Abstract:
Optical field flattener and converter having an array of gradient index rod lenses. Each gradient index rod lens is substantially in proximity with at )east one other gradient index rod lens. The array is capable of receiving electromagnetic radiation and imaging the received electromagnetic radiation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority of U.S. Provisional Application 60/673,585 entitled “OPTICAL FIELD FLATTENERS AND CONVERTERS,” filed on Apr. 21, 2005, which is incorporated by reference herein. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   This invention was made partially with U.S. Government support from the Air Force Research Lab under Contract No. F33615-99-C-1410. The U.S. Government has certain rights in the invention. 

   BACKGROUND OF THE INVENTION 
   This invention relates generally to optical field flatteners and converters. 
   A field flattener or field converter is a known device that is commonly used to flatten or modify the field curvature of an optical system, wherein the term “field converter” refers to the general case of modifying one general curve to another and the term “field flattener” refers to the special case of converting a general curve to a plane. Optical imaging elements, including but not limited to lenses, mirrors, and diffraction gratings, typically introduce a degree of field curvature into an optical system, sometimes referred to as the Petzval curvature. The presence of this optical aberration causes the focus as a function of spatial field position to deviate from a common plane, typically in a predominantly spherical or aspherical fashion. Applicable detecting arrays, including CCDs and Multiple Quantum Well (MQW) structures, however, are for the most part constrained to planar geometries due to the inherent lithographic and epitaxial fabrication technologies. This mismatch between the image locus of optical systems aberration by field curvature and these planar detector arrays results in image degradation as a function of spatial field, particularly for large fields. 
   The classic approach to compensating for this fundamental mismatch is to make use of refractive solutions in lens design, chiefly the technique originating in 1872 with C. Piazzi-Smyth in which a negative field lens is placed adjacent to the image plane and is well known in the art. When a lens is placed near a focal plane it makes little contribution to the optical power, but can have a pronounced effect on the field curvature. This Piazzi-Smyth field flattener is a standard tool used in reducing the mismatch between curved image planes and planar detectors such as the classic photographic plates and solid-state detector arrays. While this refractive field flattener approach is effective for the types of field curvatures formed in typical lens systems, it is often not capable of correcting the large field curvatures generated in extremely compact or miniaturized optical imaging systems or those generated by many dispersive elements utilized in spectrometer or hyperspectral imaging systems. 
   Current field flattening and field conversion designs are either limited in their field flattening or field conversion capabilities, are too complex or costly to fabricate, or introduce unwanted optical aberrations. 
   There is therefore a need for an optical field flattener design that is more compact in physical size than current field flatteners. 
   Furthermore, there is also a need for an optical field flattener design that is optically faster than current field flatteners. 
   Furthermore, there is also a need for an optical field flattener design that is capable of correcting larger amounts of field curvature than current field flatteners. 
   Furthermore, there is also a need for an optical field flattener design that is self-corrected for optical aberrations. 
   Still further, there is also a need for an optical field flattener design that provides a combination of the characteristics described above with superior trade-offs than have been previously attainable. 
   BRIEF SUMMARY OF THE INVENTION 
   The needs set forth above as well as further and other needs and advantages of the present invention are achieved by the embodiments of the invention described herein below. 
   In one embodiment, the optical system of this invention includes a number of gradient index rod lenses arranged in an array, where in the array, each gradient index rod lens is substantially in proximity with at least one other gradient index rod lens. The array is capable of receiving electromagnetic radiation from a source and of imaging the received electromagnetic radiation onto an image surface. The gradient index rod lenses are selected in order to image the received electromagnetic radiation onto a focal locus of said image surface and at least one surface of said array is a non-planar surface. 
   In one instance, a surface of the array that is closest to the source is a non-planar surface. In another instance, a surface of the array that is closest to the image surface is a non-planar surface. 
