1. Field of the Invention
The present invention relates to adaptive optics, and more specifically, it relates to a reference-free compensated imaging system for recovering a high resolution image of a target.
2. Description of Related Art
The basic elements of a typical (prior art) adaptive optics system 100 are shown in FIG. 1. The goal of such systems is to provide real-time compensation for propagation errors, as encountered by optical beams, as they travel through dynamic distorting paths, including turbulent atmospheres, optical pointing errors, imperfect optical elements, multi-mode optical fibers, etc.
By compensating for optical wavefront distortions, one can enhance the performance of a variety of optical systems. Examples include optical communication systems, remote sensors, precision laser-beam delivery systems for industrial and medical purposes, and compensated imaging systems such as in medical applications (ophthalmological imaging and precision surgical procedures through the eye) and microscopy. In the latter example, this implies that one can view complex objects over a distorted path with the same image quality as if the path were distortion-free. In this case, the performance of the imaging system can approach that of its theoretical diffraction limit, within the so-called isoplanatic cone.
In what follows, we first discuss a generic adaptive optical system capable of correcting for path distortions encountered by a so-called reference beam. The reference beam is typically an image-free optical source, whose function is to sample the path distortions and, thus, provide this wavefront information as input to the adaptive optical system. This discussion is followed by a description of a specific adaptive optical configuration typical of prior-art, including an example of a wavefront-error sensing device. This, in turn, is followed by a discussion of an optical compensated imaging system typical of the art. An understanding of these prior-art systems will provide perspective with regard to the exemplary embodiments of this invention that follow.
As we discuss below, compensation of wavefront phase errors enables a system to provide diffraction-limited imaging and viewing of an extended object. In general, one first samples and compensates for propagation-path errors using, a diffraction-limited reference beam. Upon compensation of wavefront errors encountered by the reference beam, the optical system can approach its theoretical diffraction-limited imaging capability of image-bearing beams that lie within the so-called isoplanatic patch, which is well known in the art.
It is to be appreciated that the compensated optical imaging system can be implemented to service a variety of imaging based applications beyond atmospheric viewing systems. Hence, when the basic imaging system is referred to as a telescope, it is to be understood that the present teachings and embodiments can also be applied, without loss of generality, to compensated microscopy systems, speckle imaging, ophthalmological systems, communications systems, and the distortion path is referred to as a dynamic atmosphere, ultrasound imaging systems and so on. Similarly, when the distortion path that imposed the wavefront distortions to be compensated is referred to as a dynamic atmosphere, it is to be understood that the teachings can also be applied, without loss of generality, to correct for propagation-path distortions such as those experienced by imperfect optical elements, and static and/or dynamic distortions due to propagation through ocular systems, skin tissue, clouds, turbid liquids, and so on. The scene-based (Shack-Hartman) wave-front sensor could also be used in a post-processing scheme such as deconvolution or to augment speckle imaging.
Turning now to FIG. 1, the goal of the prior art system is to enable one to view an optical source 110 with diffraction-limited capability. In this case, the optical source is chosen to be of spatial extent less than, or equal to, the diffraction limit of the optical system. Therefore, this source is equivalent to a point object with zero image-bearing information, analogous to a single pixel in an image. Light that emerges from this object, which is referred heretofore as a “reference beam,” 120, propagates through space, and, in general, becomes aberrated, as depicted by wavefront 120, as a result of the intervening path distortions or spatial phase errors, labeled by φ. In essence, the reference beam 120 samples the propagation path distortions between it and the optical compensation system, 100, including distortions imposed by optical elements within the compensation system itself.
At the receiver end of the link, a fraction of reference beam 120 is collected by telescope 130, which represents the input optical imaging elements of the adaptive optical receiver system 100. The collected light forms an image at the camera, or detector array, 190. In the absence of path distortions, the image at the camera plane would be in the form of an Airy disc, since the reference beam 120 is a sub-diffraction-limited point-source. However, owing to optical propagation phase distortions, φ, encountered by the reference beam on its path toward the receiver 110, the wavefronts of this beam will be aberrated, resulting in a distorted image of an Airy disc pattern at camera 190. As is known in the art, the path distortions in this scenario can stem from atmospheric turbulence, pointing and tracking errors, imperfect optical elements, thermal and mechanical perturbations, among other effects. The goal, therefore, of the adaptive optical system 100 is to compensate for such path errors so that the image quality of the reference beam at detector 190 can approach the diffraction limit.
Returning to FIG. 1, the reference beam exiting the telescope 130 will be aberrated by virtue of the deleterious path distortions, as represented by wavefront 140. In this example, the adaptive optical system consists of two optical correction elements. The first corrective element 150 is a so-called tip-tilt compensator, whose function is to compensate for overall beam pointing and tracking errors. The second corrective element 160 is a spatial phase modulator, whose function is to compensate for fine-scale optical phase errors, including focus errors and spatially complex wavefront errors. The latter can include static and/or dynamic errors resulting from atmospheric turbulence and surface and volume refractive-index irregularities of optical elements, as discussed above. Wavefront compensation element 160 can be in the form of arrays of continuous and/or discrete optical phase shifters, such as piezoelectric transducers, electro-optic elements, deformable membranes, MEMS mirrors, liquid crystal cells, photonic crystals, among other devices, as is known in the art.
