Apparatus for forming homogeneous intensity distribution with bright or dark edges

Abstract

To provide an apparatus for forming a homogeneous intensity distribution with bright or dark edges.SOLUTION: A beam homogenizer (30) for transforming a beam of laser-radiation into a flat-top intensity distribution having brighter or darker edges comprises a first lens array (32), a second lens array (14), and a positive lens (16). The first lens array (32) includes a plurality of lens elements (20) that are separated by gaps (34) having no optical power. Brighter or darker edges are produced by selecting a distance between the first and second lens arrays that is less than or greater than the focal length of lens elements in the second lens array.SELECTED DRAWING: Figure 2A

Description

[Technical field]

[0001] This application claims priority to U.S. Provisional Application No. 62 / 817,331, filed March 12, 2019, the disclosure of which is incorporated herein by reference in its entirety.

[0002] The present invention relates generally to shaping light from a laser, and more particularly to shaping light from one or more lasers into a beam of radiation having a uniform cross-section with brighter or darker edges. [Background technology]

[0003] Lasers have become an essential source for uniform illumination in a wide range of applications including surface inspection of semiconductor materials, thermal annealing of display screen glass, metal hardening, and rapid assay of biomedical fluids. A common requirement is an elongated beam of laser radiation that uniformly illuminates a line on a flat surface or plane within a volume of a transparent material. Such an elongated beam of laser radiation is generally referred to as a "line beam."

[0004] Diode lasers are efficient devices for converting electrical power into coherent optical power. For high power applications, diode laser bars with multiple diode laser emitters provide a convenient way to scale the optical power available from a single diode laser emitter. The diode laser emitters are spaced apart and arranged in a linear array. Diode laser bars inherently have an elongated emission cross section, but need to overcome the non-uniform intensity distribution resulting from emission from multiple spatially distributed diode laser emitters.

[0005] An optical device for modifying a laser beam to make the intensity uniform across the cross section of the transformed beam is generally referred to as a "beam homogenizer". A beam homogenizer often includes one or two "microlens arrays" with multiple lenslets, each of which is much smaller than the incident beam. Each microlens is an illumination source that contributes to the transformed beam. The "pitch" of the beam homogenizer is the distance between the centers of adjacent microlenses. Additional optics are required to collect and shape the multiple beams emanating from all the microlenses that intercept the incident beam. A linear array of microlenses can be used as a beam homogenizer for an elongated beam of laser radiation emitted by a diode laser bar. One example of such a beam homogenizer is described in U.S. Pat. No. 7,265,908.

[0006] Diode-pumped solid-state lasers, including fiber lasers, typically produce a Gaussian intensity distribution. A Gaussian laser beam can be converted to a nearly uniform intensity distribution by a Powell lens or equivalent optics and then homogenized using a lens array. Collimated laser beams from multiple such diode-pumped solid-state lasers can be combined and homogenized using a lens array.

[0007] Slab lasers include diode-pumped solid-state lasers, gas discharge CO2 lasers, and excimer lasers. Slab lasers essentially produce a line beam. However, the line beam may not be uniform in cross section, and the cross-sectional intensity distribution may be unstable. Lens arrays are used to homogenize the line beam from a slab laser and to combine the line beams from multiple slab lasers.

[0008] In processes involving heating a portion of a workpiece by laser illumination, a perfectly uniform cross-sectional intensity distribution may not produce uniform laser processing due to heat transfer away from the illuminated portion. Illumination with a laser beam having a higher intensity at the edge of the intensity distribution may be preferred to ensure uniform heating. In other processes requiring multiple passes of a line beam to illuminate many areas on a workpiece, adjacent passes with some overlap will ensure complete exposure but may result in differences in processing between portions illuminated once and portions illuminated twice. Illumination with a laser beam having a lower intensity at the edge of the intensity distribution may be preferred to prevent overexposure.

