Abstract:
An optical imaging head for direct engraving of flexographic printing plates, comprising at least two laser diodes ( 10, 12 ) emitting radiation in one or more wavelengths; and means for imaging the one or more wavelengths of radiation at different depths relative to a surface of the plate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Reference is made to commonly-assigned copending U.S. patent application Ser. No. 11/353,217, filed Feb. 13, 2006, entitled FLEXOGRAPHIC PRINTING PLATE PRECURSOR AND IMAGING METHOD, by Kimelblat et al., the disclosure of which is incorporated herein. 
     
     FIELD OF THE INVENTION 
       [0002]    This invention relates to an optical printing head and methods for direct engraving of sensitive flexographic printing plates by utilizing high power diode lasers. 
       BACKGROUND OF THE INVENTION 
       [0003]    Traditional flexographic printing methods prepare a printing plate by molding an elastomer such as rubber in a mold, or by photo-polymerizing a UV sensitive polymer. These methods are slow and expensive. 
         [0004]    Another technique for creating a raised pattern on an elastomer surface is to directly cut the raised pattern using the well known NdYAG or CO 2  lasers, which are currently used as light sources in the direct engraving printing systems. The laser is controlled to ablate the elastomer in recessed areas and to leave the elastomer intact in raised areas. However, conventional flexographic printing plates cannot be laser engraved quickly. This is because the laser ablates a relatively thick layer (0.5 mm-2 mm) of elastomer. Thus, a multi KW laser is required to complete a typical flexographic plate in under one hour. 
         [0005]    Another difficulty with previous attempts at laser engraving of flexographic printing surfaces with CO 2  lasers is that CO 2  lasers have a long wavelength, 10.6 microns, relative to the approximately 1 micron of the diodes lasers, which severely limits the resolution that can be achieved. In addition, due to their long wavelength of CO 2  laser, there is a need to use optical elements for the far infrared. These are quite expensive relative to optical elements that are used for the near infrared. 
         [0006]    U.S. Pat. No. 6,150,629 (Sievers), describes a laser engraving system using two lasers with different wavelengths. Each laser can be modulated independently and temporally. The patent strictly talks of using temporal modulation to achieve different effects on the plate. The lasers are combined into one beam and imaged on the plate, using an external modulation acoustic optic modulator. 
         [0007]    The Sievers methods have several disadvantages; including: 
         [0008]    The different laser beams are focused to the same depth relative to the plate surface. 
         [0009]    The different laser beams are combined into one common optical path. 
         [0010]    External modulation using acousto optic modulators. 
         [0011]    U.S. Pat. No. 4,947,023 (Minamida), describes an apparatus for roll dulling by pulse laser beam. The system utilizes a number of lasers emitting light at equal wavelengths; the lasers can be combined into one or more optical paths. The beam divergence of the lasers can be manipulated via beam expanders in order to get different spot sizes on the plate surface. The Minamida system does not use different wavelengths, or optical elements to spatially focus different wavelengths to different depths relative to the plate surface. 
         [0012]    U.S. Pat. No. 6,664,498 (Forsman), describes a method and apparatus for increasing the material removal rate in laser machining. The intention of this patent, is to process materials, such as steel, aluminum, and silicon. There is no mention of printing plates. The main idea of the patent is to use high power pulsed lasers with bursts of very short laser pulses, usually with pulse duration in the nano second range. This objective cannot be achieved with fiber diodes. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention uses high power laser diodes and/or high power fiber coupled laser diodes, and/or laser fibers, instead of the well known powerful NdYAG and CO 2  lasers that are currently used as light sources in direct engraving printing systems. The multi-beam optical head of the present invention incorporates numerous laser diodes each having relative moderate powers, of the order of 10 Watts per emitter width of 100 micron, instead of using an optical head which has just one or two powerful beams of NdYAG and CO 2  lasers that emit hundreds of Watts. 
         [0014]    The advantages of using diode lasers instead of NdYAG and CO 2  lasers are that diode lasers are compact, reliable and can be modulated directly at relative high frequencies without need for external modulators. Diode lasers are also available at different wavelengths and at high powers. No gas is used and relative low voltage is needed. 
         [0015]    Laser diodes are now already available at relative high powers of approximately 10 Watts per emitter width of 100 micron. This enables using high power diode lasers for direct engraving of new types of relatively sensitive flexographic printing plates. 
