Abstract:
An optical imaging heads that produce a plurality of light spots on light sensitive medium such as photographic film or printing plate. The optical head incorporates an array of multi-mode laser diodes optically coupled to multi-mode optical fibers, an array of micromachined supports for the optical fibers, an array of micromachined light-pipes (MLPs) aligned with the supports and with the optical fibers and means for imaging the exit aperture of each of the micromachined light-pipes on a photosensitive medium.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to optical imaging heads that produce a plurality of light spots on light sensitive medium such as photographic film or printing plate. The optical head incorporates as light source, an array of pigtailed laser diodes and a Micro Light-Pipe Array (MLPA) as a beam-shaping element.  
         BACKGROUND OF THE INVENTION  
         [0002]    Optical heads for imaging a plurality of light spots on a light sensitive medium often incorporate, as a light source, an array of pigtailed Laser Diodes (LD). Each LD is optically coupled to one end of an Optical Fiber (OF). The opposite ends of the OFs are supported in a linear array by means such as V-groove plates, as illustrated in FIG. 1. The upper and lower V-groove plates,  11  and  17  respectively, are often made by a photolithographic procedure on Si, the V-grooves  19  being etched along [111] crystallographic plane in a very tight mechanical tolerance with the fibers&#39;  18  cladding dimensions.  
           [0003]    The imaging speed in electro-optical plotters is generally limited by the power delivered by the laser beam(s) to the medium. This is especially true when the imaged medium is a thermal printing plate, where the sensitivity is typically of the order of several hundred mJ/cm 2 . In this case, the fiber-coupled diodes engaged in the array have to be powerful multi-mode LDs coupled to a multi-mode optical fiber, such as SDL-2300 manufactured by SDL Inc., of San Jose, Calif. An important characteristic of any fiber-coupled LD is the light energy distribution in the fiber&#39;s far field. Because of the multi-mode LID and the usually short length of the multi-mode fiber, the near-field and the far-field energy distributions depend on the quality of the optical coupling, the LD junction temperature (i.e. modulation data flow), the bending along the fiber length, etc. As far as the image on the photosensitive medium is obtained by imaging either the near-field or the far-field of the fiber, this non-uniform and frequently changing energy distribution of the light emerging from the fiber&#39;s end often leads to unpredictable energy distribution in the writing spot and to undesired effects on the image.  
           [0004]    A way of avoiding this effect is to use a controlled-angle diffuser as in EP 0992343 A1 to Presstek Inc. The diffuser introduces a scrambling in the angular energy distribution and thus smoothes it. This approach, however, can hardly correct non-symmetrical or doughnut-mode energy distributions.  
           [0005]    The present invention discloses an apparatus and method which successfully solve the problems described above, by using a micromachined Light-Pipe or Light-Pipe Array (MLPA) for delivering the light from a multi-mode laser source, such as multi-mode optical fiber, to a very well defined spot on the photosensitive medium.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a schematic exploded isometric view of a conventional-art V-groove assembly for supporting an array of optical fibers;  
         [0007]    [0007]FIG. 2 a  is a schematic isometric view of an optical fiber aligned in a Micro Light-Pipe by means of a V-groove according to the present invention;  
         [0008]    [0008]FIG. 2 b  is an exploded view of the assembly of FIG. 2 a;    
         [0009]    [0009]FIGS. 3 a  and  3   b  present the light energy distribution of a multi-mode optical fiber in the far field and in the exit apertures of a Micro Light-Pipe attached to it respectively;  
         [0010]    [0010]FIG. 4 schematically illustrates an exemplary optical imaging head incorporating an optical fiber as a light source and a beam-shaping Micro Light-Pipe, according to the present invention;  
         [0011]    [0011]FIG. 5 is a schematic isometric view of optical fibers aligned in an array by means of V-grooves and a Micro Light-Pipe array for beam shaping, according to the present invention;  
         [0012]    [0012]FIG. 6 schematically illustrates an exemplary optical imaging head incorporating an optical-fiber array as multiple light source and beam-shaping by means of a Micro Light-pipe Array, according to the present invention; and  
         [0013]    [0013]FIGS. 7 a  to  7   d  illustrate different channel shapes in Micro Light-Pipes.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0014]    [0014]FIG. 2 a shows a multi-mode optical fiber light source  18 , with a supporting assembly  10  and a Micro Light-Pipe (MLP)  12 . The V-groove  19  serves as a holder for the optical fiber  18 , aligning its axis  21  to coincide approximately with the MLP axis  22 . The V-groove—MLP assembly is made by upper part  11  and lower part  17 , as shown in FIG. 2 b . The MLP surface is coated with a highly reflective coating, such as Au, enhanced Al or dielectric, depending on the base material and the wavelength of the light.  
         [0015]    The light emitted from the optical fiber  18  enters the micro light-pipe  12 , where each beam experiences a number of reflections before it exits the light-pipe through its opposite side. Due to these multiple reflections, the illumination of the MLP exit aperture is relatively uniform. The uniformity, defined as  
           Edge                 Illumination       Center                 Illumination       ,                         
 
         [0016]    is proportional to the value  
           L   n     =       L   ·     NA   i         A         ,                         
 
         [0017]    called normalized length, where L is the light-pipe length, NA i  is the input beam numerical aperture and A is the cross-sectional area of the micro light-pipe. There is no precise theory of light pipes. The scrambling efficiency is usually checked experimentally, or by non-sequential ray tracing. It is, however, an empirical fact that when L n ≧4, the illumination uniformity at the MLP exit can be expected to be better than 90%. The scrambling effect of the MLP of FIGS. 2 a  and  2   b  is illustrated in FIGS. 3 a  and  3   b.    
         [0018]    [0018]FIG. 3 a  shows a doughnut-mode far-field light distribution of a multi-mode LD coupled to a multi-mode fiber with 40μ core diameter. The micro light-pipe was chosen to have a hexagonal cross section, with A=1385 μ 2  (the fiber&#39;s core  23 , FIG. 2 a , is circumscribed in the MLP&#39;s aperture A) and with length L=0.5 mm.  
         [0019]    [0019]FIG. 3 b  shows the scrambling effect of the MLP. The energy distribution at the exit aperture  14  is uniform, and as far as the this exit aperture will be imaged on the photosensitive medium, it is clear, that the resulting spot will also have uniform energy distribution, independently of the energy distribution of the light emerging from the fibers end.  
         [0020]    [0020]FIG. 4 schematically shows an optical imaging head incorporating a multi-mode fiber light source  18  and an MLP  10  (the supporting V-grooves are not shown). The exit aperture  14  of the MLP  10  is imaged by means of imaging lens  70  (preferably telecentric) on the photosensitive medium  50 , i.e. the exit aperture  14  lies in the object plane of the imaging lens  70 , while its image  60  lies on the photosensitive medium  50 , which coincides with the lens  70  image surface. Due to the relatively uniform illumination of the exit aperture  14 , as shown on FIG. 3 b , the image  60  will also feature relatively uniform distribution of illumination. Thus, a very well defined spot is achieved on the medium  50 .  
         [0021]    Reference is now made to FIG. 5, which is a schematic exploded view of an array of optical-fiber light sources with scrambling MLPs. The whole assembly  10  consists of upper and lower parts,  11  and  17  respectively. Arrays of precision V-grooves  19  are etched in both parts  11  and  17 , supporting the optical fibers  18 . The grooves  19  continue into MLPs  12 . Each MLP  12  is formed by joining two halves  12   a  and  12   b , also etched in the same upper and lower parts  11  and  17 , respectively. The keys  15  and  16  enable precise alignment of the two parts  11  and  17 . This construction allows the optical fibers&#39; cores  23  to be circumscribed very precisely into the MLP&#39;s entrance apertures. The inner surface of the parts  11  and  17  is coated with a highly reflective coating, such as bare Au, enhanced Al, dielectric, etc., depending on the base material and the wavelength of the light.  
         [0022]    It will be appreciated by any person skilled in the art, that the fiber supporting V-grooves and the light scrambling MLPs can be made as separate parts and later in the process of the assembling of the imaging system to be precisely aligned relative to each other, in order to obtained the desired position of the optical fiber relative to the MLP.  
         [0023]    [0023]FIG. 6 schematically shows an optical imaging head incorporating an array of multi-mode optical-fiber sources  18  and an array of MLPs  10  (the supporting V-grooves are not shown). The exit apertures  14  of the MLPs  12  is imaged by means of imaging lens  70  (preferably telecentric) on the photosensitive medium  50 , i.e. the exit apertures  14  of the MLPs  12  lie in the object plane of the imaging lens  70 , while their images  60  lie on the photosensitive medium  50 , which coincides with the lens  70  image surface. Due to the relatively uniform illumination of the exit apertures  14 , as shown in FIG. 3 b , the images  60  will also feature relatively uniform distribution of illumination Thus, very well defined spots are achieved on the medium  50 .  
         [0024]    PRODUCTION METHOD  
         [0025]    Micro light-pipes and arrays of MLPs such as shown in FIGS. 2 a ,  2   b  and  5  can be produced by using standard photolithographic technologies on silicon wafers. The element consists of two basic plates  17  and  11 , on which one or more V-grooves for supporting the optical fiber are etched, the V-grooves continuing into half-hexagonal grooves also etched in the same Si wafer. Here, etching along the Si [111] crystallographic planes is performed. This technology is well mastered in many companies around the world, for example in the Micro-Technology Institute in Mainz, Germany or MicroDevices Inc of Radford, Va.  
         [0026]    The grooved surfaces are coated with a highly reflective coating, for example enhanced Al or bare Au, depending on the light wavelength. The mechanical keys  15  and  16  are formed by the same photolithographic process and are used for easy alignment of the two parts  17  and  11 . By etching along the same [111] crystallographic planes, a diamond-like shape can be achieved, as illustrated in FIG. 7 b.    
         [0027]    Other shapes can be achieved and other materials can also be used. For example, shapes as illustrated in FIGS. 7 a ,  7   c  and  7   d , as well as non-symmetrical shapes can be made by the so called gray-scale photolithography, well mastered by companies like the same Micro-Technology Institute in Mainz, Germany and Rochester Photonics Corporation of Rochester, N.Y. Other than Si, materials including non-crystalline like glass, fused silica or polymers can also be used.