any suitable arrangement. Example types of controllers  230  include microprocessors, microcontrollers, digital signal processors, field programmable gate arrays, application specific integrated circuits, and discrete circuitry. In some embodiments, the operations of the controller  230  described herein may be divided and performed by two or more separate controllers  230 . 
       FIG.  3    illustrates additional details of the jitter correction system  225  according to this disclosure. As shown in  FIG.  3   , the jitter correction system  225  includes a pair of FSMs  302 - 304 , a first fold mirror  306 , a second fold mirror  308 , a deformable mirror  310  for wavefront correction, a pair of Coudé path FSMs  312 - 314 , an aperture sharing element  316 , and a high speed mirror  318 . 
     As discussed above, the BIL  215  transmits a BIL beam  320 , and the HEL  205  transmits a HEL beam  322 . Both beams  320 - 322  are aimed at the target  240 , but at slightly different locations on the target  240 . The BIL beam  320  results in the BIL spot  245  when it hits the target  240 , as discussed with respect to  FIG.  2   . 
     The FSMs  302 - 304  receive the BIL beam  320  but do not receive the HEL beam  322 . The FSMs  302 - 304  are controllable by the controller  230  and can be controlled to move in order to steer the BIL beam  320  in a direction independent of the HEL beam  322 . In particular, the FSMs  302 - 304  operate to steer the BIL beam  320  to be offset spatially and pointed angularly with respect to the HEL beam  322 . While the FSMs  302 - 304  cause the BIL beam  320  to be spatially and angularly offset from the HEL beam  322 , other components of the jitter correction system  225  keep the two beams  320 - 322  dynamically aligned, as discussed below. 
     The FSMs  302 - 304  operate in conjunction with a tracker algorithm that estimates optimal positioning of the BIL beam  320  from the TIL return. The estimate is used by the controller  230  to control the FSMs  302 - 304  to adjust the alignment of the BIL beam  320 . The optimal position of the BIL beam  320  is slightly offset from the HEL beam  322 , so that the BIL beam  320  illuminates a similar portion of the target  240 , but not exactly the same portion of the target  240  that the HEL beam  322  contacts (e.g., an offset of approximately six inches on some targets). Another reason for maintaining the BIL beam  320  offset from the HEL beam  322  is so that thermal interference from the heating of the target  240  is spatially separated from a BIL target return spot  324  in the camera  220 . The separation of the BIL target return spot  324  from the broadband thermal interference enables the ability to perform atmospheric compensation throughout the target engagement and illuminates target aimpoint features used to maintain the HEL beam aimpoint. 
     The fold mirrors  306 - 308  are separate mirrors having a similar function. The fold mirror  306  receives the BIL beam  320 , and the fold mirror  308  receives the HEL beam  322 . The fold mirrors  306 - 308  simply direct the beams  320 - 322  to the deformable mirror  310  without substantially changing any properties of the beams  320 - 322 . In contrast to the FSMs  302 - 304 , which are capable of changing orientation, the fold mirrors  306 - 308  are static mirrors. The fold mirrors  306 - 308  are representative of a beam control layout. In some embodiments, the fold mirrors  306 - 308  may be optional or their function may be implemented using other optical components. 
     The deformable mirror  310  receives the beams  320 - 322  and corrects for atmospheric wavefront errors sensed by an optional wavefront sensor (not shown). The deformable mirror  310  includes multiple actuators that move to control the shape of the surface of the deformable mirror  310 . In some embodiments, the actuators are controlled by the controller  230  based on sensor information received by the wavefront sensor. During operation of the system  200 , the beams  320 - 322  are subject to deformation. As the whole system heats up, vibrates, and flexes, the beams  320 - 322  are likely to deform. By changing the shape of its mirror surface, the deformable mirror  310  can correct the deformation of the beams  320 - 322 . In some embodiments, the deformable mirror  310  is optional in the jitter correction system  225 . 
