Abstract:
An optical imaging heads that produce a plurality of light spots on light sensitive media such as photographic film or printing plate. The optical head incorporates an array of multi-mode laser diodes as a light source, a Micro Light-Pipe Array (MLPA) as a beam-shaping element, means for reducing the divergence of the laser diode beam in the fast axis direction and means for imaging the laser diode emitters on a surface close to the micro light-pipe entrance aperture.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to optical imaging heads that produce a plurality of light spots on light sensitive media such as photographic film or printing plate. The optical head incorporates an array of laser diodes as light source 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 media may incorporate an array of laser diodes as a light source. The laser diodes array may be configured as an ordered plurality of individual laser diodes mounted on a common carriage, or as a plurality of laser emitters manufactured on a single-piece semiconductor material (such as GaAs). For brevity, the light source (whether configured out of individual laser diodes or manufactured on a single semiconductor chip) will be referred to hereunder as Individually Addressed Laser Diode Array (IALDA).  
         [0003]     The imaging speed in electro-optical plotters is generally limited by the power delivered to the medium by the laser beam(s). This is especially true when the imaged medium is a thermal or ablative printing plate, or laser-transfer material, where the sensitivity is typically of the order of several hundreds mJ/cm 2 . In order to achieve the required power, the IALDA has to be built of powerful multi-mode laser diodes (LD). Multimode LDs are characterized by the light-emitting region having a very elongated shape, typically 1 micron across and 50 to 200 microns along the array axis, with the beam divergence in the cross-emitter direction high, typically 50-60 degrees FWHM, and the beam divergence in the length direction relatively low, typically 10 degrees FWHM. For brevity, the cross-emitter direction will be referred to as the ‘fast axis’ and the emitter&#39;s length direction will be referred to as the ‘slow axis’ 
         [0004]     The near field emission pattern of multi-mode LDs is substantially rectangular. An important characteristic of multi-mode LDs is that the energy distribution of the near field in the slow axis direction is non-uniform and changes with the LD&#39;s junction temperature, as well as with the data current driving the diode. This effect is often displayed as a “hot spot” moving along the emitter&#39;s length. When the image on the photosensitive medium is formed by imaging the near field of the LD, the non-uniform and frequently changing energy distribution of its pattern leads to undesired effects, such as image density irregularities. A method and apparatus for overcoming these shortcomings of multi-mode LDs by using optical diffusers is disclosed in EP 0 992 343 A1 to Sousa U.S. Pat. No. 6,208,371 to Takeshi et al, describes an optical beam-shaping system imaging the near field of a LD.  
         [0005]     The present invention successfully solves the above mentioned shortcomings of imaging the multi-mode LD near field, by using a Micro-Light-Pipe Array (MLPA) for achieving spots with evenly distributed energy on the photosensitive medium, not depending on the LD&#39;s working conditions.  
       SUMMARY OF THE INVENTION  
       [0006]     It is an object of the present invention to provide a multiple laser-beam recording apparatus producing a plurality of high-degree identical optical spots with uniform energy distribution.  
         [0007]     Another object of the present invention is to provide a multiple laser-beam recording apparatus, which is free of image density irregularities due to non-uniform energy distribution of the LD near field.  
