Abstract:
Beam shapers, imaging heads formed using such beam shapers and methods are provided that are adapted to shape a light beam for use in imaging. In accordance with one method described herein a light beam is received and the light beam is reflectively scrambled to form generally homogenous light having a non-circular shape. The homogenous light is delivered to a circular core multimode light path that shapes the light beam so that light exiting the circular core multimode light path comprises circular shaped homogenous light.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a 111A application of U.S. Provisional Application Ser. No. 60/625,566 filed Nov. 8, 2004, entitled A PROPOSAL FOR FIBER OPTIC BEAM SHAPER. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to beam shapers, to imaging heads formed using such beam shapers and to methods adapted to shape a light beam for use in imaging. 
     BACKGROUND OF THE INVENTION 
     Optical heads for imaging a plurality of light spots on a light sensitive medium such as printing plates often incorporate, as a light source, an array of pigtailed laser diodes. Each laser diode is optically coupled to a proximal tip of a multimode optical fiber. The distal ends of the optical fibers are supported in a linear array by mechanical means such as V-groove plates. 
       FIG. 1  is a schematic representation of such a prior art imaging system comprising a light source  10 , e.g. laser and coupling optics  20  coupled to a multimode optical fiber  30  having a circular core shape. The distal end of multimode optical fiber  30  is supported by mechanical support  40 , such as micro-machined V-grooves, and emits light through a telecentric lens  50  onto an imaging surface  60 . Imaging surface  60  may be a photosensitive printing plate, including a thermally ablative printing plate. 
     An important characteristic of any fiber-coupled laser diode is the general distribution of light energy exiting from a tip face at a distal end of the multimode optical fiber. Preferably, this distribution is relatively even or homogenous. The homogeneity of the distribution depends on a variety of parameters such as the quality of the supplied laser light which can for example depend upon way the laser diode is modulated, the optical characteristics of the optical coupling between the laser diode and the optical fiber, the length of the optical fiber, the bending along the fiber, etc. which may cause non-uniform and time-dependent energy distribution of the light emerging from the distal end of the multimode optical fiber. This often leads to unpredictable energy distribution in the writing spot and to undesired effects on the image. It is well known that the radiation at the output of a regular circular multimode fiber is not steep, not homogeneous and has many modes. Furthermore, the mode structure of this radiation is dependent on the fiber lay and changes when the fiber is moved, as happens in many imaging machines. 
     Several ways of avoiding this effect are mentioned in the prior art. For example, EP 0992343 A1 to Presstek Inc. uses a controlled-angle diffuser. The diffuser introduces scrambling in the angular energy distribution and thus smoothes it. This approach, however, cannot correct asymmetrical spatial energy distributions, such as doughnut-mode energy distributions. 
     Another way is to use a non-circular hollow waveguides as described, for example, in the article entitled “Beam Homogenizer for Hollow-Fiber Delivery System of Excimer Laser Light”, published in Applied Optics, volume 42, no. 18, Jun. 20, 2003. A hollow waveguide is usually made from silicon coated with a reflecting metal layer. However, hollow wave-guides which use the principle of reflection, absorb part of the guided light converting it to heat. This wastes energy and can negatively influence the shape of the wave-guide or the direction of the wave-guide. 
     U.S. Pat. No. 6,519,387 to Sunagawa et al. claims the use of an elongated core shape at the distal end of a fiber to improve beam characteristics. However, the use of non-circular cores at the distal ends of the fibers causes the spot configuration of the scanned beam to be short in one direction rather than circular. Further, for example, it is harder to align and orient the fibers in mechanical supports such as V-grooves. 
     Thus, there is a need in the printing arts and in other fields for a system that provides homogenized steep profile of radiation from a fiber which has a circular core. This may improve the image quality of printing systems. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, an optical path for use with a laser source is provided. The optical path comprises: a beam shaper multimode optical path having a non-circular core optically coupled at one end to the laser source, said non-circular core being shaped to repeatedly reflect a light beam supplied by the laser source to homogenize the light beam; and a circular core multimode optical path optically coupled to the beam shaper multimode optical path to receive the homogenized light beam and to shape the homogenized light beam to provide a generally circular beam of homogenized light. 