   For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawings and detailed description and its scope will be pointed out in the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic sectional view of a gradient index field flattener in accordance with an embodiment of the present invention, taken along the optical axis; 
       FIG. 2A  is a schematic sectional view of a gradient index field flattener in accordance with a further embodiment of the present invention, taken along the optical axis; 
       FIG. 2B  is an isometric view of the embodiment of the present invention illustrated in  FIG. 2A ; 
       FIG. 3A  is a schematic sectional view of a gradient index field flattener in accordance with a further embodiment of the present invention, taken along the optical axis; 
       FIG. 3B  is an isometric view of the embodiment of the present invention illustrated in  FIG. 5A ; 
       FIG. 4  is a schematic sectional view of a gradient index field flattener in accordance with a further embodiment of the present invention, taken along the optical axis; 
       FIG. 5  is a schematic sectional view of a gradient index field flattener in accordance with a further embodiment of the present invention, taken along the optical axis; 
       FIG. 6A  is a schematic sectional view of a gradient index field flattener in accordance with a further embodiment of the present invention, taken along the optical axis; 
       FIG. 6B  is an isometric view of the embodiment of the present invention illustrated in  FIG. 6A ; 
       FIG. 7  is a schematic sectional view of a gradient index field flattener in accordance with a still further embodiment of the present invention, taken along the optical axis; 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In one embodiment, gradient index rod lens arrays are utilized to form high performance field flatteners and field converters. The term GRAFF is an acronym for Gradient Rod Array Field Flattener and is used to describe systems such as the systems of this invention. The GRAFF utilizes gradient index rod lenses to re-image local spatial sub-fields from one focal locus to another. Gradient index rod lenses, similar to gradient index optical fibers, are dielectric cylinders with a predetermined variation, parabolic or similar radial variation in one embodiment, of refractive index that causes the incident light to propagate in sinusoidal or other nonlinear trajectories within the element. The length of each rod lens in the array is selected such that the corresponding sub-field of one focal locus is re-imaged to the corresponding local sub-field of another, and in many configurations, each sub-field may be imaged by multiple rod lenses in the array. In this manner, a focal locus of one curvature can be re-imaged piecewise to a focal locus of another curvature, where these curvatures can vary from planar surfaces to significantly large sections of a spherical, aspherical, or other non-planar surface. The width of the rod lenses can be varied from a fraction of a millimeter to several millimeters, as required to keep the small local field sag within the depth of focus of the rod lens, and the array itself can be coupled with additional refracting elements to reduce residual optical aberrations and increase the performance of the device. 
   This invention introduces another degree of freedom in the design of tightly constrained and complex high performance optical systems. In the design of such systems, degrees of freedom are utilized to meet fixed system performance requirements, and can typically only be introduced or expanded at the cost of increased complexity. For example, additional elements or exotic materials can be introduced into a multi-element system to correct chromatic aberrations. One of these fixed system requirements is typically a flat image or dispersion field. The ability to allow the curvature of the image or dispersion field to float as an unconstrained parameter releases degrees of freedom for increasing system performance in other ways. The invention disclosed herein can be used to relay the resulting curved image field onto a planar detecting element or other non-planar image surfaces without introducing additional optical aberrations. 
   Reference is made to  FIG. 1 , which is a schematic sectional view of an embodiment of this invention  10 , taken along the optical axis  20 . In operation, electromagnetic radiation, typically in the ultraviolet, visible, and/or infrared bands, hereinafter referred to generally as light, emitted or reflected by a given object, either real or virtual, located along a non-planar object locus  30 , is imaged by an array of radial gradient index rods  50 , consisting of multiple radial gradient index rods  52 , and having curved front and back surfaces  40  and  60  respectively, onto a CCD array, phosphorescent screen, photographic film, microbolometer array, or other means of detecting light energy, hereinafter referred to generally as a detecting element  70 . 
   The surface curvatures  40  and  60  provide variable object to gradient index rod separation, gradient index rod length, and gradient index rod to image separation distances as a function of spatial field position. These variations combine to provide variable object to image conjugate distances as a function of spatial field position that are used to re-image the non-planar object locus  30  to the planar image locus  70 , thereby flattening the original object field. These curved surfaces  40  and  60  also introduce a degree of optical aberrations that can limit the optical speed and imaging quality of the device. 