The incident distorted beam 140, first encounters the tip-tilt optical component 150 followed by the spatial phase modulator 160. The beam subsequently strikes a beam splitter 165, with one output beam directed to an optical wavefront error sensor 170, and with the other output beam directed to the camera detector 190.
The telescope provides an image of the incident beam at the camera plane 190, and, furthermore, provides an image of the pupil plane at the surface of the wavefront corrective element 160. Hence, the wavefront at the incident aperture is replicated, and scaled, as needed, at the plane of 160. The number of phase-controllable elements across the aperture of 160 is determined, in part, by the so-called transverse coherence parameter, otherwise known as the Fried parameter, which is characteristic of the scale size of the turbulent atmosphere.
The spatial bandwidth of the phase modulator 160 is designed to accommodate the spatial bandwidth indicative of the wavefront distortions, 120, subject to Nyquist constraints, as is known in the art. The sampling of the wavefront sensor 170 is also designed to accommodate the wavefront distortions 120 subject to Nyquist comstraints. In the image compensation systems (to be discussed with respect to FIG. 2 below), the spatial bandwidth requirements for the corrective plate are the same, in terms of resolving the wavefront error distortions sampled by the reference beam. The imaging resolution, on the other hand, is dictated by the diffraction limit of the overall optical system. In most cases, the Fried parameter scale size of the turbulence is far greater than that of the pixel size required to faithfully image the object.
Each of the compensation elements 150 and 160 is controlled and configured in real-time using various classes of optical detectors, algorithms and electronic networks, examples of which are feedback, feed-forward and multi-dither systems, as is known in the art. One example of an optical feedback control loop is depicted in FIG. 1. It consists of a wavefront error sensor 170, a processor module 177, and a pair of electronic drivers 180 and 185 that provide control signals to tip-tilt compensator 150 and the spatial phase modulator 160, respectively. Ideally, the driver 185 will generate a spatial phase map indicative of a wavefront-reversed replica, whose phase is given by −φ. The resultant beam will therefore possess a wavefront that is a combination of the incident phase distortion, φ, with the correction phasemap, −φ, resulting in a wavefront with a net phase given as φ+(−φ)=0, indicative of an aberration-free reference beam.
The optical feedback control system is designed to correct the error 140 such that the wavefront at 190 is unaberrated. This is done by driving the wavefront error seen by 170 to a minimum relative to a known reference signal that encompasses the non-common-path errors of the optical system. Upon convergence of the servo control configuration, the resultant reference beam that strikes the camera/detector 190 will be, ideally, free of wavefront errors. In this state, the overall optical receiver system 100 will provide an image of the reference beam source 110, to its diffraction limit. Given that this system functions in real-time, dynamic path distortions can be tracked and compensated, with a residual error determined by the servo-loop gain and its bandwidth.
Turning now to FIG. 2A, a compensated image adaptive optical system 200 is shown, typical of the prior art. The goal of this system is to enable precision imaging of an extended object 205 in the presence of dynamic path distortions 220, with the resultant image viewed by camera 290. The basic adaptive optical aspect of the system functions in a manner similar to that of FIG. 1. However, in the system depicted in FIG. 2A, there are now two different input beams incident upon a telescope 230. One of the two input beams is designated as a reference beam 110, and provides the same function as that of beam 110 of FIG. 1. That is, it is in the form of a sub-diffraction-limited optical source that samples the path distortions 220. The other incident light is an image-bearing beam of object 205 whose spatial information is also distorted by the path distortions 220, and whose high-fidelity compensated image is sought.
The reference and image-bearing beams both traverse the same input optical components and propagation path, including the telescope 230, intermediate focal plane 235, a collimation component, represented by lens 245, tip-tilt compensator 150, spatial phase modulator 160, imaging optics 247. The reference beam 110 and the image-bearing beam 205 both impinge upon beam splitter 265. The beam splitter directs each respective input beam into a different direction. The incident reference beam 110 emerges from one port of the beam splitter as beam 266 and propagates along one direction; and, the incident image-bearing beam 205 emerges from the other port of the beam splitter as beam 267 and propagates along a second direction. The reference beam 266 is directed to the adaptive optical control loop, and the image-bearing beam 267 is directed to a camera/detector module 290. Beam splitter 265 partitions the reference and image beams using a variety of discrimination techniques including polarization, wavelength, spatial frequency, temporal gating, as is known in the art.