         [0016]    Direct engraving flexographic printing plates have general emissivity close to one and therefore absorb any wavelength. Laser light that impinges on the plate is absorbed by the plate, and engraves shaped holes in the plate. The present invention also includes several embodiments using optical heads and methods by which the laser light is controlled in order to enhance the direct engraving and ablating effect. For example, according to one aspect of the present invention the optical imaging head for direct engraving of flexographic printing plates comprises at least two laser diodes emitting radiation in one or more wavelengths, and means for imaging one or more wavelengths of radiation at different depths relative to a surface of the plate. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a schematic of one channel of a non-fiber optic system according to the present invention; 
           [0018]      FIG. 2  is a schematic of one channel of a fiber coupled system according to the present invention; 
           [0019]      FIG. 3  is a schematic of one channel of a non-fiber system using polarization beam combiners according to the present invention; 
           [0020]      FIG. 4  is a schematic of one channel of a fiber coupled system using polarization beam combiners according to the present invention; 
           [0021]      FIG. 5  is a ray trace and screen shots of spot sizes measured by two detectors; 
           [0022]      FIG. 6  is a schematic of one channel for a serial exposure mode according to the present invention; 
           [0023]      FIG. 7  is a schematic of one channel using two diodes having the same wavelength with fiber optics having different dimensions; 
           [0024]      FIG. 8  is a schematic of the embodiment shown in  FIG. 7  using diodes which emit at different wavelengths; 
           [0025]      FIG. 9  is a schematic of one channel of the present invention with the distal end of the fibers arranged in different object planes; 
           [0026]      FIG. 10  is a schematic of a system for measuring the relative shift in the image plane V, versus the fiber position in the object plane U; 
           [0027]      FIG. 11  is a graph showing relative shift in image plane versus fiber position U, in the object plane; 
           [0028]      FIG. 12  is an embodiment of the present invention incorporating a glass plate of thickness D and index of refraction n; 
           [0029]      FIG. 13  is an embodiment of the present invention incorporating a glass plate constructed from several zones, each having a different thickness and different or same index of refraction; 
           [0030]      FIG. 14  is an embodiment of the present invention incorporating a glass plate which has a variable profile of the index of refraction along the Y direction; 
           [0031]      FIG. 15  is a schematic showing an embodiment for confocal and auto-focus measurements useful in a diagnostics system; 
           [0032]      FIG. 16  shows conversion of a Gaussian beam profile to two types of top hat profiles by using diffractive elements; 
           [0033]      FIG. 17  shows a cross-section of a specific case where optical fibers are aligned in two V-grooves; and 
           [0034]      FIG. 18  is an expanded view of one of the V-grooves shown in  FIG. 17 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    The present invention suggests several methods by which the laser diode light is controlled in order to enhance the direct engraving and ablating effect. Referring to  FIG. 1 , the invention is described by the schemes shown for the specific case of two laser diodes  10 ,  12  that emit in two different wavelengths. By using laser diodes which emit at different wavelengths, and by controlling the dispersion of the imaging lens  16 , the focus points  18 ,  20  at the different wavelengths will be shifted one relative to each other. 
         [0036]      FIG. 2  describes the same idea as  FIG. 1  for the case of fiber coupled diodes. The two different wavelengths are combined into one fiber using a fiber optic coupler  26  instead of the beam combiner  22 . 
         [0037]    The light emitted from laser diodes is highly polarized. Therefore, by using a polarization beam combiner  24 , shown in  FIG. 3 , the power of two diodes that emit the same wavelength  10   a ,  10   b  can be combined into one path. Doing so the power of each wavelength is doubled and the overall power that impinges on the plate can be increased by four times. Polarization beam combiners are well known elements used in the optical field to couple light sources that have different configurations. Polarization beam combiners are available in both free space and fiber (mainly for single mode fibers) coupled configurations. 
         [0038]      FIG. 4  shows the same general concept as  FIG. 3  for the case of fiber coupled diodes in an embodiment using two different wavelengths. The power of two fiber coupled diodes that emit the same wavelength  10   a ,  10   b  is first combined into one fiber by using a polarization fiber-optic combiner  27 . Two additional diodes  10   c ,  10   d , which emit a second wavelength are combined into one path. 
