Robust infrared countermeasure system and method

A system and method for focusing electromagnetic energy on a moving target. Generally, the inventive system sends a pilot beam to a target and analyzes a return wavefront to ascertain data with respect to any distortions and other phase and/or amplitude information in the wavefront. This information is then used to pre-distort an output beam by so that it is focused on the target by the intervening distortions. In an illustrative embodiment, the pilot beam is provided by a beacon laser mounted off-axis with respect to the output beam. The reflected wavefront is received through a gimbaled telescope. Energy received by the telescope is detected and processed to ascertain wavefront aberrations therein. This data is used to predistort a deformable mirror to create an output beam which is the phase conjugate of the received wavefront. In a first alternative embodiment, a nonlinear optical phase-conjugate mirror is employed to generate the required wavefront-reversed replica of the received wavefront. The system further includes an arrangement for modulating the output beam to confuse the target. In a second alternative embodiment, the system is adapted to examine atmospheric distortions of starlight to predistort the output beam. The alternative embodiment offers a faster response time and a lower susceptibility to detection.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to imaging systems and methodologies. More specifically, the present invention relates to countermeasures for infrared sensing systems.

2. Description of the Related Art

Imaging systems are used for a variety of commercial, industrial and military applications. Of particular interest for military applications are infrared sensing systems and techniques. Infrared sensing systems detect heat and are therefore effective in conditions of darkness, smoke, haze and other situations where visible detection is impractical. Accordingly, many weapons have been developed which rely on infrared sensors for target acquisition and tracking and terminal guidance. The development of weapons with infrared technology has given rise to a need for countermeasures for same.

Several techniques are used in the art for infrared countermeasures or ‘IRCM’. Conventional IRCM countermeasures include sensor blinding, lethality-based approaches, and modulated lasers for “spoofing.” In most of these cases, a high-energy or high-power laser is required. In the “spoofing” approach, for example, a laser beam is temporally modulated to confuse the navigational homing system in the threat vehicle. The laser beam must be spread in angle to cover a field-of-view (FOV) of sufficient breadth to illuminate a target.

Unfortunately, the efficiency of the spoofing approach, as well as other IRCM approaches, is limited by atmospheric distortions and other aberrations that tend to impede the diffraction-limited performance of the system. As a result, most of the light “spills over” the target, and represents a loss to the system. In addition, residual beam wander, due to excess vibrations, finite servo gain on trackers, etc., creates the need to increase the angular spread of the IRCM modulated laser beam, to assure adequate target illumination. Consequently, this approach requires a high-power source which adds to the size, weight, cost and power requirements of the system and limits the performance thereof.

Hence, a need exists in the art for an improved system or method for focusing infrared energy on a moving target and maintaining it on the target throughout its flight path. A further need exists for a system for focusing infrared energy on moving target with temporal encoding or modulation to effect spoofing for countermeasure protection.

SUMMARY OF THE INVENTION

The need in the art is addressed by the system for focusing electromagnetic energy of the present invention. Generally, the inventive system sends a pilot beam to a target and analyzes a return wavefront to ascertain data with respect to any distortions thereof. This information is then used to pre-distort an output beam by so that it is focused on the target by the intervening distortions.

In an illustrative embodiment, the pilot beam is provided by a beacon laser mounted off-axis (or may be mounted on-axis) with respect to the output beam. The reflected wavefront is received through a gimbaled telescope. Energy received by the telescope is detected and processed to ascertain wavefront aberrations therein. This data is used to predistort a deformable mirror to create an output beam which is the phase conjugate of the received wavefront. The system further includes an arrangement for modulating the output beam to confuse the target.

In an alternative embodiment, the system is adapted to examine atmospheric distortions of starlight to predistort the output beam. The alternative embodiment offers a faster response time and a lower susceptibility to third-party detection of the system.

DESCRIPTION OF THE INVENTION

The illustrative application of this invention is in connection with infrared countermeasures (IRCM) with respect to incoming threats. The present invention enables the IRCM system to function with optimal efficiency. As discussed more fully below, in accordance with the adaptive optics system of the present teachings, a high-energy laser may be used to blind a sensor or to physically damage critical components on an incoming threat, with or without imposed modulation. The present invention allows for the use of a laser which operates with potentially lower power than the lasers used in the prior art. The laser is used to confuse a navigational tracking system on the threat or to blind the sensor with a gated signal.

