Abstract:
Diffractive waveplate lenses, mirrors, devices, systems and methods for performing imaging over a broad spectral band in imaging systems, such as but not limited to astronomical imaging, surveillance imaging, and in communication systems, such as laser communication systems. Corrector mirrors are used with a flat diffractive wave diffractive waveplate lens so that chromatic aberrations of the diffractive waveplate lens are reduced with the imaging system.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 62/246,923 filed Oct. 27, 2015. The entire disclosure of the application listed in this paragraph is incorporated herein by specific reference thereto. 
     
    
     GOVERNMENT LICENSE RIGHTS 
       [0002]    This invention was made with Government support under NASA Space Technology Mission Directorate contract number NNX15AL92G, awarded by NASA (National Aeronautics and Space Administration). The government has certain rights in this invention. 
     
    
     FIELD OF INVENTION 
       [0003]    This invention relates to imaging systems and, in particular, to diffractive waveplate lenses, mirrors, devices, systems and methods for performing imaging over a broad spectral band. 
       BACKGROUND AND PRIOR ART 
       [0004]    Imaging systems are ubiquitous in the modern world, primarily due to progressive improvements in digital imaging technology, allowing high quality at low cost. The cost of image sensors has been reduced drastically over the past few decades, enabling very high quality sensors in even such relatively inexpensive consumer products as smart phones. 
         [0005]    Although there has been great progress in reducing the cost of image sensors, especially for use in the visible band of the electromagnetic spectrum (i.e. the band over which human vision functions), the progress in reducing the weight and cost of the optical components that form the image on the image sensor has been less impressive. In many imaging systems, both cost and weight are dominated by the optics, with the image sensor and associated electronics contributing a relatively small fraction of the overall system cost. 
         [0006]    In many imaging systems, the portion of the optics that dominates the cost of the optics, and may even dominate the overall cost of the system, is the first optic with optical power encountered by electromagnetic radiation incident on the system. Here an optic having “power” is one that causes convergence of an incident beam. If this first optic is a refractive lens, it is generally referred to as the objective lens. If this first optic is a curved reflective mirror, it generally referred to as the primary mirror. 
         [0007]    One of the factors that drives the cost of objective lenses and primary mirrors in optical systems is the fact that these optics have curved surfaces whose shape must comply with dimensional tolerances that are difficult to achieve. 
         [0008]    Recently, it has become possible to employ a flat diffractive waveplate lens comprised of ultra-thin diffractive waveplate coatings on a thin, flat substrate, as the first optical element in some imaging systems, thus eliminating the need for a curved objective lens or curved primary mirror. See U.S. Non-Provisional patent application Ser. No. 15/189,551 filed Jun. 22, 2016, which claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 62/182,975 filed Jun. 22, 2015, which are both incorporated by reference in their entirety. 
         [0009]    This replacement has the potential to allow large cost and weight reductions for some applications. A critical advantage of using diffractive waveplate lenses is that they possess continuous structure that can be formed as micrometer thin film coatings on flat or curved shapes. Additionally, as opposed to other diffractive lenses such as Fresnel lenses, diffractive waveplate lenses are capable of near 100% diffraction efficiency in a broad band of wavelengths. 
         [0010]    A fundamental property of diffractive waveplate lenses, as well as other types of diffractive elements, is that the angle through which electromagnetic radiation is deflected by such elements is highly dependent on the wavelength. In imaging systems, this produces chromatic aberrations, which can degrade the image quality. 
         [0011]    For some applications, it has been shown that chromatic aberrations can be suppressed by means of a single corrector mirror. However, although the correction that is known to be possible with a single corrector mirror is sufficient for some applications such as laser communications, for which the optical signals are contained in a narrow spectral band, in other applications the degree of chromatic aberration correction is not sufficient. 
         [0012]    In order to obtain the noted benefits of using a diffractive waveplate lens as the objective lens of an imaging system, it is necessary to identify a technique for reducing the chromatic aberrations of the diffractive waveplate lens used as the objective lens of an imaging system. 