         [0039]    In order to roughly check and present the concept a simulation, shown in  FIG. 5 , using a specific imaging lens and two diodes which emit at 800 nm and 970 nm, was carried out. The results show the ray trace and the spot size as measured by two detectors. The detectors are located at the two focal points, which are shifted in approximately 100 micron relative to each other. The imaging lens can of course be designed in order to achieve a desired shift. 
         [0040]      FIG. 6  shows laser diodes  30   a ,  30   b  with different wavelengths located adjacent to each other. In general, the optical head (not shown) moves along the plate, in the direction indicated by arrow  31 . First, laser diode  30   b  is activated and just then the laser diode  30   a , i.e. when it reached the same pixel which was already exposed by laser diode  30   b . The embodiment described uses fiber coupled diodes. The description is just for two out of n channels. The same general idea can of course be implemented, with no fibers. 
         [0041]    Optical fibers  33   a  and  33   b  with different core diameters can be assembled on the same V-groove  35 , as shown in  FIG. 7 . This can be implemented for the same or different wavelengths. This way, by using only one imaging lens  37  one can get spots of two sizes. The figures show an example for two laser diodes  32   a  and  32   b  of the same wavelength. If optical fiber  33   b  is a 40 micron fiber and optical fiber  33   a  is a 100 micron fiber, then by using a 1×2 imaging lens  37  one gets a 20 micron spot  40   b  and a 50 micron spot  40   a.    
         [0042]      FIG. 8  shows the same concept, but now the laser diodes  41   a  and  41   b  emit at different wavelengths. Spots of different diameters  42   a  and  42   b  respectively are achieved at different locations. 
         [0043]    In  FIG. 9 , the distal tips of optical fibers  43   a  and  43   b  are assembled in the V-groove  44  in different object planes  45  and  46  respectively relative to each other. As a result, the image planes  47  and  48  of the different fiber tips respectively will be shifted one relative to the other.  FIG. 9  shows the effect just for two fibers. The optical fibers can be identical, or different, for example with different core diameters. Fibers can emit radiation at the same or different wavelengths. 
         [0044]    In any one of the embodiments of  FIGS. 6 through 9 , the optical radiation guided in the fiber can be in a filled or an underfilled state. In general, the laser diodes can be temporally modulated relative to each other in order to get different effects on the direct engraving plate. 
         [0045]      FIG. 10  describes an example of a specific measurement system in which the image position shift was measured. 
         [0046]    The graph in  FIG. 11  shows the relative shift in the image position as a function of the position U, of the distal tip of the fiber, as measured by the system of  FIG. 10 . The shift in image position V, was found by moving the position of the microscope lens in order to find the smallest spot. A laser beam analyzer was used to measure and define a spot that includes 95% of the laser beam energy. 
         [0047]    For this specific case of using a telecentric imaging lens with a magnification of  2 , it can be seen that moving the distal tip of the fiber in the object plane X mm results in a shift of 0.508 X mm in the image position. For example, moving the distal tip of the fiber 0.2 mm, from U=40 to U=40.2, causes the image to move by an absolute relative distance of 0.1 mm. 
         [0048]    As shown in  FIG. 12 , a glass plate  50  of thickness D placed between the distal tip of the fiber  52  and the imaging lens  54 , will cause the image plane to shift from image plane  47  to  48 . The shift V in the position of the image plane is a function of the thickness D of the plate and its index of refraction n. The schematic presents the effect of such a glass plate for the specific case of a single laser diode  55 . The solid rays describe the case when no plate is used and in which the rays are focused in image plane  47 . The dashed rays describe the case when a plate is used and in which the focus is shifted to image plane  48 . When a multi laser source constructed from different wavelengths is used the shift V will be a function of the wavelength due to the fact that the index of refraction n, is a function of the wavelength. 
         [0049]    The glass plate can be constructed from several zones, each having a different thickness and different or same index of refraction as depicted in  FIG. 13 . Such a structure enables moving and adjusting the glass plate in front of the fiber tips in order to get a required shift. 
         [0050]    The glass plate can also have a variable profile of the index of refraction that changes along the Y direction. This form of the glass plate is shown in  FIG. 14 . Such a structure enables to move and adjust the right zone of the glass plate in front of the fiber tips in order to get a required shift. The glass plate may also be inserted between the imaging lens and the imaged surface. 