[0009] A need exists for a beam shaping device for shaping a laser beam having an otherwise uniform cross-sectional intensity distribution with brighter or darker edges. Preferably, the optical device would have the precision, simplicity, and compactness of a lens array homogenizer. Summary of the Invention [Means for solving the problem]

[0010] In one aspect, a beam homogenizer according to the present invention comprises a first lens array having a plurality of identical lens elements. Each lens element of the first lens array has a focal length F1. Neighboring lens elements of the first lens array are separated by gaps, each gap having a common width. A second lens array is provided. Each lens element of the second lens array has a focal length F2. The first lens array and the second lens array are separated by a distance not equal to F2. A positive lens is provided and located on the optical axis. The positive lens has a focal length F3. In order, the first lens array, the second lens array, and the positive lens are arranged along the optical axis.

[0011] In another aspect, a beam homogenizer according to the present invention comprises a first lens array having a plurality of identical lens elements. Each lens element of the first lens array has a flat central portion and a curved outer portion. The central portion has no refractive power and the outer portion has a focal length F1. A second lens array is provided. Each lens element of the second lens array has a focal length F2. The first lens array and the second lens array are separated by a distance not equal to F2. A positive lens is provided, which is located on the optical axis and has a focal length F3. In order, the first lens array, the second lens array, and the positive lens are arranged along the optical axis.

[0012] In yet another aspect, the laser radiation propagates sequentially through the lens elements and gaps of the first lens array, through the second lens array, and through the positive lens onto the illumination plane. The illumination plane is located a distance F3 from the positive lens. The laser radiation propagating through the lens elements of the first lens array has a different magnification at the illumination plane than the laser radiation propagating through the gaps of the first lens array.