         [0013]    Thus, there is a need for a method and apparatus for further reducing the chromatic aberrations of diffractive waveplate lenses to an extent beyond the reduction achievable using prior art. 
       SUMMARY OF THE INVENTION 
       [0014]    A primary objective of the present invention is to provide diffractive waveplate lenses, mirrors, devices, systems and methods for performing imaging over a broad spectral band, with significantly reduced cost, size, and weight of the optical system. 
         [0015]    A secondary objective of the present invention is to provide diffractive waveplate lenses, mirrors, devices, systems and methods for reducing the chromatic aberrations of the diffractive waveplate lens used as the objective lens of an imaging system. 
         [0016]    An imaging system can include a diffractive waveplate objective lens for focusing input light, a set of diffractive waveplate coated mirrors, and an imaging sensor, wherein the diffractive waveplate objective lens and the set of diffractive corrector mirrors are configured such that all rays of light within an operating wavelength band incident on the diffractive waveplate objective lens are brought to the same system focal plane on the imaging sensor. 
         [0017]    Both said diffractive waveplate objective lens and said diffractive waveplate coated mirrors can operate in a broad band of spectrum selected from at least one of: ultraviolet, visible, near infrared, short wave infrared, mid-wave infrared, and long-wave infrared wavelengths. 
         [0018]    Both said diffractive waveplate objective lens and said diffractive waveplate coated mirrors can operate in multiple spectral bands selected from at least one of: ultraviolet, visible, near infrared, short wave infrared, mid-wave infrared, and long-wave infrared wavelengths. 
         [0019]    In the imaging system, chromatic aberrations can be reduced by a set of two diffractive waveplate coated mirrors employing a first diffractive waveplate coated mirror to bring all rays of light within an operating wavelength band incident on the diffractive waveplate objective lens to a second diffractive waveplate coated mirror; and wherein the second diffractive waveplate coated mirror brings all said rays of light to the same system focal plane. 
         [0020]    The diffractive waveplate coated mirrors can include (a) a substrate with a reflective surface, (b) a phase retardation layer conforming to the reflective surface, and (c) a diffractive waveplate coating layer conforming to said reflective surface. 
         [0021]    The phase retardation layer can be a quarter-wave plate. 
         [0022]    The imaging system can further include a mechanical structure for holding and aligning said diffractive waveplate lens and said diffractive waveplate coated mirrors. 
         [0023]    The imaging sensor can be a camera for collecting imaging data of an object within a spectral band selected from at least one of: ultraviolet, visible, near infrared, short wave infrared, mid-wave infrared, and long-wave infrared wavelengths. 
         [0024]    The imaging system can include an electronic device for displaying an output of the imaging sensor. 
         [0025]    The imaging sensor can be replaced by an eyepiece. 
         [0026]    A method for fabricating an imaging system with diffractive waveplate coated mirrors correcting chromatic aberrations of a diffractive waveplate objective lens in an imaging system, can include the steps of selecting a diameter and focal length of the diffractive waveplate objective lens, selecting axial apex spacings and the diameters of the diffractive waveplate coated mirrors so that there is no vignetting for either of the diffractive waveplate coated mirrors for the entire operating wavelength range, selecting polynomial expansion coefficients for both curvature and phase profile of a first diffractive waveplate coatings of a first diffractive waveplate coated mirror so that an objective diffractive waveplate lens is imaged onto a surface of a second diffractive waveplate coated mirror at all wavelengths within the operating bandwidth of the imaging system for paraxial rays, setting polynomial expansion coefficients for both curvature and phase profile of a second diffractive waveplate coating of a second diffractive waveplate coated mirror, by having the imaging system focal point be at a selected distance from an apex of the second mirror, and that all wavelengths within the operating band converge to the system focal point for paraxial rays, and adjusting the polynomial expansion coefficients for the curvatures and phase profiles of the first and second diffractive waveplate coated mirrors in order to bring all rays incident over an entire clear aperture of the objective lens to the same imaging system focal point. 
