Abstract:
An illuminator ( 10 ) for producing a radiation spot ( 120 ) on a workpiece ( 110 ) comprises a laser light source ( 20 ) and a beam-shearing optical system ( 30 ). Fine structure ( 160 ) generated by an integrator ( 80 ) of laser light source ( 20 ) in integrated beams ( 90 ) as a result of path length differences among coherent light beams ( 70 ) from emitters ( 60 ) of lasers ( 40 ) is sheared by the inclusion of beam-shearing optical system ( 30 ) into illuminator ( 10 ). Beam-shearing optical system ( 30 ) allows illuminator ( 10 ) to produce a radiation spot ( 120 ) in which the fine structure ( 160 ) is spread so that scanning of radiation spot ( 120 ) does not produce the striations ( 140 ) in scanned patch ( 170 ) that are obtained with prior art illuminators.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a system and method for shearing a radiation beam. In particular, the present invention relates to a system and method to reduce fine structure in radiation beams used in imaging printing media. 
       BACKGROUND OF THE INVENTION 
       [0002]    Diode lasers are used in many imaging applications as a convenient and low-cost radiation source. Material processing applications may make use of suitably coupled diode laser radiation to change the nature or character of a workpiece. Image recording and display systems may use laser diodes to provide illumination for an optical system. 
         [0003]    In one particular imaging application, a monolithic array of laser diode emitters may be used to illuminate a multi-channel light valve. A light valve generally has a plurality of individually addressable modulator sites; each site producing a beam or channel of imagewise modulated light. An image is formed by selectively activating the channels while scanning them over an image receiver. For high quality imaging it is usually necessary that channels be uniform in their imaging characteristics, a requirement that presents a difficult challenge for system designers since the illumination from a laser diode is highly astigmatic with poor overall beam quality. Consequently optical systems for gathering and formatting the light output seek to overcome the inherent limitations of the diode laser output in order to produce useable illumination. 
         [0004]    U.S. Pat. No. 5,517,359 (Gelbart) describes a method for imaging the radiation from a laser diode array having multiple emitters onto a linear light valve. The optical system superimposes the radiation line from each emitter at the plane of the light valve, thus forming a single combined illumination line. The superimposition provides some immunity from emitter failures (either partial of full). In the event of such a failure, while the output power is reduced, the uniformity of the line is not severely impacted. 
         [0005]    Even with superimposed emitters, the uniformity of the individual emitter radiation profiles still has an impact on the overall uniformity of the line. A good laser diode array can have emitters that are more than 20% non-uniform in the slow axis. When the radiation from a plurality of emitters is combined, some of the non-uniformities may offset each other but commonly 10-15% non-uniformity remains. Some light valves can accommodate this non-uniformity by balancing the output from each channel by attenuating output from channels that are more strongly illuminated. This, however, represents wastage of up to 15% of the useful light output since it is not possible to amplify weak channels. 
         [0006]    U.S. Pat. No. 6,137,631 (Moulin) describes a means for mixing the radiant energy from a plurality of emitters on a laser diode array. The mixing means comprises a plurality of reflecting surfaces placed at or downstream from a point where the laser radiation has been focused. The radiant energy entering the mixing means is subjected to multiple reflections, which makes the output distribution of the emerging radiant energy more uniform. 
         [0007]    Laser diode arrays having nineteen or more 150 μm emitters are now available with total power output of around 50 W at near-infrared wavelengths. While efforts are constantly underway to provide higher power, material and fabrication techniques still limit the power that can be achieved for any given configuration. In order to provide illumination lines with total power in the region of 100 W, an optical system designer may only be left with the option of combining the radiation from a plurality of laser diode arrays. In so combining the radiation, care has to be taken to retain the brightness of the illumination. This imposes practical limitations on the designer. 
         [0008]    U.S. Pat. No. 7,209,624 (Reynolds et al.) describes an illumination system in which the radiation from one or more laser arrays is directed into a light pipe. The light pipe mixes the individual radiation contributions from the laser arrays and forms a uniform illumination line. The pointing direction of each of the laser arrays is monitored and controlled to preserve the brightness of the composite illumination line. 
         [0009]    U.S. Pat. No. 5,923,475 (Kurtz et al.) describes an illumination system for a laser printer comprised of a laser diode array, a cross array illumination optics, a laser lenslet array, a spatial light modulator, and a fly&#39;s eye integrator which illuminates the spatial light modulator with flooded uniform light. The design seeks to provide spatially and angularly homogenized uniform flood illumination of the spatial light modulator from a plurality of diode laser emitters. 
         [0010]    The power emitted by individual diode lasers having risen as a result of technological development and it has become possible to employ ever fewer individual lasers or laser emitters to attain a given power level. While this is in general a good development, there is one aspect that complicates optical systems design. The problem is that the increased coherence-induced fine structure results when the radiation is combined for practical application, whether this is through the use of optical fiber structures, mirrors, fly eye structures or any other device that largely retains the coherence of the light. When such arrangements are employed in illuminators for imaging purposes, as in the preparation, for example, of a laser imaged lithographic plate, the fine structure may be imposed on the lithographic plate producing unwanted striations. 
