Methods and apparatus for acousto-optic non-uniformity correction and counter-countermeasure mechanisms

Examples provide a compact, dynamic non-uniformity correction mechanism and counter-countermeasure mechanism. In one example an optical imaging system includes an imaging sensor configured to receive optical radiation and to produce an image of a viewed scene from the optical radiation, an optical train including at least one optical component configured to receive the optical radiation from the viewed scene and to focus the optical radiation to the imaging sensor, and an acousto-optic modulator positioned in the optical train and having an ON state and an OFF state, the acousto-optic modulator being configured in the OFF state to pass the optical radiation, and the acousto-optic modulator being configured in the ON state to diffract the optical radiation and blur the image produced by the imaging sensor from the diffracted optical radiation.

BACKGROUND

A wide variety of optical systems include an imaging sensor that typically includes an array of photo-sensitive detectors, often termed a focal plane array (FPA). Each detector produces an output that corresponds to a pixel in the image produced by the imaging sensor. Each detector generally includes a photo-diode and electronic components for measuring the intensity of received light. An array may include many hundreds or thousands of detectors. Each detector in the array may have a slightly different sensitivity or response to same received light (i.e., light with the same wavelength, intensity, etc.). This non-uniform sensitivity yields Fixed Pattern Noise (FPN) in the images produced by the detector array. The fixed pattern noise causes some pixels in the image to be too bright, while others are too dark. This negatively impacts the Signal to Noise Ratio (SNR) of the imaging sensor, and the guidance system of a seeker when the imaging sensor is used to guide a projectile to its intended target. Therefore, to be able to produce accurate, high resolution images and accurately track a target in a seeker, it can be important to compensate for the non-uniformity in the detector outputs across the array.

A Non-Uniformity Compensation (NUC) system adjusts each pixel gain and offset to compensate for the FPN by applying a unique correction to each pixel. Typically a Two Point Non-Uniformity Compensation (Two Point NUC) is done on the ground prior to flight to make the corrections at two specific scene temperatures. However, if during flight the sensor is exposed to background scenes that are far from the corrected temperatures, the system noise will rise in proportion to the background noise. Therefore, in situ, real time, scene based NUC correction during flight may be necessary. An Adaptive Non-Uniformity Compensation (ADNUC) system adjusts each pixel dynamically to compensate for the differing sensitivity of each detector in the FPA.

Non-uniformity correction can be done by masking the array during a calibration procedure, such that every detector has the same, known input, and measuring the output from each detector. Differences in the outputs can be used to provide gain and offset calibration coefficients that are applied, for example, by altering the bias voltages at each detector, or during image processing, such that for the same known input to every detector, every pixel in the image has the same color and intensity. Conventionally, masking the array is done by placing a light-blocking shield over the array to prevent the detectors from receiving light, and the outputs are measured in this “dark state.”

Non-uniformity correction is often needed in optical systems that operate in the visible spectrum (e.g., imaging systems that produce color images of a viewed scene) or the infrared spectrum (e.g., seekers or other thermal imaging systems). In seekers, non-uniformity correction is typically done using an opaque (light-blocking) paddle that is driven by a mechanical servo to move the paddle into and out of the optical path or field-of-view of the imaging sensor. However, the need to move the paddle into and out of the field-of-view of the sensor requires bulky moving parts, and this type of mechanism can be difficult to incorporate into systems with constrained packaging requirements.

SUMMARY OF INVENTION

Aspects and embodiments are directed to a compact, dynamic non-uniformity correction mechanism and counter-countermeasure mechanism.

According to one embodiment, an optical imaging system comprises an imaging sensor configured to receive optical radiation and to produce an image of a viewed scene from the optical radiation, an optical train including at least one optical component configured to receive the optical radiation from the viewed scene and to focus the optical radiation to the imaging sensor, and an acousto-optic modulator positioned in the optical train and having an ON state and an OFF state, the acousto-optic modulator being configured in the OFF state to pass the optical radiation, and the acousto-optic modulator being configured in the ON state to diffract the optical radiation and blur the image produced by the imaging sensor from the diffracted optical radiation.

