Patent Publication Number: US-7719738-B2

Title: Method and apparatus for reducing laser speckle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 11/232,310 filed on Sep. 21, 2005 now U.S. Pat. No. 7,379,651, which is a continuation-in-part of patent application Ser. No. 10/458,390, now U.S. Pat. No. 7,306,344 filed on Jun. 10, 2003, patent application Ser. No. 11/066,605, now U.S. Pat. No. 7,301,701 filed on Feb. 25, 2005, patent application Ser. No. 11/066,616, now U.S. Pat. No. 7,318,644 filed on Feb. 25, 2005, and U.S. patent application Ser. No. 11/067,591, entitled “Light Recycler and Color Display System Including Same,” filed on Feb. 25, 2005. The aforementioned patents and application are incorporated by reference herein as though set forth in full. 

   TECHNICAL FIELD 
   The present invention relates generally to laser illumination systems. More particularly, the present invention relates to a method and apparatus for reducing laser speckle. 
   BACKGROUND 
   Due to their many advantages, which include high brightness and desirable spectral and angular beam characteristics, lasers are considered attractive light sources for various applications such as projection displays, microscopy, microlithography, machine vision and printing. However, one drawback to using lasers in these systems is speckle. Basically, speckle is an undesirable variation in the cross-sectional intensity of a laser beam. In laser projection systems, it usually makes images appear grainy and less sharp. Speckle is due to interference patterns that result from the high degree of temporal and spatial coherence of light emitted by most lasers. When such coherent light is reflected from a rough surface or propagates through a medium with random refractive index variations, speckle shows up as an uneven, random distribution of light intensity. This uneven brightness degrades the quality and usefulness of laser illumination systems. 
   The prior art describes various techniques for speckle reduction. For example, in U.S. Pat. No. 5,224,200, Rasmussen et al. propose a speckle reduction apparatus  10 , as illustrated in  FIG. 1 . The system consists of a coherence delay line in series between a laser and a homogenizer  28 . The coherence line consists of a totally reflecting mirror  24  and a partially reflecting mirror  22  separated by a distance  25  equal to an integer multiple of half the coherence length of the original laser beam. The laser beam  20  strikes the partially reflecting mirror  22  first, which transmits part of the beam and reflects the remainder toward the totally reflecting mirror  24  where it is reflected back toward the partially reflecting mirror  22 . This process continues until the reflected beam bypasses the partially reflecting mirror  22 . This final beam and the series of beams transmitted through the partially reflecting mirror  22  are focused by a lens  26  into a homogenizer  28 . Beams entering the homogenizer  28  are offset by multiples of their coherence length, leading to a reduction in their apparent coherence length, which in turn, reduces the amount of speckle. 
   In U.S. Pat. No. 5,313,479 to J. M. Florence and U.S. Pat. No. 6,594,090 B2 to Kruschwitz et al., a moving diffuser is used to remove or reduce the speckle pattern. 
   In U.S. Pat. No. 6,897,992 B2 to H. Kikuchi, the laser beam is rotated and equally divided into its S and P polarization components. After separating the S and P polarization components, an optical path difference that is at least equal to the coherence length of the laser beam is generated between the S and P polarization components through appropriate delay means. The &#39;992 patent also discloses an intensity separation means for dividing the laser beam into two or more parallel beamlets and delaying the beamlets relative to each other by an optical path difference that is at least equal to the coherence length of the laser. 
   B. Dingel et al. in “Speckle-Free Image in a Laser-Diode Microscope by Using the Optical Feedback Effect,” Optics Letters, Vol. 18, No. 7, April 1993, pp 549-551, teach a method of removing laser speckle by broadening the spectral linewidth of a laser and generating an output beam having a multimode spectrum that changes with time. This result is obtained by feeding a moderate amount of the laser light back into the cavity of the laser through the use of mirror, beam splitter and multimode fiber. 
   Although the above methods of speckle reduction are effective in some applications, they nevertheless suffer from one or more of the following disadvantages: moving or vibrating parts, low degree of compactness, long integration time, excessive loss of light energy (i.e., inefficiency), and/or lack of control over the spatial distribution of light in terms of angle and intensity. 
   Therefore, there is a need for a simple, compact, light weight, short-integration time, and efficient speckle reduction apparatus that provides control over the spatial distribution of laser light in terms of intensity and angle over a certain target area, such as the active area of a display panel. 
   SUMMARY 
   It is an advantage of the present invention to provide a relatively compact, light weight, short-integration time (or instantaneous integration time), efficient speckle reduction apparatus capable of producing an output light beam of selected cross-sectional area and spatial distribution of intensity and angle. The improved speckle reduction apparatus can efficiently couple light from laser sources (e.g., a single laser or laser array) having a variety of sizes and shapes to illumination targets of various shapes and sizes. The present invention also provides an improved method of speckle reduction. 
   Various aspects, features, embodiments and advantages of the invention are described in the following figures and detailed description, or they will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all of these aspects, features, embodiments and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims, which ultimately define the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     It is to be understood that the drawings are solely for purpose of illustration and do not define the limits of the invention. Furthermore, the components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
       FIG. 1  is a cross-sectional view of a prior art speckle reduction apparatus. 
       FIG. 2A  is a perspective view of a speckle reduction apparatus utilizing a highly reflective mirror, partially reflective mirror, light guide and an optional plano-concave lens. 
       FIG. 2B  is a cross-sectional view of the system of  FIG. 2A . 
       FIG. 3A  is a perspective view of a speckle reduction apparatus having a first structure utilizing a highly reflective mirror, partially reflective mirror, light guide, a retardation plate and an optional plano-concave lens. 
       FIG. 3B  is a cross-sectional view of the system of  FIG. 3A . 
       FIG. 4A  is a perspective view of a speckle reduction apparatus having a second structure utilizing a highly reflective mirror, partially reflective mirror, light guide, a retardation plate and an optional plano-concave lens. 
       FIG. 4B  is a cross-sectional view of the system of  FIG. 4A . 
       FIG. 5A  is a perspective view of a speckle reduction apparatus utilizing a highly reflective mirror, partially reflective mirror, light guide, an optional retardation plate, a transmissive diffuser and an optional plano-concave lens. 
       FIG. 5B  is a cross-sectional view of the system of  FIG. 5A . 
       FIG. 6A  is a perspective view of a speckle reduction apparatus utilizing a partially reflective mirror, light guide, an optional retardation plate, a reflective diffuser and an optional plano-concave lens. 
       FIG. 6B  is a cross-sectional view of the system of  FIG. 6A . 
       FIG. 7A  is a perspective view of a speckle reduction apparatus utilizing a highly reflective mirror, a partially reflective mirror, light guide, an optional retardation plate, a variable thickness plate and an optional plano-concave lens. 
       FIG. 7B  is a cross-sectional view of the system of  FIG. 7A . 
       FIG. 8A  is a perspective view of a speckle reduction apparatus utilizing a highly reflective mirror, a collimating plate, light guide, an optional retardation plate, an optional variable thickness plate and an optional plano-concave lens. 
       FIG. 8B  is a cross-sectional view of the system of  FIG. 8A . 
       FIG. 9A  is a detailed perspective view of a first collimating plate comprising micro-aperture, micro-guide and micro-lens arrays. 
       FIG. 9B  is a cross-sectional view of the collimating plate of  FIG. 9A . 
       FIG. 9C  is an exploded view of the collimating plate of  FIG. 9A . 
       FIG. 10A  is a perspective view of a second collimating plate comprising micro-aperture and micro-guide arrays. 
       FIG. 10B  is a cross-sectional view of the collimating plate of  FIG. 10A . 
       FIG. 11A  is a top view of a third collimating plate comprising micro-aperture and micro-tunnel arrays. 
       FIG. 11B  is a cross-sectional view of the collimating plate of  FIG. 11A . 
       FIG. 12A  is a perspective view of a fourth collimating plate comprising micro-aperture and micro-lens arrays. 
       FIG. 12B  is an exploded view of the collimating plate of  FIG. 12A . 
       FIG. 12C  is a cross-sectional view of the collimating plate of  FIG. 12A . 
       FIG. 13  is a cross-sectional view of a speckle reduction apparatus that receives a line of light. 
       FIG. 14A  is a top plan view of a speckle reduction apparatus in an edge-lit direct-view liquid crystal display. 
       FIG. 14B  is an exploded perspective side view of the LCD system of  FIG. 14A . 
       FIG. 14C  is a perspective view of an example of a micro-element plate. 
       FIG. 14D  is a perspective view of an example of a light guide plate. 
       FIG. 15A  is a top plan view of an illumination system utilizing red, green and blue lasers and a transmissive micro-display. 
       FIG. 15B  includes a detailed cross-sectional view of the transmissive micro-display of  FIG. 15A  and a detailed perspective view of one pixel of the transmissive micro-display. 
       FIG. 15C  is a cross-sectional view of an illumination system utilizing a tunable laser and a transmissive micro-display. 
       FIG. 15D  is a cross-sectional view of a projection system utilizing red, green and blue lasers. 
       FIG. 16  is a cross-sectional view of a liquid crystal micro-display equipped with a micro-lens array (MLA). 
       FIG. 17  is a cross-sectional view of a speckle reduction apparatus utilizing a moving diffuser at the input side. 
       FIG. 18  is a cross-sectional view of a speckle reduction apparatus utilizing a moving diffuser at the output side. 
       FIGS. 19A-B  are cross-sectional views of an exemplary transmissive micro-display system. 
   

   DETAILED DESCRIPTION 
   The following detailed description, which references to and incorporates the drawings, describes and illustrates one or more specific embodiments of the invention. These embodiments, offered not to limit but only to exemplify and teach the invention, are shown and described in sufficient detail to enable those skilled in the art to practice the invention. Thus, where appropriate to avoid obscuring the invention, the description may omit certain information known to those of skill in the art. The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment or variant described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or variants. All of the embodiments and variants described in this description are exemplary embodiments and variants provided to enable persons skilled in the art to make and use the invention, and not necessarily to limit the scope of legal protection afforded the appended claims. 
   Disclosed herein are various exemplary configurations of a laser speckle reduction apparatus that incorporates at least a light guide (e.g., a light pipe or tunnel), a highly reflective mirror (e.g., mirror plate) and a partially reflective mirror (e.g., mirror plate). In operation, the speckle reduction apparatus splits an input laser beam into many beamlets separated from each other by an optical path length difference that is preferably at least equal to the coherence length of the laser beam. The laser speckle reduction apparatus is advantageous in that it is static, i.e., it does not include any moving or vibrating parts. 
   Also disclosed are exemplary display systems embodying some of the laser speckle reduction structures described herein. 
   Turning now to the drawings,  FIGS. 2A and 2B  show perspective and cross-sectional views of a speckle reduction apparatus  200  including a light guide  45 , such as a light guide with reflective sidewalls, a highly reflective mirror  43 , a partially reflective mirror  46  and an optional plano-concave lens  42  (shown in  FIG. 2B ). The partially reflective mirror  46  is located at the exit face of the light guide  45  and the totally (or at least highly) reflective mirror  43  is located at the guide&#39;s input face. A clear aperture formed in the highly reflective mirror  46  allows the introduction of the input laser beam into the light guide  45 . 
   The highly reflective mirror  43  is preferably made of metal and/or dielectric coatings that reflect all or most of the incident light toward the partially reflective mirror  46 . The partially reflective mirror  46  is preferably made of dielectric coatings that reflect part of the incident light back toward the highly reflective mirror  43  and transmit the remainder. 
   The function of the plano-concave lens  42  is to expand the cone angle of the laser beam  40  into a desired cone angle. This lens  42  is attached to the highly reflective mirror  43  using optically transmissive adhesive. The highly reflective mirror  43  and partially reflective mirror  46  are attached to the entrance and exit faces, respectively, of light guide  45  using an appropriate adhesive. The distance L between the highly reflective mirror  43  and partially reflective mirror  46  is preferably equal to an integer multiple of half the coherence length of the input laser beam  40 . As us apparent to one of ordinary skill in the art, the coherence length of a laser is the distance over which interference will occur when the laser beam is split. 
   The light guide  45  can be solid light pipe made of optically transmissive material such as glass with polished surfaces or hollow light tunnel with reflective sidewalls and can also be straight or tapered light guide. The length of light guide ranges from few millimeters to tens of millimeters depending on the size of its entrance and exit apertures, cone angle of light propagating within the light guide  45 , coherence length of propagating light and degree of desired light uniformity. Examples of some suitable light guides are described in related U.S. patent application Ser. Nos. 10/458,390, filed on Jun. 10, 2003, and 11/066,616, filed on Feb. 25, 2005, which are incorporated herein by reference. 
   Laser light  40  enters an optional plano-concave lens  42  through a clear aperture  41  as shown in  FIG. 2 . The clear aperture  41  is formed in the totally reflective mirror  43  to permit the introduction of the laser light  40  into the light guide  45 . The size and shape of the clear aperture  41  can be circular, square, rectangular, oval or any other shape. It is also possible to have an array of clear apertures associated with an array of optional plano-concave lenses and corresponding to an array of lasers. 
   The plano-concave lens  42  is used to form a conical beam divergence symmetrically centered along the optical axis of the system  200 . Other types of lenses such as a plano-convex lens, a plano-convex micro-lens array, a plano-concave micro-lens array, holographic diffuser or non-holographic diffuser or the like can be used to perform the function of lens  42 . 
   The light source producing beam  40  can be a monochromatic laser, polychromatic laser (e.g. tunable laser) or pre-combined monochromatic lasers. There is no limitation on the laser power or wavelength which can be, for example, in the UV, visible or infrared range. The speckle reduction apparatus  200  can be used with lasers having coherence lengths ranging from small (few millimeters) to large (meters). 
   The laser light  40  exits the optional plano-concave lens  42  as a divergent beam  44  with a desired cone angle. This light  44  becomes more spatially uniform as it travels within the light guide  45 . When this light  44  initially strikes the partially reflective mirror  46 , part of it (i.e., the first beamlet) passes through and exits the speckle reduction apparatus  200  while the remainder gets reflected back toward the highly reflective mirror  43 . A small part of this reflected light exits the speckle reduction apparatus  200  through the clear aperture  41  toward the laser and the remainder gets reflected back by the highly reflective mirror  43  toward the partially reflective mirror  46 . This light strikes the partially reflective mirror  46  for the second time and a second beamlet exits speckle reduction apparatus  200  while the remainder gets reflected back toward the highly reflective mirror  43 . This process continues until the light beam  44  exits the speckle reduction apparatus  200  through the partially reflective mirror  46 . 
   Since the two mirrors  43  and  46  are separated by a distance preferably equal to an integer multiple of half the coherence length of the input laser beam  40 , the beamlets exiting the partially reflective mirror  46  are all offset by multiples of the coherence length. Thus, the beamlets exiting the speckle reduction apparatus  200  recombine incoherently, leading to a reduction in the coherence length of the output recombined beam. 
   Even if the distance between the two mirrors  43  and  46  is less than half the coherence length of the laser beam  40 , the beamlets exiting the partially reflective mirror  46  will have non-identical spatial distribution in terms of intensity and angle leading to the averaging and reduction of the speckle pattern. The optional plano-concave lens  42  increases the non-identical spatial distribution of the laser light, and thus, increases the reduction of speckle in situations where the mirror spacing is less than the coherence length. 
   In an alternative construction of the speckle reduction apparatus  200 , the sizes of the two mirrors  43  and  46  as well as the clear aperture  41  are designed to permit feeding the required amount of the laser light back into the laser cavity to broaden the spectral linewidth of the laser and produce time varying linewidth spectrum. In this case, the single-mode laser is transformed into a multi-mode laser whose multi-mode spectrum changes with time by relaxation oscillation and multiple external cavity modes, thus, leading to a reduction in the coherence length of the laser and the observed speckle pattern. This optical feedback effect is described in more detail by B. Dingel et al. in “Speckle-Free Image in a Laser-Diode Microscope by Using the Optical Feedback Effect,” Optics Letters, Vol. 18, No. 7, April 1993, pp 549-551, which is hereby incorporated by reference. 
     FIGS. 3A-B  show perspective and cross-sectional views, respectively, of another speckle reduction apparatus  300 . The apparatus  300  includes the light guide  45 , a highly reflective mirror  43 , a partially reflective mirror  46 , a retardation plate  47  and an optional plano-concave lens  42 . The retardation plate  47  is placed before the partially reflective mirror or after totally reflective mirror. The retardation plate allows the speckle reduction apparatus to deliver beamlets with various polarization components that are separated from each other by an optical path difference at least equal to the coherence length of the laser beam. 
   The distance L between the highly reflective mirror  43  and partially reflective mirror  46  is preferably equal to an integer multiple of half the coherence length of the laser beam  40 . The retardation plate  47  is used to induce a phase retardation of a desired value which leads to multiple beamlets with different polarization states and reduced speckle pattern. The wave plate is preferably a cut and polished piece of uniaxial crystal such as quartz and MgF 2 . In uniaxial crystals, light passing through the crystal experiences a different refractive index and phase delay in one crystal axis relative to the other two crystal axis. 
   In an alternative construction of the speckle reduction apparatus  300 , the retardation plate  47  is placed at the opposite end of the system  300 , between the highly reflective mirror  43  and the light guide  45 . 
     FIGS. 4A-B  illustrate a further configuration of a speckle reduction apparatus  400 . The apparatus  400  divides a polarized light component of laser beam  40  into S polarized light component and P polarized light component (as discussed below) and generates an optical path difference between the S and P polarized light components not less than the coherence length of the laser beam  40 . The two components are preferably divided into two equal S and P polarized light components. 
   The speckle reduction apparatus  400  is a special case of the speckle reduction apparatus  300  previously described above and utilizes a quarter wavelength plate  47   a  with an opening for initially introducing the polarized laser light  40  to the light guide  45  without experiencing any retardation. The plate  47   a  is placed between the highly reflective mirror  43  and the light guide  45 . The polarized laser light  44  impinges on the partially reflective mirror  46  where a first beamlet with S polarized light component (or P polarized light component depending on the laser and its orientation) is transmitted and the remainder is reflected back toward the highly reflective mirror  43 . During one round-trip in the solid light pipe or hollow light tunnel  45  the light passes twice through the quarter wavelength plate  47   a  and its polarization state is rotated by 90 degrees. Thus, the second transmitted beamlet exits with a P polarized light component, which is orthogonal to that of the first beamlet. Successive beamlets will have orthogonal polarization states alternating between the P and S states. Since the two mirrors  43  and  46  are separated by a distance preferably equal to an integer multiple of half the coherence length of the laser beam  40 , the beamlets exiting the partially reflective mirror  46  are all offset by multiples of their coherence length. Thus, the beamlets exiting the speckle reduction apparatus  400  recombine incoherently leading to a reduction in the coherence length of the recombined beam. 
   In an alternative construction of the speckle reduction apparatus  400 , the quarter wavelength plate  47   a  is placed between the partially reflective mirror  46  and the light guide  45 . In this construction, the laser beam  40  is initially rotated so that a circularly polarized light component impinges on the quarter wavelength plate  47   a , which in turn results in a first beamlet with a linearly polarized light component exiting the partially reflective mirror  46 . During one round-trip in the solid light pipe or hollow light tunnel  45 , the light passes twice through the quarter wavelength plate  47   a  and its polarization state is rotated by 90 degrees. Thus, the second transmitted beamlet exits with a polarized light component orthogonal to that of the first beamlet. Successive beamlets will have orthogonal polarization states alternating between the P and S states. 
   The laser beam  40  can be initially rotated to generate a circularly polarized light component by, for example, placing another quarter wavelength plate just before the highly reflective mirror  43 . The circularly polarized light component then enters through the aperture  41  into the speckle reduction apparatus  400 . 
     FIGS. 5A-B  show perspective and cross-sectional views, respectively, of another speckle reduction apparatus  500 . The apparatus  500  includes the light guide  45 , a highly reflective mirror  43 , a partially reflective mirror  46 , an optional retardation plate  47 , a transmissive diffuser  48   a  and an optional plano-concave lens  42 . Diffusers are a type of diffractive optical components that can take a laser beam and redistribute the light into a desired angular pattern. Diffusers can be made using different methods including holography and binary optics. Diffusers have different impact on polarized light depending on their types and materials. The transmissive diffuser  48   a  is used to induce a phase shift of a desired value which leads to transmitting beamlets with different phase shifts and reduced, averaged speckle patterns. 
   The distance L between the highly reflective mirror  43  and partially reflective mirror  46  is preferably equal to an integer multiple of half the coherence length of the laser beam  40 . 
   In an alternative construction of speckle reduction apparatus  500 , the transmissive diffuser  48   a  is placed between the optional retardation plate  47  and the light guide  45 . 
     FIGS. 6A-B  show perspective and cross-sectional views, respectively, of another configuration of a speckle reduction apparatus  600 . The apparatus  600  includes the light guide  45 , a partially reflective mirror  46 , an optional retardation plate  47 , a reflective diffuser  48   b  and an optional plano-concave lens  42 . The reflective diffuser reflects the received light beam rather than transmitting it (as in the case of the transmissive diffuser  48   a ) with the reflected light having the desired angular distribution. In addition to acting as a diffuser, the reflective diffuser  48   b  performs the function of the highly reflective mirror  43 . 
     FIGS. 7A-B  show perspective and cross-sectional views, respectively, of another speckle reduction apparatus  700 . The apparatus  700  includes the light guide  45 , a highly reflective mirror  43 , a partially reflective mirror  46 , an optional retardation plate  47 , a variable thickness plate  49  and an optional plano-concave lens  42 . The variable thickness plate  49  is used to induce a variable phase shift within the light beam which leads to transmitting beamlets with different phase shifts and reduced speckle pattern. The variable thickness plate  49  has uniform or random variations in thickness across the surface area. Such variations can be step-like or smooth and do not rearrange the angular distribution of the light beam. 
   The distance L between the highly reflective mirror  43  and partially reflective mirror  46  is preferably equal to an integer multiple of half the coherence length of the laser beam  40 . 
     FIGS. 8A-B  show perspective and cross-sectional views of another speckle reduction apparatus  800 . The apparatus  800  includes the light guide  45 , an optional retardation plate  47 , an optional variable thickness plate  49 , an optional plano-concave lens  42  and a collimating plate  50 . In addition to acting as a light collimator, the collimating plate  50  performs the function of the partially reflective mirror  46 . More specifically, the collimation plate  50  can be used as a partially reflective mirror at the exit face of the light guide  45  to provide control over the intensity and angle of delivered light at each point on the surface of the collimation plate  50 . 
   The optional variable thickness plate  49  can be replaced by an optional transmissive diffuser. It is also possible to replace both of the highly reflective mirror  43  and the optional variable thickness plate  49  with a reflective diffuser. 
     FIG. 9A  is a detailed perspective view of the collimating plate  50  of  FIGS. 8A-B . The collimating plate  50  includes an aperture plate  34   a , micro-guide array  34   b  and a micro-lens array  34   c . Each micro-lens corresponds to a micro-guide and a micro-aperture. As shown in  FIG. 9D , the aperture array  34   a  includes a plate made of a transmissive material  34   a   1  that is highly transmissive of the desired laser wavelength. The top surface of the plate has a patterned, highly reflective coating  34   a   2  applied thereto. 
   A perspective view of the micro-guide  34   b  and micro-lens  34   c  arrays is shown in  FIG. 9C . Both arrays  34   b  and  34   c  are made on a single glass plate. A cross-sectional view of the aperture  34   a , micro-guide  34   b  and micro-lens  34   c  arrays is shown in  FIG. 9B . In applications where maintaining the polarization state of the laser is important, sidewalls of the micro-guides within the micro-guide array  34   b  can be oriented so that the polarization state of the light entering and exiting the micro-guide array  34   b  is maintained. 
   Design parameters of each micro-element (e.g., micro-guide, micro-lens or micro-tunnel) within an array  34   a ,  34   b  and  34   c  include shapes and sizes of entrance and exit apertures, depth, sidewall shapes and taper, and orientation. Micro-elements within an array  34   a ,  34   b  and  34   c  can have uniform, non-uniform, random or non-random distributions and can range in number from one micro-element to millions, with each micro-element capable of being distinct in its design parameters. The size of the entrance/exit aperture of each micro-element is preferably ≧5 μm, in applications using visible light in order to avoid light diffraction phenomenon. However, it is possible to design micro-elements with sizes of entrance/exit aperture being &lt;5 μm. In such applications, the design should account for the diffraction phenomenon and behavior of light at such scales to provide homogeneous light distributions in terms of intensity, viewing angle and color over a certain area. Such micro-elements can be arranged as a one-dimensional array, two-dimensional array, circular array and can be aligned or oriented individually. In addition, the collimating plate  50  can have a smaller size than the exit face of the guide  45  and its shape can be rectangular, square, circular or any other arbitrary shape. 
   The operation of the collimating plate  50  is described as follows. Part of the light impinging on the collimating plate  50  enters through the openings of the aperture array  34   a  and the remainder is reflected back by the highly reflective coating  34   a   2 . Light received by the micro-guide array  34   b  experiences total internal reflection within the micro-guides and becomes highly collimated as it exits array  34   b . This collimated light exits the micro-lens array  34   c  via refraction as a more collimated light. In addition to this high level of collimation, collimating plate  50  provides control over the distribution of delivered light in terms of intensity and cone angle at the location of each micro-element. 
     FIGS. 10A-B  show perspective and cross-sectional views of an alternative collimating plate  60  that can be used with the speckle reduction apparatus  800 . The collimating plate includes a micro-guide array  34   b  and an aperture array  34   a.    
     FIGS. 11A-B  show top and cross-sectional views of another alternative collimating plate  70  that can be used with the speckle reduction apparatus  800 . The collimating plate  70  includes a hollow micro-tunnel array  37   b  and an aperture array  37   a . The internal sidewalls  38   b  (exploded view of  FIG. 11A ) of each micro-tunnel are coated with a highly reflective coating  39   b  ( FIG. 11B ). Part of the light impinging on collimating plate  70  enters the hollow micro-tunnel array  37   b  and gets collimated via reflection. The remainder of this light gets reflected back by the highly reflective coating  39   a  of aperture array  37   a . The advantages of collimating plate  70  are compactness and high transmission efficiency of light without the need for antireflective (AR) coatings at the entrance  38   a  and exit  38   c  apertures of its micro-tunnels. 
     FIGS. 12A-C  show perspective (integrated and exploded) and cross-sectional views of another alternative construction of a collimating plate  80  that can be used with the speckle reduction apparatus  800 . The collimating plate  80  includes an aperture array  74   a  and a micro-lens array  74   c  made on a single plate. In collimating plate  80 , the micro-lens array  74   c  performs the collimation function of delivered radiation via refraction. 
   Additional details of the construction, manufacture and operation of collimating plates, such as example collimating plates  50 ,  60 ,  70  and  80 , are given in related U.S. Pat. Nos. 7,306,344 and 7,318,644, and U.S. patent application Ser. No. 11/534,215, filed Sep. 21, 2006, all of which are incorporated herein by reference. 
     FIG. 13  illustrates a speckle reduction apparatus  1000  that receives a line of light  955  rather than a circular or elliptical laser beam(s)  951  as shown in  FIG. 13 . Shaping optics  952  convert circular or elliptical light beam(s)  951  emitted from one or more lasers  950  into a collimated line of light  953  where all rays are parallel to the optical axis (i.e. z-axis). Cylindrical lens  954  focuses this collimated line of light  953  into a rectangular aperture  960  formed in the highly reflective mirror  961 . Some examples of the shaping optics  952  are described in U.S. Pat. No. 6,323,984 B1 to J. I. Trisnadi, which is hereby incorporated by reference. The partially reflective mirror is preferably a one dimensional collimating plate  966  that reduces the cone angle (relative to the z-axis) of light exiting the speckle reduction apparatus  1000 . Optional retardation plate  965  and variable thickness plate  962  are also shown in  FIG. 13 . The length of the light guide  964  is preferably equal to an integer multiple of half the coherence length of the original laser beam(s)  951 . 
     FIG. 14A  is a top plan view of a speckle reduction apparatus  1200  used in an edge-lit direct-view liquid crystal display. In this application, a micro-element plate  1100  is utilized to uniformly distribute the light beam  1151  of laser  1150  along the edge of a light guide plate  1120 , after the light first passes through the speckle reduction apparatus  1200 . The speckle reduction apparatus  1200  includes any of the speckle reduction apparatuses  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  800  disclosed herein. 
     FIG. 14B  is an exploded perspective side view of the LCD system of  FIG. 14A . The plate  1120 , having a reflective bottom side  1121 , is usually used in a direct view liquid crystal display (LCD) to couple light from a light source into a display panel placed on top of plate  1120 . Laser light  1152  exiting the speckle reduction apparatus  1200  enters light pipe  1110  and travels toward the opposite end of light guide  1110 . Micro-element plate  1100  is attached to light guide  1110  as shown in  FIGS. 14A-B . 
   Millions of micro-elements (e.g. micro-lenses, micro-guides, micro-tunnels) formed on the surface of micro-element plate  1100  are used to couple light  1152  into light guide plate  1120 . The coupled light enters light guide plate  1120  as light  1153  and gets extracted from light guide plate  1120  in the +Y direction toward the display panel. The micro-elements are distributed non-uniformly along micro-element plate  1100  and their density increases toward the back end of the micro-element plate  1100 . Since the light intensity decreases as it travels toward the back end, this type of non-uniform micro-element distribution leads to a uniform light distribution along the edge of a light guide plate  1120 . The back end of the micro-element plate  1100  is preferably coated with a highly reflective layer  1111  to avoid light leakage. 
     FIG. 14C  shows plate  2100  as one example of micro-element plate  1100 . As light  1152  travels within light guide  1110 , it enters the micro-guides  2100   a  of plate  2100  and strikes the sidewalls of micro-guides  2100   a . Light striking the micro-guides sidewalls gets refracted toward the light guide plate  1120 . Design, operation and fabrication of the micro-element plate  1100  are described in related U.S. patent application Ser. Nos. 10/458,390, filed on Jun. 10, 2003 and 11/066,616, filed on Feb. 25, 2005. 
   It is also possible to have micro-element plate  1100  and light pipe  1110  integrated on a single plate. 
     FIG. 14D  shows plate  2120  as another possible design of light guide plate  1120 . In one implementation of plate  2120 , highly reflective white paint is applied to its back side  2120   b . Light  1153  traveling within plate  2120  is diffused upon striking the white paint. Large portion of the diffused light ends up (and after striking the white paint many times) exiting plate  2120  through its front surface  2120   a  (in the +Y direction) and enters the display panel (or the brightness enhancement films which are typically placed between the display panel and light guide plate  2120  for light collimation). 
   The plate  2120  might be implemented using micro-elements on its front  2120   a  and/or back  2120   b  surfaces and without the use of white paint. Such designs are known in the prior art. Light guide  1110 , micro-element plate  1100  and  2100 , and light guide plate  1120  and  2120  are made of optically transmissive material, such as glass or a polymer. 
     FIGS. 15A-B  illustrate an illumination system  1500  utilizing red, green and blue lasers and a transmissive micro-display. The system  1500  includes three coherent light sources  1 A,  1 B and  1 C, three dichroic beam splitters  2 A,  2 B and  2 C, a laser speckle reduction apparatus  1400  and a transmissive display  1410 . The speckle reduction apparatus  1400  includes any of the speckle reduction apparatuses  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  800  disclosed herein. 
   The three coherent light sources can be, for example, red  1 A, green  1 B and blue  1 C lasers which are gated sequentially to produce a full color display. If the three coherent light sources  1 A,  1 B, and  1 C are continuously operated rather than gated, the transmissive micro-display will utilize red, green and blue color filters in its structure to produce a full color micro-display image. The red  1 A, green  1 B and blue  1 C lasers are combined using the dichroic beam splitters  2 A,  2 B and  2 C. Element  2 A can be a highly reflective mirror or a dichroic beam splitter that reflects red. Element  2 B is a dichroic beam splitter that passes red from laser  1 A and reflects blue from laser  1 B and element  2 C is a dichroic beam splitter that passes red and blue while reflecting green from laser  1 C. 
     FIG. 15B  is a cross-sectional view of the transmissive micro-display  1410 , which can be, for example, high temperature poly silicon (HTPS) or low temperature poly silicon based micro-display. Each pixel  1411  has an open area  1411   b  where light can pass through and highly reflective area  1411   a  that reflects incident light back toward the laser speckle reduction apparatus  1400 . The wiring and transistor areas  1411   c  of each pixel  1411  are hidden behind the highly reflective area  1411   a.    
     FIG. 15C  illustrates an illumination system  1550  that utilizes a single tunable laser  1 D to produce a full color display. In this system  1550 , the three dichroic beam splitters  2 A,  2 B and  2 C of  FIG. 15A  are eliminated. 
     FIG. 15D  shows a projection system  1600  that utilizes red  1 A, green  1 B and blue  1 C lasers, three speckle reduction apparatuses  1400   a ,  1400   b  and  1400   c , three transmissive micro-displays  1410   a ,  1410   b  and  1410   c , a cross-dichroic prism  1420 , a projection lens  1430  and a screen  1440  to produce a full color display. 
   The speckle reduction apparatuses  1400   a - c  include any of the speckle reduction apparatuses  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  800  disclosed herein. 
   In this system  1600 , the lasers  1 A,  1 B and  1 C continuously illuminate their corresponding micro-displays  1410   a ,  1410   b  and  1410   c . The three transmissive micro-displays  1410   a  (red),  1410   b  (green) and  1410   c  (blue) modulate the light of the red, green and blue colors, respectively, according to image signals. The cross-dichroic prism  1420  has a multi-layer dielectric film stack  1420   a  that reflects red light and a multi-layer dielectric film stack  1420   b  that reflects blue light arranged in a form of a cross. The film stacks  1420   a  and  1420   b  are intersected by a center axis  1420   c  that extends along the y-axis. The light combined by the cross-dichroic prism  1420  is transmitted to a projection lens  1430 , which in turn projects this light (i.e. image) onto a screen  1440 . 
   The projection system  1600  of  FIG. 15D  utilizes combinations of coherent (e.g. lasers) and non-coherent (e.g. light emitting diodes (LEDs) and arc lamps) light sources for providing red, green and blue colors to the micro-display. Since blue lasers are very expensive when compared to green and red lasers (which are generally available at suitable power levels and acceptable price), this flexibility in using different types of light sources has the advantage of utilizing green and red lasers while using other non-coherent sources for the blue color. In this construction, the LED light and/or the focused arc lamp light are fed into systems  1400   a  and  1400   b  through the aperture of the highly reflective mirror. Since LED and arc lamp emit non-coherent light, systems  1400   a  and  1400   b  will not provide the speckle reduction function and will still provide the remaining benefits. The arc lamp light can be focused into the aperture of the highly reflective mirror using an elliptical reflector or a parabolic reflector followed by a focusing lens. The LED itself can be attached to the aperture of the highly reflective mirror mechanically or using suitable adhesive. The micro-displays of projection system  1600  of  FIG. 15D  can be of multiple types and resolutions. For example, the micro-display  1410   c  receiving blue light can have low resolution compared to the other two micro-displays  1410   a  and  1410   b . Due to the low sensitivity of human eye to blue color relative to green and red colors, this arrangement leads to lower cost without compromising the projection system image quality. 
   In alternative constructions of the systems  1500 ,  1550 ,  1600 , the transmissive micro-displays  1410 ,  1410   a ,  1410   b  and  1410   c  are themselves used as the partially reflective mirror in the speckle reduction apparatuses  1400 ,  1400   a ,  1400   b  and  1400   c , thus, eliminating the need for an extra partially reflective mirror  46  in the speckle reduction apparatus  200 ,  300 ,  400 ,  500 ,  600 , and  700  or a collimating plate  50 , as in speckle reduction apparatus  800 . 
   The advantages of the above illumination systems  1500 ,  1550  and  1600  include speckle removal, high compactness, and high brightness. The brightness of display systems utilizing these illumination systems  1500 ,  1550  and  1600  is higher than these of conventional systems even when the aperture ratio of the pixel  1411  is reduced. The aperture ratio is a ratio between the pixel&#39;s  1411  open area  1411   b  ( FIG. 15B ) and pixel&#39;s  1411  total area. The higher brightness is gained due to the recycling of the light that strikes the highly reflective area  1411   a  of pixel  1411  ( FIG. 15B ) within the speckle reduction apparatus  1400  until it passes through the pixel&#39;s open area  1411   b . This increased brightness is achieved without the need for a micro-lens array (MLA) to enhance the brightness of transmissive micro-displays. Each micro-lens in the MLA is used in transmissive micro-displays to enhance the display brightness by focusing received light into the aperture of the corresponding pixel. The enhanced brightness (which is almost independent of the aperture ratio) permits further reduction of the pixel and micro-display sizes and thus reduction in the micro-display and projection systems costs. 
     FIG. 16  is a cross-sectional view of a liquid crystal micro-display  1650  equipped with a micro-lens array (MLA)  1621 . The liquid crystal micro-display  1650  is itself used as a partially reflective mirror in the static speckle reduction apparatus  1400 ,  1400   a ,  1400   b  and  1400   c , thus, eliminating the need for an extra partially reflective mirror  46  in the static speckle reduction apparatus  200 ,  300 ,  400 ,  500 ,  600 , and  700  or a collimating plate  50  as in static speckle reduction apparatus  800 . 
   The liquid crystal micro-display  1650  includes at least one liquid crystal cell  1630 , a micro-lens array  1621  and a pair of polarizers  1620  and  1629 . 
   The liquid crystal cell  1630  has a transparent electrode substrate  1628 , a transparent counter substrate  1623 , and a liquid crystal layer  1626  sandwiched between both substrates  1628  and  1623 . A thin-film transistor  1635  and pixel electrode  1627  are made on the electrode substrate  1628  for each pixel. A common electrode  1625  is made on the counter substrate  1623 . A highly reflective layer  1624  is provided between the counter substrate  1623  and the common electrode  1625 . The highly reflective layer  1624  has a corresponding aperture (i.e. light transmissive opening)  1624 A for each pixel electrode  1627 . 
   Each pixel consists of one electrode  1627 , common electrode  1625 , and a liquid crystal layer  1626  sandwiched between both electrodes  1627  and  1625 . Alternatively, the highly reflective layer can be provided on the plane of incidence (i.e. the upper surface of the counter substrate  1623 ) of the liquid crystal cell  1630 , or on the electrode substrate  1628 . 
   The micro-lens array  1621  is attached to the upper side of the counter substrate  1623  via a bonding layer  1622 . The micro-lens array  1621  has a plurality of micro-lenses  1621   a . The micro-lens array  1621  includes concentric (or non-concentric) lenses that are preferably positioned at an offset from the center axis of the corresponding pixel aperture  1624 A. 
   Light entering the micro-lens array  1621  is divided into sub-beams by the plurality of micro-lenses  1621   a . Each sub-beam  1610   a  that is parallel to the clear viewing direction VD (that usually leads to the highest image contrast) is focused in the vicinity of the corresponding pixel electrode  1627  passing through the aperture  1624 A, the liquid crystal layer  1626 , pixel electrode  1627 , electrode substrate  1628 , and polarizer  1629 . Light that is not parallel to the clear viewing direction VD such as sub-beam  1610   b  having an angle of incidence symmetrical with that of the small beam  1610   a , is also converged in the vicinity of the corresponding pixel electrode  1627  where it impinges at the highly reflective layer  1624 B. This sub-beam  1610   b  gets reflected back and recycled within the static speckle reduction apparatuses  1400 ,  1400   a ,  1400   b  and  1400   c . This reflected light impinges on the liquid crystal micro-display  1650  for a second time (after being recycled once within the static speckle reduction apparatuses  1400 ,  1400   a ,  1400   b  and  1400   c ) where it has a chance of exiting along the clear viewing direction VD, thus, enhancing the micro-display and projection system contrasts. This recycling process continues until most of the light exits in clear viewing direction VD. The liquid crystal micro-display  1650  can be made to display monochrome or color images. 
   The micro-lens array (MLA)  1621  has the advantage of enhancing the contrast ratio of the transmissive micro-display. Such micro-displays are discussed by Ogawa in U.S. Pat. No. 6,195,143 B1 and by Saito, et al. in U.S. Pat. No. 6,825,889 B1, which are hereby incorporated by reference. In order to have better light coupling efficiency, it is preferable that the light shield used in the micro-displays is a highly reflective layer. 
     FIG. 17  illustrates a speckle reduction apparatus  1750  including a moving diffuser  1720  at the input of the static speckle reduction apparatus  1710 . The static speckle reduction apparatus  1710  can be any of the static speckle reduction apparatuses  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  800 ,  1000 ,  1200 ,  1400 ,  1400   a ,  140   b  and  1400   c . Three moving or vibrating diffusers are needed in case of projection system  1600  of  FIG. 15D . The diffuser  1720  is moved by means, generally designated  1730 , in its plane with a rotary, vibratory or other motion. Rotary motion can be provided by a motor or coil. Alternatively, vibratory motion can be provided using a piezo transducer, which is driven by an alternating signal source and produces the needed vibrations. 
   The moving diffuser  1720  permits further reduction of the speckle pattern or noise, which in turn simplifies the design and enhances the optical efficiency of the static speckle reduction apparatus  1710 . 
     FIG. 18  illustrates a speckle reduction apparatus  1850  utilizing a moving diffuser  1720  at the output of the static speckle reduction apparatus  1710 . The static speckle reduction apparatus  1710  and moving diffuser  1720  are described above. In this apparatus  1850 , the moving diffuser  1720  provides additional speckle reduction. In projection display systems, it is preferable to place the moving diffuser at an intermediate image plane. In long-throw projection systems, the moving diffuser is preferably placed at a position conjugate to the screen. 
   Other devices for reducing speckle can be used in connection with the static speckle reduction apparatuses  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  800 ,  1000 ,  1200 ,  1400 ,  1400   a ,  140   b  and  1400   c  disclosed herein, either at their input or output sides. Examples of these devices include the use of an electro-optic device, as described in U.S. Pat. No. 6,791,739 to Ramanujan et al., which is hereby incorporated by reference; flowing fluid diffusers; non-flowing fluid diffusers; and nutating diffusion plates as described in U.S. Pat. No. 5,534,950 to Hargis et al., which is hereby incorporated by reference. 
   A flowing fluid diffuser comprises a pair of closely spaced glass plates between which a highly turbid fluid flows. An example of such a turbid fluid is Liquid Paper®. This technique can remove speckle at modest flow rates. In non-flowing fluid diffusers, speckle is eliminated in the absence of flow or moving parts. Due the presence of particles that are sufficiently small in size suspended in a fluid diffuser. This phenomenon is caused by Brownian motion of the scattering particles. 
   The static speckle reduction apparatus  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  800 ,  1000 ,  1200 ,  1400 ,  1400   a ,  140   b  and  1400   c  can also be used in connection with a vibrating screen for further speckle reduction. 
   Alternatively, the partially reflective mirror (which can be, for example, a transmissive micro-display) can be moved via a vibratory or other motion to further reduce speckle. In this case, partially reflective mirror is positioned in close proximity to the light tunnel/pipe without being firmly attached to it. 
   A transmissive micro-display  2000  is shown in  FIG. 19A .  FIG. 19B  shows a speckle reduction apparatus  2100  utilizing the transmissive micro-display  2000  as a partially reflective plate. 
     FIG. 19A  is an enlarged cross-sectional view of part of a liquid crystal micro-display  2000 , which consists of a micro-guide array  2621 , transparent electrode substrate  2628 , a transparent counter substrate  2623 , and a liquid crystal layer  2626  sandwiched between micro-guide array  2621  and counter substrates  2623 . A thin-film transistor  2635  and pixel electrode  2627   b  are made on the electrode substrate  2628  for each pixel. A common electrode  2625  is made on the counter substrate  2623 . A highly reflective layer  2624  is provided between the electrode substrate  2628  and the thin-film transistor  2635 . The highly reflective layer  2624  has a corresponding aperture (i.e. light transmissive opening)  2624 A for each pixel electrode base  2627   a.    
   Each pixel consists of one electrode  2627   b , electrode base  2627   a , pixel electrode connector  2627   c , common electrode  2625 , and a liquid crystal layer  2626  sandwiched between both electrodes  2627   b  and  2625 . The pixel electrode connector  2627   c  and electrode base  2627   a  are both used to provide electrical contact between the thin-film transistor  2635  and pixel electrode  2627   b . The pixel electrode connector  2627   c  is deposited on the sidewalls and entrance aperture (i.e. micro-guide  2621   a  side facing the electrode base  2627   a ) of each micro-guide  2621   a.    
   The micro-guide array  2621  is first made on a carrier substrate (not shown) and then the areas between adjacent micro-guides are filled with a material  2622  that has an index of refraction preferably lower than that of the micro-guide  2621   a  material. The filling material  2622  is planarized and then back etched to more than the thickness of the thin-film transistor  2635  array. The upper side (i.e. micro-guide array  2621  side) of the carrier substrate is then attached to the upper side of the electrode substrate  2628  via a bonding layer (not shown). The micro-guide array  2621  has a plurality of micro-guides  2621   a  preferably positioned at the center axis of the corresponding pixel aperture  2624 A. The top side of the carrier substrate is removed or etched down until the pixel electrode connector  2627   c  is exposed. This step is followed by the deposition of electrode  2627   b  and making sure that electrode  2627   b  is overlapping electrode connector  2627   c  so that an electrical contact is provided to pixel electrode  2627   b.    
   Light entering the micro-guide array  2621  is divided into sub-beams by the plurality of micro-guides  2621   a . Each sub-beam  1610   a  is collimated by the micro-guide  2621   a  and delivered to the pixel electrode  2627   b . The collimation occurs via total internal reflection. When the micro-guide  2621   a  are coated with an additional reflective layer between the pixel electrode connector  2627   c  and the filling material  2622  or between the micro-guide  2621   a  sidewall itself and the pixel electrode connector  2627   c , collimation occurs via specular reflection. The collimated light passes through the pixel electrode  2627   b , the liquid crystal layer  2626 , common electrode  2625 , counter substrate  2623 , and polarizer  2620 . Light that is does not enter through aperture  2624   a  such as ray  1610   b  is reflected by the highly reflective layer  2624 B in the opposite direction. This ray  1610   b  gets reflected back and recycled within the static speckle reduction apparatus  1400  of  FIG. 19B . This reflected light impinges on the liquid crystal micro-display  2000  for a second time (after being recycled once within the static speckle reduction apparatus  1400 ) where it has a chance of passing through aperture  2624   a . This recycling process continues until most of the light exits through aperture  2624   a . The liquid crystal micro-display  2000  can be made to display monochrome or color images. 
   The advantages of the micro-display  2000  is its high optical transmission efficiency, high contrast and the elimination of screen door effect (i.e. the image appears as if it is viewed through a screen door due to the opaque inter-pixel regions) when compared to conventional liquid crystal micro-displays. 
   The reflective coatings described herein are of the specular type and can be a metallic coating, dielectric coating, cold mirror coating, dichroic mirror coating, or a combination of these. The light guide  45  can be straight, tapered, cylindrical, square, rectangular, or spherical. The length of light guide  45  ranges from few millimeters to tens of millimeters depending on the laser source size, size of tunnel&#39;s entrance and exit apertures, cone angle of light propagating within the guide  45  and degree of desired light uniformity delivered by the speckle reduction apparatuses  200 ,  300 ,  400 ,  500 ,  600 ,  700 , and  800 . The entrance and exit apertures of the light guide  45  can be independent in terms of size and shape and can have different sizes and different shapes such as square, rectangular, circular, trapezoidal, polygonal, asymmetrical and even irregular shapes. 
   While one or more specific embodiments of the invention have been described above, it will be apparent to those of ordinary skill in the art that many more embodiments are possible that are within the scope of the invention. Further, the foregoing summary, detailed description and drawings are considered as illustrative only of the principles of the invention. Since other modifications and changes may be or become apparent to those skilled in the art, the invention is not limited the exact constructions and operations shown and described above, and accordingly, all suitable modifications and equivalents are deemed to fall within the scope of the invention, the invention being defined by the claims that follow.