         [0008]     Still another object of the present invention is to provide a high energy-efficient multiple laser-beam recording apparatus free of image density irregularities due to non-uniform energy distribution on the LD near field.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1   a  is a schematic isometric view of an IALDA and a beam-shaping MLPA according to the present invention;  
         [0010]      FIGS. 1   b  and  1   e  schematically illustrate an exemplary optical imaging head incorporating the IALDA and beam-shaping MLPA of  FIG. 1   a;    
         [0011]      FIG. 2   a  is a schematic isometric view of an IALDA with an anamorphic correcting lens and a beam-shaping MLPA according to the present invention;  
         [0012]      FIGS. 2   b  and  2   c  schematically illustrate an exemplary optical imaging head incorporating the IALDA with anamorphic correcting lens and beam-shaping MLPA of  FIG. 2   a;    
         [0013]      FIG. 3   a  is a schematic isometric view of an IALDA with a correcting and imaging lens system with virtual emitter image and a beam-shaping MLPA according to the present invention;  
         [0014]      FIGS. 3   b  and  3   c  schematically illustrate an optical imaging head incorporating the IALDA with correcting and imaging lens system with virtual emitter image and beam-shaping MLPA of  FIG. 3   a;    
         [0015]      FIG. 4   a  is a schematic isometric view of an IALDA with a correcting and imaging lens system and a beam-shaping MLPA according to the present invention;  
         [0016]      FIGS. 4   b  and  4   c  schematically illustrate an optical imaging head incorporating the IALDA with correcting and imaging lens system and beam-shaping MLPA of  FIG. 4   a;    
         [0017]      FIGS. 5   a  and  5   b  show the energy distribution of light in the entrance and exit apertures of a micro light-pipe respectively;  
         [0018]      FIG. 6   a  is a schematic isometric view of an IALDA and a tapered beam-shaping MLPA according to the present invention;  
         [0019]      FIGS. 6   b  and  6   c  schematically illustrate an exemplary optical imaging head incorporating the IALDA and tapered beam-shaping MLPA of  FIG. 6   a;    
         [0020]      FIG. 7   a  is a schematic isometric view of an IALDA with an anamorphic correcting lens and a funnel-type beam-shaping MLPA according to the present invention;  
         [0021]      FIGS. 7   b  and  7   c  schematically illustrate an exemplary optical imaging head incorporating the IALDA with anamorphic correcting lens and funnel-type beam-shaping MLPA of  FIG. 7   a;    
         [0022]      FIG. 8  is an exploded isometric view of a micro-machined MLPA;  
         [0023]      FIGS. 9   a  to  9   d  illustrate different channel shapes in micro-machined MLPA according to the present invention;  
         [0024]      FIG. 10  is a schematic isometric view of an IALDA and a bulk-type beam-shaping MLPA according to the present invention;  
         [0025]      FIG. 11  is a schematic isometric view of an external-drum-type electro-optical plotter with optical imaging head incorporating an IALDA and a beam-shaping MLPA according to the present invention; and  
         [0026]      FIG. 12  is a schematic isometric view of a flatbed-type electro-optical plotter with an optical imaging head incorporating an IALDA and a beam-shaping MLPA according to the present invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0027]     There are generally two types of light-pipes: Bulk and Hollow.  
         [0028]     The bulk-type light-pipe is a rod of transparent material with a polygonal cross-section (triangular, rectangular, etc.). The index of refraction of the material forming the light-pipe is higher than the index of refraction of the surrounding material. A typical example is glass rod in air. This type of light-pipes employ the principle of Total Internal Reflection (TIR) on the interface of the two materials—in the above example on the interface glass—air.  
         [0029]     The hollow light-pipes are tubes with a polygonal cross section (triangular, rectangular, etc.), made of transparent or nontransparent material, with their internal walls coated with a highly reflective coating. This type of light-pipes work on reflection from the reflective coating.  
         [0030]     In all preferred embodiments described below, a hollow light-pipe is taken as an example. It will be, however, appreciated by any person skilled in the art, that same performance can be achieved by using bulk-type light-pipes.  
         [0031]      FIG. 1   a  shows an IALDA light source  20  and an MLPA  10 , aligned in parallel with the array of laser emitters  21 . The number of the channels  12  in the MLPA corresponds to the number of the laser diodes  21  in the IALDA and the MLPA is placed in close proximity to the IALDA, in order to avoid optical crosstalk between the channels. The Micro Light-Pipes (MLP)  12  are hollow and their internal surface is coated with a highly reflecting coating, such as Au, enhanced Al or dielectric, depending on the base material and the wavelength of the light. The light emitted from each diode  21  enters the corresponding MLP  12  trough its entrance aperture  13 . Inside the MLP  12  each beam experiences a number of bounces from its walls before it exits from the opposite side through the exit aperture  14 . Due to these multiple reflections, the illumination of the MLP exit aperture is relatively uniform The uniformity, defined as  
           Edge   ⁢           ⁢   Illumination       Center   ⁢           ⁢   Illumination       ,         
 depends on the value  
           L   n     =       L   ·     NA   i         n   ·     A           ,         
 called the MLP normalized-length, where L is the light-pipe length, NA i  is the numerical aperture of the input beam n is the index of refraction of the MLP (n=1 for a hollow MLP), and A is the cross-sectional area of the MLP. 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%. 