         [0011]    Thus, there exists a requirement for a means to reduce or remove the fine structure from such illumination systems. 
       SUMMARY OF THE INVENTION 
       [0012]    Briefly, according to one aspect of the present invention an illuminator includes a laser light source and a beam-shearing optical system, wherein light from the laser light source is directed through the beam-shearing optical system. The illuminator has a laser light source which includes one or more emitters extending in a first direction. The laser light source is arranged to emit one or more light beams about an optical axis perpendicular to the first dimension. The anomorphic optical system has one or more planes of symmetry containing the optical axis. At least one light beam from the laser light source is directed through the beam-shearing optical system. A lest one of the planes of symmetry is oriented at an angle of between 0 and +/−90 degrees with respect to the first dimension. 
         [0013]    The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The present invention will be more readily understood from the detailed description of exemplary embodiments presented below considered in conjunction with the attached drawing: 
           [0015]      FIG. 1  shows an illuminator comprising a laser light source and a beam-shearing optical system; 
           [0016]      FIG. 2  shows a scanned patch made with an illuminator of the prior art; 
           [0017]      FIG. 3  shows a scanned patch made with an illuminator of the present invention; 
           [0018]      FIG. 4  shows an illuminator comprising a beam-shearing optical system, the beam-shearing optical system comprising an anomorphic optical system based on a pair of crossed cylindrical lenses and a spherical lens; 
           [0019]      FIG. 5  shows an illuminator comprising a beam-shearing optical system based on optical Fourier transformation; and 
           [0020]      FIG. 6  shows a scanned patch made with an illuminator of the present invention with its resultant sheared light beams directed through a spatial light modulator. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    In  FIG. 1 , showing a preferred embodiment of the present invention, an illuminator  10  comprises a laser light source  20  and a beam-shearing optical system  30 . Laser light source  20  comprises one or more lasers  40 , collector system  50 , and integrator  80 . Each laser  40  comprises one or more emitter  60 . Light beams  70  from emitters  60  are collected into integrator  80  by collector system  50  to produce integrated beam  90 . 
         [0022]    Collector system  50  can be any optical element capable of suitably collecting the light from emitter  60 . An example is provided by U.S. Pat. No. 5,517,359. Integrator  80  can be any suitable integrator, including but not limited to a light pipe as in U.S. Pat. No. 6,137,631 or U.S. Pat. No. 7,209,624, or a fly&#39;s eye integrator as described in U.S. Pat. No. 5,923,475, or the like. 
         [0023]    The light beams  70  from emitter  60  are collected by collector system  50  and integrated by integrator  80  to produce integrated beam  90 . Due to the coherent nature of the light from emitter  60 , the combined light beams  70  from emitter  60  traverse different path lengths and are therefore out of phase at the exit of laser light source  20 , thereby creating fine structure in integrated beam  90 . The more singular the modal structure of emitter  60 , the deeper the modulation producing the fine structure. 
         [0024]    Beam-shearing optical system  30  spreads the fine structure across the beam, thereby reducing any striations that may be created by the unsheared integrated beam  90  produced by laser light source  20 . In a preferred embodiment, light beams  70  are coupled into collector system  50  such that integrated beam  90 , if focused onto a workpiece  110 , produces a radiation spot  120  with a very large aspect ratio in which the dimension W of radiation spot  120  in the plane of lasers  40  is much larger than the dimension H of radiation spot  120  that is perpendicular to the plane of lasers  40 . 
         [0025]    Turning to  FIG. 2 , representing the prior art, no beam-shearing optical system  30  is incorporated in illuminator  10 ′, and radiation spot  120  has fine structure  130 . When radiation spot  120  produced in this way is scanned in direction Z, it produces striations  140  in scanned patch  150 . 
         [0026]      FIG. 3  shows an embodiment of the present invention, wherein beam-shearing optical system  30  is incorporated in illuminator  10 , such that radiation spot  120  has sheared fine structure  160 . When radiation spot  120  produced in this way is scanned in direction Z, it produces scanned patch  170  with much reduced or no striations. 
         [0027]    In a preferred embodiment of the present invention, shown in  FIG. 1 , beam-shearing optical system  30  is an anomorphic optical system, including but not limited to an anomorphic optical system comprising at least one reflective and one refractive element. Emitter  60  extends in a first dimension and is arranged to emit one or more light beams about an optical axis perpendicular to the first dimension. At least one light beam from emitter  60  is directed through beam-shearing optical system  30 , beam-shearing optical system  30  having one or more planes of symmetry containing the optical axis and at least one of the planes of symmetry is oriented at an angle of between 0 and +/−90 degrees with respect to the first dimension. 
         [0028]    In a preferred embodiment of the present invention, shown in  FIG. 4 , beam-shearing optical system  30  is anomorphic and comprises a pair of crossed cylindrical lenses  180  and  190 , together with a spherical lens  200 , positioned on the optical axis of illuminator  10 . Beam-shearing optical system  30  is rotated at a small angle θ different from 0 degrees with respect to the fine structure in integrated beam  90  emitted by laser light source  20 . In this embodiment of the present invention, the degree of shear is determined by the magnitude of the angle θ. For θ=0 there is no shear. 
