Abstract:
An optical head comprising an array of laser diodes (LDA) where each member of the LDA having a fast axis direction and a slow axis direction of beam divergence, a spatial light modulator (SLM), an imaging lens, and a Micro-Scramblers array (MSA) where each member of the MSA has exit apertures and each member of the MSA is associated with a member of the LDA for scrambling the light only in the direction of the slow axis of the member of the LDA. The optical head can be incorporated into an external-drum electro-optical plotter or a flat bed electro-optical plotter.

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 a printing plate. The optical head incorporates an array of laser diodes (LDA) as a light source, a Micro-Scramblers Array (MSA) as a beam-shaping element, a Spatial Light Modulator (SLM) for spatial modulation of the light emitted from the LDA in accordance with data representing the image to be obtained, and an imaging lens for imaging the SLM on the light sensitive medium.  
         BACKGROUND OF THE INVENTION  
         [0002]    Multi-channel optical imaging systems are well known in the art and are currently being used in a variety of applications, including imaging of different printing media. Such systems often utilize a LDA and a SLM as a light source. In a typical offset platesetter for example, the LDA is used as a light source for illuminating the SLM, which, in turn, spatially modulates the light in accordance with the pattern to be imaged. The SLM on the other hand, is imaged by an imaging lens on the photosensitive medium, such as an offset printing plate, and thus the desired pattern is achieved.  
           [0003]    A LDA can be an array of ordered individual laser emitters, a laser-diode bar, or a laser-diode stack. For simplicity, all these options will be referred to as LDA hereinbelow. A laser diode bar (linear array of laser diode emitters) will be used as an example in the explanations below (FIG. 1 a ). It will, however, be appreciated by any person skilled in the art, that the embodiments described may be easily scaled up in Y direction to a two-dimensional array such as a diode laser bars stack. All the conclusions drawn for a linear array will be valid for a two-dimensional array. In many applications, such as imaging of thermal offset printing plates, the individual emitters of the LDA are expected to deliver a significant amount of light power, namely, from several hundred milliwatts to several watts. In such cases, multimode laser diode emitters are used. FIG. 1 a  illustrates a conventional art LDA  310 . The individual emitters  311  are usually with length          between 50 and 150 μm and height          of approximately 1 μm. The pitch          of the emitters  311  can be of several hundred micrometers. All references to X, Y and Z directions hereinbelow refer to the coordinate system illustrated in FIG. 1 b . The emission properties of the emitters  311  are different in the directions X (parallel to the LDA length) and Y (perpendicular to the LDA length). The beam divergence in the Y direction is usually between 50 and 90 degrees full angle, while the beam divergence in X direction is usually well below  20  degrees full angle. It is commonly accepted for the Y-axis to be termed “fast axis” and the X-axis to be termed “slow axis”. Thus, the orientation of the LDA  310  in an optical system is defined in the XYZ coordinate system by the fast and slow axes. The Z-axis is considered an optical axis of the optical system.  
           [0004]    An important part of optical imaging heads utilizing LDA and SLM is the illumination system of the SLM. There are a number of prior art works tackling the issue of illuminating an SLM by LDA, for example U.S. Pat. No. 5,900,981 to Oren et al. and U.S. Pat. No. 5,333,077 to Legar et al.  
           [0005]    [0005]FIGS. 2 a  and  2   b  schematically illustrate an optical system  20  for illumination of SLM  16 . FIG. 2 a  schematically illustrates the propagation of the beams in the X-Z plane (the plane of the slow axis) and FIG. 2 b  schematically illustrates the propagation of the beams in the Y-Z plane (the plane of the fast axis). The main idea in this, as in most illumination systems, is to image each individual emitter  11  on the SLM  16  by overlapping their images, so that each point on the SLM  16  receives light from each emitter  11 . This method of illumination provides a high degree of redundancy, namely, partial or total loss of emitting abilities of one or more emitters  11  of LDA  10  will result in decreased illumination power, but will hardly affect the power distribution over SLM  16 .  
           [0006]    An important characteristic of LDA-SLM imaging systems is the uniformity of the SLM illumination. The illumination method of FIGS. 2 a  and  2   b  provides illumination which, in each point of the SLM  16 , is a sum of the contributions of the individual emitters  11  of the LDA  10 . However, as was already mentioned, a wide range of contemporary applications requires high-power LDAs of multimode emitters. The near field spatial power distribution of a multimode laser emitter along the slow axis is highly non-uniform and variable over time, depending on parameters such as current through the diode, junction temperature, etc. In a laser diode bar there is a tendency to similarity in the near field power distribution between the individual emitters. FIGS. 3 a  and  3   b  illustrate the spatial power distribution along the slow axis of two 150 μm long emitter members of a commercially available laser diode bar and clearly indicate this similarity. The illumination pattern of the SLM  16  is an overlap of the images of the individual emitters  11 . Typical power distribution of such illumination pattern along the slow axis direction is illustrated in FIG. 3 c . It can be seen that the distribution approximately resembles the power distribution of the individual LDA members and therefore is highly non-uniform.  
           [0007]    The non-uniformity of the SLM illumination leads to uneven power distribution among the writing beams, causing undesired image density irregularities. The nonuniformity of the SLM illumination can be compensated by using a SLM with calibration capabilities for each channel. In this case, all channels are equalized to the least illuminated one. The result of such calibration is an even power distribution among the writing channels, at the expense of energy loss. The amount of energy lost is illustrated in FIG. 3 c  by the hatched area and in some cases can reach several tens of percents.  
           [0008]    Another method of obtaining a uniform illumination of the SLM is disclosed in U.S. Pat. No. 6,137,631 to Moulin. The inventor suggests using a glass blade as a mixing means for obtaining substantially uniform illumination. The proposed embodiments include a focusing lens for delivering the light emitted by each emitter of the LDA to the entrance aperture of a single (common to all emitters) mixing glass blade, which should have a significant length (It is well known to any person skilled in the art that good mixing capabilities can be obtained when the length of the blade is at least ten times greater than its aperture diagonal). These features increase the complexity and the cost of the system. The main disadvantage, however, is the significant loss of brightness in the slow axis direction (by factor          /         , FIG. 1 a ), making the proposed embodiments low energy efficient.  
         SUMMARY OF INVENTION  
         [0009]    The present invention provides a multi-beam, LDA-SLM based optical imaging head with a high degree of illumination uniformity of the SLM.  
           [0010]    The present invention additionally provides a high energy-efficient LDA-SLM based, multiple laser-beam recording apparatus, free of image density irregularities due to non-uniform illumination of the SLM.  
           [0011]    In a first embodiment, the present invention comprises an optical head. The optical head comprises an array of laser diodes (LDA) where each member of the LDA having a fast axis direction and a slow axis direction of beam divergence, a spatial light modulator (SLM), an imaging lens; and a Micro-Scramblers array (MSA) where each member of the MSA has exit apertures and each member of the MSA is associated with a member of the LDA for scrambling the light only in the direction of the slow axis of the member of the LDA.  
           [0012]    In another embodiment, the present invention comprises an external-drum electro-optical plotter comprising an optical head. The optical head comprises an array of laser diodes (LDA) where each member of the LDA having a fast axis direction and a slow axis direction of beam divergence, a spatial light modulator (SLM), an imaging lens; and a Micro-Scramblers array (MSA) where each member of the MSA has exit apertures and each member of the MSA is associated with a member of the LDA for scrambling the light only in the direction of the slow axis of the member of the LDA.  
           [0013]    In yet another embodiment, the present invention comprises a flatbed electro-optical plotter comprising an optical head. The optical head comprises an array of laser diodes (LDA) where each member of the LDA having a fast axis direction and a slow axis direction of beam divergence, a spatial light modulator (SLM), an imaging lens; and a Micro-Scramblers array (MSA) where each member of the MSA has exit apertures and each member of the MSA is associated with a member of the LDA for scrambling the light only in the direction of the slow axis of the member of the LDA.  
           [0014]    In another embodiment, the present invention is directed to a method of producing a plurality of writing spots on a photosensitive medium, comprising the steps of (i) providing an array of laser diode emitters (LDA), wherein said LDA having exit apertures and wherein each member of said LDA having a fast axis direction and a slow axis direction of beam divergence; (ii) providing an array of Micro Scramblers (MSA), wherein said MSA having exit apertures and wherein each of said individual Micro Scramblers associated with one of said laser diode emitters; (iii) providing a spatial light modulator (SLM); (iv) providing optical means for imaging the exit apertures of said MSA; and (v) imaging the exit aperture of each of said Micro Scramblers on said photosensitive medium.  
           [0015]    There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject matter of the claims appended hereto.  
           [0016]    In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.  
           [0017]    As such, those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.  
           [0018]    Further, the purpose of the foregoing abstract is to enable the U.S. Patent and Trademark Office and the public generally, and especially the scientists, engineers and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The abstract is neither intended to define the invention of the application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way.  