 
         [0034]      FIGS. 1   b  and  1   c  schematically show an optical imaging head  100  incorporating an IALDA  20 , represented by a limited number of emitters  21 , and an MLPA  10 .  FIG. 1   b  illustrates the beams propagation in a plane coinciding with the emitters&#39; fast axis, while  FIG. 1   c  illustrates the beams propagation in a plane coinciding with emitters&#39; slow axis. The exit aperture  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  lie in the object plane of the imaging lens  70 , while their images  60  lie on the photosensitive medium  50 , which coincides with the image plane of lens  70 . As far as all light spots  60  are images of substantially identical objects—the exit apertures  14  of the MLPs  12 —they too will be substantially identical. Due to the relatively uniform illumination of the exit apertures  14 , their images  60  will also feature a relatively uniform distribution of illumination. Thus, substantially identical light spots with uniform energy distribution are achieved on the medium  50 .  
         [0035]     A very important parameter of the imaging head in electro-optical plotters is the depth of focus; higher depth of focus requires lesser mechanical accuracy. The depth of focus is in direct dependence to the numerical aperture NA im  of lens  70  image plane, such that the higher NA im , the lower is the depth of focus. The numerical aperture (NA) at the image side of lens  70  is  
           NA   im     =       NA   o     K       ,       
 
 where K&lt;1 (usually) is the magnification of imaging lens  70  and NA o  is the object side NA, i.e. the NA of the beam emerging from the MLP  12  exit aperture  14 . Assuming that the MLP is a prism, i.e. all its walls are parallel to a central axis  16  ( FIGS. 1   b ,  1   c ), it can be proven out of simple geometrical considerations, that a beam entering the MLP  12  at a certain angle with respect to the axis  16 , will exit the MLP at the same angle with respect to the axis  16 , but with altered direction due to the MLP scrambling effect. In other words, the numerical aperture at the MLP exit will be equal in all directions and will be NA o =NA i , and hence  
         NA   im     =         NA   i     K     .         
 
 It is therefore apparent that for achieving bigger depth of focus, the designer&#39;s goal will be to work with as low as possible NA i  and as low as possible MLP  12  dimension a, allowing for higher K values. The minimum value for a is determined by the beam divergence in the slow axis direction and the distance d between the IALDA  20  and the MIA  10  ( FIG. 1 ), and it is obviously achieved when d≅0. In this case, all energy emitted by the emitter will enter the MLP, but also NA i  will have its maximum value corresponding to the emitter&#39;s beam divergence in the fast axis direction. In other words, working with a low numerical aperture and accommodating all emitted energy are contradictory conditions in the embodiment of  FIG. 1   a - 1   c . Depending on the specific requirements of the optical system, a compromise between these two parameters should be made. As an example, in the optical system of  FIGS. 1   a - 1   c  the cross sectional dimension a of the MLPs is chosen to be approximately the length l of the LD emitters, while the NA i  is chosen to correspond to the FWHM divergence angle of the LD in the emitter&#39;s slow axis direction. The latter is done by proper choice of the IALDA-MLPA distance d and the MLPs&#39; cross sectional dimension h: NA i =sin(atan(h/d)). It is important to point out that the minimum value for h depends also on the mutual displacement of the emitters  21  in the fast axis direction (often call “smile” of the array—dashed lines  21   a  in  FIG. 1   a ). This configuration, however, is far from being optimal, because, as mentioned above, the fast axis divergence angle is much larger than the slow axis one, and all the energy emitted in this direction outside NA i  is lost 
 
         [0038]     The loss of energy described above is avoided, to a great extent, in the optical system presented in  FIGS. 2   a ,  2   b  and  2   c.    