         [0027]    The imaging system can be a telescope. The imaging system can be augmented reality glasses. The imaging system can be a binocular. 
         [0028]    An imaging system, can include a light source, a beam shaping system for forming an illuminated area, a diffractive waveplate objective lens for focusing the light collected from said illuminated area, a set of diffractive waveplate coated mirrors, and an imaging sensor, wherein the diffractive waveplate objective lens and the set of diffractive waveplate coated mirrors are configured so that all rays of light within an operating wavelength band incident on the diffractive waveplate objective lens are brought to the same system focal point on the imaging sensor. 
         [0029]    The light source can emit radiation at wavelengths within a spectral band selected from at least one of ultraviolet, visible, near infrared, short wave infrared, mid-wave infrared and long-wave infrared wavelengths. 
         [0030]    The light source can be a laser, which emits radiation at wavelengths within the spectral band of the imaging sensor, selected from at least one of ultraviolet, visible, near infrared, short wave infrared, mid-wave infrared, and long-wave infrared wavelengths. 
         [0031]    The beam shaping system can include diffractive waveplate optics. 
         [0032]    The diffractive waveplate optics can be electrically switched between states with and without optical power producing variable beam divergence or variable beam profile. 
         [0033]    The imaging sensor can include a camera for collecting imaging data of the object within a spectral band selected from at least one of ultraviolet, visible, near infrared, short wave infrared, mid-wave infrared, and long-wave infrared wavelengths, wherein the camera captures imaging data when signals emitted by the laser light source of and reflected by the imaged object or scene are collected by said camera. 
         [0034]    Further objectives and advantages of this invention will be apparent from the following detailed description of the presently preferred embodiments which are illustrated schematically in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0035]      FIG. 1  illustrates an imaging system employing a single diffractive waveplate lens to image an object at a great distance from the imaging system. 
           [0036]      FIG. 2  illustrates an imaging system employing a diffractive waveplate lens as the objective lens of the imaging system, and including a single diffractive waveplate coated mirror to reduce chromatic aberrations. 
           [0037]      FIG. 3  illustrates an imaging system employing a diffractive waveplate lens as the objective lens of an imaging system, and two diffractive waveplate coated mirrors to reduce chromatic aberrations, with intermediate focal points between the two diffractive waveplate coated mirrors. 
           [0038]      FIG. 4  illustrates a specific example of an imaging system employing a diffractive waveplate lens as the objective lens of an imaging system, and two diffractive waveplate coated mirrors to reduce chromatic aberrations. 
           [0039]      FIG. 5  illustrates the shape of the first diffractive waveplate coated mirror in the specific example presented in  FIG. 4 . 
           [0040]      FIG. 6  illustrates the shape of the second diffractive waveplate coated mirror in the specific example presented in  FIG. 4 . 
           [0041]      FIG. 7  illustrates the results of a polychromatic optical ray trace for the specific optical system design example presented in  FIG. 4 , demonstrating that the system is diffraction-limited over a significant portion of the visual spectral band. 
           [0042]      FIG. 8  illustrates the results of an optical ray trace for the specific example presented in  FIG. 4 , for several different wavelengths within the visual spectral band, demonstrating that the system is diffraction-limited over a significant portion of the visual spectral band. 
           [0043]      FIG. 9  illustrates the design process that was employed in producing the optical system design illustrated in  FIG. 4 . 
           [0044]      FIG. 10  illustrates the structure of a diffractive waveplate coated mirror, showing the diffractive waveplate coating and the quarter-wave coating. 
           [0045]      FIG. 11  is a schematic of an active imaging concept employing an imaging system of the present invention, that is, a system in which the illumination is artificially provided by a light source such as a laser or light emitting diodes. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0046]    Before explaining the disclosed embodiments of the present invention in detail it is to be understood that the invention is not limited in its applications to the details of the particular arrangements shown since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation. 