           [0019]    These together with other objects of the invention, along with the various features of novelty which characterize the invention, are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated exemplary embodiments of the invention.  
           [0020]    Other objects of the present invention will be evident to those of ordinary skill, particularly upon consideration of the following detailed description of exemplary embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:  
         [0022]    [0022]FIG. 1 a  is a schematic illustration of a conventional LDA;  
         [0023]    [0023]FIG. 1 b  presents the coordinate system used throughout the following description;  
         [0024]    [0024]FIGS. 2 a  and  2   b  schematically illustrate a conventional SLM illumination system;  
         [0025]    [0025]FIGS. 3 a  and  3   b  illustrate typical near field power distribution of a laser Diode near field;  
         [0026]    [0026]FIG. 3 c  illustrates typical prior art SLM illumination power distribution along the slow axis;  
         [0027]    [0027]FIGS. 4 a  and  4   b  illustrate the beams propagation in an optical imaging head incorporating LDA-SLM system with an array of one-dimensional micro-scramblers, according to the present invention;  
         [0028]    [0028]FIGS. 4 c  and  4   d  are isometric views of exemplary arrays of one-dimensional micro-scramblers according to the present invention;  
         [0029]    [0029]FIGS. 5 a  through  5   c  present the power distribution along the slow axis of the emitters near field, at the exit aperture of the one-dimensional micro-scrambler and at the SLM, respectively, according to the present invention;  
         [0030]    [0030]FIG. 6 schematically illustrates a typical design of an optical imaging head incorporating LDA-SLM system with an array of 1D MSs according to the present invention;  
         [0031]    [0031]FIG. 7 is a schematic isometric view of an external-drum-type electro-optical plotter with an optical imaging head incorporating an LDA, SLM, and a one-dimensional micro-scrambler according to the present invention; and  
         [0032]    [0032]FIG. 8 is a schematic isometric view of a flatbed-type electrooptical plotter with an optical imaging head incorporating an LDA, SLM, and a one-dimensional micro-scrambler according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0033]    In the following detailed description, numerous specific details are set forth regarding the system and method and the environment in which the system and method may operate, etc., in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known components, structures and techniques have not been shown in detail to avoid unnecessarily obscuring the subject matter of the present invention. Moreover, various examples are provided to explain the operation of the present invention. It should be understood that these examples are exemplary. It is contemplated that there are other methods and systems that are within the scope of the present invention. Also, the same reference numerals are used in the drawings and in the description to refer to the same elements to simplify the description.  
         [0034]    The SLM used in imaging systems can modulate the light beam&#39;s amplitude, phase, or direction of propagation (diffractive SLM) and can have one-dimensional or two-dimensional modulating properties. The fundamental principles of the present invention can be applied to any of the different SLM types. As an example, in the following explanations a one-dimensional SLM will be used. It will be appreciated by any person skilled in the art, that the same principles can be applied to two-dimensional SLMs, by simple system scaling in the Y direction.  
         [0035]    [0035]FIGS. 4 a  and  4   b  schematically illustrate one exemplary SLM illumination system of the present invention. FIG. 4 a  presents the beams propagation in the X-Z plane (the slow axis plane), and FIG. 4 b  presents the beams propagation in the Y-Z plane (the fast axis plane). LDA  110  has a number of multimode emitting members  111 , of length          and height          . The pitch of the array is          . An array  130  of Micro-Scramblers  131  (MS) is placed in close proximity to the exit apertures of the emitters  111 . The MS  131  is designed to scramble the light only in the slow axis direction and therefore it is referred to herein as 1D MS (one dimensional micro-scramblers). Preferably, the number of the 1D MS  131  in the array  130  equals the number of the emitters  111  in the LDA  110 . The LDA  110  and the MS array  130  are precisely aligned so that an entrance aperture  134  (FIG. 4 c ) of each 1D MS  131  is positioned in front of each emitter  111 . The thickness of the 1D MS  131  is then chosen to be the same or slightly bigger than the length of the emitters  111 : a≅         . The light emitted from each emitter  111  enters the corresponding entrance aperture  134  of the 1D MS  131 . Inside the 1D MS  131 , each beam experiences a number of bouncings from the vertical walls of the MS  131 , before exiting on the opposite side through the exit aperture  135  (FIG. 4 c ). Due to these multiple reflections, the light is spatially scrambled in the slow axis direction and the illumination of the exit aperture  135  is relatively uniform in the slow axis direction. At the same time, the height          of the 1D MS  131  is chosen big enough, so that the beam propagation in the fast axis direction is altered only by the Snell refraction (FIG. 4 b ). For example, if the beam divergence in the fast axis direction is α (the full angle), and the length of the MS  131  is          , then          &gt;2.tan(α/2).         .  
         [0036]    [0036]FIGS. 4 a  and  4   b  also illustrate one exemplary illuminating system  120 . It is designed of anamorphic fast axis imaging lens  121  and a slow axis imaging system—anamorphic lens array  122  and anamorphic lens  123 . The object plane of the fast axis lens  121  coincides with the plane of the emitters  111 . The image plane of the fast axis lens  121  coincides with the SLM  116 .  
         [0037]    The slow axis imaging part consists of array  122  of anamorphic collimating microlenses  122   a  (FIG. 4 a ) and the field lens  123 . Preferably, the number of the microlenses  122   a  in the array  122  equals the number of the emitters  111  of the LDA  110 . Each microlens  122   a  is assigned to a specific emitter  111 . Lens  123  is also anamorphic and is common for all emitters. The object plane of the lens system  122 - 123  coincides with the plane of the exit apertures  135  of the 1D MSs  131 , while the image plane coincides with the SLM  116 . The power distribution in the slow axis direction of the SLM  116  illumination is uniform, because it results from imaging the exit apertures  135  of the 1D MSs  131 , which illumination is relatively uniform as will be shown below.  
         [0038]    Examples of a multi-channel 1D MS are schematically illustrated in FIGS. 4 c  and  4   d.    
         [0039]    [0039]FIG. 4 c  presents a periodic structure with period          s containing optical windows  131  and spacers  132 . The transparent optical windows  131  have thickness a=         , length          , height          and index of refraction n. The entrance aperture of the windows  131  is denoted by  134  and the exit aperture is denoted by  135 . The spacers  132  can be of the same or different material as the windows  131  and with approximately the same height          and length          . The windows  131  and the spacers  132  are bonded together with optical cement, glue, etc.  133  of thickness c. The thickness c and the index of refraction n1 of the optical cement layers  133  are chosen so that the condition for Total Internal Reflection (“TIR”) on the interface  131 - 133  is fulfilled for all beams emitted in the slow axis direction. The thickness of the spacer  132  is then chosen to fulfill the condition a+b+c==         s=         , where          is the LDA pitch (FIGS. 1 a ,  4   a ). The light beams entering the entrance aperture  134  of the window  131  experience a number of TIRs from its vertical walls before exiting on the opposite side through the exit aperture  135 . Due to these multiple reflections, a beam entering the window  131  at a particular X-coordinate value          1 of the entrance aperture  134 , will exit the window at a different X-coordinate value          2 of the exit aperture  135  (FIG. 4 a ). Moreover, depending of the number of TIRs the beam experiences (odd or even), the propagation direction of the beam regarding the slow axis can be reversed. Thus, the light is spatially scrambled in the slow axis direction and the illumination of the exit aperture  135  is relatively uniform in this direction. At the same time, the height          of the windows  131  is chosen big enough, so that the beam propagation in the fast axis direction is altered only by the Snell refraction (FIG. 4 b ). In other words, the windows  131  act as 1D MS.  
         [0040]    [0040]FIG. 4 d  illustrates another approach to designing an array of 1D MS according to the present invention. The array  230  of 1D MS is a stack of double-sided mirrors  231  and spacers  232 . In this embodiment, the optical beams experience multiple reflections by the mirrors&#39; reflective surfaces  237  and propagate in the free space between them. In other words, the space between two neighboring mirrors  231  in the array  230  constitutes a 1D MS with length          , height          , and with entrance and exit apertures  234  and  235 , respectively. The pitch of such an array is          s=a+b=         , where          is the LDA pitch (FIG. 4 a ) and a and b are the thickness of the spacers  232  and the mirrors  231 , respectively. In this embodiment, the distance between the mirrors (thickness of the spacers) includes the thickness of the bonding material (glue, optical cement, etc.), and equals the LDA emitter length: a=         . The operation of the 1D MSs of FIG. 4 d  is similar to that of FIG. 4 c , with the following differences:  
         [0041]    1) The multiple bouncing from the 1D MS vertical walls is due not to TIR but a reflection from mirrors  231 ;  
         [0042]    2) The beam propagation between the reflections is in free space and therefore is not altered in the fast axis direction; and  
         [0043]    3) It can be expected that this design will have relatively higher energy losses due to the non-ideal reflective coating of the mirrors  231 .  
