Patent Description:
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.

<CIT> discloses a beam control system and method including, an arrangement for receiving a first beam of electromagnetic energy; measuring wavefront aberrations in the first beam with a wavefront sensor; and removing global tilt from the measured wavefront aberrations to provide higher order aberrations for beam control. In an embodiment, the invention uses a traditional (quad-cell) Shack-Hartmann wavefront sensor to measure wavefront aberrations. An adaptive optics processor electronically removes the global tilt (angular jitter) from this measurement leaving only the higher-order Zernike components. These higher-order aberrations are then applied to wavefront control elements, such as deformable mirrors or spatial light modulators that correct the tracker image and apply a conjugate distortion to the wavefront of the outgoing HEL beam. A track error (angular jitter) component is supplied by a separate fine track sensor. This jitter error is then applied by the adaptive optics processor to a fast steering mirror, which corrects jitter in the tracker image and applies a compensating distortion to the LOS of the HEL beam.

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 aspect, the present disclosure provides a jitter correction system comprising: at least one fast steering mirror 'FSM' configured to receive a beacon illuminator 'BIL' beam transmitted by a BIL and steer the BIL beam to be spatially and angularly offset from a high energy laser 'HEL' beam transmitted by a HEL, wherein the HEL beam is aimed at a first location on an airborne target, the BIL beam is aimed at a second location on the target, and the second location is offset from the first location; and at least one Coude path FSM configured to simultaneously receive both the HEL beam and the BIL beam and 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 second aspect, the present disclosure provides a system comprising: a high energy laser 'HEL' configured to transmit a HEL beam aimed at a first location on an airborne target; a beacon illuminator 'BIL' configured to transmit a BIL beam aimed at a second location on the target, wherein the second location is offset from the first location; and a jitter correction system according to the first aspect.

In a third aspect, the present disclosure provides a method comprising: transmitting, by a high energy laser 'HEL', a HEL beam aimed at a first location on an airborne target; transmitting, by a beacon illuminator 'BIL', a BIL beam aimed at a second location on the target, wherein the second location is offset from the first location; steering, by at least one fast steering mirror 'FSM', the BIL beam to be spatially and angularly offset from the HEL beam; and simultaneously receiving, by at least one Coud6 path FSM, both the HEL beam and the BIL beam and steering 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.

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> illustrates examples of both kinds of atmospheric jitter. As shown in <FIG>, a focusing Gaussian HEL uplink beam <NUM> is transmitted from a laser source <NUM> to a target <NUM>. The diffuse surface of the target <NUM> spreads the beam <NUM>, which results in a diverging downlink beam <NUM>, which is a spherical wave or plane wave. Both the uplink beam <NUM> and the downlink beam <NUM> pass through cells <NUM> of optical turbulence, which act as small lenses that distort the beams <NUM>-<NUM>.

As indicated by the arrows of the beam paths, the uplink beam <NUM> and the downlink beam <NUM> interact with different cells <NUM> 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 <NUM> is a focusing Gaussian beam, and the downlink beam <NUM> is a spherical wave or plane wave.

The optical turbulence in the atmosphere degrades the effect of the HEL beam <NUM> 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 <NUM> going up in the atmosphere is different than the jitter of the downlink beam <NUM>. In addition, as the target <NUM> heats up from the HEL beam <NUM>, 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> illustrates an example system <NUM> for correcting for atmospheric jitter and high energy laser broadband interference according to this disclosure. As shown in <FIG>, the system <NUM> includes a HEL <NUM>, a TIL <NUM>, a BIL <NUM>, a camera <NUM>, a jitter correction system <NUM>, and a controller <NUM>.

The HEL <NUM> is configured to generate a high energy laser beam that is aimed toward a particular location on a target <NUM>. The TIL <NUM> is configured to illuminate the target <NUM> with an illumination beam <NUM>, and can be used to measure the distance and angle of the target <NUM> relative to the HEL <NUM>. In some embodiments, the TIL <NUM> generates an illumination light at a wavelength of approximately <NUM>. However, this wavelength is merely one example, and in other embodiments, the illumination light could be at a longer or shorter wavelength.

The BIL <NUM> is configured to generate a more focused illumination spot <NUM> on the target <NUM>. A particular intended location on the target <NUM> is selected to be illuminated by the BIL spot <NUM>. For example, it may be predetermined to illuminate a particular feature on the nose of the target <NUM>, 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>, the BIL spot <NUM> is subject to optical turbulence <NUM> in the atmosphere, which results in uplink jitter of the BIL spot <NUM>. The actual location of the BIL spot <NUM> on the target <NUM> relative to the intended or expected location of the BIL spot <NUM> on the target <NUM> is used to determine the uplink jitter. In some embodiments, the BIL spot <NUM> is at a wavelength of approximately <NUM>. However, this wavelength is merely one example, and in other embodiments, the BIL spot <NUM> could be at a longer or shorter wavelength. The wavelength of the BIL spot <NUM> is close to the wavelength of the HEL <NUM>; thus, the two experience approximately the same uplink jitter.

The camera <NUM> is a high-speed SWIR camera co-boresighted with the HEL <NUM>. The camera <NUM> is configured to receive and process images from the target <NUM>. In particular, the camera <NUM> receives images that illustrate motion of the BIL spot <NUM> caused by atmospheric jitter. In some embodiments, one camera <NUM> is used for both TIL tracking of the target <NUM> and tracking of the BIL spot <NUM>. In other embodiments, these functions can be performed by separate cameras <NUM>.

The jitter correction system <NUM> is disposed in the optical path of the HEL <NUM> and the BIL <NUM>, 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 <NUM> operates to ensure that both beams reach the target <NUM> 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 <NUM> allows precise, independent pointing of multiple beams. Further details regarding the jitter correction system <NUM> are provided below with respect to <FIG>.

The controller <NUM> performs multiple algorithms and control operations to correct atmospheric jitter and compensate for HEL broadband interference. The controller <NUM> 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 <NUM>. For example, the controller <NUM> could denote at least one processor <NUM> configured to execute instructions obtained from at least one memory <NUM>. The controller <NUM> 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 <NUM> 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 <NUM> described herein may be divided and performed by two or more separate controllers <NUM>.

<FIG> illustrates additional details of the jitter correction system <NUM> according to this disclosure. As shown in <FIG>, the jitter correction system <NUM> includes a pair of FSMs <NUM>-<NUM>, a first fold mirror <NUM>, a second fold mirror <NUM>, a deformable mirror <NUM> for wavefront correction, a pair of Coudé path FSMs <NUM>-<NUM>, an aperture sharing element <NUM>, and a high speed mirror <NUM>.

As discussed above, the BIL <NUM> transmits a BIL beam <NUM>, and the HEL <NUM> transmits a HEL beam <NUM>. Both beams <NUM>-<NUM> are aimed at the target <NUM>, but at slightly different locations on the target <NUM>. The BIL beam <NUM> results in the BIL spot <NUM> when it hits the target <NUM>, as discussed with respect to <FIG>.

The FSMs <NUM>-<NUM> receive the BIL beam <NUM> but do not receive the HEL beam <NUM>. The FSMs <NUM>-<NUM> are controllable by the controller <NUM> and can be controlled to move in order to steer the BIL beam <NUM> in a direction independent of the HEL beam <NUM>. In particular, the FSMs <NUM>-<NUM> operate to steer the BIL beam <NUM> to be offset spatially and pointed angularly with respect to the HEL beam <NUM>. While the FSMs <NUM>-<NUM> cause the BIL beam <NUM> to be spatially and angularly offset from the HEL beam <NUM>, other components of the jitter correction system <NUM> keep the two beams <NUM>-<NUM> dynamically aligned, as discussed below.

The FSMs <NUM>-<NUM> operate in conjunction with a tracker algorithm that estimates optimal positioning of the BIL beam <NUM> from the TIL return. The estimate is used by the controller <NUM> to control the FSMs <NUM>-<NUM> to adjust the alignment of the BIL beam <NUM>. The optimal position of the BIL beam <NUM> is slightly offset from the HEL beam <NUM>, so that the BIL beam <NUM> illuminates a similar portion of the target <NUM>, but not exactly the same portion of the target <NUM> that the HEL beam <NUM> contacts (e.g., an offset of approximately six inches on some targets). Another reason for maintaining the BIL beam <NUM> offset from the HEL beam <NUM> is so that thermal interference from the heating of the target <NUM> is spatially separated from a BIL target return spot <NUM> in the camera <NUM>. The separation of the BIL target return spot <NUM> 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 <NUM>-<NUM> are separate mirrors having a similar function. The fold mirror <NUM> receives the BIL beam <NUM>, and the fold mirror <NUM> receives the HEL beam <NUM>. The fold mirrors <NUM>-<NUM> simply direct the beams <NUM>-<NUM> to the deformable mirror <NUM> without substantially changing any properties of the beams <NUM>-<NUM>. In contrast to the FSMs <NUM>-<NUM>, which are capable of changing orientation, the fold mirrors <NUM>-<NUM> are static mirrors. The fold mirrors <NUM>-<NUM> are representative of a beam control layout. In some embodiments, the fold mirrors <NUM>-<NUM> may be optional or their function may be implemented using other optical components.

The deformable mirror <NUM> receives the beams <NUM>-<NUM> and corrects for atmospheric wavefront errors sensed by an optional wavefront sensor (not shown). The deformable mirror <NUM> includes multiple actuators that move to control the shape of the surface of the deformable mirror <NUM>. In some embodiments, the actuators are controlled by the controller <NUM> based on sensor information received by the wavefront sensor. During operation of the system <NUM>, the beams <NUM>-<NUM> are subject to deformation. As the whole system heats up, vibrates, and flexes, the beams <NUM>-<NUM> are likely to deform. By changing the shape of its mirror surface, the deformable mirror <NUM> can correct the deformation of the beams <NUM>-<NUM>. In some embodiments, the deformable mirror <NUM> is optional in the jitter correction system <NUM>.

The Coud6 path FSMs <NUM>-<NUM> simultaneously receive the HEL beam <NUM> and the BIL beam <NUM> from the deformable mirror <NUM>. The FSMs <NUM>-<NUM> operate to overcome atmospheric jitter to keep both beams <NUM>-<NUM> still (or substantially still) on the target <NUM>. The Coude path FSMs <NUM>-<NUM> keep the HEL beam <NUM> and the BIL beam <NUM> aligned through the optical assembly, while stabilizing the BIL beam <NUM> from atmospheric disturbances estimated from the BIL return and processed in the camera <NUM> and controller <NUM>, and while allowing separate control of the beams <NUM>-<NUM> based on return images received by the camera <NUM>. The Coude path FSMs <NUM>-<NUM> simultaneously steer both beams <NUM>-<NUM> the same amount. However, because the BIL beam <NUM> is steered slightly by the FSMs <NUM>-<NUM> upfront, the Coudé path FSMs <NUM>-<NUM> allow the BIL beam <NUM> and the HEL beam <NUM> to be pointed in slightly different directions, thereby maintaining the offset at the target <NUM>. The Coudé path FSMs <NUM>-<NUM> provide an independent atmospheric correction to the HEL beam <NUM> and the BIL beam <NUM> through the BIL spot that is not provided by the high speed mirror <NUM>. The high speed mirror <NUM> only corrects for atmospheric jitter as seen by the TIL <NUM> and TIL return processed by the camera <NUM> and the controller <NUM>, that is separated from the BIL return in time.

In one aspect of operation, the BIL target return spot <NUM> reflects off the target <NUM> and is returned to the camera <NUM>. The BIL target return spot <NUM> moves with the atmospherically introduced uplink jitter and is distorted from atmospheric wavefront errors. A control algorithm executed by the controller <NUM> estimates the uplink jitter from the BIL target return spot <NUM> 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 <NUM>-<NUM> under control of the controller <NUM>. The movement of the Coudé path FSMs <NUM>-<NUM> to compensate for the uplink jitter of the beams <NUM>-<NUM> 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 <NUM> is a beam splitter that reflects the beams <NUM>-<NUM> to the high speed mirror <NUM> while allowing the BIL target return spot <NUM> to pass through to the camera <NUM>, and the resulting image is processed by the controller <NUM>. The aperture sharing element <NUM> could have any suitable structure configured to allow some beams to reflect while allowing other beams to transmit.