[0013] In yet another aspect, the laser radiation propagates, in sequence, through the first lens array, through the second lens array, and through the positive lens onto the illumination plane, which is located a distance F3 from the positive lens. The laser radiation on the illumination plane is transformed into a homogenized intensity distribution having brighter or darker edges in an otherwise uniform intensity distribution. The present invention provides, for example, the following: (Item 1) 1. A beam homogenizer comprising: a first lens array having a plurality of identical lens elements, each lens element of the first lens array having a focal length F1, adjacent lens elements of the first lens array being separated by gaps, each gap having a common width; a second lens array, each lens element of the second lens array having a focal length F2, the first lens array and the second lens array being separated by a distance unequal to F2; and a positive lens located on an optical axis having a focal length F3, the first lens array, the second lens array, and the positive lens being arranged along the optical axis, in that order; A beam homogenizer comprising: (Item 2) 2. The beam homogenizer of claim 1, wherein the first lens array and the second lens array are separated by a distance greater than F2. (Item 3) 2. The beam homogenizer of claim 1, wherein the first lens array and the second lens array are separated by a distance that is less than F2. (Item 4) 2. The beam homogenizer of claim 1, wherein the first lens array and the second lens array are separated by a distance F2±δ, where δ is less than 10% of F2. (Item 5) 5. The beam homogenizer according to claim 4, wherein δ is less than 5% of F2. (Item 6) 6. The beam homogenizer of any one of claims 1 to 5, wherein the gaps between adjacent lens elements of the first lens array are empty gaps or air gaps. (Item 7) 6. The beam homogenizer of any one of claims 1 to 5, wherein the gaps between adjacent lens elements of the first lens array are made from the same material as the lens elements of the first lens array. (Item 8) 8. The beam homogenizer of claim 7, wherein the material from which the gaps and the lens elements of the first array are made is transparent glass or polymer. (Item 9) 9. The beam homogenizer of any one of items 1 to 8, wherein the gaps between adjacent lens elements of the first lens array fixedly connect the adjacent lens elements. (Item 10) 10. The beam homogenizer of any one of items 1 to 9, wherein the gaps between adjacent lens elements of the first lens array have no optical power. (Item 11) Item 11. The beam homogenizer of item 10, wherein the gaps between adjacent lens elements of the first lens array are in the form of slabs having plane-parallel faces. (Item 12) Item 11. The beam homogenizer of item 10, wherein the gaps between adjacent lens elements of the first lens array are in the form of wedges having non-parallel planar surfaces. (Item 13) 6. The beam homogenizer of any one of items 1 to 5, wherein the gaps between adjacent lens elements of the first lens array are occupied by optical elements having a focal length at least 5 times longer than F1. (Item 14) 14. The beam homogenizer according to any one of items 1 to 13, wherein an image having a repeated pattern of wider and narrower bands located in an image plane at a distance F2 from the second lens array is imaged onto an illumination plane located at a distance F3 from the positive lens. (Item 15) Item 15. The beam homogenizer of any one of items 1 to 14, wherein the first lens array is configured and arranged to be translated longitudinally parallel to the optical axis. (Item 16) 1. A beam homogenizer comprising: a first lens array having a plurality of identical lens elements, each lens element of the first lens array having a flat central portion and a curved outer portion, the central portion having no optical power and the outer portion having a focal length F1; a second lens array, each lens element of the second lens array having a focal length F2, the first lens array and the second lens array being separated by a distance unequal to F2; and a positive lens located on an optical axis having a focal length F3, the first lens array, the second lens array, and the positive lens being arranged along the optical axis, in that order; A beam homogenizer comprising: (Item 17) Item 17. The beam homogenizer of item 16, wherein the first lens array is constructed and arranged to be translated longitudinally parallel to the optical axis. (Item 18) 1. A beam homogenizer for laser radiation, comprising: a first lens array having a plurality of identical lens elements, adjacent lens elements of the first lens array being separated by gaps, each gap having a common width; a second lens array, each lens element of the second lens array having a focal length F2, the first lens array and the second lens array being separated by a distance unequal to F2; and a positive lens having a focal length F3, wherein the laser radiation propagates sequentially through lens elements and gaps of the first lens array, through the second lens array, through the positive lens, and onto an illumination plane, the illumination plane being located a distance F3 from the positive lens; wherein laser radiation propagating through the lens elements of the first lens array has a different expansion at the illumination plane than laser radiation propagating through the gaps in the first lens array. (Item 19) Item 19. The beam homogenizer of item 18, wherein the first lens array and the second lens array are separated by a distance greater than F2, and the laser radiation propagates through lens elements of the first lens array, thereby having a smaller magnification than laser radiation propagating through the gaps of the first lens array. (Item 20) Item 19. The beam homogenizer of item 18, wherein the first lens array and the second lens array are separated by a distance that is less than F2, and the laser radiation propagates through lens elements of the first lens array, thereby having a greater magnification than laser radiation propagating through the gaps of the first lens array. (Item 21) 1. A beam homogenizer for laser radiation, comprising: a first lens array having a plurality of identical lens elements, adjacent lens elements of the first lens array being separated by gaps, each gap having a common width; a second lens array, each lens element of the second lens array having a focal length F2, the first lens array and the second lens array being separated by a distance unequal to F2; and a positive lens having a focal length F3, wherein the laser radiation propagates sequentially through the first lens array, through the second lens array, and through the positive lens onto an illumination plane, the illumination plane being located at a distance F3 from the positive lens, and wherein the laser radiation on the illumination plane is transformed into a homogenized intensity distribution having brighter or darker edges in an otherwise uniform intensity distribution; A beam homogenizer comprising: [Brief description of the drawings]

[0014] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

[0015] [Figure 1A]1A is a plan view diagrammatically illustrating a prior art beam homogenizer having a first lens array, a second lens array, and a positive lens. Collimated laser radiation propagating parallel to the optical axis is incident on the first lens array and propagates through the beam homogenizer.

[0016] [Figure 1B] Figures 1B and 1C are plan views that diagrammatically illustrate imaging through the beam homogenizer of Figure 1 A. Inverted and magnified images of each lens element in the first lens array are formed and superimposed at the illumination plane. [Figure 1C] Figures 1B and 1C are plan views that diagrammatically illustrate imaging through the beam homogenizer of Figure 1 A. Inverted and magnified images of each lens element in the first lens array are formed and superimposed at the illumination plane.