         [0044]    The designs of FIGS. 4 c  and  4   d  are only exemplary ones. It will be appreciated by any person skilled in the art that there are other methods of designing an array of 1D micro-scramblers. For example, the sidewalls  137  of the windows  131  of FIG. 4 c  may be coated with a highly reflecting coating (Gold, Aluminum, Silver, etc.) prior to bonding the array. In this case, the 1D MS will operate not on TIR but on simple reflection from the reflective coating, similar to the design of FIG. 4 d.    
         [0045]    [0045]FIG. 5 a  illustrates the scrambling capabilities of one 1D MS element  131  of the array  130  of FIG. 4 c , or the array  230  of FIG. 4 d , by presenting the near field power distributions in the slow axis direction of a 150 μm long and 1 μm wide multimode emitter  111   a  (thin line) and of the exit aperture  135  of the 1D MS  131  (thick line). In this example, the light scrambler  131  was chosen with dimensions a×         ×         =0.15×2×2 mm and was placed at a distance of 10 μm from the emitter. FIGS. 5 b  and  5   c  illustrate the spatial power distribution at the exit aperture of the one 1D MS element  131  of the array  130  and at the SLM, respectively, according to the present invention.  
         [0046]    [0046]FIG. 6 schematically illustrates a typical design of an optical imaging head incorporating LDA-SLM system with an array of 1D MSs according to the present invention. The head  300  comprises a light source, e.g., LDA  310 , an array of 1D MSs  330 , illumination optics  320 , SLM  316  and an imaging lens  340 . The light from the individual emitters of the LDA  310  is scrambled in the slow axis direction by the 1D MSs of the array  330 . In this example, one 1D MSs of the array  330  is assigned to each emitter of the LDA  310 . The illumination optical system  320  accepts the light emerging from the exit apertures of the 1D MSs ( 135  and  131  respectively in FIG. 4 a ) and conveys it to SLM  316 . The illumination optical system  320  can be, for example, as illustrated in FIG. 4 a . Because of the involvement of the 1D MS array  330 , the illumination of the SLM  316  has a uniform energy distribution, close to the desired square distribution illustrated in FIG. 3 c . Thus, all the energy reaching the SLM  316  can be used in the imaging process. Consequently, the light reaching the SLM  316  is spatially modulated by the SLM. The imaging lens  340 , preferably telecentric, produces a real image of the SLM on the imaging medium  350 .  
         [0047]    Optical imaging head as described above in reference to FIG. 6 can be used, as mentioned hereinabove, in electro-optical plotters for offset plates, laser transfer media, etc. FIG. 7 illustrates an exemplary design of such an electro-optical plotter. The photosensitive medium (offset plate, etc.)  801  is wrapped around a rotating drum  800 . Optical head  804 , incorporating an LDA-SLM system of the present invention with array 1D MSs, produces a plurality of spots  803  on the photosensitive medium  801 . The drum rotates with substantially constant speed in the direction indicated by arrow  805 , while the optical head  804  moves parallel to the drum axis (not shown) in the direction marked by arrow  806 . The system is being driven by a central processor  809 , which by means of control unit  807  synchronizes the two movements  806  and  805 , and the data transfer between the image data bank  808  and the optical head  804 . The digital equivalent of the image to be written on the photosensitive medium is stored in the image data bank  808 , from where it is transferred to the optical head  804 , which by means of producing a plurality of light spots  803  on the photosensitive medium  801 , forms the desired image  802 .  
         [0048]    [0048]FIG. 8 illustrates an electro-optical plotter of flatbed type, with optical head  903  incorporating LDA-SLM system with array 1D MSs. The photosensitive medium  904  is placed on a flat surface of an X-Y scanning engine  900 . The digital equivalent of the image to be written on the photosensitive medium is stored in the image data bank  808 , from where it is transferred to the optical head  903 , which by means of producing a plurality of light spots  901  on the photosensitive medium  904 , forms the desired image  902 . The scanning movement of the optical head  903  in two perpendicular direction  905  and  906 , is controlled by a central processor  809 , through control unit  807 . The CPU  809  also synchronizes the data flow from the image data bank  808  to the optical head  903  with the scanning movements  905  and  906 .  
         [0049]    Although the invention has been described and illustrated in the foregoing exemplary embodiments, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the details of construction and combination and arrangement of processes and equipment may be made without departing from the spirit and scope of the invention as claimed below.