Patent Publication Number: US-6989938-B2

Title: Wavefront aberrator and method of manufacturing

Description:
RELATED APPLICATION INFORMATION 
     This application is a divisional of U.S. application Ser. No. 09/875,447, filed Jun. 4, 2001, now U.S. Pat. No. 6,813,082, which claims priority to U.S. Provisional Patent Application No. 60/253,418, filed Nov. 27, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     In traditional optical systems having reflecting and refracting surfaces, it is common to assume that the light passing through the system is limited to paraxial rays, specifically, rays that are near the optical axis and are sustained within small angles. However, practical optical systems rarely are limited to paraxial rays, and thus, the actual images assumed by gaussian optics often depart from the “perfect” image. This departure from the “perfect” image results in the introduction of distortion into the optical system, called aberrations. These aberrations are most problematic in small focal ratio optical systems where the angle from the optical axis is larger. 
     In a monochromatic optical system with only spherical surfaces, there are five (5) basic ray and wave aberrations, namely, spherical aberrations, coma, astigmatism, curvature of field, and distortion. Optical systems for use with multi-chromatic light have an additional source of distortion, namely, chromatic aberration. 
     Because the distortion introduced by aberrations into an optical system significantly degrades the quality of the images on the image plane of such system, there are significant advantages to the reduction of those aberrations. Various techniques are often used to minimize the aberrations. For example, in order to minimize spherical aberrations or coma, a lens may be “bent” to have different radii of curvature on opposite sides while maintaining a constant focal length, such as is contemplated by using the Coddington shape factor. Also, a pair of lenses, where one glass lens has a positive focal length, and the other made from a different glass has a negative focal length, are used together to correct spherical aberrator. One example of this technique is the “doublet” lens in which the two lenses have the same radius of curvature on the facing sides, and are cemented together. 
     Despite the available techniques to minimize the various aberrations, it is often difficult to simultaneously minimize all aberrations. In fact, corrections to an optical system to minimize one type of aberration may result in the increase in one of the other aberrations. Typically, one may decrease coma, at the expense of increasing spherical aberrations. Moreover, because it is often necessary to measure the aberrations only after an optical system is constructed due to additional aberrations from manufacturing or assembly tolerances, the creation of an optical system with minimal aberration typically requires several reconstructions before a suitable system is developed. 
     In complex optical systems, in addition to traditional aberration correction, it is often advantageous to create an optical element which generates a unique wavefront phase profile. Typically, these unique optical elements have been created by sophisticated grinding and polishing of traditional lenses. However, this method of manufacturing a unique optical element requires a significant amount of time and expertise, and results in a high cost of manufacturing the optical element. 
     Consequently, a need exists for the creation of an optical element which can generate a unique wavefront phase profile, and that can simultaneously minimize the chosen aberrations within an optical system. 
     SUMMARY OF THE INVENTION 
     The wavefront aberrator of the present invention includes a pair of transparent windows, or plates, separated by a layer of a monomers and polymerization initiators, such as epoxy. This epoxy exhibits a variable index of refraction as a function of the extent of its curing. Curing of the epoxy may be made by exposure to light, such as ultraviolet light. The exposure to light may be varied across the surface of the epoxy to create a particular and unique wavefront retardation profile such that when an ideal plane wave passes through the wavefront aberrator, a predetermined change of the wavefront profile can be affected by the wavefront aberrator device. Conversely, if a distorted wavefront is known, such as by measuring the wavefront with a Hartmann/Shack sensor, a correction of such aberrated or distorted wavefront aberration may be achieved by first producing a complementary wavefront aberrator device such that passing the abnormal wavefront through the wavefront aberrator device, a plane wave emerges. 
     One method of creating the wavefront aberrator of the present invention includes the exposure of the epoxy to an array of light emitting diodes (LEDs). These LEDs may be selectively illuminated such that different portions of the epoxy are exposed to different levels of illumination. This variance in illumination results in the creation of a wavefront aberrator having a varying index of refraction across its surface, and may include the formation of multiple sub-regions, where the index of refraction of the cured epoxy in a sub-region has a constant index of refraction, with the index of refraction varying between adjacent sub-regions. 