     In another aspect of the invention, an optical imaging head is provided. The optical imaging head comprises: a plurality of laser sources; a plurality of optical paths each optical path having: beam shaper multimode optical fiber having a non-circular core optically coupled to at least one of said laser sources at a proximate face of the optical path; and a circular core multimode optical fiber optically coupled, at a first end, to the beam shaper multimode optical fiber and defining, at a second end, a distal face of the optical path. A mechanical support positions the distal face of each optical path at a predetermined location relative to other distal faces of the other ones of the plurality of optical paths. Wherein beam shaper multimode optical fiber reflects the beam of light from the at least one of said laser sources to provide a homogenous light beam and wherein said circular core multimode optical fiber shapes the homogeneous light beam into a circular shaped homogeneous light beam at the distal face. 
     In yet another aspect of the invention, a method for processing a light beam is provided. In accordance with this method, a light beam is received and the light beam is reflectively scrambled to form generally homogenous light having a non-circular shape. The homogenous light is delivered to a circular core multimode light path that shapes the light beam so that light exiting the circular core multimode light path comprises a circular shaped homogenous light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a prior-art system; 
         FIG. 2  is a schematic representation of one embodiment an optical path; 
         FIG. 3  is a schematic representation of another embodiment of an optical path; 
         FIGS. 4A–4C  are schematic cross-sectional views of exemplary optical fibers and a splice used in the embodiment of an optical path shown in  FIG. 3 ; 
         FIG. 5  is a schematic representation of the cross section and refractive index profile of an exemplary fiber optic beam shaper which has a hexagonal core and two layers of cladding; 
         FIG. 6  illustrates a qualitative example of beam profile for a prior art multi-mode circular core optical fiber; 
         FIG. 7  illustrates a qualitative example of a beam profile for a light beam that has passed through a rectangular fiber optic beam shaper; 
         FIG. 8  illustrates another qualitative example beam profile for a light beam that has passed through a circular core optic fiber; and 
         FIG. 9  illustrates an example of a beam profile for a beam that has passed through a hexagonal fiber optic beam shaper. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  is a schematic representation of a first embodiment of an optical path  100 . In the embodiment of  FIG. 2 , a laser source  110  is coupled, by coupling optics  120  to optical path  100 . Optical path  100  has a proximate face  102  for receiving a light beam from laser source  110  and coupling optics  120  and a distal face  104  from which light that has passed through optical path  100  is emitted. In the embodiment of  FIG. 2 , optical path  100  comprises a beam shaper multimode optical path  106  in the form of fiber optic beam shaper  170  that homogenizes the light beam and a circular core multimode optical path  108  in the form of a circular core multimode optical fiber  130  that shapes the homogenized light from fiber optic beam shaper  170  to provide a shaped and homogenized light. In the embodiment of  FIG. 2 , fiber optic beam shaper  170  comprises a multimode optical fiber with a non-circular core shape which scrambles the light beam supplied by laser source  110 . This scrambling effect is achieved due to the multiple reflections that a light beam experiences while traveling through the non-circular core of the fiber optic beam shaper  170 . The degree of scrambling is controlled by defining parameters such as the shape, the length and the numerical aperture of fiber optic beam shaper  170 . 
     Fiber optic beam shaper  170  is typically formed from drawn silica. However, other materials can also be used. As will be discussed in greater detail below, fiber optic beam shaper  170  can optionally have a cladding. The cladding may be constructed to have an absorbing or depressed coating at any place along and around the cladding, in order to omit leaky or cladding modes that can cause undesired effects. 
     In the embodiment of  FIG. 2 , fiber optic beam shaper  170  works on the principle of total internal reflection, in which no power is absorbed, unlike hollow wave-guides, which use the principle of reflection and therefore absorb part of the guided light, converting it to heat. This difference is an especially important consideration when an optical path is used to carry relatively high power laser light beams. In certain embodiments, the following aspects of a fiber optic beam shaper  170  may be advantageous:
         1. Fiber optic beam shaper  170  is a non-circular silica fiber, while a hollow light pipe is usually made from silicon coated with a metal layer. A silicon with a metal layer is typically less durable than a silica fiber.   2. Fiber optic beam shaper  170  can be used in situations where it is very difficult to use a hollow light pipe. For example, when the fiber tips are angled polished or angled cleaved (this is usually done in order to prevent back reflection from the fiber tip) it is practically very difficult to align a hollow light pipe relative to the angled cleaved fiber tips. Fiber optic beam shaper  170  thus enables one to angle polish the fiber tips very easily.   3. Fiber optic beam shaper  170  can be made with any core shape. As a practical matter, hollow light pipes can be constructed just in a limited number of shapes.       