In one example the acousto-optic modulator includes an acousto-optic material configured to support an acoustic wave, a piezo-electric transducer coupled to the acousto-optic material and configured to generate the acoustic wave in the acousto-optic material in response to an RF signal applied to the piezo-electric transducer, and an acoustic absorber coupled to the acousto-optic material. The acousto-optic material may be sandwiched between the piezo-electric transducer and the acoustic absorber. In one example the acousto-optic material is Germanium. In other examples other acousto-optic materials can be used. For example, the acousto-optic material may be any one of Lithium Niobate, Gallium Phosphide, a chalcogenide glass, fused Silica, quartz, and Tellurium Oxide. In one example the optical train includes a window, and the acousto-optic modulator is integrated with the window. The optical radiation can be infrared radiation, for example. In one example the optical imaging system is a seeker.

The optical imaging system may further comprise a controller coupled to the acousto-optic modulator, the controller being configured to dynamically switch the acousto-optic modulator between the ON state and the OFF state. In one example the optical imaging system further comprises a photosensor coupled to the controller, the photosensor configured to receive a laser beam from the viewed scene and to produce a signal in response to receiving the laser beam, and the controller being further configured to receive the signal from the photosensor and to switch the acousto-optic modulator into the ON state in response to receiving the signal from the photosensor. In one example the controller is configured to produce non-uniformity calibration coefficients based on outputs from the imaging sensor when the acousto-optic modulator is in the OFF state, and to adjust the image produced by the imaging sensor from the optical radiation when the acousto-optic modulator is in the OFF state to remove fixed pattern noise from the image.

According to another embodiment an infrared seeker system comprises an imaging sensor sensitive to infrared radiation and configured to receive the infrared radiation from a viewed scene and to produce an image from the infrared radiation, an optical train including at least one optical component configured to receive the infrared radiation from the viewed scene and to focus the infrared radiation to the imaging sensor, and an acousto-optic modulator positioned in the optical train and having an ON state and an OFF state, the acousto-optic modulator being transparent to the infrared radiation in the OFF state, and the acousto-optic modulator being configured in the ON state to diffract the infrared radiation and blur the image produced by the imaging sensor from the diffracted infrared radiation.

In one example the acousto-optic modulator includes an acousto-optic material configured to support an acoustic wave, a piezo-electric transducer coupled to the acousto-optic material and configured to generate the acoustic wave in the acousto-optic material in response to an RF signal applied to the piezo-electric transducer, and an acoustic absorber coupled to the acousto-optic material. In one example the acousto-optic material is Germanium. In other examples other acousto-optic materials can be used. In one example the at least one optical component includes at least one lens, and the optical train further includes a filter and a window, the filter and the at least one lens being positioned between the window and the imaging sensor, and wherein the acousto-optic modulator is integrated with the window. The filter may be configured to pass the infrared radiation in a spectral band of interest including the mid-wave infrared spectral band and the long-wave infrared spectral band and to block optical radiation outside the spectral band of interest.

In one example the infrared seeker system further comprises a controller coupled to the acousto-optic modulator, the controller being configured to dynamically switch the acousto-optic modulator between the ON state and the OFF state. The infrared seeker system may further comprise a photosensor coupled to the controller, the photosensor configured to receive a direct or indirect laser beam from the viewed scene and to produce a signal in response to receiving the laser beam, the controller being further configured to receive the signal from the photosensor and to switch the acousto-optic modulator into the ON state in response to receiving the signal from the photosensor. In one example the controller is configured to produce non-uniformity calibration coefficients based on outputs from the imaging sensor when the acousto-optic modulator is in the OFF state, and to adjust the image produced by the imaging sensor from the infrared radiation when the acousto-optic modulator is in the OFF state to remove fixed pattern noise from the image.

Another embodiment is directed to an optical imaging system with counter-countermeasure capability. The optical imaging system may comprise an imaging sensor configured to receive optical radiation and to produce an image of a viewed scene from the optical radiation, an optical train including at least one optical component configured to receive the optical radiation from the viewed scene and to focus the optical radiation to the imaging sensor, an acousto-optic modulator positioned in the optical train and having an ON state and an OFF state, the acousto-optic modulator being configured in the OFF state to pass the optical radiation, and the acousto-optic modulator being configured in the ON state to diffract the optical radiation and blur the image produced by the imaging sensor from the diffracted optical radiation, a controller coupled to the acousto-optic modulator, the controller being configured to dynamically switch the acousto-optic modulator between the ON state and the OFF state, and a photosensor coupled to the controller, the photosensor configured to receive a laser beam from the viewed scene and to produce a signal in response to receiving the laser beam, the controller being further configured to receive the signal from the photosensor and to switch the acousto-optic modulator into the ON state in response to receiving the signal from the photosensor.