The high speed mirror <NUM> is a fine track mirror that receives the beams <NUM>-<NUM> and reflects the beams <NUM>-<NUM> for transmission to the target <NUM>. The high speed mirror <NUM> also receives and stabilizes the BIL target return spot <NUM>. The BIL target return spot <NUM> is also stabilized through the Coud6 path FSMs <NUM>-<NUM>, in addition to the stabilization provided by the high speed mirror <NUM>. The Coude path FSMs <NUM>-<NUM> provide residual uplink correction and opto-mechanical correction after the correction from the high speed mirror <NUM> is applied. The Coude path FSMs <NUM>-<NUM> provide correction based on the difference between the uplink jitter and the downlink jitter, while the high speed mirror <NUM> only provides downlink atmospheric correction.

The controller <NUM> operates to ensure that both beams <NUM>-<NUM> are pointed at the light of sight of interest, that the BIL beam <NUM> is offset from the HEL beam <NUM>, and that both beams <NUM>-<NUM> are maintained at the desired location on the target <NUM>. The controller <NUM> performs these functions by controlling movement of the FSMs <NUM>-<NUM> and the Coudé path FSMs <NUM>-<NUM> based on return images received at the camera <NUM>. In particular, based on images received at the camera <NUM>, the controller <NUM> controls operation of the FSMs <NUM>-<NUM> to adjust the offset of the BIL beam <NUM> from the HEL beam <NUM>, in order to maintain a constant offset. In addition, the controller <NUM> controls operations of the Coud6 path FSMs <NUM>-<NUM> to reduce or eliminate movement of the beams <NUM>-<NUM> on the target <NUM> 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 <NUM>, because the Coudé path FSMs <NUM>-<NUM> are common for the two beams, any errors or uncertainty caused by the Coude path FSMs <NUM>-<NUM> would be the same for the BIL beam <NUM> and the HEL beam <NUM>, and thus would be easier to address and correct.

Although <FIG> and <FIG> illustrate one example system <NUM> for correcting for atmospheric jitter and high energy laser broadband interference according to this disclosure, various changes may be made to <FIG> and <FIG>. In general, the makeup and arrangement of the system <NUM> 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 <NUM> 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> illustrates an example method <NUM> for correcting for atmospheric jitter and high energy laser broadband interference according to this disclosure. For ease of explanation, the method <NUM> is described as being performed using the system <NUM> of <FIG> and <FIG>. However, the method <NUM> could be used with any other suitable device or system.

At step <NUM>, a HEL transmits a HEL beam aimed at a first location on an airborne target. This may include, for example, the HEL <NUM> transmitting the HEL beam <NUM> toward the target <NUM>.

At step <NUM>, 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 <NUM> transmitting the BIL beam <NUM> toward the BIL spot <NUM> on the target <NUM>.

At step <NUM>, 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 <NUM>-<NUM> steering the BIL beam <NUM>. This may also include the controller <NUM> controlling movement of the FSMs <NUM>-<NUM> to adjust the offset of the BIL beam <NUM> based on the BIL target return spot <NUM> received at the camera <NUM> and resulting images processed by the controller <NUM>.

At step <NUM>, at least one Coude 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 <NUM>-<NUM> steering the HEL beam <NUM> and the BIL beam <NUM>. This may also include the controller <NUM> controlling movement of the Coudé path FSMs <NUM>-<NUM> to correct for the atmospheric jitter, based on the BIL target return spot <NUM> received at the camera <NUM> and resulting images processed by the controller <NUM>. The Coudé path FSMs <NUM>-<NUM> may provide correction based on the difference between the uplink jitter and the downlink jitter.

Although <FIG> illustrates one example of a method <NUM> for correcting for atmospheric jitter and high energy laser broadband interference, various changes may be made to <FIG>. For example, while shown as a series of steps, various steps shown in <FIG> 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.

Claim 1:
A jitter correction system (<NUM>) comprising:
at least one fast steering mirror 'FSM' (<NUM>, <NUM>) configured to receive a beacon illuminator 'BIL' beam (<NUM>) transmitted by a BIL (<NUM>) and steer the BIL beam to be spatially and angularly offset from a high energy laser 'HEL' beam (<NUM>) transmitted by a HEL (<NUM>), wherein the HEL beam is aimed at a first location on an airborne target (<NUM>), the BIL beam is aimed at a second location (<NUM>) on the target, and the second location is offset from the first location; and
at least one Coud6 path FSM (<NUM>, <NUM>) configured to simultaneously receive both the HEL beam and the BIL beam and 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.