     An alternative method of creating the wavefront aberrator of the present invention includes the exposure of the epoxy to an array of LEDs through a demagnifier lens. In this manner, the LEDs may create a curing pattern which is then focussed onto the surface of the epoxy to create a similar, yet smaller, version of the curing pattern to provide for reduced-sized wavefront aberrators. 
     Yet another alternative method of creating the wavefront aberrator of the present invention includes the creation of a curing pattern by the transmission of light through a liquid crystal display (LCD). A non-coherent light source may be positioned adjacent to a diffuser to create a diffused light source. This diffused light may then be transmitted through a LCD containing a curing pattern, and onto a wavefront aberrator. As the epoxy is exposed, the curing pattern on the LCD creates the desired refractive index profile. New patterns may be generated by changing the pattern on the LCD. 
     A sensor may be placed beneath the wavefront aberrator to monitor the transmitted image of the curing pattern. The output of this sensor may be used to actively modulate the transmission of light through the LCD to create a wavefront aberrator having a desired refractive index profile, and to provide for an active monitor and control of the curing of each sub-region of the wavefront aberrator. 
     Another alternative method of creating the wavefront aberrator of the present invention includes the creation of a curing pattern by the selective illumination of portions of the epoxy using a point light source, such as a laser. This selective illumination may be accomplished by rastering a portion of the surface of the epoxy, varying the speed and/or intensity of the light source to vary the curing of the epoxy. Alternatively, the light source could trace particular curing patterns directly onto the wavefront aberrator at various speeds and/or intensities of light, such as by raster or vector scanning the curing pattern onto the aberrator. Also, a positive or negative, or “contact print,” containing a particular wavefront retardation design may be positioned adjacent the wavefront aberrator and exposed to a diffused or collimated light to create the desired refractive index profile. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the wavefront aberrator of the present invention showing a layer of epoxy sandwiched between an upper transparent cover and a lower transparent cover; 
         FIG. 2  is a top view of the wavefront aberrator of the present invention showing a circular barrier within the epoxy layer, and the epoxy layer formed within the circular barrier confining the epoxy within a predetermined volume and having a variety of refractive index profiles between different sub-regions; 
         FIG. 3  is a cross-sectional view of the wavefront aberrator of the present invention taken along line  3 — 3  of  FIG. 1 , and showing the positioning of the epoxy layer between the upper and lower transparent covers; 
         FIG. 4  is a system for manufacturing the wavefront aberrator of the present invention showing a computer controlled light emitting diode (LED) array panel generating a curing pattern which is directed through a diffuser element onto a wavefront aberrator to selectively cure the epoxy to create a particular, pre-determined refractive index profile; 
         FIG. 5  is a system for manufacturing the wavefront aberrator of the present invention showing an LED array panel generating a curing pattern which is directed through a demagnifier element and onto a wavefront aberrator to cure the epoxy to create a particular refractive index profile; 
         FIG. 6  is a system for manufacturing the wavefront aberrator of the present invention showing a computer controlled liquid crystal display (LCD) generating a curing pattern such that when the LCD is exposed light, light corresponding to the curing pattern is transmitted through the LCD and onto the wavefront aberrator to create a particular refractive index profile; 
         FIG. 7  is a system for manufacturing the wavefront aberrator of the present invention showing point light source that is moved across the surface of the wavefront aberrator at varying speeds and with varying intensities to selectively cure the epoxy to create a particular refractive index profile or arrangement of sub-regions; 
         FIG. 8  is a side view of an alternative embodiment of the wavefront aberrator of the present invention incorporating a transparent cover formed in the shape of a lens; and 
         FIG. 9  is a side view of an alternative embodiment of the wavefront aberrator of the present invention formed with a salt window as the lower transparent cover which, when dissolved, provides for an exposed epoxy layer facilitating post curing treatment of the epoxy. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to  FIG. 1 , a perspective view of a preferred embodiment of the wavefront aberrator of the present invention is shown and generally designated  100 . Aberrator  100  includes a lower transparent cover  102 , an epoxy layer  104 , and an upper transparent cover  106 . 