DETAILED DESCRIPTION

As discussed above, typical non-uniformity correction (NUC) in seekers is done with a mechanical servo-driven paddle that is moved into and out of the field-of-view of the sensor. This approach can be difficult to implement due to space constraints and adds significant complexity to the system. Aspects and embodiments provide purely electronically driven solid state NUC method that may be simpler, less expensive, more robust, and allow implementation on a wider variety of systems with space constraints than conventional NUC methods. In addition, aspects and embodiments provide a counter-countermeasure (CCM) mechanism for use in seekers or other optical systems.

FIG. 1illustrates an example of an optical system, such as a seeker, for example, in which the NUC or CCM mechanisms and methods of various embodiments may be implemented. The optical system100includes a plurality of optical elements that direct and focus incident optical radiation110onto an imaging sensor120. As used herein, the term optical radiation refers to non-ionizing electromagnetic radiation in the one or more optical regions of the electromagnetic spectrum, including the visible, infrared, or ultraviolet spectral bands, that can be manipulated (e.g., reflected, refracted, focused, etc.) by conventional optical elements, such as mirrors or lenses. The imaging sensor120can include any type of optical detector that receives the incident optical radiation110and produces an electrical output signal indicative of at least one property of the received optical radiation (such as, but not limited to, intensity or color). In one example, the imaging sensor120is a multi-pixel focal plane array sensor. In the example illustrated inFIG. 1, the optical elements that direct and focus the optical radiation110onto the detector120include three lenses,132,134, and136; however, in other examples any combination of lenses, mirrors, or both can be used. The optical system100may also include a filter140configured to spectrally filter the incident optical radiation110. In one example the filter140can be configured to limit the spectral range of the optical radiation reaching the imaging sensor120to one or more spectral bands of interest.

In certain applications, such as seekers, where the imaging sensor120is operating in the thermal infrared spectral region, the sensor and at least some of the optical elements can be housed within a cooled chamber (not shown), such as a Dewar, for example, to reduce thermal noise. This cooled chamber includes a window that is optically transparent to infrared optical radiation to allow the incident optical radiation to enter the cooled chamber and reach the imaging sensor120. In other applications, even of cooling (also referred to as “cold shielding”) is not required, the detector and optionally other components can be housed within a protective chamber that also includes an optically transparent window. Accordingly, as shown inFIG. 1, in certain embodiments the optical system100includes a window150that is optically transparent in at least one spectral band of interest.

Certain seekers used in tactical missiles, for example, are packaged within the nose cone of the missile. Accordingly, in certain examples, the optical system100can include a dome160positioned in front of (i.e., towards the viewed scene) of the window150. In other applications, the dome160may represent any outer packaging of the optical system100, not limited to a component of a missile or other flight projectile.

The optical radiation110from a viewed scene passes through the dome160, through the window150, is filtered by the filter140, and directed and focused by the lenses132,134,136onto the imaging sensor120. As discussed above, the optical radiation110may include infrared wavebands or visible light. According to one embodiment, the imaging sensor120includes an array of n×m infrared detectors, n and m being integer numbers. Each detector has slightly different sensitivity to infrared radiation. As discussed above, this non-uniform sensitivity causes sensor fixed pattern noise, which is manifested in the image by some pixels being too dark and some being too bright.

According to one embodiment, an acousto-optic modulator is used to provide non-uniformity correction in the optical system100. An example of an acousto-optic modulator200is illustrated inFIG. 2. As shown inFIG. 3, the acousto-optic modulator200can be positioned in the optical system100between the dome160and the imaging sensor120, and receives the optical radiation110. In one example the acousto-optic modulator200can be implemented in the window150, as discussed further below; however, in other examples the acousto-optic modulator can be positioned anywhere along the optical train300on the scene or object-space side of the imaging sensor120to receive the incident optical radiation110before it reaches the imaging sensor120. The acousto-optic modulator200has an ON state in which it acts on the optical radiation110, as discussed below, and an OFF state in which it is substantially transparent to the optical radiation, allowing the optical radiation to pass through and reach the imaging sensor120.