         [0029]    In an embodiment of the present invention shown in  FIG. 5 , an illuminator  210  comprises a laser light source  220  and beam-shearing optical system  230 . Beam-shearing optical system  230  is an optical image processing system based on an optical Fourier transformation. Beam-shearing optical system  230  comprises Fourier transformer  240 , filter  250 , and a reverse Fourier transformer  260 . Fourier transformer  240  performs a Fourier transform in the y-direction and is a unit magnifier in the x-direction, x and y being mutually perpendicular and both perpendicular to the optical axis of illuminator  210 . Reverse Fourier transformer  260  performs a reverse Fourier transform in the y-direction and is a unit magnifier in the x-direction. 
         [0030]    Filter  250  comprises two crossed cylindrical lenses with their axes at 45 degrees with respect to the x and y axes, and the x and y axes are both perpendicular to the optical axis of illuminator  210 . In incorporating beam-shearing system  230  into illuminator  210 , the x axis of beam-shearing optical system  230  is aligned parallel to the fine structure in the light emitted by laser light source  220 . In this embodiment, the degree of shear is determined by the optical power of the crossed cylindrical lenses of filter  250 . As with the embodiment of  FIG. 4 , the light emitted by illuminator  210 , can be focused onto a workpiece  110 , to produce a radiation spot  120  with a very large aspect ratio in which the dimension W of radiation spot  120  in the plane of lasers (that is, the slow direction of the laser) is very much larger than the dimension H of radiation spot  120  that is perpendicular to the slow direction of the lasers. 
         [0031]      FIG. 6  shows that sheared light beam  100 , directed first to a spatial light modulator  105 , which imagewise modulates integrated beam  90 , and can then made to impinge on workpiece  110  through suitable focusing optics (not shown). Workpiece  110  may be a media to be imaged, including but not limited to a printing plate, which printing plate can be, but is not limited to, a lithographic printing plate or a flexographic printing plate. The media to be imaged can also be a gravure cylinder. Without the beam-shearing optical system of the present invention in the illuminator, the fine structure of integrated beam  90  would be imposed on the media. 
         [0032]    Returning to  FIG. 3 , the method of the present invention comprises, irradiating a scanned patch  170  on a workpiece  110 . The irradiating comprises scanning a radiation spot  120  having a fine structure  160  having linear extent in a first direction K, scanning in a second direction Z, the second direction Z having a component perpendicular to the first direction K in the plane of workpiece  110  at the radiation spot  120 . In a preferred embodiment of the present invention, fine structure  160  comprises lines oriented in the first direction K. 
         [0033]    Fine structure  160  is produced by distorting the integrated light beams  90  from a laser light source  20  to form radiation spot  120 . In a preferred embodiment the distorting is shearing. In a preferred embodiment of the present invention, the shearing of integrated beams  90  comprises anamorphically distorting integrated beams  90 . In a further preferred embodiment of the present invention, shearing of integrated beams  90  comprises Fourier transforming integrated beams  90 . More specifically, the shearing of integrated beams  90  can comprise anamorphically magnifying an intensity distribution in integrated beams  90  at one of a real and a virtual plane, or optically processing an intensity distribution in integrated beams  90  at a real or virtual plane by means of Fourier transformation According to one aspect, the invention comprises irradiating a patch on a work piece with an illumination spot having fine structure, forming the illumination spot using light beams from a light source, shearing the light beams used to form the illumination spot, and scanning the spot along a first direction in the plane of the workpiece, the first direction being at an angle of between 0 and 90 degrees with respect to any sheared fine structure in the illumination spot. The sheared light beams can be directed through a spatial light modulator to impinge on the work piece to imagewise modulate the illumination spot. 
         [0034]    The important features of the invention have been outlined in order that the present contribution of the present invention to the art may be better appreciated. Those skilled in the art will appreciate that the conception on which this disclosure is based may readily be utilized as a basis for the design of other apparatus and methods for carrying out the several purposes of the invention. It is most important, therefore, that this disclosure be regarded as including such equivalent apparatus and methods as do not depart from the scope of the invention. 
       PARTS LIST 
       [0000]    
       
           10  illuminator 
           10 ′ illuminator 
           20  laser light source 
           30  beam-shearing optical system 
           40  laser(s) 
           50  collector system 
           60  emitter(s) 
           70  light beam 
           80  integrator 
           90  integrated beam 
           100  sheared light beam 
           105  spatial light modulator 
           110  workpiece 
           120  radiation spot 
           130  fine structure 
           140  striations 
           150  scanned patch 
           160  fine structure 
           170  scanned patch 
           180  cylindrical lens 
           190  cylindrical lens 
           200  spherical lens 
           210  illuminator 
           220  laser light source 
           230  beam-shearing optical system 
           240  Fourier transformer 
           250  filter 
           260  reverse Fourier transformer