     The Coudé path FSMs  312 - 314  simultaneously receive the HEL beam  322  and the BIL beam  320  from the deformable mirror  310 . The FSMs  312 - 314  operate to overcome atmospheric jitter to keep both beams  320 - 322  still (or substantially still) on the target  240 . The Coudé path FSMs  312 - 314  keep the HEL beam  322  and the BIL beam  320  aligned through the optical assembly, while stabilizing the BIL beam  320  from atmospheric disturbances estimated from the BIL return and processed in the camera  220  and controller  230 , and while allowing separate control of the beams  320 - 322  based on return images received by the camera  220 . The Coudé path FSMs  312 - 214  simultaneously steer both beams  320 - 322  the same amount. However, because the BIL beam  320  is steered slightly by the FSMs  302 - 304  upfront, the Coudé path FSMs  312 - 314  allow the BIL beam  320  and the HEL beam  322  to be pointed in slightly different directions, thereby maintaining the offset at the target  240 . The Coudé path FSMs  312 - 314  provide an independent atmospheric correction to the HEL beam  322  and the BIL beam  320  through the BIL spot that is not provided by the high speed mirror  318 . The high speed mirror  318  only corrects for atmospheric jitter as seen by the TIL  210  and TIL return processed by the camera  220  and the controller  230 , that is separated from the BIL return in time. 
     In one aspect of operation, the BIL target return spot  324  reflects off the target  240  and is returned to the camera  220 . The BIL target return spot  324  moves with the atmospherically introduced uplink jitter and is distorted from atmospheric wavefront errors. A control algorithm executed by the controller  230  estimates the uplink jitter from the BIL target return spot  324  seen on the camera image and determines corrections needed to compensate for the jitter. The corrections are implemented by movement of one or more of the Coudé path FSMs  312 - 314  under control of the controller  230 . The movement of the Coudé path FSMs  312 - 314  to compensate for the uplink jitter of the beams  320 - 322  can introduce artificial motion into the line of sight correction performed by the TIL FSM (not shown), and can be corrected with image processing to stabilize the TIL image prior to track processing. 
     The aperture sharing element  316  is a beam splitter that reflects the beams  320 - 322  to the high speed mirror  318  while allowing the BIL target return spot  324  to pass through to the camera  220 , and the resulting image is processed by the controller  230 . The aperture sharing element  316  could have any suitable structure configured to allow some beams to reflect while allowing other beams to transmit. 
     The high speed mirror  318  is a fine track mirror that receives the beams  320 - 322  and reflects the beams  320 - 322  for transmission to the target  240 . The high speed mirror  318  also receives and stabilizes the BIL target return spot  324 . The BIL target return spot  324  is also stabilized through the Coudé path FSMs  312 - 314 , in addition to the stabilization provided by the high speed mirror  318 . The Coudé path FSMs  312 - 314  provide residual uplink correction and opto-mechanical correction after the correction from the high speed mirror  318  is applied. The Coudé path FSMs  312 - 314  provide correction based on the difference between the uplink jitter and the downlink jitter, while the high speed mirror  318  only provides downlink atmospheric correction. 
     The controller  230  operates to ensure that both beams  320 - 322  are pointed at the light of sight of interest, that the BIL beam  320  is offset from the HEL beam  322 , and that both beams  320 - 322  are maintained at the desired location on the target  240 . The controller  230  performs these functions by controlling movement of the FSMs  302 - 304  and the Coudé path FSMs  312 - 314  based on return images received at the camera  220 . In particular, based on images received at the camera  220 , the controller  230  controls operation of the FSMs  302 - 304  to adjust the offset of the BIL beam  320  from the HEL beam  322 , in order to maintain a constant offset. In addition, the controller  230  controls operations of the Coudé path FSMs  312 - 314  to reduce or eliminate movement of the beams  320 - 322  on the target  240  due to atmospheric jitter. 
     In theory, it would be possible to have two or more FSMs point the HEL beam and two or more different FSMs separately point the BIL beam. However, in such a system, there would be more elements to correlate to keep the HEL beam and the BIL beam together. Each set of mirrors would result in a different error or different uncertainty, thus making correlation much more difficult in a dynamic environment. In the jitter correction system  225 , because the Coudé path FSMs  312 - 314  are common for the two beams, any errors or uncertainty caused by the Coudé path FSMs  312 - 314  would be the same for the BIL beam  320  and the HEL beam  322 , and thus would be easier to address and correct. 