Referring toFIG. 2, the acousto-optic modulator200includes an acousto-optic material210, an acoustic absorber220, and a piezoelectric transducer230. In the illustrated example, the acousto-optic material210is positioned between the acoustic absorber220and the piezoelectric transducer230. The piezo-electric transducer can be bonded to the acousto-optic material210. When an RF signal is applied to the piezo-electric transducer230, a sinusoidal diffraction grating is generated from the acoustic wave in the acousto-optic material210, and each wavelength of the incident light110spreads out over multiple diffraction orders, as shown inFIG. 2. By varying the acoustic frequency applied to the acousto-optic material210via the piezo-electric transducer230, a range of varying grating periods can be generated. Thus diffracted light from a single grating produced with a unique RF frequency for a wideband radiation smears or blurs the image. Alternatively, the diffracted light can be swept across the detectors of the imaging sensor120by varying the RF frequency, resulting in a blurred image. This blurred image can provide a neutral background that can be used to calibrate the imaging sensor120and provide non-uniformity correction. For example, when an RF signal is applied to the acousto-optic modulator200to turn the acousto-optic modulator ON and provide the blurred image, the output of each detector (intensity or irradiance measurement) ideally should be the same. However, as discussed above, in reality there will be differences in the outputs from each detector. These differences can be recorded by a controller and processing electronics310coupled to the imaging sensor120and used to provide non-uniformity calibration coefficients to adjust the real images produced by the imaging sensor120when the acousto-optic modulator200is OFF to remove the fixed pattern noise and compensate for the differing sensitivity of each detector in the array. Scene-based non-uniformity correction compensates for non-uniformities caused by changes in the background of the imaging sensor120during flight of a seeker, for example.

The acousto-optic material210can be any of a variety of materials capable of responding to the piezo-electric transducer230to support an acoustic wave and also having suitable optical properties for use in the optical system100. For example, when the acousto-optic modulator200is OFF, the acousto-optic material210should be highly transparent to the optical radiation110so as not to degrade the imaging performance of the optical system100. Examples of acousto-optic materials210that can be used in embodiments of the acousto-optic modulator210include Germanium (for mid-wave and long-wave infrared applications), Lithium Niobate (for mid-wave infrared applications), Gallium Phosphide (for mid-wave and long-wave infrared applications), a chalcogenide glass, such as AMTIR (amorphous material transmitting infrared radiation; for mid-wave infrared applications), fused Silica (for mid-wave infrared applications), quartz (for mid-wave infrared applications), or Tellurium Oxide (for mid-wave infrared applications).

In certain embodiments and applications, particularly in infrared seekers, the window150is made of Germanium, and therefore, the window150can be adapted to also function as the acousto-optic modulator. Germanium is a suitable acousto-optic (AO) material210with a relatively high figure of merit, and can be used in both mid-wave and long-wave infrared applications. Thus, advantageously, optical systems using a Germanium window can be easily configured to include the acousto-optic modulator200without requiring significant modifications to the optical train300or arrangement or packaging of the optical components.

The controller310can be coupled to the acousto-optic modulator200and configured to control application of the RF signal (e.g., when the RF signal is applied and the frequency of the applied RF signal) to the piezo-electric transducer230to produce the blurred image, and thus allow dynamic calibration of the imaging sensor120, “on demand”. The acousto-optic modulator200can be configured to operate at a repetition rate (i.e., rate at which the acousto-optic modulator can be turned ON and OFF) on the order of kilohertz (kHz), which is sufficiently fast to perform dynamic non-uniformity correction during operation of the optical system100in many applications (e.g., during flight of a seeker). The efficiency of the acousto-optic modulator200is proportional to the acoustic power, the figure of merit of the acousto-optic material210, and the geometry of the acousto-optic modulator (e.g., size/shape/arrangement of electrodes of the piezo-electric transducer230), and is inversely proportional to wavelength. The controller can dynamically turn the acousto-optic modulator200ON and OFF, periodically, at random, or at the command of a user or other system/component.