FIG. 1is a simplified block diagram of a typical infrared counter-measure laser system implemented in accordance with conventional teachings. As shown inFIG. 1, the typical conventional IRCM system10′ includes a gimbaled telescope12′ adapted to receive light14′ emanating from a target. The received beam14′ is directed to a tracker18′ by a beamsplitter16′. The tracker18′ includes a detector (not shown) and provides control signals to the telescope12′ effective to cause the telescope12′ to track a target. The output of the tracker18′ is also input to a processor20′. The processor20′ also receives signals from an ultraviolet (UV) or infrared (IR) missile warning sensor22′. In response, the processor20′ outputs a signal to control the gimbaled telescope12′ as well as to control a laser24′ effective to cause the laser24′ to output a high power output beam in the direction of the target. The output of the laser24′ may be modulated by the processor to spoof the target.

As mentioned above, this conventional approach suffers from the shortcoming that atmospheric distortions and other aberrations tend to impede the diffraction-limited performance of the system. As a result, most of the light “spills over” the target, and represents a loss to the system. In addition, residual beam wander, due to excess vibrations, finite servo gain on trackers, etc., creates the need to increase the angular spread of the IRCM modulated laser beam, to assure target illumination. Consequently, this approach requires a high-power source which adds to the cost and limits the performance of the system.

Hence, a need has existed in the art for an improved system or method for focusing infrared energy on a moving target and maintaining it on the target throughout its flight path. This need is addressed by the system and method of the present invention.

FIG. 2is a simplified optical schematic and electrical block diagram of an infrared counter-measure system implemented in accordance with the teachings of the present invention. The system10includes a beacon laser12which may be mounted off-axis or on axis with respect to a telescope22discussed more fully below. Those skilled in the art will appreciate that the system10is adapted to operate with a coarse tracker such as that shown in FIG.1. The coarse tracker directs the system10toward a detected target. The system10subsequently performs fine tracking as discussed more fully below.

In accordance with the present teachings, the beacon laser12outputs a pilot beam14in the direction of a target18. The pilot beam is reflected as a ‘glint’ by the target18. A glint is a highly reflective (i.e., specular) feature17on the target18which typically has a lateral dimension on the order of, or less than, the diffraction limit of the optical system. The glint spreads out as a return wavefront20which is distorted by the atmosphere, shown generally at16. The distorted wavefront20is then received by first and second lenses24and26, respectively, of a telescope22. The optical design of the telescope22can include mirrors and/or diffractive optical elements instead of the lenses. The telescope focuses the received wavefront20onto a gimbaled mirror28. The mirror28directs the reflected wavefront20to a second mirror32via a laser amplifier30. The laser amplifier is adapted to amplify energy at the wavelength of the beacon laser12. In practice, the laser amplifier may be an optical parametric amplifier or other suitable laser amplifier known in the art depending on the operating wavelength of the beacon laser12.

In the illustrative embodiment, the second mirror32is a conventional low reflectance beamsplitter. A portion of the received wavefront20is directed to a tracker34. The tracker34may be implemented in the same manner as the tracker18′ of FIG.1. That is, the tracker34includes a detector and control logic (not shown) for providing tilt control and focus correction for the mirror28.

A second beamsplitter36directs a portion of the received beam to a closed-loop IRCM system controller48. The controller48provides modulation control as discussed below.

Another portion of the received wavefront is directed to a wavefront error sensor40by a third beamsplitter38. As discussed more fully below, the wavefront error sensor detects aberrations in the received wavefront and provides control signals for a deformable mirror42effective to cause the mirror to reflect energy from a readout laser44as a phase conjugate of the received wavefront. The mirror42provides a wavefront-reversed replica of the incoming beam, which retraces the path of the incident beam. The conjugator42, in essence, provides for the “fine” angle tuning of the return beam, thereby compensating for small-angle dynamic beam wander and relative platform motion, as well as compensation of dynamic higher-order wavefront errors. The mirror42can be in the form of a self-pumped phase conjugate mirror (PCM), an adaptive closed-loop system (e.g., a spatial light modulator, a liquid crystal light valve (LCLV), micro-electrical-mechanical (MEMS) array, with a wavefront error sensor, a four-wave mixer, a double-pumped PCM, or a stimulated Brillouin scattering (SBS) cell.

The wavefront error sensor40may be implemented in accordance with techniques well known in the art including shearing interferometers, Shack-Hartmann systems etc.

The output of the readout laser44is modulated directly or by an electro-optic shutter46under control of the IRCM controller48mentioned above. The shutter46and the IRCM controller48may be of conventional design and construction. The IRCM controller48detects the modulation scheme or is fed target type data from an external tracking system or a database (not shown).

In operation, a “two-pass” approach is employed, as opposed to the “singlepass” approach in the prior IRCM art. In the present invention, the threat is first actively illuminated by the broad-angle (a large field-of-view, FOV) beacon beam14. A fraction of the light scatters from the seeker of the missile18and is collected at the telescope22. This glint return, which has been distorted by the intervening optical path (i.e., the atmosphere16), is subsequently conjugated, amplified and, by virtue of the wavefront-reversal property of the conjugator42, is automatically redirected back to the threat. The conjugation process thereby compensates for dynamic atmospheric distortions, and beam wander, resulting in a diffraction-limited laser beam incident back on the target. System efficiency is enhanced over the prior art, since the corrected, return beam is amplified, and automatically directed back to the target as a diffraction-limited beam with little or no “spillover”; losses incurred in the large-area beacon illuminator are minimal, since this is in the low-energy leg of the sequence of two “passes”.

In one embodiment of the present invention, the conjugate beam is also temporally modulated with, as an example, amplitude information (e.g., pulse-position encoding) as to “spoof” (or, “jam”) the incoming object (e.g., a missile), thereby confusing its navigational homing system. Moreover, by modulating the phase-conjugator mirror42directly, a compact system can be realized.

In traditional IRCM systems, the broad-angle high-power laser source (or laser amplifier) is modulated, typically, at kHz rates, to achieve the desired countermeasure. In this invention, while it is still possible to modulate the laser amplifier, it is also possible to modulate the conjugate beam instead, which is in a low-power (optical) leg of the system, and, in many cases, is easier to implement. This is effected by the modulator46. Thus, the system has more flexibility to accommodate different modulation formats or rates, as well as different types of lasers and amplifiers, that may not easily adapt well to the required direct modulation formats. Moreover, since the readout laser can be chosen to possess a single transverse (or spatial) mode, a variety of compact, low-cost modulators can be used for the modulator46.

In another embodiment of the present invention, no temporal encoding of the conjugate beam is required. In this case, the diffraction-limited and amplified return beam propagates back to the target with sufficient flux to blind or otherwise damage critical components.

The front-end beam steering arrangement (which may also be implemented with a MEMS deflector or an optical phased array) provides “coarse” angle tracking of the target and therefore bore-sighting of the beam into the conjugator's field-of-view (FOV).

Given the conjugation property of the system, the effective angular spread (or focus) of the return beam will dynamically adapt to the field-of-view of the target as it approaches the IRCM system, thereby optimally directing (i.e. “auto-targeting”) the counter-measure beam to the target during flight.

The optional optical amplifier30provides greater return flux to the target. Finally, the optical detector and modulation signal processor48provides information as to the effectiveness of the IRCM system, as well as potential updating of the modulation format, as needed, for optimal IRCM system performance.

The invention provides for precision “auto-targeting” of the threat vehicle, as well as modulation of the laser beam onto the moving platform, which is actively illuminated by a broad-angle beacon beam. The broad-angle beam illuminates the general region where a target is assumed to exist. Since the modulation is in a low-power leg of the system, and not constrained to be in the high-power leg (such as in the broad-area illuminator, laser amplifier, etc.), the system is more flexible, in that other candidate optical amplifiers, lasers, and modulators can he employed, relative to the prior art. (In this connection, the ‘low-power’ leg can be the readout beam from the laser44along the beam path61.) Moreover, given the relative low-power of the readout laser, low-cost, high precision, compact conjugation devices (42) can be employed, such as MEM phase-shifting arrays, with minimal thermal loading.