     Although  FIGS.  2  and  3    illustrate one example system  200  for correcting for atmospheric jitter and high energy laser broadband interference according to this disclosure, various changes may be made to  FIGS.  2  and  3   . In general, the makeup and arrangement of the system  200  are for illustration only. Components could be added, omitted, combined, or placed in any other configuration according to particular needs. For example, while the jitter correction system  225  includes two FSMs for BIL beam offset and two Coudé path FSMs for jitter control, this is merely one example. Other embodiments could include more or fewer FSMs for BIL beam offset and more or fewer Coudé path FSMs for jitter control. 
       FIG.  4    illustrates an example method  400  for correcting for atmospheric jitter and high energy laser broadband interference according to this disclosure. For ease of explanation, the method  400  is described as being performed using the system  200  of  FIGS.  2  and  3   . However, the method  400  could be used with any other suitable device or system. 
     At step  401 , a HEL transmits a HEL beam aimed at a first location on an airborne target. This may include, for example, the HEL  205  transmitting the HEL beam  322  toward the target  240 . 
     At step  403 , a BIL transmits a BIL beam aimed at a second location on the target, where the second location is offset from the first location. This may include, for example, the BIL  215  transmitting the BIL beam  320  toward the BIL spot  245  on the target  240 . 
     At step  405 , at least one FSM steers the BIL beam to be spatially and angularly offset from the HEL beam. This may include, for example, the FSMs  302 - 304  steering the BIL beam  320 . This may also include the controller  230  controlling movement of the FSMs  302 - 204  to adjust the offset of the BIL beam  320  based on the BIL target return spot  324  received at the camera  220  and resulting images processed by the controller  230 . 
     At step  407 , at least one Coudé path FSM simultaneously receives both the HEL beam and the BIL beam and steers the HEL beam and the BIL beam to correct for atmospheric jitter of the HEL beam and the BIL beam while maintaining the offset of the BIL beam from the HEL beam. This may include, for example, the Coudé path FSMs  312 - 314  steering the HEL beam  322  and the BIL beam  320 . This may also include the controller  230  controlling movement of the Coudé path FSMs  312 - 314  to correct for the atmospheric jitter, based on the BIL target return spot  324  received at the camera  220  and resulting images processed by the controller  230 . The Coudé path FSMs  312 - 314  may provide correction based on the difference between the uplink jitter and the downlink jitter. 
     Although  FIG.  4    illustrates one example of a method  400  for correcting for atmospheric jitter and high energy laser broadband interference, various changes may be made to  FIG.  4   . For example, while shown as a series of steps, various steps shown in  FIG.  4    could overlap, occur in parallel, occur in a different order, or occur multiple times. Moreover, some steps could be combined or removed and additional steps could be added according to particular needs. 
     It may be advantageous to set forth definitions of certain words and phrases used throughout this patent document. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrase “associated with,” as well as derivatives thereof, means to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, have a relationship to or with, or the like. The phrase “at least one of,” when used with a list of items, means that different combinations of one or more of the listed items may be used, and only one item in the list may be needed. For example, “at least one of: A, B, and C” includes any of the following combinations: A, B, C, A and B, A and C, B and C, and A and B and C. 
     The description in the present application should not be read as implying that any particular element, step, or function is an essential or critical element that must be included in the claim scope. The scope of patented subject matter is defined only by the allowed claims. Moreover, none of the claims is intended to invoke 35 U.S.C. § 112(f) with respect to any of the appended claims or claim elements unless the exact words “means for” or “step for” are explicitly used in the particular claim, followed by a participle phrase identifying a function. Use of terms such as (but not limited to) “mechanism,” “module,” “device,” “unit,” “component,” “element,” “member,” “apparatus,” “machine,” or “system” within a claim is understood and intended to refer to structures known to those skilled in the relevant art, as further modified or enhanced by the features of the claims themselves, and is not intended to invoke 35 U.S.C. § 112(f). 
     While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims.