System and method to detect signatures from an underwater object

Embodiments of the invention are directed to detecting an underwater object from an air-based system. The air-based system is associated with at least one controller. A broadband acousto-optic signal detection device is associated with the air-based platform. The broadband acousto-optic signal detection device is configured to emit a laser beam at an underwater object. The laser beam terminates at the underwater object and reflects back as a return laser beam. The broadband acousto-optic signal detection device is configured to detect and receive the return laser beam.

FIELD OF THE INVENTION

Embodiments of the invention generally relate to mitigating signal distortions caused by air and random air-water interface turbulence.

It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not to be viewed as being restrictive of the invention, as claimed. Further advantages of this invention will be apparent after a review of the following detailed description of the disclosed embodiments, which are illustrated schematically in the accompanying drawings and in the appended claims.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Significant signal distortion occurs due to turbulent flow conditions in both atmospheric and random air-water interface regions. Embodiments of the invention mitigate the signal distortions by integrating a Laser Doppler Vibrometer (LDV) with an Adaptive Optics (AO) system for sensing and detecting signatures from an underwater object by sensing the acoustic signals originated from the underwater object. As described herein, embodiments of the invention are directed to non-transitory signals. The LDV signal is enhanced while it passes through a complex random air-water interface as well as atmospheric turbulence. Hence, embodiments of the invention can be considered a remote sensing technique that stabilizes and aims a laser beam onto an object underneath the water and detects a reflected beam back from the object.

Embodiments of the invention use an optical technique of remotely detecting the signature of the presence of an underwater object by measuring the acoustic signals originated from the object. Embodiments can detect underwater objects having a reflective surface. Embodiments are equally applicable to both stationary and moving underwater objects. The method of detection is based on stabilizing a laser beam from an airborne platform which otherwise is distorted when propagating through atmospheric turbulence and random air-water interface turbulence. The airborne platform is sometimes referred to as an air-based platform. Embodiments employ a sensor that integrates the LDV and the AO system, both of which are co-located on the airborne platform. A person having ordinary skill in the art will recognize that stabilizing means keeping the laser beam fixed in the presence of atmospheric and water distortions.

Embodiments can detect acoustic signals from many objects and sources due to a detection range from about a few hertz to tens and hundreds of kilohertz. A person having ordinary skill in the art will recognize that the underwater object can be referred to as an object, object of interest, and target, without detracting from the merits or generalities of embodiments of the invention.

Embodiments of the invention detect a broadband acousto-optic signal from an underwater object by stabilizing a transmitted laser beam propagated through a distorted medium. The distorted medium has both atmospheric turbulence and random air-water interface turbulence. The laser beam is stabilized by applying an adaptive optics (AO) technique to a laser Doppler vibrometer (LDV) output beam. The LDV vibrometer output beam is used to detect an acoustic optical signal generated by the underwater object.

The distorted medium interferes with the acoustic signals generated by the underwater object. The resultant acoustic-optical signal generated by the underwater object is sensed and then is routed back to the LDV after the AO system corrects the distortions and stabilizes the return beam. The wavefront sensor senses the resultant acoustic-optical signal and stabilizes the return beam. The wavefront sensor is physically-connected by a wire to both the fast steering mirror and the deformable mirror. The wavefront sensor provides feedback to both the fast steering mirror and the deformable mirror.

The corrections of the distorted laser beam in both the atmospheric turbulence and the random air-water interface turbulence is performed by applying the AO technique. The AO technique measures the quantitative distortions with the wavefront sensor. Lower-order modes are mitigated with at least one fast-steering mirror (FSM). Higher-order modes are mitigated using at least one deformable mirror (DM) by correcting the distortions using phase-conjugation. The lower and higher order modes are processed with a feedback control system.

Although embodiments of the invention are described in considerable detail, including references to certain versions thereof, other versions are possible. Examples of other versions include performing the tasks in an alternate sequence or hosting embodiments on different platforms. Therefore, the spirit and scope of the appended claims should not be limited to the description of versions included herein.

Block Diagram of a System with Functional Components—FIGS. 1, 2, &3

In the accompanying drawings, like reference numbers indicate like elements. In the block diagram inFIG. 1, a system according to embodiments of the invention is depicted. Reference character10depicts a system, according to embodiments of the invention, as well as its operating environment. The system10is configured for sensing and detecting an acoustic signal generated by an underwater object, which can also be referred to as a source of the reflected beam, source, and the like.

The system10includes an Optical-based Sensor100, an LDV in this illustration, and an AO system200. The LDV100is used to make non-contact measurements of a surface of the object400. The vibration amplitude and frequency are extracted from the Doppler shift of the laser beam frequency due to the relative motion of the object generating the vibration.