     The shape of aberrator  100  is shown in  FIG. 1  having square covers  102  and  106 . It is to be appreciated, however, that the shape of the aberrator  100  shown in  FIG. 1  is merely exemplary, and that any shape may be used while not departing from the present invention. Also, for purposes of illustration, the transparent covers  102  and  106  are shown in  FIG. 1  as being substantially planar. However, it is to be appreciated that the covers  102  and  106  may be curved to provide a non-planar aberrator  100 . 
     Referring to  FIG. 2 , a top view of the wavefront aberrator  100  of the present invention is shown. A barrier, such as circularly-shaped barrier  108 , may be positioned within the epoxy layer  104  to retain the placement of the epoxy  104  between the upper transparent cover  106  and the lower transparent cover  102 . 
     Epoxy  104  is, in a preferred embodiment, a light-curable resin comprised of monomers and polymerization initiators. The refractive index of the resin changes as the resin is cured, and it varies between locations within the resin layer depending on the extent of curing of the epoxy. The extent of curing is determined by the percentage of cross-linking between the monomers within the epoxy. Suitable resins include VLE-4101 UV-Visible Light Cure Epoxy, available from Star Technology, Inc., or Optical Adhesive #63, U.V. Curing, available from Norland Products, Inc. Typically, these resins are curable by exposure to UV or visible light radiation in the range of 300 to 550 nanometers (300–550 nm). Generally, the present invention applies to any type of epoxy that exhibits an index of refraction change upon curing and the corresponding curing light source may have wavelengths ranging between 300 nm and 3000 nm. 
     It is to be appreciated that many other suitable resins exist which exhibit a similar change in its index of refraction upon exposure to light. Other monomers that polymerize into long-chain molecules using photo-initiators may be used in the present invention. For example, a suitable monomer may be chosen from the family of epoxides, urethanes, thiol-enes, acrylates, cellulose esters, or mercapto-esters, and a broad class of epoxies. Also, for example, a suitable photo-initiator may be chosen from alpha cleavage photoinitiators such as the benzoin ethers, benzil ketals, acetophenones, or phosphine oxides, or hydrogen abstraction photoinitiators such as the benzophenones, thioxanthones, camphorquinones, or bisimidazole, or cationic photoinitiators such as the aryldiazonium salts, arylsulfonium and aryliodonium salts, or ferrocenium salts. Alternatively, other photoinitiators such as the phenylphosphonium benzophene salts, aryl tert-butyl peresters, titanocene, or NMM may be used. 
     In the present invention, a light source containing a particular wavelength irradiates the monomer layer which activates the photo-initiator and begins the curing process within the epoxy. The curing process results in a corresponding change of the index of refraction within the resin. However, it is also to be appreciated that terminating the exposure to the particular wavelength of light ceases the curing of the epoxy, and ceasing the change of the index of refraction exhibited by the epoxy. In this manner, a aberrator  100  of the present invention may be formed by exposing certain portions of the resin  104  to a light source which varies with time and position, resulting in an aberrator having a varied index of refraction across its surface. 
     From  FIG. 2 , a variety of refractive index profiles are shown to be formed in resin layer  104 . More specifically, different refractive index profile is illustrated by regions  110 ,  112 , and  114 , such that aberrator  100  includes three (3) distinct levels of refractive index. 
     It is to be appreciated that the incorporation of three (3) different levels of refractive index in  FIG. 2  is merely exemplary, and the present invention contemplates the incorporation of any number of refractive index profiles, and that those different profiles may be formed within epoxy layer  104  to have virtually any shape or local curvature. Moreover, the epoxy layer  104  may be considered to be an array of pixels, such as pixels  109 ,  111 ,  113 , which may each be selectively illuminated and cured to exhibit a particular index of refraction. 