         [0039]      FIG. 2   a  is a schematic isometric view of an IALDA  120  with anamorphic correcting lens  130  and beam-shaping MLPA  110 . The difference between the systems of  FIG. 1   a  and  FIG. 2   a  is in the lens  130 , placed between the IALDA  120  and the MLPA  110 . For simplicity of the illustration, a cylindrical lens is shown. It will be, however, appreciated by any person skilled in the art, that anamorphic lenses of different types can be used with the same success. The function of the anamorphic lens  130  in the optical system is illustrated in  FIGS. 2   b  and  2   c , which are illustrations of the beams propagation in an optical imaging head  105 , in a plane coinciding with the fast axis direction, and the beams propagation in a plane coinciding with the slow axis direction, respectively. The power of the anamorphic lens  130  in the fast axis direction is chosen such that the beam divergence in the fast axis direction beyond the lens  130  will approximately equal the beam divergence in the slow axis direction (in  FIGS. 2   a  and  2   b  both values are designated by NA i ). Thus, the numerical aperture NA i  of the beam that enters the MLP  112  entrance aperture  113  contains most of the energy emitted by the diode. In  FIGS. 2   a - 2   c , the lens  130  is chosen to be common for all the emitters of the array. It will be, however, appreciated by any person skilled in the art, that other solutions may be implemented, for example a micro-lens array. Since the role of the lens  130  is only to decrease the beam divergence in the fast axis direction and to direct as much energy as possible to the MLP for a given NA, no constraint for imaging with minimum optical aberrations are placed here. This makes the design of the system easy and flexible in the choice of the lens  130 .  
         [0040]     The imaging lens  170  is preferably telecentric and images the exit apertures  114  of the MLPs  113 . As the apertures  114  are substantially identical objects with very even spatial energy distribution, their resulting images  160  on the photosensitive medium  150  will also be substantially identical with even spatial energy distribution.  
         [0041]     The system of  FIGS. 2   a - 2   c  still does not provide optimum performance in terms of efficiency. Adding the lens  130  increases the distance d ( FIG. 2   a ) between the IALDA  120  and the MLPA  110 . Therefore, as can be seen in  FIG. 2   c , it is necessary to increase the cross-sectional dimension a of the MLP array to a value a&gt;l+2d.NA i , in order to accommodate the entire energy emitted in the slow axis direction. This increased dimension is marked as a1 and leads to a higher demagnification ratio (smaller K) of lens  170 , hence to a larger image size numerical aperture NA im  and a reduced depth of focus. The minimum value for the cross sectional dimension h is determined by the array smile and the magnification of the anamorphic lens  130 ; the spot  121   b  produced by lens  130  from the most displaced emitter  121   a , should be within the entrance aperture  113   a  of the corresponding MLP ( FIG. 2   a ).  
         [0042]     The increase in the cross sectional dimensions a of the MLPs, which is necessary in the system of  FIGS. 2   a - 2   c  for collecting the entire emitted energy, can be avoided by employing an optical system that produces an image of the emitters in close vicinity to the entrance aperture of the MLPs. Because of the emitter&#39;s different beam divergence in the fast and slow axis directions, such system should have different power in these two directions. In the examples below ( FIGS. 3   a - 3   c  and  4   a - 4   c ), the numerical aperture of the beam NA i  at the entrance of the MLP is chosen to be approximately identical in all directions and approximately equal to the numerical aperture NA s  of the emitter in the slow axis direction: NA i ≅NA s . From the preservation of Etendue principle, it follows that the magnification in the slow axis direction will be approximately 1, while the magnification in the fast axis direction will be approximately NA f /NA s &gt;1. The emitters&#39; image length l1 ( FIG. 3   c ) will equal approximately the emitters&#39; length l, and from considerations of preserving the brightness it will follow that the MLPs&#39; dimension a in the slow axis direction will be: a≅l 1≅l. It will be appreciated by any person skilled in the art that other efficient configurations are also possible: a≅l 1&lt;l; NA i &gt;NA s  or a≅l 1&gt;l; NA i &lt;NA s . The designer, however, should bear in mind that the numerical aperture NA o  of the beam exiting the MLP will be identical in all directions and will correspond to the maximum angle of the entrance beam with respect to the MLPs&#39; axis, and in conjunction with the magnification K of the imaging lens  270 , will determine the system&#39;s depth of focus.  
