Patent Description:
High-power laser systems are being developed for a number of military and commercial applications. One example use of high-power laser systems involves capturing images of remote objects, such as drones or other aircraft. Another example use of high-power laser systems involves focusing high-power lasers to achieve high energy accumulation on remote objects to produce certain effects. In these and other types of systems, beam directors are often needed to track moving objects and to direct or focus laser beams onto the moving objects. This typically occurs in the presence of atmospheric disturbances, aero-optic effects, and other disturbances.

Conventional beam directors often use "on-axis" telescope designs in which a secondary mirror is placed physically in front of a primary mirror. Such telescopes are referred to as "obscured" telescopes since the central portion of the primary mirror is obscured by the secondary mirror. While suitable for use at low laser powers, on-axis telescope designs typically require accommodations for use at high laser powers to avoid illuminating the secondary mirror and its associated support structures to high-power laser beams, which can damage or degrade the performance of those components. Moreover, the obscuration associated with on-axis telescopes reduces the effectiveness of focused beams on remote objects.

Conventional laser imaging systems and high energy laser (HEL) focusing may also require the use of a "beacon" laser to be focused to a small spot on the object and a sensor to estimate the wavefront error of the optical path from the object to the sensor. However, this approach is complicated by the need for a separate beacon laser, as well as by blurring of the outgoing beacon laser beam by atmospheric disturbances, aero-optic effects, and other disturbances.

"<NPL>, discloses simulation methods for high energy laser systems incorporating scaling law codes into engagement codes.

<CIT> discloses a high power laser beam delivery system including a rotary turret platform rotatable along multiple axes for aiming of a high power laser beam. The system further includes a turret payload device coupled to the rotary turret platform that is a truncated sphere and configured to rapidly deploy from a vehicle and stow within the vehicle. The system further includes at least two conformal windows in a spherical side of the turret payload. The system further includes an off-axis telescope coupled to the turret payload, having an articulated secondary mirror for correcting optical aberrations, and configured to reflect the high power laser beam to a target through the first of the at least two conformal windows. The system further includes an illuminator beam device coupled to the turret payload and configured to detect atmospheric disturbance between the system and the target by actively illuminating the target to generate a return aberrated wavefront through the first of the at least two conformal windows. The system further includes a coarse tracker coupled to the turret payload, positioned parallel to and on an axis of revolution of the off-axis telescope, and configured to detect, acquire, and track the target through the second of the at least two conformal windows.

<CIT> discloses a high-energy beam precompensated by a process including receiving a high-energy beam from a source and energy from a target. The target energy includes wavefront aberrations related to atmospheric and other external disturbances encountered along a distance separating the target. A correction signal is determined responsive to the high-energy beam and the target energy. The correction signal is also configured to pre-compensate for wavefront aberrations related to the atmospheric and other external disturbances and to cancel aberrations introduced by the adaptive optics techniques. A wavefront of the outcoupled high-energy beam is adjusted responsive to the determined correction signal. A beam control system includes three adaptive optics servo loops and an aperture-sharing element. The arrangement is adapted to self-cancel internal optical-path-difference errors in the outcoupled beam and to pre-compensate the outcoupled beam according to a conjugate of the wavefront aberrations related to atmospheric and other external disturbances.

This disclosure provides a high-performance beam director for high-power laser systems and other systems.

In a first aspect, the present disclosure provides an apparatus comprising: a wavefront sensor configured to receive coherent flood illumination that is reflected from a remote object and to estimate wavefront errors associated with the coherent flood illumination; and a beam director optically coupled to the wavefront sensor and comprising a telescope and an auto-alignment system, the auto-alignment system configured to adjust at least one first optical device in order to alter a line-of-sight of the wavefront sensor; wherein the wavefront errors estimated by the wavefront sensor include a wavefront error resulting from the adjustment of the at least one first optical device; and wherein the auto-alignment system is further configured to generate auto-alignment illumination that passes through the telescope, is reflected off a mirror, and returns through the telescope to the auto-alignment system so that the auto-alignment system is able to use the auto-alignment illumination to provide an indication of internal line-of-sight errors within the apparatus and to adjust one or more components of the auto-alignment system to compensate for the internal line-of-sight errors within the apparatus, the mirror outside an optical path of the coherent flood illumination.

In a second aspect, the present disclosure provides an apparatus comprising: a wavefront sensor configured to receive coherent flood illumination that is reflected from a remote object and to estimate wavefront errors associated with the coherent flood illumination; and a beam director optically coupled to the wavefront sensor and comprising a telescope and an auto-alignment system, the auto-alignment system configured to adjust at least one optical device in order to alter a line-of-sight of the wavefront sensor; wherein the wavefront errors estimated by the wavefront sensor include a wavefront error resulting from the adjustment of the at least one optical device; and wherein the auto-alignment system comprises: at least one illumination source configured to generate auto-alignment illumination; a plurality of optical elements configured to direct the auto-alignment illumination and the coherent flood illumination towards the telescope along a common optical path through the beam director, wherein the optical elements comprise at least one fast steering mirror; a mirror optically positioned after the telescope and configured to reflect at least some of the auto-alignment illumination back towards the telescope and along the common optical path through the beam director; at least one detector configured to receive the reflected auto-alignment illumination and to detect a position or an angle-of-arrival of the reflected auto-alignment illumination; and a controller configured to adjust the at least one fast steering mirror based on the detected position or angle-of-arrival in order to adjust at least part of the common optical path and to adjust the line-of-sight of the wavefront sensor.

In a third aspect, the present disclosure provides a system comprising at least one illumination source configured to generate coherent flood illumination; and the apparatus of the first or second aspect.

In a particular embodiment, the system further includes a high-energy laser (HEL) source configured to generate HEL illumination, and the telescope includes a focusing mechanism. The wavefront errors estimated by the wavefront sensor include a wavefront error resulting from the focus mechanism.

In a particular embodiment, the telescope comprises a primary mirror, a secondary mirror, and a positioner configured to move the secondary mirror; and the system further comprises: a range sensor configured to estimate a distance to the remote object; and a controller configured to cause the positioner to move the secondary mirror to an initial location based on the estimated distance.

In a fourth aspect, the present disclosure provides a method comprising: receiving coherent flood illumination that is reflected from a remote object at a wavefront sensor; estimating wavefront errors associated with the coherent flood illumination using the wavefront sensor; and adjusting a line-of-sight of the wavefront sensor using a beam director comprising a telescope and an auto-alignment system, the auto-alignment system adjusting at least one first optical device in order to alter the line-of-sight of the wavefront sensor; wherein the wavefront errors estimated by the wavefront sensor include a wavefront error resulting from the adjustment of the at least one first optical device; and wherein the auto-alignment system generates auto-alignment illumination that passes through the telescope, is reflected off a mirror, and returns through the telescope to the auto-alignment system so that the auto-alignment
system is able to use the auto-alignment illumination to provide an indication of internal line-of-sight errors within an apparatus and to adjust one or more components of the auto-alignment system to compensate for the internal line-of-sight errors within the apparatus, the mirror outside an optical path of the coherent flood illumination.

In a particular embodiment, the wavefront errors also include a wavefront error caused by disturbances along a path of the coherent flood illumination; and further comprising: focusing high-energy laser 'HEL' illumination on the remote object by moving a secondary mirror of the telescope; measuring a wavefront error resulting from movement of the secondary mirror using the wavefront sensor; and correcting the wavefront error resulting from movement of the secondary mirror using a deformable mirror.

For a more complete understanding of this disclosure, reference is made to the following description, taken in conjunction with the accompanying drawings, in which:.

Coherent imaging systems rely on interference between a local oscillator (LO) beam and a reflected or return beam from a coherently illuminated object. Coherent imaging provides great value for interrogating distant objects. Specific elements of this value include improved performance with low light levels, three-dimensional (3D) imaging, correction of optical aberrations, and evaluation of intra-object motion. For example, coherent imaging generally involves photon-limited detection, which allows operation with lower illumination power than direct detection methods. In addition, 3D images may be obtained by combining coherent images at different wavelengths, and optical aberrations (such as wavefront errors) may be determined and corrected with coherent imaging.

Coherent signatures, however, are often very dynamic, such as due to movement of the object being illuminated or movement of the laser system performing the illuminating. Beam directors are often needed to track a moving object and to direct or focus a laser beam onto the moving object. While various approaches have been developed for providing these types of beam directors, those approaches can suffer from a number of problems.

<FIG> illustrates an example coherent imaging system <NUM> in accordance with this disclosure. Those skilled in the art will recognize that, for simplicity and clarity, some features and components are not explicitly shown, including those illustrated in connection with later figures. For example, the entire optical system (including all mirrors, lenses, beam splitters, beam combiners, transmitters/receivers, apertures, electromechanical shutters, etc. and their associated light paths) is not illustrated in <FIG>. Such features, including those illustrated in later figures, will be understood to be equally applicable to the coherent imaging system <NUM> of <FIG>.

The coherent imaging system <NUM> is used here to image a target object <NUM> and does not include the target object <NUM> itself. A master oscillator <NUM> produces a frequency signal, which passes through a modulator <NUM> and a power amplifier <NUM> in order to generate coherent flood illumination <NUM> directed toward the object <NUM> to be imaged. Reflected or return illumination <NUM> reflected off the object <NUM> is received through an exit pupil <NUM> and focused through an imaging pupil <NUM> onto a short-wave infrared (SWIR) focal plane array (FPA) <NUM>. The SWIR FPA <NUM> operates under the control of at least one computer or processing device <NUM>, which is coupled to the master oscillator <NUM>. Also coupled to the master oscillator <NUM> is a local oscillator (LO) <NUM>, which drives an illumination source (not explicitly shown) directing illumination <NUM> onto the SWIR FPA <NUM>.

In the system <NUM> of <FIG>, the target object <NUM> is flood illuminated with the coherent flood illumination <NUM> at a frequency derived from the master oscillator <NUM>. The reflected or return illumination <NUM> interferes with the illumination <NUM> based on the frequency of the LO <NUM>, and this interference may be determined based on total light detected using the FPA <NUM>. This detection method may be referred to as spatial heterodyne or digital holography (DH). Such interference imaging enables photon-noise limited detection and phase processing that also allows 3D imaging, aberration determination/correction, and vibration imaging.

The system <NUM> in <FIG> may further include a ranging sensor <NUM>. The ranging sensor <NUM> can be used to estimate the initial distance or range to the target object <NUM>. As described in more detail below, the estimated range to the target object <NUM> can be used to help initially focus a telescope that directs illumination to and receives illumination from the target object <NUM>. The ranging sensor <NUM> includes any suitable structure for identifying a target range, such as a RADAR or LADAR.

<FIG> illustrates another example coherent imaging system in accordance with this disclosure, and <FIG> illustrates an example coherent imaging system combined with a high-energy laser (HEL) system in accordance with this disclosure. For simplicity and clarity, some components of <FIG> and/or later figures are not shown in <FIG> and <FIG>, while additional components not illustrated in <FIG> are shown in <FIG> and <FIG>. It will be understood that all features illustrated in the figures may be employed in any of the embodiments described here. Omission of a feature or component from a particular figure is for purposes of simplicity and clarity and not meant to imply that the feature or component cannot be employed in the embodiments described in connection with that figure.

As shown in <FIG>, a coherent imaging system <NUM> includes an optical power amplifier <NUM> that is driven by the master oscillator <NUM> and generates the coherent flood illumination <NUM>, which reflects off the object <NUM> and is received as the reflected or return illumination <NUM>. In the embodiment shown in <FIG>, the coherent light from the optical power amplifier <NUM> passes through an auto-alignment system <NUM>, a telescope <NUM>, and an output window <NUM>. Examples of the auto-alignment system <NUM> are described in more detail below. The reflected or return illumination <NUM> similarly passes through the window <NUM>, the telescope <NUM>, and the auto-alignment system <NUM>.

A portion 106a of the reflected or return illumination <NUM> is focused on the SWIR FPA <NUM>. Another portion 106b of the reflected or return illumination <NUM> is focused on a Doppler sensor <NUM>. The Doppler sensor <NUM> has a single pixel or multiple pixels that measure Doppler frequency of the illumination <NUM> relative to the LO illumination <NUM>. The LO illumination <NUM> is carried along a light conduit (such as a fiber) to an exit point <NUM>. The sampling rate for the Doppler sensor <NUM> can be greater than the greatest (longitudinal) velocity for the target object <NUM> divided by the wavelength used for the flood illumination <NUM> (or other illumination source(s)).

The output of the Doppler sensor <NUM> is received by the computer or processing device <NUM>, which determines a Doppler shift (or offset) of the reflected or return illumination <NUM> relative to the flood illumination <NUM>. That Doppler shift is representative of the longitudinal velocity (or, stated differently, the longitudinal component of the total velocity) of the object <NUM> relative to the coherent imaging system <NUM>. The Doppler shift can be measured and updated on a regular basis to account for longitudinal acceleration by the object <NUM> or the imaging system. The Doppler shift is employed by a Doppler-shifted LO <NUM>, which generates a Doppler-shifted version of the output frequency from the LO <NUM>. The Doppler-shifted output from the LO <NUM> is carried along a light conduit (such as a fiber) to an exit point <NUM>, which provides the output as Doppler-shifted LO illumination <NUM>.

A value based on a measurement (such as an inverse) of the Doppler shift may be applied to the frequency signal from the master oscillator <NUM> by an electro-optic modulator (EOM) <NUM>. The output of the EOM <NUM> is received by the Doppler-shifted LO <NUM>, which produces the Doppler-shifted LO illumination <NUM>. For a pulsed system, the Doppler sensor <NUM> can also measure the target range so that the SWIR FPA <NUM> may be triggered when a return pulse from the object <NUM> is present.

The SWIR FPA <NUM> receives the Doppler-shifted LO illumination <NUM> superimposed with the portion 106a of the reflected or return illumination <NUM> to offset Doppler effects on the imaging. As a result, the interference pattern can be recorded with a low bandwidth focal plane array, such as an FPA operating with a <NUM> to <NUM> global shutter.

The computer or processing device <NUM> processes various data to generate one or more images of the target object <NUM>. The computer or processing device <NUM> can use the images in any suitable manner, such as by presenting the images to one or more operators or other users on at least one display device <NUM>.

Various optical devices are used in the coherent imaging system <NUM> to support the transport or modification of optical signals. For example, a beam splitter <NUM> can be used to separate the reflected or return illumination <NUM> into the different portions 106a-106b. A beam combiner <NUM>, which may be polarized to pass the portion 106b of the reflected or return illumination <NUM>, effectively superimposes the portion 106b of the reflected or return illumination <NUM> and the LO illumination <NUM>. Lenses <NUM>-<NUM> are used to focus illumination onto the FPA <NUM> and Doppler sensor <NUM>, respectively, while a lens <NUM> is used to expand the illumination <NUM>.

The telescope <NUM> is used to direct the flood illumination <NUM> towards the target <NUM> and receive the reflected or return illumination <NUM> from the target <NUM>. The telescope <NUM> denotes any suitable telescope. In some embodiments, the telescope <NUM> denotes an off-axis telescope, although other types of telescopes could be used. The window <NUM> allows passage of various illumination used by the imaging system. The window <NUM> includes any suitable structure that is substantially transparent to at least the wavelengths used by the imaging system.

The imaging system further includes a mirror <NUM>, which is used to reflect auto-alignment illumination <NUM> from the auto-alignment system <NUM> back to the auto-alignment system <NUM>. As described in more detail below, the auto-alignment system <NUM> uses the auto-alignment illumination <NUM> to provide an indication of internal light-of-sight errors or other errors within the imaging system, which could be caused by factors such as shock, vibrations, or thermal variations in the imaging system. The auto-alignment system <NUM> can then adjust one or more components of the auto-alignment system <NUM> to compensate for these errors. In some embodiments, the mirror <NUM> denotes an annular mirror that is substantially normal to the telescope's optical axis and that has a central opening through which the coherent flood illumination <NUM> and the reflected or return illumination <NUM> can pass.

As discussed above, coherent imaging methods have great value for obtaining image-based information from distant objects, including lower illumination power and determination of and correction for optical aberrations (such as wavefront errors). In addition, wavelength selectivity allows for simultaneous imaging of multiple bands. Accordingly, digital holography may be employed to simultaneously image and observe a high-energy laser (HEL) beam spot on a distant non-cooperative target.

As shown in <FIG>, a coherent imaging system <NUM> is similar to the embodiment of <FIG> but is extended to support the use of a projected laser spot (an HEL hitspot <NUM>) on the target object <NUM>. In this example, an HEL <NUM> produces high-power laser illumination (HEL illumination) <NUM> for projecting the HEL hitspot <NUM> on the object <NUM>. The HEL <NUM> may operate based on a signal from an HEL master oscillator <NUM>, which is separate from the master oscillator <NUM>. The HEL illumination <NUM> is processed within the auto-alignment system <NUM> and is directed toward the object <NUM> via the telescope <NUM> and the window <NUM> to form the HEL hitspot <NUM> on the object <NUM>.

An HEL beam return <NUM> reflected off the object <NUM> follows a similar path as the reflected or return illumination <NUM> and impinges upon the SWIR FPA <NUM>. Also, HEL LO-modulated illumination <NUM> is carried along a light conduit (such as a fiber) to an exit point <NUM> and is based on an output of a Doppler-shifted HEL LO <NUM> is superimposed on the SWIR FPA <NUM> along with the HEL beam return <NUM>. The Doppler-shifted HEL LO <NUM> may receive a signal from an EOM <NUM> based on an output of the Doppler sensor <NUM> and produce an output based on the EOM signal.

With this configuration, the HEL LO illumination <NUM> is spatially, angularly, and spectrally offset from the Doppler-shifted LO illumination <NUM>. The HEL LO illumination <NUM> is emitted onto the SWIR FPA <NUM>, together with the reflected or return illumination <NUM>, the HEL beam return <NUM>, and the LO illumination <NUM>. The gating or global shutter speed for gating the SWIR FPA <NUM> may be set based upon the pulse timing for the reflected or return illumination <NUM>.

Because the HEL illumination <NUM> is typically continuous wave (CW) rather than pulsed, the system <NUM> offers flexibility in modifying the detector integration time for the HEL imaging. There may also be orders of magnitude difference in the power levels of the HEL beam return <NUM> and the reflected or return illumination <NUM>. To prevent saturation of the SWIR FPA <NUM> by the HEL beam return <NUM>, various techniques (including spectral and polarization attenuation) may be employed, or the detector integration time may be adaptively decreased when the HEL beam return <NUM> is present. The combined illumination received at the SWIR FPA <NUM> is processed by the computer or processing device <NUM> to generate an image of the object <NUM> and an image of the projected laser spot on the object <NUM> (if in fact the HEL illumination <NUM> is reflected off the object <NUM>). The superposition of the two images can be shown on the display device <NUM>.

The telescope <NUM> in <FIG> includes a focusing mechanism <NUM>. As described in more detail below, the focusing mechanism <NUM> can be used to focus the HEL illumination <NUM> onto the target <NUM> in order to create the HEL hitspot <NUM>. Moreover, the auto-alignment system <NUM> operates to help compensate for line-of-sight shifts and aberrations typically created when a telescope changes its focus. Any suitable focusing mechanism <NUM> can be used in a telescope. In some embodiments, the mirror <NUM> again denotes an annular mirror that is substantially normal to the telescope's optical axis and that has a central opening through which the coherent flood illumination <NUM>, the reflected or return illumination <NUM>, and the HEL illumination <NUM> can pass.

As described in more detail below, the imaging system supports an agile beam director with active focus and light-of-sight (LOS) control, co-boresighted SWIR imagery, and adaptive optical wavefront error (WFE) correction.

The beam director operates in conjunction with a digital holographic sensor (such as the ones used in <FIG> and <FIG>) or other wavefront sensor to enable an unobscured off-axis telescope to be dynamically focused onto moving target objects. The adaptive optical WFE correction corrects for both internal optical aberrations and external wavefront errors caused by atmospheric disturbances and aero-optical effects. The beam director architecture employs digital holographic or other wavefront sensing to provide co-boresighted enhanced imagery and estimates of WFE correction, and a secondary mirror of an off-axis telescope can be moved to adjust the focus, nominally correct the LOS error shift, and minimize induced aberrations. If a deformable primary mirror is used, the conic constant of the primary mirror can be changed to correct for induced aberrations. If a deformable mirror is used elsewhere (not as the primary mirror), the deformable mirror can be used to correct for induced aberrations. An auto-alignment scheme is used to maintain pointing accuracy as the telescope is focused. This approach can operate effectively without requiring a separate "beacon" laser (only the illuminating laser or lasers are needed), which can significantly reduce the potential size, weight, cost, and power of the system.

Although <FIG> illustrate examples of coherent imaging systems (with and without HEL capabilities), various changes may be made to <FIG>. For example, various components in <FIG> could be combined, further subdivided, omitted, or rearranged and additional components could be added according to particular needs. As a particular example, the computer or processing device <NUM> could be subdivided into a number of controllers and processing devices for performing different functions. Also, any of the components shown in one or more of these figures could be used in others of these figures. In addition, while <FIG> illustrate example environments in which a beam director could be used, beam directors could be used in any other suitable system.

As noted above, on-axis telescopes are often used in beam directors. Unobscured off-axis telescope designs may be preferable over on-axis telescopes since they could provide the highest irradiance on a target object <NUM>, but dynamically focusing such telescopes using conventional approaches can be difficult. For example, moving the secondary mirror of an unobscured off-axis telescope to focus outgoing HEL illumination typically introduces line-of-sight shifts and aberrations.

The approaches described in this patent document use a digital holographic sensor or other wavefront sensor to provide an actively-illuminated SWIR image of a target object, a target range, and an estimate of two-dimensional (2D) WFE that is used for active WFE compensation. A pulsed SWIR illuminator laser is generally used, but tight focus on the target object <NUM> is not required. The active auto-alignment system <NUM> maintains the LOS as the focus of the telescope is varied and may help to correct for WFE and other errors.

<FIG> illustrate an example auto-alignment system <NUM> for a coherent imaging system or other system and related details in accordance with this disclosure. For ease of explanation, the auto-alignment system <NUM> may be described as being used in the coherent imaging systems of <FIG>. However, the auto-alignment system <NUM> could be used in any other suitable system to provide high-performance line-of-sight control or other control.

As shown in <FIG>, the auto-alignment system <NUM> is used in conjunction with a digital holographic sensor <NUM>, which provides active SWIR imaging, ranging, and WFE estimation. The holographic sensor <NUM> here is similar to those shown in <FIG> and <FIG>, although some modifications have been made to the placement of components in the holographic sensor <NUM>. Also, the holographic sensor <NUM> includes lenses 403a-403b and an additional beam combiner <NUM>. The lenses 403a-403b operate to expand inputs (the LO illumination <NUM> and the Doppler shifted LO illumination <NUM>) from the local oscillators <NUM>, <NUM>, which could denote point sources whose outputs are expanded in order to create larger beams that cover most or all of the detection surfaces of the FPA <NUM> and Doppler sensor <NUM>. The additional beam combiner <NUM> combines the portion 106a of the reflected or return illumination <NUM> with the Doppler-shifted LO illumination <NUM>. Note that the same modifications used in <FIG> could be made in <FIG> and <FIG>, or the arrangement shown in <FIG> and <FIG> could be used in <FIG>. Also note that any other suitable digital holographic sensor or other wavefront sensor could be used in an auto-alignment system.

The auto-alignment system <NUM> here supports the use of an off-axis telescope <NUM> that includes a primary mirror <NUM> and a secondary mirror <NUM>. The secondary mirror <NUM> is not located in the optical path between the primary mirror <NUM> and the output window <NUM> of the system, which is where the secondary mirror would be located in an on-axis telescope. Each mirror <NUM> and <NUM> includes any suitable optical device for reflecting optical signals. In this example, both mirrors <NUM> and <NUM> are paraboloid mirrors.

A positioner <NUM> is coupled to and can move the secondary mirror <NUM>. In some embodiments, the positioner <NUM> can move the secondary mirror <NUM> in up to three ways. For example, the positioner <NUM> could move the secondary mirror <NUM> in the "x" direction (perpendicular to the plane of <FIG>), the "y" direction (up and down in <FIG>). The positioner <NUM> could also rotate the secondary mirror <NUM> around one or more of the three axes (xyz), although this is typically done in the plane of <FIG>. These movements can be done to help change the focus of the optical system and to reduce aberrations. Note that all three directions of movement need not be supported and that a subset of these movements could also be supported. The positioner <NUM> includes any suitable structure(s) for moving a mirror, such as a hexapod six-axis positioner.

Various mirrors <NUM>-<NUM> are used to direct optical signals to and from the primary and secondary mirrors <NUM> and <NUM> and through gimbal axes. Each mirror <NUM>-<NUM> includes any suitable optical device for reflecting optical signals, such as plane mirrors. In some embodiments, one or more of these mirrors (such as the mirrors <NUM> and <NUM>) could denote fast steering mirrors (FSMs), which can be rotated to support LOS adjustment and correction of pupil wander. In this example, the mirrors <NUM> and <NUM>, <NUM>-<NUM> and the positioner <NUM> are located within a dashed box <NUM> and can be collectively moved to adjust the elevation of the optical system with respect to a target object <NUM>. Similarly, the mirrors <NUM> and <NUM>, <NUM>-<NUM> and the positioner <NUM> are located within a dashed box <NUM> and can be collectively moved to adjust the azimuth of the optical system with respect to a target object <NUM>. Gimbals <NUM> can be used to adjust these components to alter the elevation and azimuth of the laser system. Each gimbal <NUM> can denote any suitable structure for moving components in one axis.

Other components in the auto-alignment system <NUM> include a mirror or beam splitter <NUM>, an aperture sharing element (ASE) <NUM>, and a transmit-receive mirror <NUM>. The mirror or beam splitter <NUM> reflects optical signals to and from the mirror <NUM>. If a component auto-alignment (AA) subsystem <NUM> is included in the system, element <NUM> can be implemented as a beam splitter in order to provide a portion of various signals to the component AA measurement subsystem <NUM>. As explained below, the component AA measurement subsystem <NUM> helps to provide alignment between an HEL laser, an SWIR laser, and the receiver line-of-sight. The ASE <NUM> allows an aperture to be shared by multiple transmitted and received optical signals or by both low-power and high-power optical signals. The mirror <NUM> implements a transmit/receive combiner and passes the transmit beam (frequently via a hole in the mirror) and reflects signals to and from the digital holographic sensor <NUM>. The mirror <NUM> could denote any suitable optical device for reflecting optical signals, such as a plane mirror.

Imaging optics <NUM> (such as one or more lenses) are optically positioned between the mirror <NUM> and a beam splitter <NUM>. The imaging optics <NUM> alter incoming and outgoing optical signals, including the reflected or return illumination <NUM> and the auto-alignment illumination <NUM>, which is generated by a gimbal AA subsystem <NUM>. In particular, the beam splitter <NUM> directs the reflected or return illumination <NUM> towards the digital holographic sensor <NUM> and directs the auto-alignment illumination <NUM> between the gimbal AA subsystem <NUM> and the imaging optics <NUM>.

An additional mirror <NUM> reflects the coherent flood illumination <NUM> from a diverger <NUM> into the optical path to the window <NUM>. The diverger <NUM> receives input from the optical power amplifier <NUM> (which in this example is an SWIR source) and causes the input to diverge into suitable flood illumination <NUM>. The diverger <NUM> includes any suitable structure for diverging or diffusing optical signals. An optional pupil relay <NUM> could be included between the mirror <NUM> and the ASE <NUM> to improve pupil imaging in the system. The pupil relay <NUM> forms a real image of the system pupil. Note that the pupil relay <NUM> could be used in other locations, such as between the mirror <NUM> and the digital holographic sensor <NUM>. While that position may not allow the mirror <NUM> to reside at a pupil plane, this may be acceptable since SWIR illumination is used primarily on-axis.

The architecture in <FIG> employs the digital holographic sensor <NUM> and provides a comprehensive layout that supports active focus and LOS control, co-boresighted SWIR imaging, ranging, and adaptive optics compensation for atmospheric turbulence, aero-optics effects, and other disturbances <NUM>. The coherent flood illumination <NUM> here is directed through the window <NUM> towards a target object <NUM> using various lenses and other optical devices shared between the larger system and the auto-alignment system <NUM>. The HEL illumination <NUM> can also be directed through the window <NUM> towards a target object <NUM> using the various lenses and other optical devices shared with the auto-alignment system <NUM>. The reflected or return illumination <NUM> is received through the window <NUM> and directed to the digital holographic sensor <NUM> using the various lenses and other optical devices shared with the auto-alignment system <NUM>.

As described below, the digital holographic sensor <NUM> can be used to measure external WFEs and other errors caused by the external disturbances <NUM>, as well as WFEs caused by optical aberrations between the digital holographic sensor <NUM> and the window <NUM>. These WFEs can then be corrected by altering the optical properties of elements between the digital holographic sensor <NUM> and the window <NUM>, such as by changing the surface properties of one or more deformable mirrors. The gimbal AA subsystem <NUM> can be used to measure internal errors created within the auto-alignment system <NUM> or within the larger system, such as line-of-sight errors. The auto-alignment illumination <NUM> from the gimbal AA subsystem <NUM> is directed to the mirror <NUM>, which is in front of the primary mirror <NUM> and reflects the auto-alignment illumination <NUM> back to the gimbal AA subsystem <NUM>. The mirror <NUM> is located outside the optical path of the coherent flood illumination <NUM>, the reflected or return illumination <NUM>, and the HEL illumination <NUM>. The auto-alignment illumination <NUM> is used to provide an indication of internal light-of-sight errors or other errors within the system. These errors can then be corrected by modifying the optical properties of one or more elements along the beam path, such as by controlling the tilt of one or more steering mirrors.

The function of the telescope <NUM> of <FIG> and <FIG> can be assigned to the optically-coupled primary mirror <NUM> and secondary mirror <NUM> in <FIG>, where the secondary mirror <NUM> is moved with the positioner <NUM> to focus the outgoing HEL illumination <NUM>. Initially, the secondary mirror <NUM> could be moved along a predetermined path to a position that is based on an estimated range to a target object <NUM>. If present, the component AA measurement subsystem <NUM> measures auto-alignment beams (not shown) from the HEL, SWIR laser, and receiver on a common detector. This measurement can then be used to adjust one or more alignment mirrors (not shown) to force the HEL, SWIR laser and SWIR sensor to be co-aligned. The HEL <NUM> and the digital holographic sensor <NUM> have a common optical path starting after the ASE <NUM>, and the shared optical path can be all reflective (achromatic) except for the ASE <NUM> and the window <NUM>. The gimbal AA subsystem <NUM> measures beams sent along the optical path from the digital holographic sensor <NUM> through the main telescope <NUM> and thus facilitates the correction of internal errors within the system. The digital holographic sensor <NUM> functions as both an SWIR imager and a wavefront sensor, which facilitates correction for atmospheric and aero-optical effects on the HEL illumination <NUM>.

One or more of the mirrors shared between the larger system and the auto-alignment system <NUM> can denote a deformable mirror. For example, in some embodiments, the primary mirror <NUM> could represent a deformable mirror. As a particular example, actuators in the primary mirror <NUM> could be used to alter the conic constant of the primary mirror <NUM> as the distance to the target object <NUM> varies. The actuators could be controlled by the computer or processing device <NUM> or by another controller. The secondary mirror <NUM> can be moved axially when the distance to the target object <NUM> varies for focus, and the secondary mirror <NUM> can decenter or tilt to compensate the line of sight. Adjusting the conic constant of the primary mirror <NUM> can correct substantially all aberrations induced by moving the secondary mirror <NUM>, which frees the dynamic range of the deformable mirror to be used for correction of atmospheric and aero-optical effects. Such an approach can be used to achieve extremely small wavefront errors, such as wavefront errors of about <NUM> to about <NUM> waves RMS.

In other embodiments, one or more deformable mirrors may be located at any suitable location(s) within the dashed boxes <NUM>-<NUM> of <FIG>. One example position where a deformable mirror can be located is at a pupil, which in <FIG> is at the primary mirror <NUM> or in the fold near the secondary mirror <NUM>. As such, the mirror <NUM> could denote a deformable mirror, and the primary and secondary mirrors <NUM> and <NUM> could denote simple paraboloid mirrors. The secondary mirror <NUM> can be moved in the "z" direction for focus and can be offset in the "y" direction (without tilt) to adjust the line of sight and to reduce aberrations. The deformable mirror <NUM> could be used to correct residual wavefront errors. Such an approach can be used to achieve small wavefront errors, such as wavefront errors of about <NUM> to about <NUM> waves RMS. It is also possible to use multiple deformable mirrors, such as one to correct for internal wavefront errors caused by components of the laser system and another to correct for external wavefront errors caused by the disturbances <NUM>.

<FIG> illustrates an example implementation of the gimbal AA subsystem <NUM>, which is used to support LOS adjustments of the laser system. As shown in <FIG>, the gimbal AA subsystem <NUM> includes an auto-alignment illumination source <NUM>, which denotes a suitable source of optical signals used for auto-alignment purposes, such as a point source. Illumination <NUM> from the illumination source <NUM> (which can form the auto-alignment illumination <NUM> of <FIG>) is provided through two beam combiners <NUM> and <NUM> to a path length adjuster <NUM>. The illumination <NUM> passes through the path length adjuster <NUM> and then through the optical path of the laser system to the mirror <NUM>. The mirror <NUM> reflects the illumination <NUM> back to the path length adjuster <NUM>, and the illumination <NUM> passes through the beam combiners <NUM> and <NUM> again.

The beam combiners <NUM> and <NUM> provide different portions of both the outgoing and incoming illumination <NUM> onto two position sensitive detectors (PSDs) <NUM> and <NUM>. The PSD <NUM> effectively receives an image of the illumination <NUM> from the source <NUM> and as reflected from the mirror <NUM>, and the PSD <NUM> effectively receives an image of the laser system's pupil. The path length adjuster <NUM> operates here to adjust the optical path length traversed by the illumination <NUM> so that the image of the source <NUM> and its conjugate (the image after traversing the optical path to and from the mirror <NUM>) is located on the front focal plane of the PSD <NUM>. The fast steering mirrors <NUM> and <NUM> could be controlled so that images are substantially centered on the PSDs <NUM> and <NUM>.

The path length adjuster <NUM> in <FIG> denotes a component that alters the length of an optical path traversed by optical signals and could be implemented in any suitable manner. Ideally, the path length adjuster <NUM> does not introduce substantial angular or spatial offset in the optical signals. <FIG> illustrates one example implementation in which the path length adjuster <NUM> can be formed using two prisms <NUM> and <NUM>. The prisms <NUM> and <NUM> are substantially optically transparent to the illumination <NUM> passing through the path length adjuster <NUM>. The prisms <NUM> and <NUM> are substantially matched so that they effectively implement a plate having a variable thickness. As one prism slides relative to the other prism, the plate thickness varies, which introduces a change in the optical path length. The spacing between the prisms <NUM> and <NUM> may not vary substantially as one or more of the prisms <NUM> and <NUM> move. Each prism <NUM> and <NUM> could be formed from any suitable material(s), such as glass.

In the approach shown in <FIG>, the AA illumination <NUM> could be processed to form an annular beam or could overlap with (but be wider than) the coherent flood illumination <NUM>, reflected or return illumination <NUM>, and HEL illumination <NUM>. In either case, the AA illumination <NUM> reflected off the mirror <NUM> is spatially separated from the coherent flood illumination <NUM>, reflected or return illumination <NUM>, and HEL illumination <NUM>. An example of this spatial offset is shown in <FIG>, where a pupil <NUM> is defined as having two footprints <NUM> and <NUM>. The footprint <NUM> denotes the area in which the AA illumination <NUM> is reflected from the mirror <NUM> and returns to the gimbal AA subsystem <NUM>. The footprint <NUM> denotes the area in which the coherent flood illumination <NUM>, reflected or return illumination <NUM>, and HEL illumination <NUM> pass.

The separation of the footprints <NUM> and <NUM> allows two independent optical corrections to occur within the auto-alignment system <NUM>. First, compensation can occur for internal sensor effects, such as focus errors and induced aberrations created within the telescope <NUM> itself. The telescope <NUM> can be focused nominally to a target range by moving the secondary mirror <NUM> to a predicted position associated with that range. WFE of the telescope <NUM> can be compensated for the given telescope configuration using one or more deformable mirrors as described above. Second, compensation can occur for external effects (such as atmospheric and aero-optics) using wavefront measurements made by the digital holographic sensor <NUM>. The coherent flood illumination <NUM>, reflected or return illumination <NUM>, and HEL illumination <NUM> can require both corrections, but the AA illumination <NUM> does not require correction for external disturbances since the AA illumination <NUM> does not experience those external disturbances. As a result, the illumination <NUM> reflected from the mirror <NUM> can be spatially separated so that corrections for external disturbances do not affect the illumination <NUM>.

As noted above, it is possible to use one or multiple deformable mirrors in order to support optical corrections in the auto-alignment system <NUM>. As noted above, a single mirror (such as the primary mirror <NUM> or the mirror <NUM>) could denote a deformable mirror. <FIG> illustrates an example in which multiple deformable mirrors <NUM> and <NUM> could be used. The deformable mirrors <NUM> and <NUM> could denote any pair of mirrors shared between the larger system and the auto-alignment system <NUM>, and those mirrors need not be adjacent to one another in the optical path. In this example, the deformable mirror <NUM> can be used to provide internal WFE correction, while the deformable mirror <NUM> can be used to provide external WFE correction. The illumination <NUM> in the footprint <NUM> does not require external WFE correction, so an annular mask <NUM> can be used with the deformable mirror <NUM>. The annular mask <NUM> redirects illumination within the footprint <NUM> without modification of that illumination by the deformable mirror <NUM>, while at the same time allowing illumination in the footprint <NUM> to be modified by the deformable mirror <NUM>. This helps to maintain the spatial separation between the footprints <NUM> and <NUM> near the pupil image to keep the illumination <NUM> from being influenced by the external WFE corrections.

Also as noted above with respect to <FIG>, the PSDs <NUM> and <NUM> operate to detect positions or angles of arrival of the incoming illumination <NUM>. The PSD <NUM> ideally obtains an image of the point source (source <NUM>), while the PSD <NUM> ideally obtains an image of a pupil (defined by the mirror <NUM>). The PSDs <NUM> and <NUM> can be high-frame-rate imaging sensors. Lateral effect devices could also be used to implement the PSDs <NUM> and <NUM>, with suitable coding and decoding schemes. <FIG> illustrates one example coding and decoding scheme in which the gimbal AA subsystem <NUM> includes two sources 902a and 902b of the illumination <NUM>. The sources 902a and 902b both provide illumination to a beam combiner <NUM>, which combines the illumination from both sources to generate the illumination <NUM>. Masks <NUM> on the beam combiner <NUM> can be used to shape the illumination from the sources 902a and 902b into a suitable format for the illumination <NUM>. <FIG> illustrates an example formatting of the illumination <NUM>, where different halves or other portions <NUM> and <NUM> of the illumination <NUM> are formed using illumination from different sources 902a and 902b.

The illumination from each source 902a and 902b can be temporally modulated or otherwise altered so that the centroid of the illumination from each source 902a and 902b can be determined. This allows, for example, the system to measure the centroids of the individual illuminations using the PSD <NUM>. The PSD <NUM> can also be used to measure the offset of the centroid in the pupil image, which is formed by the illumination from both sources 902a and 902b. The fast steering mirrors <NUM> and <NUM> could be adjusted to move the centroid of the combined image as needed, while focusing of the illumination <NUM> can be adjusted to move the centroids of the individual illuminations from the sources 902a and 902b as needed.

Although <FIG> illustrate one example of an auto-alignment system <NUM> for a coherent imaging system or other system and related details, various changes may be made to <FIG>. For example, optical paths and their associated optical devices could vary widely while still supporting the same or similar functionality described above. In general, any suitable optical devices can be placed in any suitable optical paths to support the auto-alignment, focusing, and error correcting functionality of the larger system or the auto-alignment system <NUM>. Also, the computer or processing device <NUM> could provide all of the control functionality used to adjust various devices within the larger system or the auto-alignment system <NUM>, or separate controllers can be provided for adjusting different devices or different groups of devices within the larger system or the auto-alignment system <NUM>.

<FIG> illustrates an example method <NUM> for beam direction employing wavefront sensing in high-power laser systems and other systems in accordance with this disclosure. For ease of explanation, the method <NUM> is described with respect to the auto-alignment system <NUM> of <FIG> and <FIG> operating in the coherent imaging systems of <FIG>. However, the method <NUM> could be used in any other suitable manner.

As shown in <FIG>, an estimated range to a target object is identified at step <NUM>, and a secondary mirror of a telescope is moved to a position based on the estimated range at step <NUM>. This could include, for example, a separate or combined subsystem using a RADAR, LADAR, or other ranging sensor <NUM> to estimate the range to a target object <NUM>. This could also include using the Doppler sensor <NUM> in a pulsed laser system to measure the target range. Any other suitable technique could be used to identify an estimated range to a target. This could further include using the positioner <NUM> to move the secondary mirror <NUM> to a specified position associated with the estimated range. In some embodiments, a lookup table or other data structure could associate estimated ranges with positions for the secondary mirror <NUM>.

Coherent flood illumination, HEL illumination, or both are transmitted through the optical system (including the telescope) at step <NUM>, and reflected coherent flood illumination, HEL illumination, or both are received and processed using a digital holographic sensor or other wavefront sensor at step <NUM>. This could include, for example, the optical power amplifier <NUM> generating the coherent flood illumination <NUM> or the HEL <NUM> generating the HEL illumination <NUM>. This could also include directing superimposed LO illumination <NUM> and reflected or return illumination <NUM> onto the Doppler sensor <NUM> and directing superimposed Doppler-shifted LO illumination <NUM> and reflected or return illumination <NUM> (and optionally HEL LO-modulated illumination <NUM> and HEL beam return <NUM>) onto the FPA <NUM>. In addition, this could include the computer or processing device <NUM> or other controller analyzing the outputs from the Doppler sensor <NUM> and the FPA <NUM>. If necessary, external disturbances and errors are corrected at step <NUM>. This could include, for example, the computer or processing device <NUM> or other controller using WFE measurements from the digital holographic sensor <NUM> or other wavefront sensor to perform phase correction on the coherent flood illumination <NUM>, the reflected or return illumination <NUM>, or the HEL illumination <NUM> using at least one deformable mirror. This ideally corrects for any WFE or other external errors.

Auto-alignment illumination is transmitted through the optical system (including the telescope) at step <NUM>, and reflected auto-alignment illumination is received and processed at step <NUM>. This could include, for example, the illumination source <NUM> generating the illumination <NUM>. This could also include multiple sources 902a and 902b generating illumination that is combined to produce the illumination <NUM>. This could further include transporting the illumination <NUM> through the optical system (as the auto-alignment illumination <NUM>) and reflecting at least some of the illumination <NUM> from the mirror <NUM>, where at least the reflected portion of the illumination <NUM> is spatially separate from coherent flood illumination <NUM>, reflected or return illumination <NUM>, and HEL illumination <NUM>. In addition, this could include the computer or processing device <NUM> or other controller analyzing the outputs from the PSDs <NUM> and <NUM> to identify whether spot and pupil images are centered. If necessary, internal disturbances and errors are corrected at step <NUM>. This could include, for example, the computer or processing device <NUM> or other controller adjusting the fast steering mirrors <NUM> and <NUM>, the positioner <NUM> for the secondary mirror <NUM>, or other optical devices so that the spot and pupil images are substantially centered. This ideally corrects for any focus, line-of-sight, or other internal errors.

There might be instances when the line between internal and external errors is blurred since the two are coupled and both types of errors will likely be registered in the digital holographic sensor <NUM>. For example, changing focus can lead to a line-of-sight change. Having the focus/line-of-sight adjustment and the wavefront correction optically coupled in the disclosed manner allows auto-alignment and focusing to occur alongside internal and external wavefront error correction. This is achieved by combining the use of a digital holographic sensor <NUM> and an auto-alignment system <NUM> (and the focus mechanism <NUM> if applicable). The auto-alignment system <NUM> corrects for internal errors within the laser system, while outputs from the digital holographic sensor <NUM> can be used to correct for internal and external wavefront errors.

Claim 1:
An apparatus (<NUM>) comprising:
a wavefront sensor (<NUM>) configured to receive coherent flood illumination (<NUM>) that is reflected from a remote object (<NUM>) and to estimate wavefront errors associated with the coherent flood illumination; and
a beam director optically coupled to the wavefront sensor and comprising a telescope (<NUM>) and an auto-alignment system (<NUM>), the auto-alignment system configured to adjust at least one first optical device (<NUM>-<NUM>) in order to alter a line-of-sight of the wavefront sensor;
wherein the wavefront errors estimated by the wavefront sensor include a wavefront error resulting from the adjustment of the at least one first optical device; and
wherein the auto-alignment system is further configured to generate auto-alignment illumination (<NUM>) that passes through the telescope, is reflected off a mirror (<NUM>), and returns through the telescope to the auto-alignment system so that the auto-alignment system is able to use the auto-alignment illumination to provide an indication of internal line-of-sight errors within the apparatus and to adjust one or more components of the auto-alignment system to compensate for the internal line-of-sight errors within the apparatus, the mirror outside an optical path of the coherent flood illumination.