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

A system includes a high energy laser (HEL) configured to transmit a HEL beam aimed at a first location on an airborne target. The system also includes a beacon illuminator laser (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. The system also includes at least one fast steering mirror (FSM) configured to steer the BIL beam to be spatially and angularly offset from the HEL beam. The system also includes at least one Coudé 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.

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 aimed at a first location on an airborne target or sensor. The system also includes a beacon illuminator laser (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. The system also includes at least one fast steering mirror (FSM) configured to steer the BIL beam to be spatially and angularly offset from the HEL beam. The system also includes at least one Coudé path FSM configured to simultaneously receive both the HEL beam and the BIL beam and steer the HEL beam and the BIL beam in the same manner 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 transmitted by a BIL and steer the BIL beam to be spatially and angularly offset from a HEL beam transmitted by a HEL, wherein the HEL beam is aimed 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. The jitter correction system also includes at least one Coudé 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 third embodiment, a method includes transmitting, by a HEL, a HEL beam aimed at a first location on an airborne target. The method also includes transmitting, by a BIL, a BIL beam aimed at a second location on the target, wherein the second location is offset from the first location. The method also includes steering, by at least one FSM, the BIL beam to be spatially and angularly offset from the HEL beam. The method also includes simultaneously receiving, at least one Coudé 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.

DETAILED DESCRIPTION

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.1illustrates examples of both kinds of atmospheric jitter. As shown inFIG.1, a focusing Gaussian HEL uplink beam101is transmitted from a laser source110to a target120. The diffuse surface of the target120spreads the beam101, which results in a diverging downlink beam102, which is a spherical wave or plane wave. Both the uplink beam101and the downlink beam102pass through cells103of optical turbulence, which act as small lenses that distort the beams101-102.

As indicated by the arrows of the beam paths, the uplink beam101and the downlink beam102interact with different cells103of 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 beam101is a focusing Gaussian beam, and the downlink beam102is a spherical wave or plane wave.

The optical turbulence in the atmosphere degrades the effect of the HEL beam101by 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 beam101going up in the atmosphere is different than the jitter of the downlink beam102. In addition, as the target120heats up from the HEL beam101, 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.2illustrates an example system200for correcting for atmospheric jitter and high energy laser broadband interference according to this disclosure. As shown inFIG.2, the system200includes a HEL205, a TIL210, a BIL215, a camera220, a jitter correction system225, and a controller230.

The HEL205is configured to generate a high energy laser beam that is aimed toward a particular location on a target240. The TIL210is configured to illuminate the target240with an illumination beam235, and can be used to measure the distance and angle of the target240relative to the HEL205. In some embodiments, the TIL210generates 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 BIL215is configured to generate a more focused illumination spot245on the target240. A particular intended location on the target240is selected to be illuminated by the BIL spot245. For example, it may be predetermined to illuminate a particular feature on the nose of the target240, 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 inFIG.2, the BIL spot245is subject to optical turbulence238in the atmosphere, which results in uplink jitter of the BIL spot245. The actual location of the BIL spot245on the target240relative to the intended or expected location of the BIL spot245on the target240is used to determine the uplink jitter. In some embodiments, the BIL spot245is at a wavelength of approximately 1005 nm. However, this wavelength is merely one example, and in other embodiments, the BIL spot245could be at a longer or shorter wavelength. The wavelength of the BIL spot245is close to the wavelength of the HEL205; thus, the two experience approximately the same uplink jitter.

The camera220is a high-speed SWIR camera co-boresighted with the HEL205. The camera220is configured to receive and process images from the target240. In particular, the camera220receives images that illustrate motion of the BIL spot245caused by atmospheric jitter. In some embodiments, one camera220is used for both TIL tracking of the target240and tracking of the BIL spot245. In other embodiments, these functions can be performed by separate cameras220.

The jitter correction system225is disposed in the optical path of the HEL205and the BIL215, 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 system225operates to ensure that both beams reach the target240in 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 system225allows precise, independent pointing of multiple beams. Further details regarding the jitter correction system225are provided below with respect toFIG.3.