         [0043]      FIG. 3   a  is a schematic isometric view of an optical system consisting of an IALDA  220 , an anamorphic correcting lens  230 , a lenslet array  240  and a understood with the help of  FIGS. 3   b  and  3   c , which are illustrations of the beams propagation in an optical imaging head  200 , in a plane coinciding with the emitter&#39;s fast axis direction, and the beams propagation in a plane coinciding with the emitter&#39;s slow axis direction, respectively: The anamorphic lens  230  is designed so as to create virtual images  222  of the LD emitters  221 . Because of the anamorphic properties of the lens  230 , the NA beyond the lens (designated by NA v  in  FIGS. 3   b ,  3   c ) is identical in all directions perpendicular to the system axis of symmetry. The virtual image  222  serves as an object for lens  240 , which produces a real image  223  in the vicinity of the entrance aperture  213  of the MLP  212 . Because of the imaging properties of the lens combination  230 - 240  and the controlled NA, the dimensions of the image  223 , as explained above, can be made to equal the MLP dimension a. Thus, the energy losses are minimized and no increase in the MLP cross sectional dimension is required.  
         [0044]     The anamorphic lens  230  can be produced by extrusion, a method used by Bluesky Research Inc., of San Jose, Calif. The imaging lens  270  is preferably telecentric and images the exit apertures  214  of the MLPs  212 . As the apertures  214  are substantially identical objects with a very even spatial energy distribution, their resulting images  260  on the photosensitive medium  250  will also be substantially identical, with an even spatial energy distribution.  
         [0045]     The minimum value for the cross sectional dimension h of the MLP&#39;s  212  is determined by the array smile and the magnification of lens system  230 - 240  in the fast axis direction; the image  221   b  of the most displaced emitter  221   a  should be within the entrance aperture  213   a  of the corresponding MLP ( FIG. 3   a ). It will be understood that some loss of energy will occur for displaced emitters. This loss can be compensated by choosing the individual emitters&#39; working regimes such as to obtain the same power yield for each channel of the optical head  200 .  
         [0046]     Another system producing an image of the emitters in proximity to the MLP entrance aperture is illustrated in  FIGS. 4   a - 4   c . Here, the anamorphic lens  330  has its focal plane approximately coinciding with the entrance aperture  313  of the MLP and decreases the numerical aperture in the fast axis direction to a value close to the numerical aperture in the slow axis direction. The array  340  is an array of anamorphic lenses with power only in the slow axis direction. For each emitter  321 , there is a member  341  of the array  340  associated with it. The imaging planes of anamorphic lenses  330  and  340  coincide. Thus, a real image of the emitter  321  is produced in close vicinity to the MLP entrance aperture  313 , with numerical aperture approximately identical in all directions.  
         [0047]     It will be also appreciated that the same optical effect can be achieved by designing the lens  340  not as a lenslet array, but as an assembly of bulk optical elements. It will also be appreciated that the anamorphic lens  330  and the focusing lens  340  can be combined in a single lenslet array of anamorphic elements. As in the previously described systems, the imaging lens  370  is preferably telecentric and images the exit apertures  314  of the MLPs  312 . As the apertures  314  are substantially identical objects with very even spatial energy distribution, their resulting images  360  on the photosensitive medium  350  will also be substantially identical with even spatial energy distribution. The minimum value for the cross sectional dimension h of the MLPs  312  is determined by the array smile and the magnification of lens system  330 - 340  in the fist axis direction; the image  321   b  of the most displaced emitter  321   a  should be within the entrance aperture  313   a  of the corresponding MLP ( FIG. 4   a ). The loss of energy due to the displacement can be compensated, as in the previously described systems, by choosing the individual emitter&#39;s working regimes such as to obtain the same power yield for each channel of the optical head  300 .  