In the compensated imaging system 200, the reference beam 266 exiting beam splitter 265 is directed to an adaptive optical processor in a manner analogous to that described with respect to FIG. 1. However, as opposed to FIG. 1, in the compensated imaging system of FIG. 2A, light from the incident reference beam 110 does not strike the camera 290. The sole purpose of the reference beam in this case is to provide path-distortion information to the wavefront error sensor 270 in the servo-loop so that, upon correction of the distortions imposed posed onto the reference beam, the image-bearing beam can be viewed with little or no distortion. The feedback loop, operationally, is similar to that of FIG. 1, namely, the raw wavefront-error information is inputted into processor 175 (see 277 in FIG. 2A), which, in turn provides error correcting information to drivers 180 and 185, the outputs of which provide signals to the tip-tilt compensator and the spatial phase modulator, 150 and 160, respectively.
The reference beam 266 emerging from beam splitter 265 passes through an intermediate image plane 255, followed by lens 249, which transforms the beam to a pupil plane. The beam is then scaled by the telescope (lenses 247 and 249) to satisfy the spatial bandwidth constraints of the wavefront-error sensor (WFS) 270. In this system, the WFS is a so-called Shack-Hartmann class of configuration. As is known in the art the Shack-Hartmann WFS consists of a lenslet array 271 and a detector array 273, the latter positioned at the focal plane of the lenslets. This pair of elements provides a spatial mapping of the local tilt phase errors across the overall pupil-plane aperture, that characterize the path-distorted incident reference wavefront 110. As known in the art, the required number of lenslets is a function of the square of the ratio of the input aperture size to that of the coherence (Fried) parameter indicative of the incident wavefront distortions. Under these constraints, it is assumed that the incident wavefront can be described as a series of plane-wave segments, each with a different tilt, or phase slope, and all concatenated together. Hence, each plane-wave segment is considered as a diffraction-limited beamlet, each with a different tilt angle.
FIGS. 2B and 2C, respectively, illustrate the basic prior-art principles of the Shack-Hartmann WFS, as applied to an aberrated wavefront 220, and a distortion-free wavefront 221. The WFS, identical in both FIGS. 2B and 2C, consists of a lenslet array 271 and a multi-pixel detector array 273, the latter positioned at the focal plane of the lenslets. FIG. 2B depicts the operation of the WFS assuming an input reference beam whose wavefront is aberrated. Each plane-wave segment of the input beam 222 is incident upon a different lenslet in the array 271. In the presence of no wavefront phase errors beyond the wavefront sensor's Nysquist limit, a nearly diffraction-limited sinc-squared pattern will appear at each respective focal plane. However, since each plane-wave segment is comprised of a tilted wavefront, the sinc-squared pattern at each respective focal plane at the detector array 273 win be spatially shifted, with the lateral shift increasing with the slope of the local tilt. In most systems, especially atmospheric compensation systems, the spots will not form diffraction-limited pattern due to phase errors beyond. Nyquist which distort each individual spot. A “beam's eye view” at the detector surface 273, in the presence of the aberrated bean is shown in 274. Note that the array of focused spots is does not precisely overlap the grid-pattern. This is indicative of a typical aberrated beam, whose local tilts are randomly distributed. Therefore, each spot at the plane 274 has a correspondingly different offset in the (x,y) plane relative to the grid pattern. As is known in the art, the camera or ccd array 273 will require a sufficient number and density of resolvable detector pixels to measure the offset in spot position to ascertain the local tilt error with sufficient precision.
FIG. 2C depicts the operation of the WFS assuming an input reference beam whose wavefront aberrations have been corrected. In the ideal case, the input beam 221 is a perfect plane wave, with a corresponding equi-phase surface across the entire input aperture to the WFS. As in FIG. 2B, each resolvable plane-wave segment of the input beam 223 is incident upon a different lenslet in the array 271. As before, an Airy disc pattern will appear at each respective focal plane along the detector surface 273. However, since each plane-wave segment has the same tilt (ideally, zero degrees with respect to the optical axis), each respective Airy pattern at the focal plane at the detector array 273 will be centered on its respective grid location. The “beam's eve view” at the detector surface 273, in the presence of the compensated reference beam, is shown in 274. Note that the array of focused spots precisely overlaps the grid-pattern. This is indicative of an ideal plane wave, whose local tilts are identical, and a wavefront sensor with no internal aberrations. In actual implementations, there will be internal wavefront sensor error that must be measured and removed to produce reference spot/sub-image default positions. Therefore, each spot at the plane 274 has a zero spatial offset in the (x,y) plane relative to the grid pattern. It is the goal of the servo-loop adaptive optical system to drive an aberrated beam (comprised of is finite number of tilted plane-wave segments) to a converged wavefront whose differential tilts approach zero.
It is important to emphasize that the WFS detects only the reference beam, which, by definition, does not contain image information, other than the spatial information resulting from the intervening propagation-path distortions. Hence, based on the prior art, in order to realize an image-compensation adaptive optics system, a reference beam must be present in addition to the image-bearing beam. However, in many applications, a diffraction-limited reference beam will not always be present or practical, even in cooperative scenarios (whereby, knowledge of the existence of a reference beam or of an observer is not a drawback). And, in certain cases, a reference beam optical source may be undesirable for strategic considerations, since detection of a reference optical source by a third party can reveal the presence and/or location of a covert observer. For these and other considerations, it is desirable to realize a compensated imaging system without the need for a cooperative reference beam