         [0047]    In the Summary above and in the Detailed Description of Preferred Embodiments and in the accompanying drawings, reference is made to particular features (including method steps) of the invention. It is to be understood that the disclosure of the invention in this specification does not include all possible combinations of such particular features. For example, where a particular feature is disclosed in the context of a particular aspect or embodiment of the invention, that feature can also be used, to the extent possible, in combination with and/or in the context of other particular aspects and embodiments of the invention, and in the invention generally. 
         [0048]    In this section, some embodiments of the invention will be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in alternative embodiments. 
         [0049]    The present invention relates to imaging systems employing a diffractive waveplate lens as the objective lens of the system, and employing two diffractive waveplate coated mirrors to bring radiation over a wide range of wavelengths to the same focal point. 
         [0050]    A list of components will now be provided.
     101  input electromagnetic radiation     102  diffractive waveplate lens     103  focal point     104  photodetector or photodetector array     105  radiation at a first wavelength     106  radiation at a wavelength shorter than the first wavelength     201  input electromagnetic radiation     202  diffractive waveplate objective lens     203  diffractive waveplate coated mirror     204  focal point     205  photodetector or photodetector array     206  radiation at a first wavelength     207  radiation at a wavelength shorter than the first wavelength     300  imaging system     301  incident electromagnetic radiation     302  diffractive waveplate objective lens     303  first diffractive waveplate coated mirror     304  second diffractive waveplate coated mirror     305  system focal point     306  photodetector or photodetector array     307  radiation at a first wavelength     308  intermediate focal point for radiation at a first wavelength     309  radiation at a wavelength shorter than the first wavelength     310  intermediate focal point for radiation at a wavelength shorter than the first wavelength     311  electronic processing and display     401  incident electromagnetic radiation     402  diffractive waveplate objective lens     403  first diffractive waveplate coated mirror     404  intermediate focal points for radiation over a range of wavelengths     405  second diffractive waveplate coated mirror     406  system focal point     1000  diffractive waveplate coated mirror     1020  substrate of mirror     1021  shaped mirror surface with a reflective coating     1022  phase retardation film     1023  diffractive waveplate coating     1101  light source     1105  light emitted by the light source     1110  beam shaper     1120  illuminating beam     1130  illuminated target     1140  light scattered from target     1150  light collected by the imaging system     1160  imaging system   
 
         [0095]      FIG. 1  illustrates a single diffractive waveplate lens collecting input electromagnetic radiation emanating from a distant on-axis target. The input radiation  101  is incident on the diffractive waveplate lens  102 , and the effect of the diffractive waveplate lens on the input radiation is to bring the radiation at a selected wavelength to a focal point  103  on a photodetector or photodetector array  104 . Radiation  105  at a first wavelength is brought to a sharp focal point  103 , but radiation  106  at a shorter wavelength is not brought to the same focal point  103 . This blurring of the focal point for radiation at all wavelengths except one limits the usefulness of the illustrated imaging system to a narrow spectral bandwidth. 
         [0096]      FIG. 2  illustrates an optical design as described in U.S. Non-Provisional patent application Ser. No. 15/189,551 filed Jun. 22, 2016, which claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 62/182,975 filed Jun. 22, 2015, entitled “Diffractive Mirrors and Diffractive Telescopes with Corrected Temporal Dispersion which are both incorporated by reference in their entirety In the optical design illustrated in  FIG. 2 , a diffractive waveplate lens is used for collecting the radiation from an input collimated beam, and a single diffractive waveplate coated mirror is used to correct for chromatic aberrations. 
         [0097]    In  FIG. 2 , input radiation  201  is incident on a diffractive waveplate objective lens  202 , which converges the radiation towards a focus. The converging radiation is collected by a diffractive waveplate coated mirror  203 , which collects the radiation and directs it to a focal point  204  on a photodetector or photodetector array  205 . Radiation  206  of a first wavelength and radiation  207  at a nearby shorter wavelength is all brought to the same focal point  204 . However, although the range of wavelengths over which this correction technique can effectively collect and focus radiation is sufficient for some applications such as laser communications, it is insufficient for other applications such as some imaging applications. 