         [0048]     Reference is now made to  FIGS. 5   a  and  5   b , illustrating the light scrambling of the optical system of  FIGS. 4   a ,  4   b  and  4   c .  FIG. 5   a  shows the light distribution in the real image  323  of LD emitter  321 , in the entrance aperture  313  of the MLP  312 . The length of emitter  321  in this example was 80 microns and the diameter of the aperture  313  was also chosen to be 80 microns. The MLP was chosen to be with a hexagonal cross section and with length L=1.5 mm.  FIG. 5   b  shows the energy distribution in the same MLP exit aperture  314 . It is obvious, that as far as the spot  360  on the photosensitive medium  350  ( FIGS. 4   b  and  4   c ) is an image of the MLP exit aperture  314 , the light energy distribution in it will be similar to that in the aperture  314 , i.e. relatively uniform. Same results can be obtained with the optical systems of  FIGS. 1   a - 1   c ,  2   a - 2   c , and  3   a - 3   c.    
         [0049]     In all the previous embodiments, the MLP used is a prism, i.e. it does not alter the beam angle with respect to the optical axis. There is another type of light-pipe, known as tapered, which can be used not only for scrambling the light energy, but also for altering the numerical aperture of the beam. These light-pipes have a shape of a truncated pyramid, tapered in one or more directions. In the direction in which the light-pipe is tapered, the beam angle with respect to the axis will be changed.  
         [0050]     Reference is now made to  FIG. 6   a , presenting an IALDA  420 , working in conjunction with a MLPA  410  of tapered MLPs  412 . The dimension h1 of the MLP  412  entrance aperture  413  and the corresponding dimension h2 of the exit aperture  414 , are chosen to be different: h1&lt;h2. Thus, because the MLPs  412  are tapered in the fast axis direction, the fist axis numerical aperture NA f1  of the beam at the entrance of the MLPs will be decreased at the exit to a value  
         NA   f2     =       NA   f1     ⁢       h1   h2     .           
 
 Thus, by proper choice of h1, the numerical aperture of the beam at the exit aperture  414  can be made identical in all directions. The embodiment of  FIG. 6   a  can be integrated in an optical imaging head  400 , shown in  FIGS. 6   b  and  6   c . The possibility of altering the beam&#39;s numerical aperture, allows for choosing the MLPs&#39; dimension in the slow axis direction a=l, where l is the emitter length and for minimizing the IALDA−MLPA distance d≅0 without loss of energy due to system geometry and without loss of depth of focus, in contrast to the optical head of  FIGS. 1   a - 1   c.  
 
         [0052]     The optical imaging head of  FIGS. 6   a - 6   c  is an improved variant of the imaging head of  FIGS. 1   a - 1   c . In this embodiment, however, there still could be significant energy losses. Because of the high entrance NA in the fast axis direction, the beam experiences more reflections in a tapered MLP than in a regular MLP, which leads to increased losses of energy in the reflective coating. If the tapered MLPs are of bulk-type, then the high entrance numerical aperture can lead to unfulfilled conditions for TIR and hence, once again, to energy losses.  
         [0053]     These drawbacks of the previously described embodiment are avoided in the system presented in  FIGS. 7   a - 7   c , which is an improved variant of the embodiment of  FIGS. 2   a - 2   c . In this preferred embodiment, the MLPs  512  of the array  510  are designed as ‘funnels’, consisting of two parts, I and II, with lengths L1 and L2 respectively, as shown in  FIG. 7   a . Part I is a tapered MLP with entrance aperture  513 , with dimensions l1×h1 in the fast and slow axis directions respectively, and exit aperture  515  with dimensions l2×h2 in the fast and slow axis directions respectively. Part II is a regular MLP with identical entrance and exit apertures,  515  and  514  respectively. Imaging head  500 , with array  510  of funnel type MLPs, illustrates the beam propagation in the fast axis direction. The correcting anamorphic lens  530  reduces the beam NA in the fast axis direction to a value approximately equal to the NA in the slow axis direction: NA f ≅NA s  and images the emitter  521  in close proximity to the part I of the MLP  512  exit aperture  515 . The dimension h1 of the part I entrance aperture in the fast axis direction and the tapering angle α are chosen such that the MLPs in this part do not alter the NA in the fast axis direction, but allow for accommodation of the entire energy emitted in this direction, even when the emitter is displaced to some extent in vertical direction. Part I of the MLPs does not scramble the light in the fast axis direction. The light in this direction is scrambled by part II of the MLPs. The length L1 is chosen so that at the given angle α, the resulting height at aperture  515  is h2. h2 is a parameter determining the spot  560  dimension on the photosensitive medium  550  in the fast axis direction.  
         [0054]      FIG. 7   c  illustrates the beam propagation in the slow axis direction. The dimension l1 of the part I entrance aperture in this direction is chosen such that the MLPs in this part accommodate the full energy emitted in numerical aperture NA s , accounting for the beam divergence and the distance between the IALDA  520  and the MLPA  510 . Thus, the distance between the correcting anamorphic lens  530  and the MLPA  510  can be made approximately zero: d≅0. The light in this direction is scrambled in both parts I and II of the MLPs. The tapering angle β can be chosen to be zero and in this case l1=l2 and the full energy is accommodated with loss of brightness in this direction. If β is not zero, then l1≠l2 and the NA in this direction is altered by factor l1/l2 and there is once again a loss of brightness. As far as l1 depends on the distance d1 between the IALDA  520  and the MLPA  510 , it is advisable to use correcting lens  530  with as small as possible cross section dimension, for example Luneburg type lens with diameter 60μ to 100μ produced by DORIC LENSES of Canada  
         [heading-0055]     Production Method  
         [0056]     Hollow MLPAs can be produced by using standard photolithography technology on silicon wafers, or deep X-ray lithography on polymers. Both technologies are well mastered in many companies around the world, for example in the Institute for Micromechanics in Mainz, Germany or MicroDevices Inc of Radford, Va.  
         [0057]      FIG. 8  is an exploded isometric view of a MLPA  610 . The array consists of two base plates  617  and  611 , in which half-hexagonal grooves are etched. The grooved surfaces are coated with a highly reflective coating, for example enhanced aluminum or bare gold, depending on the LD wavelength. The mechanical keys  615  and  616  are formed by the same photolithography process and are used for easy alignment of the two parts  617  and  611 . By etching along the Si crystallographic planes, a diamond-like shape can be achieved as illustrated in  FIG. 9   b.    
         [0058]     Other shapes can be achieved and other materials can also be used. For example, shapes as illustrated in  FIGS. 9   a ,  9   c  and  9   d , as well as non-symmetrical shapes can be made by using lithography or micro-molding techniques.  
         [0059]     An additional method for producing MLPAs is by shaping a coherent bundle of optical fibers. This technology involves the steps of: 
        1. Arranging the fibers in a coherent bundle;     2. Heating the bundle to a predetermined temperature; and     3. Extrusion under predetermined pressure.        
 
         [0063]     This technology is mastered for example in Schott Optical Fibers of Germany (www.schott.com). The resulting optical element is a coherent bundle of optical fibers with hexagonal cross-section. Such optical elements are usually used as image tapers or extenders. However, if a thin plate of the bundle is cut-off and optically polished on both sides, it can be used as an MLPA, as illustrated in  FIG. 10 . The initial dimensions of the fibers&#39; cladding  712   a  and core  712   b  and the production process parameters (temperature, pressure, extrusion, etc.) are chosen such as to achieve a certain distance p between each two or three or four, etc. fibers, to match the pitch of emitters  721  of IALDA  720 . Thus, a small number (cross-hatched) of elements  712  in the cut-off plate  700  constitute the MLPA  710 .  
         [0064]     Another method of producing MLP is by means of standard glass technology, to form macro rods with desired cross section shape and then by extrusion to reduce the cross sectional dimension to a desired value. Such techniques are widely used for producing anamorphic lenses with small cross sectional dimensions.  
         [0065]     Optical imaging heads incorporating IALDA and MLPA can be used, as mentioned hereinabove, in electro-optical plotters for offset plates, laser transfer mediums, etc.  FIG. 11  illustrates the basic design of such electro-optical plotter. The photosensitive medium (offset plate, etc.)  801  is wrapped around a rotating drum  800 . Optical head  804 , incorporating IALDA and MLPA, produces a plurality of spots  803  on the photosensitive medium  801 . The drum rotates with substantially constant speed in the direction shown 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 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 .  
         [0066]      FIG. 12  illustrates an electro-optical plotter of flatbed type, with optical head  903  incorporating IALDA and MLPA. 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 .