FIG. 3is an optical schematic and electrical block diagram of an alternative embodiment of the system for focusing electromagnetic energy of the present invention. The system50is adapted to use starlight to predistort an output beam thereby reducing the time required for beam optimization and the susceptibility for detection. In the embodiment ofFIG. 3, light52,54and56from first, second and third stars58,60and62, respectively, is received by the first and second lenses24and26of the telescope22and amplified by an optional amplifier30before illuminating a deformable mirror42. In the receive mode, the mirror42is adapted to accurately reflect the received beam to the beamsplitters36and38. The first beamsplitter36directs a portion of the received wavefront to a wavefront error sensor60adapted to receive the starlight data and detect aberrations therein. The wavefront error sensor60then provides control signals71to the deformable mirror42effective to focus a readout beam from a laser source44onto a target18. The image processor60effectively provides phase correction data (signal71) to the deformable mirror42so that diffraction-limited compensated imaging of the target18can be realized. The image processor60also provides for target recognition and classification and subsequently enables the readout laser beam44to be directed to the target with diffraction limited performance in conjunction with the dual-axis mirror driven by signal72from the processor60and the optical relay system70, that images the readout optical beam from the dual-axis mirror49onto the surface of the deformable mirror42. Coarse targeting data may be supplied by an external system if needed. A modulator46acting under control of an IRCM system48may be used to provide for modulation of the output beam as per the embodiment ofFIG. 2. Adual axis mirror49operates under control of the wavefront sensor60to locate the output beam on the target. The wavefront error sensor60may be implemented in accordance with techniques known in the art.

Those skilled in the art will appreciate that in accordance with the present teachings, inasmuch as, no active beacon illumination is employed, adaptive optical correction and imaging are performed in a passive covert manner. Hence, the embodiment ofFIG. 3offers the additional advantage of providing a countermeasure before the system50is discovered by a threat, thereby further improving the efficacy and survivability of the system.

FIG. 4is an optical schematic and electrical block diagram of a second alternative embodiment of the system for focusing electromagnetic energy of the present invention. In the embodiment ofFIG. 4, a nonlinear optical phase conjugate mirror150is used in place of elements40and42in FIG.2. Hence, a beacon laser112outputs a pilot beam114in the direction of a target18. The pilot beam is reflected as a glint by the target18which spreads as a return wavefront20as per the case in FIG.2. The distorted wavefront is then directed to a laser amplifier130by a mirror128. A portion of the received wavefront is directed to a tracker134by a second mirror132. The tracker134may be implemented in the same manner as the tracker18′ ofFIG. 1, or28ofFIG. 2. Asecond beamsplitter136directs a portion of the received beam to a closed-loop IRCM controller148. As discussed above, the closed-loop IRCM controller148detects the incoming modulation scheme from the target, if any, and/or is fed target data from an external tracking system and/or database (not shown). The controller148can also provide control signals for modulating the phase-conjugate return beam via an external modulator152.

Most of the received beam from the target passes through the beamsplitter136and is incident upon a phase-conjugate mirror (PCM)150. The PCM150generates a wavefront-reversed replica of the incident beam, using all-optical processing in a nonlinear optical element, without the need for a wavefront error sensor processor or a deformable mirror (elements40and42, respectively, of FIG.2). The PCM150can either be a passive device (e.g., a nonlinear element) or a combination of a passive device (e.g., a nonlinear optical element) in conjunction with a readout laser.

One class of a nonlinear optical processor that comprises an externally pumped PCM is an optical four-wave mixer element (e.g., a semiconductor laser diode medium, a doped solid-state crystal or glass in a bulk or waveguide configuration, a nonlinear optical material with a thermal nonlinearity, an atomic vapor, etc.). Other examples of PCMs include double-pumped PCMs and self-pumped PCMs (e.g., photorefractive crystals, SBS cells using long multi-mode optical fibers or liquid-filled cells).

The required IRCM modulation signal is determined by the processor148, and can be imposed directly to the conjugate mirror,150by the controller148or can be imposed onto the external modulator152through which the phase-conjugate beam traverses on its way back through the system and, ultimately, back to the target.

In the former case, modulation of the phase-conjugate beam can be imposed onto the readout laser beam (not shown, but can be incorporated into element150) in the case of four-wave mixing, double-pumped PCMs or a modulated seed beam in the case of SBS.

In the case of certain self-pumped PCMs (which do not require separate readout lasers within element150), such as photorefractive crystals, the modulation can be imposed globally onto the crystal via electro-optic methods, by applying the modulation signal149, directly onto the PCM crystal itself. (See U.S. Pat. No. 4,767,195 issued Aug. 30, 1988 to D. M. Pepper and entitled SYSTEM AND METHOD FOR ENCODING INFORMATION ONTO AN OPTICAL BEAM, the teachings of which are incorporated herein by reference.)