   Reference is made to  FIG. 2A , which is a schematic sectional view of a further embodiment of this invention  100 , taken along the optical axis  20 . In operation, light located along a non-planar object locus  110 , is imaged through an array of radial gradient index rods  130 , consisting of multiple radial gradient index rods  132 , and having a substantially planar front surface  120  and curved back surface  140 . The light then propagates through a refractive element  150 , which is in contact with the gradient index rod array  130 , with a curved front surface  140 , which is coincident with the back surface of the gradient index rod array  130 , and a substantially planar back surface  160 , onto a detecting element  170 . The substantially planar surfaces  120  and  160  reduce the optical aberrations of the system, while the curved contact surface  140  provides the variable object to gradient index rod separation, gradient index rod length, and gradient index rod to image separation distances as a function of spatial field position required to flatten the field. 
   Reference is made to  FIG. 2B , which is an isometric view of this same embodiment  100 . The gradient index rods  132  that comprise the gradient index rod array  130  are arranged in a packing configuration, the preferred embodiment of which is a hexagonally packed array, to provide the required degree of spatial field coverage. The gradient index rod array  130  and refracting element  150  are typically combined to form a cemented doublet configuration. 
   It should be realized that a non-refractive or diffractive optical element may also be used in place of the refractive element  150  of embodiment  100 . 
   Reference is made to  FIG. 3A , which is a schematic sectional view of a further embodiment of this invention  200 , taken along the optical axis  20 . In operation, light located along a non-planar object locus  210 , is imaged through an array of radial gradient index rods  230 , consisting of multiple radial gradient index rods  232 , and having a substantially planar front surface  220  and curved back surface  240 . The light then propagates through a refractive element  250 , which is in contact with the gradient index rod array  230 , with a curved front surface  240 , which is coincident with the back surface of the gradient index rod array  230 , and a substantially planar back surface  260 , onto a detecting element  270 . The substantially planar surfaces  220  and  260  reduce the optical aberrations of the system, while the curved contact surface  240  provides the variable object to gradient index rod separation, gradient index rod length, and gradient index rod to image separation distances as a function of spatial field position required to flatten the field. 
   Reference is made to  FIG. 3B , which is an isometric view of this same embodiment  200 . The gradient index rods  232  that comprise the gradient index rod array  230  are arranged in a packing configuration, the preferred embodiment of which is a linearly packed array cemented between two structural substrates  234 , to provide the required degree of spatial field coverage, the preferred embodiment of which is a narrow slit field suitable for line imaging systems. The gradient index rod array  230  and structural substrates  234  are typically combined as a single element that is in turn combined with refracting element  250  to form a cemented doublet configuration. 
   A greater degree of field curvature correction can be attained by introducing some degree of curvature to one or more of the substantially planar surfaces. Reference is made to  FIG. 4 , which is a schematic sectional view of a further embodiment of this invention  300 , taken along the optical axis  20 . In operation, light located along a non-planar object locus  310 , is imaged through an array of radial gradient index rods  330 , consisting of multiple radial gradient index rods  332 , and having a curved front and back surfaces  320  and  340  respectively. The light then propagates through a refractive element  350 , which is in contact with the gradient index rod array  330 , with a curved front surface  340 , which is coincident with the back surface of the gradient index rod array  330 , and a substantially planar back surface  360 , onto a detecting element  370 . The slight curvature of surface  320  provides a greater degree of field curvature correction without introducing significant optical aberrations of the system. 
   A still greater degree of field curvature correction can be attained by introducing some degree of curvature to all surfaces. Reference is made to  FIG. 5 , which is a schematic sectional view of a further embodiment of this invention  400 , taken along the optical axis  20 . In operation, light located along a non-planar object locus  410 , is imaged through an array of radial gradient index rods  430 , consisting of multiple radial gradient index rods  432 , and having a curved front and back surfaces  420  and  440  respectively. The light then propagates through a refractive element  450 , which is in contact with the gradient index rod array  430 , with a curved front surface  440 , which is coincident with the back surface of the gradient index rod array  430 , and a curved back surface  460 , onto a detecting element  470 . The curvatures of surfaces  420  and  460  provide a still greater degree of field curvature correction, which is balanced against the optical aberrations that are introduced to the system. 