FIG. 2illustrates the functional components of the LDV100and includes a combination of beam splitters (abbreviated as BS and depicted as reference character114) and a mirror (abbreviated as M and depicted with reference character112) for directing and receiving the optical beams, a Bragg Cell102, and a Photo Detector103. The LDV100is configured as a combination of modules, which together produce a Doppler frequency shift, proportional to object velocity, the laser light wavelength and the angle between the laser beam direction and the vibration velocity vector. The LDV100probes and penetrates through a distorting medium300, sometimes referred to as a distorted medium. In embodiments of the invention, the distorting medium300includes both atmospheric and random air-water surface (interface) distortions. A beam then reflects back (a reflected beam) from the object400(sometimes referred to as a vibrating object). The LDV100detects the acoustic signal via surface perturbation caused by the impinging acoustic pressure field. The incoming laser signal containing the acoustic signal information which is already generated by the object400pass through an Adaptive Optics (AO) system200, where it is corrected for the random distortions (caused by air-water interface, and the water surface, and the atmospheric conditions) of the laser signal to be detected by the Photo Detector103. Reference character100A depicts the outgoing laser beam/signal from the LDV100. Reference character100B depicts the incoming laser beam/signal back to the LDV100. Reference character200A depicts the outgoing laser beam/signal from the AO system200. Reference character200B depicts the return laser beam/signal from the object400, through the distorted medium300, and back into the AO200. Reference character300A depicts the laser beam/signal out of the distorted medium300and to the object400. Reference character300B depicts the laser beam/signal returned (reflected) from the object400and back into the distorted medium300.

FIG. 3provides the overall block diagram of the combined LDV100and AO system200showing the different modules and how they are connected with each other as well as the operating environment in block diagram form. The AO system200is used to compensate air-water interface-induced random image distortions and includes an integrated Beacon Illuminator (BILL)202, Track Illuminator (TILL)201, and a wavefront sensor203that can be used for an object that provides BILL return from the object.

As shown inFIG. 3the AO system200includes a fast steering mirror204, a deformable mirror205, and a Wavefront Sensor203for mitigating the air-water surface distortions. The AO system200includes a combination of beam splitters (abbreviated as BS and depicted as reference character214) and mirrors (abbreviated as M and depicted with reference character212) for directing and receiving the optical beams. The AO system200mathematically takes the complex conjugate of the random phase caused by air-water interface allowing the laser beam to penetrate through the water by cancelling with the complex conjugate. The return optical signal reflected from the object400and distorted back again by the air-water surface is mitigated when reaching back to the LDV100for sensing and optically detecting the acoustic signal generated and originated at the object. The AO system feedback control is depicted by reference character206, which represents the plurality of computer executable instructions used for signal processing.

As shown inFIGS. 1 & 3, the distorted medium is represented by300where three laser beams are shown with reference characters201A,202A, &100A. The laser beam from the TILL201is depicted by reference character201A. The laser beam from the BILL202is depicted with reference character202A. The laser beam from the LDV100is depicted with reference character100A. The three laser beams201A,202A, and100A simultaneously propagate through the distorting medium300, illuminate the underwater object400and reflect back after performing the distortion corrections where the corrected LDV laser signal (the return laser back to the LDV)100B is reflected back towards the LDV100where the signal is interferometrically processed for acoustic signal sensing/detection.

Working Laboratory System Example and Test Results—FIGS. 4A Through 4D

Significant laboratory testing to prove the feasibility of embodiments of the invention was conducted with a water tank equipped with a fan to produce air-water surface randomness.FIGS. 4A through 4Dare directed to a laboratory testing system to detect an underwater object, according to embodiments of the invention. Metrics to measure the success of embodiments of the invention are graphically displayed inFIGS. 4A through 4C. The AO improvements are measured using the Strehl ratio. In general, the Strehl ratio is the percent return (reflection) from an object. In embodiments of the invention, the AO improvement is measured by the Strehl ratio, defined by the acoustic-optical signal increase at the AO-on mode compared to the signal at AO-off mode. The metric Strehl ratio also is sensitive to the different acoustic signal bandwidth generated by the under-water object/source, thus providing a means of optimizing the presence or absence of the source by optimizing the observed signal strength. Embodiments of the invention can provide the results for AO gain in DB of the detected LDV signal within the various broad frequency range of a few Hz to 10s to 100s of kHz.

FIG. 4Dillustrates a physical laboratory system, according to embodiments of the invention, and is depicted by reference character460. The system460used an air-based platform462to detect an underwater object478. The air-based platform462was positioned about 36 inches above the underwater object478. The air-based platform462positioning in the laboratory system460can be suspension from a ceiling, elevation from scaffolding or ladder, an elevated structure, or attachment to a drone. The air-based platform462was in electrical communication with an electronic processor, sometimes referred to as a computer464. Reference character465is generically used for the electrical communication between the air-based platform462and the computer464. The computer464was used for signal processing functions and for displaying, in conjunction with a visual display screen, the graphs inFIGS. 4A, 4B, &4C. The air-based platform462included a broadband acousto-optical signal detection device (not shown for ease of viewing). A mirror466was attached to the air-based platform462and was used for directing and receiving optical beams. Reference character467is generically used for the attachment of the mirror466to the air-based platform462. The underwater object478was in a glass water tank468filled with approximately twelve inches of water476. The underwater object478had reflective tape on its upper surface, the surface closest to the air-based platform462. Free space above the water surface is air472. The water tank468had a glass bottom470resting on an optical table480. A water surface distorter474, sometimes referred to as a wind generator such as, for example, a fan, was positioned above the water surface and was used to create a distortion zone475, sometimes referred to as a distorted region, or distorting region, or distorted medium. During testing, the water surface distorter474was set to a wind speed of about 7 miles per hour.

The transmitted and reflected laser beams are not specifically shown inFIG. 4Dfor ease of viewing and because the beams are discussed with respect toFIGS. 1, 2, 3, &5. The broadband acousto-optic detection device was configured to emit a laser beam (such as reference character506inFIG. 5), sometimes referred to as a transmitter beam, transmitted beam, or reference beam at the underwater object478. The laser beam506was a continuous beam, such as an unpulsed beam. The laser beam506can also be a pulsed beam if desired for blanking purposes. The laser beam506selected was a blue-green laser having a wavelength range of about 400 to 500 nanometers because of its ability to penetrate water.

The laser beam506(FIG. 5) terminated at the underwater object478in the glass water tank468and reflected back as a return laser beam512(FIG. 5), which is also referred to as a reflected laser beam, and a signal beam. The broadband acousto-optic signal detection device was configured to detect and receive the return laser beam512.

Graphical representations of the test results obtained from the system illustrated inFIG. 4Dare shown inFIGS. 4A through 4C, as a computer display screen, rendering visual verification that the system performs as expected. The takeaway from viewingFIGS. 4A through 4Cis that practicing embodiments of the invention, as disclosed herein with the AO-on, yields improved, quantifiable, results when compared to the AO-off conditions.

The left graph ofFIG. 4A, depicted by reference character400A, is an exemplary graphical representation on a computer display screen of an LDV signal amplitude in arbitrary units (a.u.) with the AO-off for signal sensing at a frequency of 5000 Hz. The right graph ofFIG. 4A, depicted by reference character400B, is an exemplary graphical representation of an LDV signal on a computer display screen of an LDV signal amplitude in arbitrary units (a.u.) with the AO-on at a frequency of 5000 Hz. The right graph400B ofFIG. 4A(the AO-on condition) is the result when embodiments of the invention are practiced. The right graph400B ofFIG. 4depicts the LDV signal amplitude using embodiments of the invention as disclosed herein, and further described with respect toFIG. 4D. The takeaway fromFIG. 4Ais that practicing the embodiments of the invention, with the AO-on as disclosed herein, results in an LDV signal amplitude that is about 9 to 10 times greater than during AO-off conditions.

The left graph ofFIG. 4B, depicted by reference character440A, is an exemplary graphical representation of the average Strehl ratio results for the AO-off conditions in an environment having a 7 mile per hour fan-generated wind speed. The right graph ofFIG. 4B, depicted by reference character440B, is an exemplary graphical representation of the average Strehl ratio results for the AO-on conditions in an environment having a 7 mile per hour fan-generated wind speed. The Strehl ratio is a metric for the effectiveness of an AO system. The AO-off conditions440A correspond to open loop conditions. Conversely, the AO-on conditions440B, corresponding to practicing embodiments of the invention as discussed herein, are closed loop conditions. A comparison of theFIG. 4Bgraphs shows that the AO-on (closed loop) conditions (440B), corresponding to embodiments configured as disclosed herein, yield a far greater Strehl ratio than the AO-off (closed loop) conditions (440A). The takeaway fromFIG. 4Bis that practicing the embodiments of the invention with the AO-on, as disclosed herein, results in a far greater percentage return (greater Strehl ratio) than during AO-off conditions.

FIG. 4Cis an exemplary graphical representation of the AO gain in decibels (dB) of the LDV amplitude for the frequency range of about 20 Hz to about 10 kHz, according to some embodiments of the invention, and is depicted by reference character450.FIG. 4Cshows the AO gain (dB) experienced during AO-on conditions compared to AO-on conditions, and mathematically defined as:

This parameter shows the range in which the system can detect a specific signal. In laboratory measurements, it was from about 6 to about 17 dB gain for a corresponding frequency range of about 20 Hz to about 10 kHz. The takeaway fromFIG. 4Cis that significant improvement in signal detection occurs in the AO-on conditions due to the AO optimization of the LDV signal in the distorted medium.

Referring toFIG. 5, an air-based system to detect an underwater object is depicted using reference character500. The system500includes an air-based platform502associated with at least one controller (not depicted on the drawings). The air-based platform502can be manned or unmanned. The controller is at least one computer and can be referred to as a non-transitory computer readable medium. Embodiments of the invention are directed to non-transitory signals. A broadband acousto-optic signal detection device504is associated with the air-based platform502. The broadband acousto-optic detection device504is configured to emit a laser beam506, sometimes referred to as a transmitter beam, transmitted beam, or reference beam at an underwater object508. The laser beam506can be continuous beam, such as an unpulsed beam. The laser beam506can also be a pulsed beam if desired for blanking purposes. The laser beam506is sometimes referred to as a reference beam. The laser beam506selected is a blue-green laser having a wavelength range of about 400 to 500 nanometers, because of its ability to penetrate water.

Reference character510is used to generically depict the water. The laser beam506terminates at the underwater object508and reflects back as a return laser beam512, which is also referred to as a reflected laser beam, and a signal beam. The broadband acousto-optic signal detection device504is configured to detect and receive the return laser beam512.

In embodiments, the broadband acousto-optic signal detection device504is a Laser Doppler Vibrometer (LDV)100coupled to an adaptive optics (AO) system/device200. The LDV100houses and is mechanically-associated with a laser source101, which emits and transmits the laser beam506. The LDV100produces a Doppler shift in the return laser beam512. The LDV100includes an interferometer that is used to measure frequency difference between the laser (reference) beam506and the return (signal) beam512. The process is sometimes referred to as interferometrically processing for acoustic signal sensing/detection. The return laser beam/signal beam512comes back to the AO system200. The reference beam reference beam506remains in the LDV100.

A distorted medium300(depicted inFIG. 3), sometimes referred to as a distorting medium, is located between the air-based platform502and the underwater object508. The distorted medium300includes an atmospheric turbulence zone514and an air-water interface turbulence zone516. Each of the zones514&516are characterized by turbulent flow conditions.

The AO system/device200includes a wavefront sensor (WFS)203that is electrically-connected with a fast steering mirror (FSM)204and a deformable mirror205. The WFS203measures quantitative distortions caused by both the atmospheric turbulence514and the air-water interface turbulence516. A person having ordinary skill in the art will recognize that lower and upper modes, often called Zernike polynomials, are used in the optics field. The FSM204is used to mitigate the lower-order modes. The deformable mirror205uses phase-conjugation to undo the distortions and mitigate the higher-order modes. A feedback control system206is generically shown and is electrically-associated with the non-transitory computer readable medium. The feedback control system206and its associated computer executable instructions are included for signal processing of the lower and higher-order modes. For the AO system200, the WFS203is the feedback control mechanism for the deformable mirror205and the FSM204. Information gained after signal processing determines how to keep the beam still or stabilized in the presence of atmospheric turbulence and air-water interface turbulence.

Embodiments of the invention can be applied and used for any laser-based broad classes of sensors as long as matching AO system accepting the input laser wavelength is used. Additionally, embodiments of the invention are applicable to other configurations such as optical communications between the air-based/airborne platform and underwater terminals, and between under-water terminals with the system placed at both ends.

Embodiments of the invention are useful for the visual verification of the reflected beam512by visually verifying the Strehl ratio, which can be displayed on a visual display screen, sometimes referred to as a display monitor, and which is included in the embodiments (electrically-associated with the computer and the air-based platform502). The display monitor/visual display screen is included in embodiments of the invention and is a tangible medium for displaying output, such as the respective LDV signal amplitudes inFIG. 4A, the respective Strehl ratios inFIG. 4B, and the AO gain inFIG. 4C. Other tangible outputs are possible without detracting from the merits or generality of embodiments of the invention. As such, in the embodiments, the tangible outputs may be shown and/or represented as a visual display screen depiction, hard copy printouts, as well as other media using the information such as, for example, a computer having computer-readable instructions that is configured to use output from embodiments of the invention.