         [0098]      FIG. 3  illustrates a preferred embodiment of the present invention, in which two diffractive waveplate coated mirrors are used to bring all the radiation within a broad operating band of wavelengths to the same focal point. In  FIG. 3 , incident radiation  301  from a distant on-axis target is incident on a diffractive waveplate objective lens  302 , which converges the radiation towards the focal point. A first diffractive waveplate coated mirror  303  collects the radiation and diffracts it towards a second diffractive waveplate coated mirror  304 . The second diffractive waveplate coated mirror  304  focuses radiation within the operating band of the system to a system focal point  305  on a photodetector or photodetector array  306 . 
         [0099]    The diffractive waveplate objective lens  302 , diffractive waveplate coated first mirror  303 , and diffractive waveplate coated second mirror  304  are spaced and configured such that radiation  307  at the long wavelength limit of the operating wavelength band is brought to an intermediate focal point  308  between the two mirrors  303  and  304 . In addition, the diffractive waveplate objective lens  302 , diffractive waveplate coated first mirror  303 , and diffractive waveplate coated second mirror  304  are spaced and configured such that radiation  309  at the short wavelength limit of the operating wavelength band is brought to an intermediate focal point  310  between the two mirrors  303  and  304 . Finally, the entire system configuration is such that wavelengths over the broad operating wavelength band is all brought to the same system focal point  305 . The imaging sensor, which may be a CCD camera, may be further connected to an electronic processing and display system  311 . 
         [0100]    An exemplary embodiment of the subject invention is illustrated and described in  FIG. 4 . The exemplary embodiment is that of a large-aperture astronomical telescope, but the design principles disclosed in describing the exemplary embodiment also apply to a wide variety of imaging systems, such as but not limited to surveillance imaging systems, augmented reality displays, virtual reality displays, communications including laser communications systems, and the like. 
         [0101]    In  FIG. 4 , radiation  401  is incident on a diffractive waveplate objective lens  402 , and is converged towards the first diffractive waveplate coated mirror  403 . For the specific design used for illustration, the diameter of the objective lens  402  is 20 meters, and the distance from the diffractive waveplate objective lens  402  and the first diffractive waveplate coated mirror  403  is 410 meters. The first diffractive waveplate coated mirror  403  brings radiation of all wavelengths within the broad operating wavelength band of the system to focal points in the region  404  of  FIG. 4 . For this specific design used for illustration, the distance between the first diffractive waveplate coated mirror  403  and the second diffractive waveplate coated mirror  405  is 50 meters. The second diffractive waveplate coated mirror  405  brings radiation of all wavelengths within the broad operating wavelength band of the system to the same focal point  406 , defined as the system focal point. 
         [0102]      FIG. 5  illustrates the shape and size of the first diffractive waveplate coated mirror for the system described in  FIG. 4 . The diameter of this mirror is 4 meters, and the maximum sag distance from the center to the edge of the clear aperture is 22 millimeters. In this context, “sag distance” is the distance parallel to the axis of the mirror from a plane tangent to the apex of the mirror to the surface of the mirror. 
         [0103]      FIG. 6  illustrates the shape and size of the second diffractive waveplate coated mirror. The diameter of this mirror is 2.4 meters, and the maximum sag distance from the center to the edge of the clear aperture is 150 millimeters. 
         [0104]      FIG. 7  illustrates the result of tracing rays of optical radiation with wavelengths from 0.50 micrometers to 0.58 micrometers through the exemplary optical system illustrated in  FIG. 4 . For the on-axis case, the RMS radius of the rays at the focal plane is 0.5 micrometers, compared with the Airy radius of 1.1 micrometers. Since the system focal length is 34 m, this Airy radius corresponds in the object space to an RMS radius of 15 nanoradians and an Airy radius of 32 nanoradians. This predicted angular resolution is far superior to that of any existing telescope. 
         [0105]    The curve described in  FIG. 8  as applying to the case of “Two corrector mirrors” illustrates the result of tracing rays of optical radiation with wavelengths from 500 nm to 650 nm through the exemplary optical system illustrated in  FIG. 4 . The design of the optical system, including the spacing between the elements, was held constant as the wavelength was varied for the calculations for which results are shown in  FIG. 8 . As is evident from  FIG. 8 , the system is diffraction limited for all wavelengths from 520 nm through 600 nm, since the geometric point spread function is smaller than the Airy disk radius for this range of wavelengths. For reference, an additional curve labeled “One corrector mirror” is shown in  FIG. 8 , illustrating the point spread function radius as a function of wavelength for a design employing only a single diffractive waveplate coated mirror, as is described in the previously referenced as described in U.S. Non-Provisional patent application Ser. No. 15/189,551 filed Jun. 22, 2016, which claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 62/182,975 filed Jun. 22, 2015, entitled “Diffractive Mirrors and Diffractive Telescopes with Corrected Temporal Dispersion which are both incorporated by reference in their entirety 
         [0106]      FIG. 8  clearly demonstrates that the appropriate employment of a second diffractive waveplate coated mirror provides much broader diffraction-limited wavelength coverage than designs with only a single diffractive waveplate coated mirror. 
         [0107]    The method  900  described in  FIG. 9  can be used to create the exemplary optical system design illustrated in  FIG. 4  through  FIG. 8 . The method described by  FIG. 9  comprises steps  901  through  907 . It is assumed that the operating wavelength band has already been selected prior to beginning the design. The expression “design wavelength” will be used here to mean the wavelength at the center of the operating wavelength band. 
         [0108]    The first step  901  is to select the diameter and focal length of the objective lens, a diffractive waveplate lens, at a design wavelength. There are various considerations in this selection, such as the required angular resolution and light collecting power. 
         [0109]    The second step  902  is to select the axial apex spacings and the diameters of the two diffractive waveplate coated mirrors. The diameter of the first diffractive waveplate coated mirror can be determined, as an example, by requiring that all rays from a distant on-axis target, for the entire operating wavelength range, that are within the diameter of the objective lens, be captured by the first diffractive waveplate coated mirror. The diameter of the second diffractive waveplate coated mirror can be determined, for example, by requiring that it capture the edge ray for the design wavelength, and for a distant on-axis target, assuming that the first diffractive waveplate coated mirror does not deviate any rays at the design wavelength. 
         [0110]    The third step  903  is to select initial coefficients describing both the curvature and the diffractive waveplate coatings of the first diffractive waveplate coated mirror. This can be done by requiring that the mirror initially be spherical, that the combination of the curvature and the diffractive coating be such that the first diffractive waveplate coated mirror image the diffractive waveplate lens on the second diffractive waveplate coated mirror for paraxial rays at all wavelengths within the operating spectral bandwidth. 
         [0111]    The fourth step  904  is to set the initial coefficients describing both the curvature and the phase profile of the diffractive waveplate coatings of the second diffractive waveplate coated mirror. This can be done by requiring that the mirror initially be spherical, that the system focal point be at a specified distance from the apex of the mirror, and that paraxial rays for all wavelengths within the operating wavelength band converge to the system focal point. The distance between the apex of the second diffractive waveplate coated mirror and the system focal point  406  in  FIG. 4  can be set to a value determined by the requirement that the mirror be manufacturable, and that the field of view over which the optical system achieves acceptable resolution be acceptably large. 