     There are a variety of commercially available non-circular and optionally depressed fibers that can be adapted for the purpose of providing fiber optic beam shaper  170 . 
     In the embodiment of  FIG. 2 , fiber optic beam shaper  170  is spliced or tapered at splicing point  180  to multimode optical fiber  130  having a circular core. In this embodiment, the splicing is provided by applying the circular core of multimode optical fiber  130  so that it circumscribes the non-circular shaped core of fiber optic beam shaper  170 . The homogenized light that enters circular core multimode optical fiber  130  by way of such a splice emerges from an end of circular core multimode optical fiber that defines in this embodiment, distal face  104  of optical path  100 . This light takes the form of a circular shaped beam of homogenized light. The circular shaped light beam of homogenized light emerging from distal face  104  is then imaged by an optical system  150  on an imaging surface  160 , such as a photosensitive or thermally sensitive imaging surface or other image-receiving surface. Optical system  150  can comprise, for example, a telecentric lens. 
     Laser source  110  may be one of an individually addressable laser diode array or one of a number of fiber coupled diodes or any other conventional source of laser light of a type that can be carried by an optical path for use in imaging. 
     In the embodiment of  FIG. 2 , a splice is described as being used to join fiber optic beam shaper  170  and multimode optical fiber  130 . In one embodiment, the splice can take the form of a taper. A splice is more reliable relative to optical connectors which are used for example by prior art U.S. Pat. No. 6,519,387. For example, a splice ensures a much lower attenuation of the optical power. Furthermore, the splice can also ensure that the numerical aperture of the radiation will not change. By providing a circular core at the distal face of multimode optical fiber  130  it is easier to align a plurality of such fibers in mechanical supports such as V-grooves of the type that are used to form imaging head  140  as illustrated in  FIG. 2 . This is due to the fact that there is no need to define the orientation of multimode optical fibers  130 , as in the case of elongated-core-shaped fiber tips. 
       FIG. 3  is a schematic representation of another embodiment of an optical path  100 . In this embodiment, fiber optic beam shaper  170  is spliced at points  180  and  181  between two multimode fibers  130  and  190  each having circular cores. In this embodiment, multimode fiber  190  optically couples fiber optic beam shaper  170  to laser source  110  and coupling optics  120  at a proximate face  102  so that optical path  100  will receive a light beam supplied by laser source  110 . Fiber optic beam shaper  170  homogenizes the supplied light beam as is described generally above. As is also described generally above, circular core multimode optical fiber  130  carries and shapes the homogenized light to the distal face  104  of optical path  100  so that homogenous light having a circular profile emerges. It will be appreciated that the use of conventional circular core multimode fibers  130  and  190  simplifies the integration of optical path  100  into an imaging head  140 , especially where such imaging head  140  uses multiple optical paths. 
       FIGS. 4A–4C  are schematic representations of the cross-sections of fiber optic beam shaper  170  and multimode optical fiber  130  of  FIG. 3  along cutting line A—A, at splicing point  180 , and along cutting line B—B, respectively. In  FIG. 4A , fiber optic beam shaper  170  is shown having a hexagonal core  200  and circular cladding  210 . In  FIG. 4C , multimode optical fiber  130  is shown having a circular core  220  and cladding  230  and in  FIG. 4B , both fibers  130  and  170  are shown spliced, so that hexagonal core  200  of fiber optic beam shaper  170  is circumscribed by a circumference  235  of circular core  220  of multimode optical fiber  130 . 
     Coupling optics  120  used in the embodiments of  FIGS. 2 and 3  may be constructed from refractive or reflective optical elements such as mirrors or lenses. Furthermore, coupling optics  120  may be formed to be part of fiber optic beam shaper  170  or multimode fiber  190 . This can be done by shaping the proximal tip of fiber optic beam shaper  170  or multimode fiber  190 , for example, by forming a chisel shaped fiber tip. 