FIGS. 4A-4Care graphs showing sample theoretical predictions of a single wavelength irradiance (light intensity over area) at the imaging sensor120for different conditions of the acousto-optic modulator200. The results illustrated inFIGS. 4A-Ccorrespond to an example of the optical system100shown inFIG. 1with an example of the acousto-optic modulator200used as the window150, the acousto-optic material210being Germanium.FIG. 4Ashows the irradiance measurement when the acousto-optic modulator200was OFF. In this case, the incident optical radiation110is focused on the imaging sensor120producing a single peak410.FIG. 4Bshows the irradiance measurement with the acousto-optic modulator200ON and operated at an acoustic frequency (i.e. frequency of the acoustic wave generated in the acousto-optical material210by the piezo-electric transducer230) of 1.5 MHz. In this case, the optical radiation110is diffracted by the acousto-optic modulator200, producing multiple peaks420spread over the array of detectors of the imaging sensor120. In this example, the distance between a main peak420aand a first “sidelobe” or “harmonic” peak420bcorresponds to two detectors in the array, or two pixels in the image. As discussed above, this diffraction and spreading of a wideband of wavelengths over the array of detectors causes the image produced to be blurry, such that it can be used as a neutral background for non-uniformity correction.FIG. 4Cshows the irradiance measurement for another example in which the acousto-optic modulator200was ON and operated at an acoustic frequency of 2.5 MHz. In this instance, the diffraction of the optical radiation110again causes blurring of the image produced at the imaging sensor120, and the distance between the main peak430aand a first “sidelobe” or “harmonic” peak430bcorresponds to four detectors in the array, or four pixels in the image. Rapidly sweeping the diffracted orders across the imaging sensor120by varying the RF frequency is another way to blur the image, as discussed above.

Thus, aspects and embodiments provide a dynamic non-uniformity correction mechanism that can be easily incorporated into optical imaging systems, including infrared seekers, without requiring significant alterations to the optical configuration of the system. The acousto-optic modulator can be controlled such that the output of the imaging sensor alternates between focused and blurred images, with the blurred images being used as part of the dynamic non-uniformity correction process to help discriminate scene and target from fixed pattern noise. The acousto-optic modulator according to various embodiments can be incorporated into an existing optical component, such as the window as discussed above, and may be significantly easier to include in optical systems with tight packaging constraints and more reliable/robust than conventional mechanical non-uniformity correction mechanisms.

According to another embodiment, the acousto-optic modulator200can also (or alternatively) be configured to operate as a counter-countermeasure mechanism.FIG. 5is a block diagram of another example of the optical system100including an embodiment of the acousto-optic modulator200and components configured to provide counter-countermeasure functionality. In this example, the optical system100further includes a photosensor320configured to detect a countermeasure beam330. The photosensor320may be a photodiode, for example. The countermeasure beam330may be a laser beam designed to damage the imaging sensor120or otherwise prevent the imaging sensor120from obtaining useful images of a target or scene. Upon detection of the countermeasure beam330, the photosensor320may provide a signal to the controller310to direct the controller to activate the acousto-optic modulator200. Activation of the acousto-optic modulator200blurs the image formed at the imaging sensor120, as discussed above, preventing focusing of incident radiation on the imaging sensor. Thus, the countermeasure beam330can be dissipated or prevented from being focused onto the imaging sensor120by the blurring/diffraction action of the acousto-optic modulator200, thereby protecting the imaging sensor.

In embodiments in which the acousto-optic modulator200is configured to provide a counter-countermeasure mechanism, the acousto-optic modulator200can also function to provide non-uniformity correction as discussed above, or may be a dedicated counter-countermeasure device. The acousto-optic modulator200can be incorporated into the window150or another optical component of the optical imaging system100, or may be a separate component included in the optical train300. In the block diagram illustrated inFIG. 5, the photosensor320is shown offset from the optical train300; however, this is for ease of illustration only. The photosensor320may be incorporated anywhere within the optical train300, or with the dome160, and may be on-axis or off-axis with respect to the optical axis of the optical imaging system100to collect scattered laser beam.