         [0051]    An example of the concept is shown in  FIG. 15 . Optical detectors  60   a  and  60   b , are optically coupled to laser diodes  62   a ,  62   b  by fiber optic couplers  61   a  and  61   b  and optical fibers  63   a  through  63   d , respectively. The laser radiation that impinges on the printing plate is partially reflected backwards and detected by optical detectors  60   a  and  60   b . The signal at optical detector  60   a , V 1 , and the signal at optical detector  60   b , V 2 , are proportional to the position of the printing plate. When the plate is in position A the signal at optical detector  60   a  will be at its maximum value, and when the plate is in position B, the signal at optical detector  60   b  will be at its maximum value. Hence, signals V 1  and V 2  can be used to adjust the imaging lens at a desired distance relative to the position of the printing plate. Furthermore, the signals V 1  and V 2  can be used to inspect and diagnose the printing plate after or during the exposure to the laser beam. 
         [0052]    When a single mode diode is used, the Gaussian profile of the beam can be converted, utilizing diffractive optic elements, to be top hat, as depicted in  FIG. 16  Such diffractive elements are made by several companies, including: www.holoor.co.il/website/data/index.html. 
         [0053]    The light source described by  FIGS. 1-16  can be a diode laser and/or a fiber coupled diode laser. The different configurations in which the fibers are aligned relative to each other described by  FIGS. 1-16  can be done relative to the slow and/or fast axis of the printing drum (the slow and fast axis are well known parameters to any one skilled in the printing art). 
         [0054]    The fibers can be aligned in space in any configuration relative to each other; for example, in a mechanical support, such as a V-groove  65 , shown in  FIGS. 17 and 18 . The fibers may be aligned one adjacent to the other and/or one above the other, where two or more V-grooves are aligned one on top of the other in a sandwich configuration. The specific case of a sandwich configuration can be seen in  FIG. 17 . 
         [0055]    By using diodes at different wavelengths several advantages can be obtained for direct engraving. By a proper optical design the depth of focus can be increased while keeping a relative good spot size which will fit the direct engraving quality. 
         [0056]    The diodes can be spatially (by using fibers with different core diameters, or by positioning the distal tips of the fibers at different object planes) and/or temporally modulated relative to each other in order to get different effects on the direct engraving plate. For example, by initiating the first diode before the second diode, etc. 
         [0057]    When using laser diodes which emit light at different wavelengths, the multi color light source can be tailored to the special optical and thermal characteristics of a direct engraving printing plate, such as the printing plate described in commonly-assigned copending U.S. patent application Ser. No. 11/353,217. 
         [0058]    The advantages of the present invention are: 
         [0059]    Increasing both the power and depth of focus while keeping a relatively good spot size which will be adequate for the quality needed for direct engraving. 
         [0060]    The lasers can be temporally modulated, simultaneously or relative to each other in order to get a better thermal effect on special engraving plates. 
         [0061]    Direct modulation of the lasers does not require external modulation. 
         [0062]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
       Parts List  
       [0000]    
       
           10  laser diode 
           10   a  wavelength λ 2  P polarization 
           10   b  wavelength λ 2  S polarization 
           10   c  wavelength λ 1  P polarization 
           10   d  wavelength λ 1  S polarization 
           12  laser diode 
           16  imaging lens 
           18  focus point 
           20  focus point 
           22  beam combiner 
           24  polarization beam combiner 
           26  fiber optic coupler 
           27  polarization fiber-optic combiner 
           30   a  laser diode 
           30   b  laser diode 
           31  direction of head movement 
           32   a  laser diode 
           32   b  laser diode 
           33   a  optical fiber 
           33   b  optical fiber 
           35  V-groove 
           37  imaging lens 
           40   a  spot with 50 microns diameter 
           40   b  spot with 20 microns diameter 
           41   a  laser diode 
           41   b  laser diode 
           42   a  spot with 50 microns diameter 
           42   b  spot with 20 microns diameter 
           43   a  optical fiber 
           43   b  optical fiber 
           44  V-groove 
           45  object plane 
           46  object plane 
           47  image plane 
           48  image plane 
           50  glass plate 
           52  optical fiber 
           54  imaging lens 
           55  laser diode 
           60   a  optical detector 
           60   b  optical detector 
           61   a  fiber optic coupler 
           61   b  fiber optic coupler 
           62   a  laser diode 
           62   b  laser diode 
           63   a  optical fiber 
           63   b  optical fiber 
           63   c  optical fiber 
           63   d  optical fiber 
           65  V-groove