         [0112]    At this point in the design process, paraxial rays from a distant on-axis target for all wavelengths within the operating wavelength band all intersect the first diffractive waveplate coated mirror at the same radius, and all pass through the system focal point. However, other rays will in general not satisfy either of these two criteria. Specifically, rays of various wavelengths within the operating wavelength band, incident on the objective lens at radii other than at radii near the center of this lens, will not all pass through the system focal point. The purpose of the optimization step  905  in  FIG. 9  is to adjust the curvatures and the phase profiles of the diffractive waveplate coatings on the two diffractive waveplate coated mirrors in order to minimize the root mean square a (RMS) radius of rays at all radii within the clear aperture of the system reaching the system focal point. This optimization can be performed with an optical design program such as Zemax. In this optimization, rays of radiation at various wavelengths within the operating wavelength band, incident on the objective lens throughout its clear aperture from a distant on-axis target, are traced through the optical system. The sag of each of the two diffractive waveplate coated mirrors, and the phase profile of the diffractive waveplate coatings on each of those mirrors, are all expanded in polynomial expansions in the radial coordinate, and the coefficients of the polynomial expansions in the radial coordinate of the sags of each mirror, and of the phase profiles of the diffractive waveplate coatings on the two diffractive waveplate coated mirrors, are adjusted so as to minimize the polychromatic spot size at the system focal point. In accordance with step  906  in  FIG. 9 , optimization stops when some predetermined performance is achieved. Here the criterion for the performance to be acceptable is that the polychromatic RMS spot size at the system focal point is reduced to a predetermined level, for example to below the diffraction limit. 
         [0113]    If the optimization step  905  of  FIG. 9  does not result in sufficient reduction of the RMS spot size at the focal point, step  907  of  FIG. 9  allows for the possibility to try alternative axial spacings and diameters of the diffractive waveplate coated mirrors. 
         [0114]    At the conclusion of the design process described above, and outlined in  FIG. 9 , it was found that it was possible to arrive at a design of a telescope with a 20 meter aperture diameter, with diffraction-limited performance over a spectral band that includes a significant portion of the visible spectrum, or more specifically, over a band of wavelengths from 520 nm to 600 nm. The same design technique can be used for other applications by starting with different initial conditions, but using the same design method outlined in  FIG. 9 . 
         [0115]    An example of a diffractive waveplate coated mirror shown in  FIG. 10  comprises a mirror  1020 , a phase retarder coating  1021  conforming the mirror shape, and diffractive waveplate coating  1023  conforming to the mirror shape. The diffractive waveplate coating may comprise multiple layers for broadband or multispectral performance. Both the mirror shape and the structure of the diffractive waveplate coating are determined for each application following the method summarized in  FIG. 9 . 
         [0116]    In the layout of  FIG. 3 , there is no central obscuration because the optical path is unfolded. In the layout of  FIG. 4 , on the other hand, the reflected beam from the first diffractive waveplate coated mirror  403  is folded out of the path of the incoming beam from the objective lens  402 , but the photodetector or photodetector array  406  would partially obscure rays from the first diffractive waveplate coated mirror  403  to the second diffractive waveplate coated mirror  405 . However, an artisan with ordinary skill in the art of optical design could employ slight variations of the method of  FIG. 9  to produce designs with various foldings of the optical path, but with essentially equivalent optical performance. 
         [0117]    In an alternative embodiment of the current invention shown in  FIG. 11 , the target  1130  being imaged is illuminated by a light source  1101  that may contain wavelengths in more than a single spectral band. In some embodiments, the light source may be a laser. The light  1105  emitted by the light source is shaped by beam shaper  1110 . Examples of beam shapers include lenslet arrays and diffusers. In a preferred embodiment, beam shapers are thin film diffractive waveplate films coated, for example, at the output window of the laser. In another preferred embodiment, the beam shapers are reconfigurable, particularly, electrically switchable between different optical conditions, for example, between different beam divergent conditions or different shapes. Some of the rays of light  1120  from the output of the beam shaper are scattered by the target. Some of the scattered light  1140  is collected by the imaging system  1160 . The advantage of such “active imaging” is that imaging can be performed at long distances and it does not rely as much on ambient light. 
         [0118]    While the invention has been described, disclosed, illustrated and shown in various terms of certain embodiments or modifications which it has presumed in practice, the scope of the invention is not intended to be, nor should it be deemed to be, limited thereby and such other modifications or embodiments as may be suggested by the teachings herein are particularly reserved especially as they fall within the breadth and scope of the claims here appended.