The controller230performs multiple algorithms and control operations to correct atmospheric jitter and compensate for HEL broadband interference. The controller230can 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 system225. For example, the controller230could denote at least one processor231configured to execute instructions obtained from at least one memory232. The controller230may include any suitable number(s) and type(s) of processors or other computing or control devices in any suitable arrangement. Example types of controllers230include microprocessors, microcontrollers, digital signal processors, field programmable gate arrays, application specific integrated circuits, and discrete circuitry. In some embodiments, the operations of the controller230described herein may be divided and performed by two or more separate controllers230.

FIG.3illustrates additional details of the jitter correction system225according to this disclosure. As shown inFIG.3, the jitter correction system225includes a pair of FSMs302-304, a first fold mirror306, a second fold mirror308, a deformable mirror310for wavefront correction, a pair of Coudé path FSMs312-314, an aperture sharing element316, and a high speed mirror318.

As discussed above, the BIL215transmits a BIL beam320, and the HEL205transmits a HEL beam322. Both beams320-322are aimed at the target240, but at slightly different locations on the target240. The BIL beam320results in the BIL spot245when it hits the target240, as discussed with respect toFIG.2.

The FSMs302-304receive the BIL beam320but do not receive the HEL beam322. The FSMs302-304are controllable by the controller230and can be controlled to move in order to steer the BIL beam320in a direction independent of the HEL beam322. In particular, the FSMs302-304operate to steer the BIL beam320to be offset spatially and pointed angularly with respect to the HEL beam322. While the FSMs302-304cause the BIL beam320to be spatially and angularly offset from the HEL beam322, other components of the jitter correction system225keep the two beams320-322dynamically aligned, as discussed below.

The FSMs302-304operate in conjunction with a tracker algorithm that estimates optimal positioning of the BIL beam320from the TIL return. The estimate is used by the controller230to control the FSMs302-304to adjust the alignment of the BIL beam320. The optimal position of the BIL beam320is slightly offset from the HEL beam322, so that the BIL beam320illuminates a similar portion of the target240, but not exactly the same portion of the target240that the HEL beam322contacts (e.g., an offset of approximately six inches on some targets). Another reason for maintaining the BIL beam320offset from the HEL beam322is so that thermal interference from the heating of the target240is spatially separated from a BIL target return spot324in the camera220. The separation of the BIL target return spot324from 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 mirrors306-308are separate mirrors having a similar function. The fold mirror306receives the BIL beam320, and the fold mirror308receives the HEL beam322. The fold mirrors306-308simply direct the beams320-322to the deformable mirror310without substantially changing any properties of the beams320-322. In contrast to the FSMs302-304, which are capable of changing orientation, the fold mirrors306-308are static mirrors. The fold mirrors306-308are representative of a beam control layout. In some embodiments, the fold mirrors306-308may be optional or their function may be implemented using other optical components.

The deformable mirror310receives the beams320-322and corrects for atmospheric wavefront errors sensed by an optional wavefront sensor (not shown). The deformable mirror310includes multiple actuators that move to control the shape of the surface of the deformable mirror310. In some embodiments, the actuators are controlled by the controller230based on sensor information received by the wavefront sensor. During operation of the system200, the beams320-322are subject to deformation. As the whole system heats up, vibrates, and flexes, the beams320-322are likely to deform. By changing the shape of its mirror surface, the deformable mirror310can correct the deformation of the beams320-322. In some embodiments, the deformable mirror310is optional in the jitter correction system225.

The Coudé path FSMs312-314simultaneously receive the HEL beam322and the BIL beam320from the deformable mirror310. The FSMs312-314operate to overcome atmospheric jitter to keep both beams320-322still (or substantially still) on the target240. The Coudé path FSMs312-314keep the HEL beam322and the BIL beam320aligned through the optical assembly, while stabilizing the BIL beam320from atmospheric disturbances estimated from the BIL return and processed in the camera220and controller230, and while allowing separate control of the beams320-322based on return images received by the camera220. The Coudé path FSMs312-214simultaneously steer both beams320-322the same amount. However, because the BIL beam320is steered slightly by the FSMs302-304upfront, the Coudé path FSMs312-314allow the BIL beam320and the HEL beam322to be pointed in slightly different directions, thereby maintaining the offset at the target240. The Coudé path FSMs312-314provide an independent atmospheric correction to the HEL beam322and the BIL beam320through the BIL spot that is not provided by the high speed mirror318. The high speed mirror318only corrects for atmospheric jitter as seen by the TIL210and TIL return processed by the camera220and the controller230, that is separated from the BIL return in time.

In one aspect of operation, the BIL target return spot324reflects off the target240and is returned to the camera220. The BIL target return spot324moves with the atmospherically introduced uplink jitter and is distorted from atmospheric wavefront errors. A control algorithm executed by the controller230estimates the uplink jitter from the BIL target return spot324seen 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 FSMs312-314under control of the controller230. The movement of the Coudé path FSMs312-314to compensate for the uplink jitter of the beams320-322can 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 element316is a beam splitter that reflects the beams320-322to the high speed mirror318while allowing the BIL target return spot324to pass through to the camera220, and the resulting image is processed by the controller230. The aperture sharing element316could have any suitable structure configured to allow some beams to reflect while allowing other beams to transmit.

The high speed mirror318is a fine track mirror that receives the beams320-322and reflects the beams320-322for transmission to the target240. The high speed mirror318also receives and stabilizes the BIL target return spot324. The BIL target return spot324is also stabilized through the Coudé path FSMs312-314, in addition to the stabilization provided by the high speed mirror318. The Coudé path FSMs312-314provide residual uplink correction and opto-mechanical correction after the correction from the high speed mirror318is applied. The Coudé path FSMs312-314provide correction based on the difference between the uplink jitter and the downlink jitter, while the high speed mirror318only provides downlink atmospheric correction.

The controller230operates to ensure that both beams320-322are pointed at the light of sight of interest, that the BIL beam320is offset from the HEL beam322, and that both beams320-322are maintained at the desired location on the target240. The controller230performs these functions by controlling movement of the FSMs302-304and the Coudé path FSMs312-314based on return images received at the camera220. In particular, based on images received at the camera220, the controller230controls operation of the FSMs302-304to adjust the offset of the BIL beam320from the HEL beam322, in order to maintain a constant offset. In addition, the controller230controls operations of the Coudé path FSMs312-314to reduce or eliminate movement of the beams320-322on the target240due 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 system225, because the Coudé path FSMs312-314are common for the two beams, any errors or uncertainty caused by the Coudé path FSMs312-314would be the same for the BIL beam320and the HEL beam322, and thus would be easier to address and correct.

AlthoughFIGS.2and3illustrate one example system200for correcting for atmospheric jitter and high energy laser broadband interference according to this disclosure, various changes may be made toFIGS.2and3. In general, the makeup and arrangement of the system200are 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 system225includes 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.4illustrates an example method400for correcting for atmospheric jitter and high energy laser broadband interference according to this disclosure. For ease of explanation, the method400is described as being performed using the system200ofFIGS.2and3. However, the method400could be used with any other suitable device or system.

At step401, a HEL transmits a HEL beam aimed at a first location on an airborne target. This may include, for example, the HEL205transmitting the HEL beam322toward the target240.

At step403, 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 BIL215transmitting the BIL beam320toward the BIL spot245on the target240.

At step405, at least one FSM steers the BIL beam to be spatially and angularly offset from the HEL beam. This may include, for example, the FSMs302-304steering the BIL beam320. This may also include the controller230controlling movement of the FSMs302-204to adjust the offset of the BIL beam320based on the BIL target return spot324received at the camera220and resulting images processed by the controller230.

At step407, 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 FSMs312-314steering the HEL beam322and the BIL beam320. This may also include the controller230controlling movement of the Coudé path FSMs312-314to correct for the atmospheric jitter, based on the BIL target return spot324received at the camera220and resulting images processed by the controller230. The Coudé path FSMs312-314may provide correction based on the difference between the uplink jitter and the downlink jitter.

AlthoughFIG.4illustrates one example of a method400for correcting for atmospheric jitter and high energy laser broadband interference, various changes may be made toFIG.4. For example, while shown as a series of steps, various steps shown inFIG.4could 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.