     Referring now to  FIG. 3 , a cross-sectional view of the wavefront aberrator of the present invention taken along line  3 — 3  of  FIG. 1  is shown. Epoxy layer  104  is sandwiched between the upper transparent cover  106  and the lower transparent cover  102 , and held in place by barrier  108 . The enclosed volume of epoxy layer  104  is determined by the size of the barrier  108 , and the distance between the upper transparent cover  106  and the lower transparent cover  102 . In a preferred embodiment, the thickness  116  of the epoxy layer  104  is approximately 0.005 inches (0.125 mm), and the thicknesses  118  of the upper transparent cover  106  is approximately 0.025 inches (0.625 mm), and the thicknesses  120  of the lower transparent cover  102  is approximately 0.025 inches. 
     In a preferred embodiment, upper transparent cover  106  and lower transparent cover  104  are formed from a rigid transparent material, such as glass or plastic. While glass provides a stable platform for the formation of the refractive index profile, such rigidity is not necessary. In fact, covers  102  and  106  may be made from a flexible material, such as a transparent polymer. A suitable transparent polymer may include, but not be limited to, mylar film, polycarbonate film, or acetate film. Use of such materials results in a flexible aberrator having a distinct refractive index profile. 
     METHODS OF MANUFACTURING 
     Referring to  FIG. 4 , a system for manufacturing the wavefront aberrator of the present invention is shown and generally designated  130 . System  130  includes a light emitting diode (LED) array panel  132  having a number of diodes  135 ,  137 , separated from adjacent diodes by a distance  134 , and controlled by a computer  136  through interface  138 . In a preferred embodiment, the distance  134  between diodes  135  and  137  varies, and may typically be approximately 0.125 inches (3.175 mm), though alternative distances may be used. A diffuser element  140  may be placed between LED array panel  132  and wavefront aberrator  100  to diffuse the light emitted by the LED array panel  132  to create a smoother refractive index profile. 
     In operation, once a desired refractive index profile is determined, computer  136  determines a particular pattern to be illuminated in the LED array panel  132  thereby generating a curing pattern which is directed through diffuser element  140  onto a aberrator  100 . By selectively illuminating particular LEDs  135  and  137 , for example, within the LED array panel  132 , the epoxy (not shown this Figure) is selectively cured. This selective curing creates a pre-determined, particular refractive index profile corresponding to the time of exposure of the epoxy as well as the intensity of the exposure. This selective curing results in an aberrator with areas having different indices of refraction. Thus, by varying the intensity and period of illumination of LEDs  135  and  137 , for example, the aberrator may be formed to exhibit the desired refractive index profile. 
     Referring now to  FIG. 5 , a system for manufacturing the wavefront aberrator  100  of the present invention is shown in a side view and generally designated  150 . System  150  includes an LED array panel  132  where each LED  151  generates a light beam  154  having an diverging angle  152 , and the LEDS collectively generate a curing pattern which is directed through a demagnifier imaging element  156  which focusses the curing pattern into light pattern  158  and onto a wavefront aberrator  100  to cure the epoxy (not shown this Figure) within the aberrator  100  to create a particular wavefront profile as shown in  FIG. 2 . Alternatively, the curing pattern can be magnified, instead of demagnified, to produce a larger area aberrator device. 
       FIG. 6  depicts a system for manufacturing the wavefront aberrator  100  of the present invention and is generally designated  170 . System  170  includes a light source  172  adjacent a diffuser  174  which smooths the light beams  178  and creates uniform intensity light rays  180 . Light rays  180  pass through a computer controlled LCD  176  which acts as a spatial light intensity modulator and generates a curing pattern  182  such that when the LCD is exposed light rays  180  from light source  172 , light corresponding to the curing pattern  182  is transmitted through the LCD  176  and onto the wavefront aberrator  100  to create a particular refractive index profile  184 . 
     In a preferred embodiment, light source  172  of system  170  is a constant fluence light having a constant intensity across the illuminated surface of the light. For example, light source  172  may contain an array of LEDs, or any other suitable source of illumination. The optical transmissive properties of the LCD can be controlled by applying a variable electrical voltage to an array of electrodes on an LCD device. This provides for the spatial and temporal variation of the intensity of light transmitted through the LCD device to selectively cure the resin  104  in the aberrator  100 . 