[0017] [Figure 2A] 2A-2C are plan views diagrammatically illustrating one preferred embodiment of a beam homogenizer according to the present invention. The beam homogenizer of the present invention comprises a first lens array, a second lens array, and a positive lens. Each lens element of the first lens array is separated by a gap having no optical power. The distance between the first lens array and the second lens array is slightly greater than the focal length of each lens element of the second lens array. [Figure 2B] 2A-2C are plan views diagrammatically illustrating one preferred embodiment of a beam homogenizer according to the present invention. The beam homogenizer of the present invention comprises a first lens array, a second lens array, and a positive lens. Each lens element of the first lens array is separated by a gap having no optical power. The distance between the first lens array and the second lens array is slightly greater than the focal length of each lens element of the second lens array. [Figure 2C]2A-2C are plan views diagrammatically illustrating one preferred embodiment of a beam homogenizer according to the present invention. The beam homogenizer of the present invention comprises a first lens array, a second lens array, and a positive lens. Each lens element of the first lens array is separated by a gap having no optical power. The distance between the first lens array and the second lens array is slightly greater than the focal length of each lens element of the second lens array.

[0018] [Figure 3A] Figures 3A and 3B are plan views diagrammatically illustrating another preferred embodiment of a beam homogenizer according to the present invention, which is similar to the embodiment of Figures 2A-2C, but in which the distance between the first lens array and the second lens array is slightly less than the focal length of each lens element of the second lens array. [Figure 3B] Figures 3A and 3B are plan views diagrammatically illustrating another preferred embodiment of a beam homogenizer according to the present invention, which is similar to the embodiment of Figures 2A-2C, but in which the distance between the first lens array and the second lens array is slightly less than the focal length of each lens element of the second lens array.

[0019] [Figure 4A] FIG. 4A diagrammatically illustrates the intensity distribution in the illumination plane for a beam of laser radiation propagating through the beam homogenizer of FIGS. 2A-2C.

[0020] [Figure 4B] FIG. 4B diagrammatically illustrates the intensity distribution in the illumination plane for a beam of laser radiation propagating through the beam homogenizer of FIGS. 3A and 3B.

[0021] [Figure 5A] 5A and 5B are plan views that diagrammatically illustrate the effect of diffraction on a beam of laser radiation propagating through the beam homogenizer of FIGS. 2A-2C. [Figure 5B]5A and 5B are plan views that diagrammatically illustrate the effect of diffraction on a beam of laser radiation propagating through the beam homogenizer of FIGS. 2A-2C.

[0022] [Figure 6A] FIG. 6A diagrammatically illustrates a camera image of the intensity distribution in the illumination plane for a beam of laser radiation propagating through the example beam homogenizer of FIGS. 3A and 3B.

[0023] [Figure 6B] FIG. 6B is a graph that diagrammatically illustrates the intensity distribution along line AA in FIG. 6A. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Referring now to the drawings, in which like components are designated by like numbers, FIG. 1A diagrammatically illustrates a prior art beam homogenizer 10 having a first linear lens array 12, a second linear lens array 14, a positive lens 16, and an optical axis 18. Each identical lens element 20 of the lens array 12 has a focal length F1, each identical lens element 22 of the lens array 14 has a focal length F2, and the lens 16 has a focal length of F3. Each lens element 20 and 22 has a common width d. The lens array 12 and the lens array 14 are separated by a distance F2. The drawing depicts rays 24 of laser radiation propagating through the beam homogenizer 10. In this example, the laser radiation incident on the lens array 12 is collimated and propagating parallel to the optical axis 18.

[0025] 1B and 1C diagrammatically illustrate imaging through the beam homogenizer 10. An inverted magnified image 26 of the laser radiation incident on each lens element 20 is formed in the illumination plane 28, which is located at a distance F3 from the lens 16. The inverted magnified images 26 of all illuminated lens elements 20 are superimposed in the illumination plane 28 and have a width D. FIG. 1B depicts the imaging of light rays originating from the center of each lens element 20. FIG. 1C depicts the imaging of light rays originating from a location displaced by a distance X1 from the center of each lens element 20. These light rays are projected to a location in the illumination plane 28 displaced by a distance X2 from the optical axis 18. The imaging magnification is the ratio [ka] It is.

[0026] The lens array 12 does not directly affect homogenization, and the focal length F1 does not determine the magnification. The purpose of the lens array 12 is to increase the acceptance angle for the rays 24 incident on the beam homogenizer 10. Although the incident rays 24 are depicted parallel to the optical axis 18, they can be incident up to a maximum angle with respect to the optical axis while still projecting overlapped images onto the illumination plane 28. At the maximum acceptance angle, the peripheral rays focused by each lens element 20 can be incident on the edge of the corresponding lens element 22. There is a compromise between this maximum acceptance angle and the area of ​​each lens element 22 that is illuminated. Reducing F1 relative to F2 increases this area, thereby reducing the vulnerability of the lens elements 22 to optical damage, while also reducing the maximum acceptance angle.

[0027] 2A-2C diagrammatically illustrate one preferred embodiment of a beam homogenizer 30 according to the present invention. The beam homogenizer 30 is similar to the beam homogenizer 10, except that neighboring lens elements 20 of the first lens array 32 are separated by the same gap 34, and the separation between the first lens array 32 and the second lens array 14 is a distance F2+δ. Each lens element 20 has a common width d1, each gap 34 has a common width d2, and each lens element 22 has a common width d=d1+d2. The distance δ is small compared to the focal length F2. For example, δ is preferably less than 20% of F2. More preferably, δ is less than 10% of F2, and most preferably less than 5% of F2.

[0028] The gaps 34 are depicted as empty gaps or "air gaps." The gaps 34 may also be made of transparent glass or polymer. The gaps 34 are most preferably made of the same material as the lens elements 20, have plane-parallel surfaces, and therefore have no refractive power. In general, it may be practical to mill or mold the lens array 32 from a single piece of material, with each gap fixedly connecting neighboring lens elements. FIG. 2A depicts a ray 24 of laser radiation propagating through the lens elements 20. FIG. 2B depicts a ray 36 of laser radiation propagating through the gap 34. FIG. 2C has a close-up view of both the ray 24 (solid lines) and the ray 36 (dashed lines) around the illumination plane 28 and around the lens array 32.

[0029] To explain imaging through the beam homogenizer 30 of the present invention, it is useful to consider ray matrix analysis, or ABCD matrix analysis, as is known in the optical arts. First, referring to FIG. 1C, the ray matrix for focusing by lens 16 and propagation between lens 16 and illumination plane 28 is:

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number

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number

number

[0030] When δ=0, which corresponds to an “ideal imaging” of the beam homogenizer 10, Equation 5 simply becomes:

number

[0031] 2A, inverted images 26 of all illuminated lens elements 20 are superimposed in an illumination plane 28. The images 26 are [ka] Therefore, the width [ka] 2B, the inverted images 40a and 40b of all illuminated gaps 34 are also superimposed in the illumination plane 28. The two gap images are split according to the lens elements 22 of the lens array 14 on which the respective light rays are incident. Both gap image 40a and gap image 40b have [ka] Therefore, each gap image is magnified by [ka] 2C, the smaller absolute magnification of image 26 produces dark gaps between image 26 and gap images 40a and 40b.

[0032] It is insightful to consider the beam homogenizer 30 in terms of the ideal imaging described above. In the beam homogenizer 30, an image plane 38 located at a distance F2 from the lens array 14 is precisely imaged onto the illumination plane 28 by the lens array 14 and the lenses 16. The image plane 38 is illuminated by converging light rays 24 focused by the lens elements 20 and parallel light rays 36 propagating through the gap 34. The illumination of the image plane 38 is a repeating pattern of wider and narrower bands, which is depicted in FIG. 2C. The illumination of the image plane 38 is [ka] , is projected and superimposed by lens array 14 and lens 16 onto illumination plane 28 with a magnification of .

[0033] The two presented descriptions of the beam homogenizer 30, one using a ray matrix and the other in terms of ideal imaging, are equivalent. The homogenizer produces a uniform or "flat-topped" intensity distribution in the illumination plane, with reduced intensity towards the edges of the otherwise uniform intensity distribution. The relative location and width of the dark gaps in the intensity distribution are controlled by selecting the focal length F1, the gap width d2, and the distance δ.

[0034] 3A and 3B diagrammatically illustrate another preferred embodiment of a beam homogenizer 50 according to the present invention. Beam homogenizer 50 is similar to beam homogenizer 30, except that the separation between lens array 32 and lens array 14 is a smaller distance F2-δ. The ray matrix analysis for beam homogenizer 50 is the same as for beam homogenizer 30. Image 26 is [ka] The width is expanded by [ka] , on the other hand, width [ka] 3B. Thus, for ray 24 incident on lens element 20, the absolute magnification is greater than for ray 36 incident on gap 34. This greater magnification creates an overlap between image 26 and gap images 40a and 40b. The overlap creates two bright peaks toward the edge of the otherwise uniform intensity distribution. For convenience of illustration, the image on illumination plane 28 is depicted in FIG. 3B with the rays propagating through lens element 20 and the rays propagating through gap 34 differentiated by their respective left and right offsets.

[0035] From an ideal imaging perspective, the image plane 38 is located in the path of the light rays 24 incident on the lens elements 20 and the light rays 36 incident on the gaps 34, prior to the lens array 32. Again, for ease of illustration, the illumination of the image plane 38 by the light rays 24 and 36 are distinguished by their respective left and right offsets. The image plane 38 is uniformly illuminated, but the portion of the plane illuminated by the light rays 36 (right offset) is imaged through both the lens elements 20 and the gaps 34. For example, the dashed light rays 52 depicted on the lens elements 20 result from the portion of the image plane illuminated by the light rays 36. The portion imaged through both the lens elements and the gaps appears brighter when projected onto the illumination plane 28. The beam homogenizer 50 thereby produces a uniform intensity distribution in the illumination plane with increased intensity towards the edges.

[0036] FIG. 4A schematically illustrates a nominally uniform intensity distribution within the illumination plane 28 having a dark gap towards the edge of the intensity distribution produced by the beam homogenizer 30. The overall intensity distribution is depicted by the thick line, and the lower intensity distribution produced by the light rays propagating through the lens element 20 and the gap 34 is depicted by the thinner line. Similarly, FIG. 4B schematically illustrates a nominally uniform intensity distribution within the illumination plane 28 having a bright peak towards the edge of the intensity distribution produced by the beam homogenizer 50. Note that the overall width D of the intensity distribution is independent of the gap width d2 and the distance δ. Since D > D1 + 2D2, a dark gap appears in the intensity distribution of FIG. 4A. Since D < D1 + 2D2, a bright peak appears in the intensity distribution of FIG. 4B.

[0037] A beam homogenizer having a lens array 32 configured and arranged to be longitudinally translated parallel to the optical axis 18 can be adjusted to produce a continuum of intensity distributions having a darker edge (in the case of FIG. 4A) or a brighter edge (in the case of FIG. 4B) or a completely uniform intensity. These intensity distributions can have a constant overall width D and an intensity that is also constant over most of the distance D2 at the edge. Mechanical devices for precise linear translation of the optical system are commercially available, and a detailed description of such devices is not necessary for understanding the principles of the present invention. An intensity distribution with a darker edge can have a slightly higher intensity over the distance D1 between the dark gaps. An intensity distribution with a brighter edge can have a slightly lower intensity over the distance D1 between the bright peaks. When the lens array 32 has a gap 34 with a width d2 that is small compared to the width d1 of the lens element 20, these intensity differences are relatively small. As an example, the ratio

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[0038] 5A and 5B diagrammatically illustrate some effects of diffraction on the beam homogenizer 30 of FIGS. 2A-2C. Diffraction can occur at the edges of each lens element 20 of the lens array 32 (this is the depicted example). Light rays propagating through the gaps 34 and near neighboring lens elements 20 become angularly dispersed. Light rays passing very close to the lens elements 20 become dispersed over a circumferential angle φ. Note that the edge intensity distribution caused by diffraction is not uniform, rather, the diffracted intensity decreases with increasing angle. The circumferential angle φ is differentiated here to illustrate this effect of diffraction. Also, note that the diffracted light rays can be directed toward one of two lens elements 22, either the lens element 22 corresponding to the diffractive lens element 20 or another lens element 22 in the neighborhood of the corresponding lens element. Both of these lens elements 22 are located within the circumferential angle φ.

[0039] FIG. 5A depicts only the diffracted rays propagating through corresponding lens elements 22 of lens array 14. These rays are projected onto the same range of locations in illumination plane 28 as in the absence of diffraction. FIG. 5B depicts only the diffracted rays propagating through neighboring lens elements 22. These rays are projected onto a range of locations in illumination plane 28 that would be outside the nominally uniform intensity distribution in the absence of diffraction. Overall, diffraction reduces the illumination slightly over a distance D2 inside each edge of the nominally uniform intensity distribution, and weakly illuminates a distance D2 outside each edge of the intensity distribution. Thus, edge contrast is reduced by diffraction.

[0040] FIG. 6A diagrammatically illustrates a camera image of an intensity distribution obtained using the experimental beam homogenizer 50. The image is expanded in the vertical dimension relative to the horizontal dimension. The beam homogenizer was partially illuminated with a collimated beam from an excimer laser. The lens array 32 has 25 lens elements 20, and most of these lens elements were illuminated. The lens array 32 is proportionally [ka] , with a width d=d1+d2 of a few millimeters. FIG. 6B is a graph illustrating diagrammatically the measured intensity distribution along the line AA superimposed on FIG. 6A. FIGS. 6A and 6B are shown to the same horizontal scale. The measured intensity distribution has bright peaks at each edge and relatively uniform intensity between the bright peaks. By translating the lens array 32 vertically and varying the distance δ, we demonstrated a continuous evolution from a perfectly uniform intensity distribution when δ was about 0 mm to an intensity distribution that can be described as having bright edges when δ was about 9 mm.

[0041] Although gaps in the first lens array having no refractive power have been illustrated and described herein, a requirement for producing an intensity distribution with dark gaps or bright peaks is a "gap" 34 in the first lens array having a refractive power substantially less than the refractive power of the lens elements 20 in the first lens array 32. For example, the gap 34 may be occupied by a weaker lens element, the weaker lens element having a focal length at least 5 times longer than the focal length F1 of the lens element 20. Increasing the focal length of the weaker lens element increases the contrast between the dark gaps or bright peaks and the uniform portions of the intensity distribution.

[0042] The gaps 34 in the first lens array, which has no refractive power, may have the form of a slab with plane-parallel faces. Such a slab may not modify the angle of the light rays propagating through it. Alternatively, the gaps 34 may have the form of a wedge, which has a plane but is not parallel. The wedge angle may be another variable that may be selected to position dark gaps or bright peaks in an otherwise uniform intensity distribution, either toward or away from the optical axis.

[0043] Yet another preferred embodiment of the beam homogenizer device for producing the intensity distribution depicted in Figures 4A and 4B may have a first lens array 32 similar to the beam homogenizer of Figures 3A and 2A, but replaced by a different inventive lens array. This different inventive lens array may be fabricated from the prior art lens array 12 by machining the center portion of each lens element flat. The resulting lens array has a plurality of identical lens elements, each of which may have a flat central portion and a curved outer portion. The central portion may have no refractive power, and the outer portion may have a focal length F1. The light rays passing through the flat portion may be imaged onto the illumination plane with a different magnification than the light rays passing through the curved portion. The advantage of this beam homogenizer embodiment is that the first lens array may be fabricated by simply grinding and polishing a prior art lens array that is commercially available, e.g., Thorlabs (Newton, New Jersey). A disadvantage of this embodiment is that it has a smaller acceptance angle for the light rays incident on the beam homogenizer.

[0044] The ray matrix analysis presented herein above assumes thin lenses and uses the paraxial approximation. Analysis using these assumptions is sufficient to explain the principles and operation of the present invention. Those skilled in the art will recognize that precise calculations must properly account for the geometry of the optical elements, and will recognize when to include these additional parameters.

[0045] In short, a beam of laser radiation can be transformed into a homogenized intensity distribution with brighter or darker edges within an otherwise uniform intensity distribution using the beam homogenizer of the present invention. The beam homogenizer includes a first lens array having a plurality of identical lens elements separated by gaps having a common width. A second lens array has a plurality of identical lens elements with a focal length F2. The first and second lens arrays are separated by a distance close to, but not equal to, F2. A separation distance less than F2 produces a bright peak, and a separation distance greater than F2 produces a dark gap, which is located toward the edge of the intensity distribution.

[0046] The present invention is described above in terms of preferred and alternative embodiments. However, the present invention is not limited to the embodiments described and depicted herein. Rather, the present invention is limited only by the claims appended hereto.

Claims

[Claim 1] The invention described in this specification.

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