       FIG. 5  is a schematic representation of the cross section and refractive index profile of a fiber which has a hexagonal core and two cladding layers. Such a structure may advantageously be used as a fiber optic beam shaper  170  of the present invention. As illustrated in  FIG. 5 , this embodiment of fiber optic beam shaper  170  has a hexagonal non-circular core  240 , a first cladding  250  and an outer cladding  260 . In  FIG. 5 , n1 through n3 represent the indices of refraction for, respectively, core  240 , first cladding  250 , and outer cladding  260 . In this embodiment, n1 is the highest index of refraction and n2 is the lowest. Outer cladding  260  may be produced of transparent material, such as glass, in which embodiment it will serve as a depressed cladding. Alternatively, outer cladding  260  may be produced of a light absorbing material. Both embodiments ensure that only the non-circular core  240  will guide the radiation. 
     The technology of constructing fibers with non-circular cores and fibers with one or more layer of clad is well mastered by many companies around the world, for example by Nufern (www.nufern.com). These companies, for example, draw fibers with non-circular cross sections in order to pump fiber lasers. It will be appreciated that generally speaking, the number of modes in a light beam carried by an optical fiber is dependent, amongst others, on the size of the core. The larger the core size, the more modes carried by the fiber. This means that a light beam in a fiber having a large core size is typically more homogeneous and steeper than a similar light beam in an optical fiber having a smaller core size.  FIGS. 6–9  illustrate qualitative measurements of optical fibers that have different core diameters and numerical apertures and therefore do not represent the real case but rather present illustrative representations of the extent of homogenization of a light beam in general fibers with different shapes of cores. A real and a fair comparison which shows just the scrambling effect due to the core shape, can be obtained by measuring the beam profile for a light beam carried by optical fibers that have identical core dimensions but different core shapes, as described for example in  FIGS. 4A ,  4 B and  4 C. With this understanding,  FIGS. 6–9  will now be described. 
       FIG. 6  illustrates one qualitative example of a beam profile of a light beam emerging from a distal face of a circular core multimode fiber  290 . As can be seen from  FIG. 6 , there are a number of localized high intensity areas of the beam, e.g. areas  300 ,  302 ,  304 ,  306 ,  308 ,  310 ,  312 , while there are also regions  314  and  316  that have relatively low beam intensity. Such an uneven distribution of intensity can cause unintended effects when the light beam is put to uses such as in printing. It will be appreciated that such a distribution is highly unpredictable in that, as noted above, the distribution can be time variant and can also vary as a function of changes in the lay of the prior art multimedia fiber. 
       FIG. 7  illustrates a qualitative example beam profile of a light beam that has passed through a rectangular core multimode optical fiber  292 . As can be seen from the distribution of light and dark areas representing different light intensities, the beam profile is substantially more homogeneous. However, as is also illustrated in this example, defects in a fiber used to form rectangular core multimode optical fiber  292  can create minor variations  320 ,  322 , in the distribution of light that can be avoided, for example, through careful selection and testing of rectangular core multimode optical fiber  292 . 
       FIG. 8  illustrates another qualitative example of a beam profile of light provided by a conventional circular core multimode fiber  294 . 
       FIG. 9  illustrates another qualitative example of a beam profile of a light beam as emitted by a hexagonal core multimode optical fiber  296 . As shown by the more even distribution of light and dark areas in  FIG. 9 , this light is steeper and more homogeneous than the light supplied by a conventional circular core multimode fiber. 
     It will be appreciated that the qualitative examples shown in  FIGS. 6–9 , are for the purpose of example only and are not limiting. 
     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 
     
         
           10  laser source 
           20  coupling optics 
           30  multimode optical fiber 
           40  support 
           50  telecentric lens 
           60  imaging surface 
           100  optical path 
           102  proximate face 
           104  distal face 
           106  beam shaper multimode optical path 
           108  circular core multimode optical path 
           110  laser source 
           120  coupling optics 
           130  multimode optical fiber 
           140  imaging head 
           150  optical system 
           160  imaging surface 
           170  fiber optic beam shaper 
           180  splicing point 
           181  splicing point 
           190  multimode fibers 
           200  hexagonal core 
           210  cladding 
           220  circular core 
           230  cladding 
           235  circumference 
           240  non-circular core 
           250  first cladding 
           260  outer cladding 
           290  circular core multimode optical fiber 
           292  rectangular core multimode optical fiber 
           294  circular core multimode optical fiber 
           296  hexagonal core multimode optical fiber 
           300  high intensity area 
           302  high intensity area 
           304  high intensity area 
           306  high intensity area 
           308  high intensity area 
           310  high intensity area 
           312  high intensity area 
           314  low intensity area 
           316  low intensity area 
           320  variations 
           322  variations