     As an addition to system  170 , a detector  185  may be placed beneath aberrator  100  to detect the transmitted image  186  through aberrator  100 . A feedback interface  188  may connect sensor  185  to computer  189 , which may in turn control LCD panel  176 . In this manner, a refractive index profile may be determined in the computer  189 , implemented in the LCD  176 , and verified in sensor  185 , thereby ensuring the appropriate wavefront profile was created in aberrator  100 . Sensor  185  may include a intensity imager, such as a CCD or a wavefront sensor, such as a Shack-Hartmann sensor. 
     Although panel  176  is discussed above as a LCD panel, an alternative embodiment could incorporate a photographic negative or positive that may be used to form the refractive index profile  184  in aberrator  100 . In this manner, light source  172  would present a constant source of illumination, and the photographic negative or positive containing the refractive index profile  182  would control the spatial and intensity level of illumination reaching aberrator  100  to create the proper refractive index profile  184 . 
     Referring now to  FIG. 7 , an alternative system for manufacturing the wavefront aberrator  100  of the present invention is shown and generally designated  190 . System  190  includes a beam scan unit  195  having a laser unit  191  generating a laser beam  193  which forms a point light source (“spot”)  192  on aberrator  100  which may include a laser intensity control (not shown). Spot  192  is moved across the surface of the aberrator  100  in a rastering path shown by dashed lines  194 ,  196 , and  198 , at varying speeds and with varying intensities to selectively cure the epoxy  104  to create a particular refractive index profile  212  having areas  214 ,  215 , and  216 , with different indices of refraction. 
     Alternatively, a spot  200  may be formed and moved across aberrator  100  in paths  202 ,  204  and  206 . Yet another alternative method of forming refractive index profile  212  includes the formation of spot  210  in the center of aberrator  100 , and movement of the spot along an outwardly spiraling path  212 . Also, a particular refractive index profile  212  may be traced, or circumscribed in a predetermined area, by laser beam  193  directly forming the boundaries between the areas  214 ,  215 , and  216 , for example. In an alternative embodiment, laser beam  193  may remain stationary and the aberrator device  100  may be moved relative to the laser beam  193  such that the spot  210  moves across the surface of the aberrator. Specifically, aberrator  100  may be moved in directions  220  and/or  222  to move the spot  210  across the surface of the aberrator. 
     ALTERNATIVE EMBODIMENTS 
       FIG. 8  is a side view of an alternative embodiment of the wavefront aberrator  100  of the present invention incorporating a transparent cover  232  formed in the shape of a lens having a face  233  showing a lens with position focusing power. Alternatively, a lens with negative focussing power and with cylindrical (astigmatism) power may also be incorporated. Sandwiched between face  233  and a transparent cover  236  is a layer  234  of index-changing epoxy. Transparent cover  232  has a spherical refractive surface  238  which functions an optical element. Thus, the cover  232  in combination with epoxy layer  234 , provides for an optical element having both focusing and wavefront phase profile characteristics. 
     An alternative embodiment of the wavefront aberrator of the present invention is shown in  FIG. 9  and generally designated  240 . Aberrator  240  includes an upper transparent window  242  and an adjacent layer  244  of index-changing epoxy. A lower transparent window  246  (shown in dashed lines) is formed from a dissolvable salt. Once the refractive index profile has been formed in the layer  244  of epoxy, the salt window  246  may be dissolved. The dissolving nature of window  246  provides for an exposed epoxy layer facilitating post curing treatment of the epoxy if necessary. Alternatively, windows  242  and  246  may be made of organic materials which are dissolvable in organic solvents. 
     USES FOR THE PRESENT INVENTION 
     The present invention may be used to correct aberrations in virtually any optical system. For instance, the present invention may be particularly useful to correct inherent static aberrations in optical imaging systems, such as telescopes, binoculars, or microscopes. The present invention may also be particularly useful by incorporating aberration corrections into eyepieces of optical systems such as telescopes, binoculars, or microscopes. 
     The aberrator of the present invention may also be used to correct static aberrations in laser beams or associated optics for use in laser ranging, detection, scanning, communication, or tracking instruments. This listing of uses for the present invention is merely exemplary, and is not intended to limit the scope of the invention whatsoever. 
     While there have been shown what are presently considered to be preferred embodiments of the present invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope and spirit of the invention.