   Reference is made to  FIG. 6A , which is a schematic sectional view of a further embodiment of this invention  500 , taken along the optical axis  20 . In operation, light located along a non-planar object locus  510 , propagates through a first refractive element  530 , having a substantially planar or curved front surface  520  and curved back surface  540 . The light is then imaged through an array of radial gradient index rods  550 , consisting of multiple radial gradient index rods  552 , with a curved front surface  540 , which is coincident with the back surface of the first refractive element  530 , and a curved back surface  560 . The light then propagates through a second refractive element  570 , which is in contact with the gradient index rod array  550 , with a curved front surface  560 , which is coincident with the back surface of the gradient index rod array  550 , and a planar or curved back surface  580 , onto a detecting element or image field  590 . The curved contact surfaces  540  and  560  provide the variable object to gradient index rod separation, gradient index rod length, and gradient index rod to image separation distances as a function of spatial field position required to convert the curved object field  510  to the desired curved image field  590 . The planar or curved surfaces  520  and  580  can be optimized to correct the optical aberrations of the system. 
   Reference is made to  FIG. 6B , which is an isometric view of this same embodiment  500 . The gradient index rods  552  that comprise the gradient index rod array  550  are arranged in a packing configuration, the preferred embodiment of which is a linearly packed array cemented between two structural substrates  554 , to provide the required degree of spatial field coverage, the preferred embodiment of which is a narrow slit field suitable for line imaging systems. The gradient index rod array  550  and structural substrates  554  are typically combined as a single element that is in turn combined with refracting elements  530  and  570  to form a cemented triplet configuration. 
   It should be realized that a non-refractive or diffractive optical element may also be used in place of the refractive element  530  of embodiment  500 . 
   Reference is made to  FIG. 7 , which is a schematic sectional view of a still further embodiment of this invention  600 , taken along the optical axis  20 , and illustrates a modification of the embodiment  500  where a slit or other method of extracting a line image, hereinafter referred to generally as a slit element  654 , is located at an intermediate image locus located within the unmodified gradient index rod array  550 . In operation, light located along a non-planar object locus  510 , propagates through a first refractive element  530 , having a substantially planar or curved front surface  520  and curved back surface  540 . The light is then imaged through a first array of radial gradient index rods  650 , consisting of multiple radial gradient index rods  652 , with a curved front surface  540 , which is coincident with the back surface of the first refractive element  530 , onto a slit element  654 . The light is then imaged through a second array of radial gradient index rods  656 , consisting of multiple radial gradient index rods  658 , with a curved back surface  560 . The light then propagates through a second refractive element  570 , which is in contact with the second gradient index rod array  656 , with a curved front surface  560 , which is coincident with the back surface of the second gradient index rod array  550 , and a substantially planar or curved back surface  580 , onto a detecting element or image field  590 . The curved contact surfaces  540  and  560  provide the variable object to gradient index rod separation, gradient index rod length, and gradient index rod to image separation distances as a function of spatial field position required to convert the curved object field  510  to the desired curved image field  590 . The substantially planar or curved surfaces  520  and  580  can be optimized to correct the optical aberrations of the system. The light imaged at the slit element  654  may be multiple in occurrence due to the possibility of multiple imaging paths through the first array of gradient index rods  650 . These multiple images, however, remain within the same plane along which the slit element  654  is oriented. The second array of gradient index rods  656  recombines these multiple images at the detecting element or image field  590 . 
   It should be realized that any aperture shape may be used in place of the slit element  654  of embodiment  600 . It should further be realized that any optical element, including a diffractive element such as a grating, may be used in place of the slit element  654  of embodiment  600 . 
   Although the invention has been described with respect to various embodiments, it should be realized this invention is also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims.